BACKGROUND OF THE INVENTION Pressure devices, such as clamps, vises, separators, or jacks, are well known in the art.

Typically, such devices include a stationary member and a movable member which is slideably mounted on the stationary member. The two members are generally connected by a long bolt which is rotatably mounted on the movable member and engages a threading in the stationary member, or vice versa. This configuration allows continuous adjustment of the position of the movable member relative to the stationary member. The grip-size of a clamp or vise or the height of a jack is adjusted to the size of the gripped/supported object by rotation of the bolt in the threading to provide relative linear advance therebetween. The bolt is typically rotated by a handle connected thereto.

To provide the clamp, vise, separator, or jack with sufficient grip and/or support pressure capability and to ensure"locked", i. e. non-slip, grip or support, the threading on the bolt generally has a very small helix angle, typically on the order of 8.5 degrees or less. Such a small helix angle results in a very low linear advance rate of the bolt, typically on the order of 4-5 millimeters per rotation of the handle (in medium size vises). Due to the low linear advance rate, a large number of handle rotations are typically required in adjusting the pressure device to fit the dimensions of various objects. Thus, the process of adapting the pressure device to a given object is often very tedious and time consuming.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a screw-driven mechanism having a dual advance rate which is automatically shifted, in response to changes of pressure, from a first, high advance rate, mode of operation to a second, low advance rate, mode of operation and vice versa. In some aspects of the present invention, the mechanism is used to drive a pressure device for gripping or supporting an object. Such devices include, but are not limited to, clamps, jacks, separators and vises.

In the first mode of operation, the device is quickly adjustable to given object dimensions, thus saving valuable operating time. In the second mode of operation, the device provides firm, self-locking, high pressure grip/support of the given object. The provision of two, substantially independent, modes of operation enables optimization of predetermined parameters in each mode of operation.

In preferred embodiments of the present invention, the mechanism includes a thread lock, which engages a screw thread in the mechanism and causes the mechanism to switch automatically from the first to the second mode in response to force exerted on or by the pressure device. In a first group of preferred embodiments of the present invention, a tilting element provides the thread locking for automatically shifting between the modes of operation. In a second group of preferred embodiments of the present invention, a sliding mechanism provides the thread locking. Other types of thread locking elements may also be used, including, for example, an electrically-actuated element, which locks the thread responsive to a signal from a pressure or force sensor.

In preferred embodiments of the invention, the pressure device includes first and second elements, having respective first and second jaws, which are interconnected by first and second threaded members, which are also referred to herein as first and second screws.

The jaws may comprise, for example, the engaging elements of a vise or of a jack. The first threaded member is rotatably mounted on the first element and has a first threaded portion which engages a first threading of the second threaded member. A second threading of the second threaded member engages a second threaded portion which is mounted on the second element. The helix angle of the second threading matches the helix angle of the second threaded portion and is larger than the helix angle of the first threading which matches the helix angle of the first threaded portion. Thus, relative motion between the first threaded portion and the first threading provides a lower linear advance rate than relative motion between the second threading and the second threaded portion. Preferably, the first threading

of the second member is an internal threading which matches an external threading on the first threaded portion. The second threading is preferably an external threading which matches an internal threading on the second threaded portion.

When the force between the first and second jaws is below a predetermined threshold, the second member rotates relative to the second threaded portion, while there is no relative rotation between the first and second members which rotate together. This implements the first mode of operation, whereby the relatively large helix angle of the second threading provides the high advance rate of the first element relative to the second element. When the pressure between the first and second jaws exceeds the predetermined threshold, there is no relative rotation between the second member and the second threaded portion, i. e. the second member is locked with the second threaded portion, while the first member rotates relative to the second member. This implements the second mode of operation, whereby the relatively small helix angle of the first threading provides the low advance rate of the first element relative to the second element.

In preferred embodiments of the present invention, the first and second threaded members have a shared thread axis. Further, in a preferred embodiment of the present invention which is in the first group described above, the second threaded portion is tiltable, in response to a predetermined change in the jaw pressure or force, about an axis generally perpendicular to the thread axis. In a preferred embodiment of the present invention which is in the second group described above, the second threaded portion is slideable, in response to a predetermined change in the jaw pressure or force, along an axis generally perpendicular to the thread axis. Alternatively, the second threaded portion is slideable, in response to a predetermined change in the jaw pressure, along the thread axis.

Thus, in these preferred embodiments, in a first, unlocked, position of the mechanism, the thread axis of the second threaded portion coincides with the thread axis of the second member, and the second member advances through the second portion at the high linear advance rate. An urging means, such as a spring, preferably applies a predetermined counteracting force which maintains the second threaded portion in the unlocked position.

Alternatively, in a preferred embodiment, the counteracting force results from the weight of the second threaded portion. When the second threaded portion is in the unlocked position, the first threaded portion does not rotate relative to the first threading due to a predetermined mechanical resistance applied therebetween, for example, by friction or by the force of a spring.

In preferred embodiments which are in the first group above, when the jaw pressure exceeds a predetermined threshold, the second threaded portion experiences a tilting force which overpowers the counteracting force and tilts the second threaded portion to a second, locked position. In the second-position, the thread axis of the second threaded portion is tilted with respect to the thread axis of the second member and, thus, movement of the second threaded portion relative to the second threaded member is locked. When additional rotational force is applied to the first member, to overcome the predetermined mechanical resistance, the first threaded portion advances through the first threading of the second member at the low linear advance rate.

In preferred embodiments which are in the second group above, when the jaw pressure exceeds a predetermined threshold, the second threaded portion experiences a sliding force which overcomes the counteracting force and slides the second threaded portion to a second, locked position. In the second position, the thread axis of the second threaded portion is offset with respect to the thread axis of the second member; alternatively, the second threaded portion slides along the thread axis. Thus, movement of the second threaded portion relative to the second threaded member is locked. When additional rotational force is applied to the first member, to overcome the predetermined mechanical resistance, the first threaded portion advances through the first threading of the second member at the low linear advance rate.

In a preferred embodiment of the invention, the second element is associated with a stationary part of the pressure device, for example the base of a clamp or the base of a jack.

Further, in this preferred embodiment of the invention, the first element is associated with a movable part of the device, such as the moving part of the clamp or the moving part of the jack.

There is therefore provided, in accordance with a preferred embodiment of the present invention, a dual advance rate pressure device including: a first element having a first jaw; a second element having a second jaw ; a first member having a first threaded portion engaging a first threading; a second member having a second threading; a second threaded portion, engaging said second threading and mounted on said second element so as to be movable, in response to a predetermined force acting between said first and second jaws, from a first, unlocked, position, in which the thread axis of said second threaded portion substantially coincides with the thread axis of said second threading,

to a second, locked, position in which the thread axis of said second threaded portion is shifted with respect to the thread axis of the second threading; and a mechanical resister which prevents relative rotation between said first member and said second member when said second threaded portion is in said first position.

There is further provided, in accordance with a preferred embodiment of the present invention, a dual advance rate vise device including: a first element having a first jaw; a second element having a second jaw; a first member rotatably mounted on said first element and having a first threaded portion; a second member having a first threading engaging said first threaded portion and a second threading; a second threaded portion, engaging said second threading and mounted on said second element so as to be movable, in response to a predetermined force acting between said first and second jaws, from a first, unlocked, position, in which the thread axis of said second threaded portion substantially coincides with the thread axis of said second threading, to a second, locked, position in which the thread axis of said second threaded portion is shifted with respect to the thread axis of the second threading preventing relative motion between-said second threading and said second threaded portion; and a mechanical resister which prevents relative rotation between said first threaded portion and said first threading when said second threaded portion is in said first position.

There is also provided, in accordance with a preferred embodiment of the present invention, a dual advance rate separator device including: a first element having a jaw; a second element having a second jaw; a first member having a first threaded portion rotatably engaging a first threading associated with said first element; a second member rotatably mounted on the first member and having a second threading; a second threaded portion, engaging said second threading and mounted on said second element so as to be movable, in response to a predetermined force acting between said first and second jaws, from a first, unlocked, position, in which the thread axis of said

second threaded portion substantially coincides with the thread axis of said second threading, to a second, locked, position in which the thread axis of said second threaded portion is shifted with respect to the thread axis of the second threading preventing relative motion between said second threading and said second threaded portion; and a mechanical resister which prevents relative rotation between said first member and said second member when said second threaded portion is in said first position.

Preferably, the separator device includes a jack.

Preferably, the above devices include a counteracting device which urges said second threaded portion to said first position.

Alternatively, the counteracting device includes at least one spring.

Preferably, the advance rate between said first and second jaws in said first unlocked position is controlled at least by the linear advance rate of said second threading, and preferably the advance rate between said first and second jaws in said second locked position is controlled by the linear advance rate of said first threading.

Alternatively, the second threaded portion is tiltably mounted on the second element, and tilts in response to the predetermined force.

Preferably, the second threaded portion is slideably mounted on the second element, and slides in response to the predetermined force.

Alternatively, the second threaded portion includes a locking tooth, and the second threading includes a plurality of grooves, said tooth and grooves mutually engaging in response to the predetermined force.

Preferably, the mechanical resister includes a spring acting generally axially between the first member and the second member.

Alternatively, the mechanical resister includes a spring acting generally radially between the first member and the second member.

Further alternatively, the mechanical resister includes a spring acting generally torsionally between the first member and the second member.

Alternatively or additionally, the mechanical resister includes a clutch, which releasably links the first member and the second member.

There is further provided, in accordance with a preferred embodiment of the present invention, a screw-operated linear advance mechanism including: first and second screws having respective first and second coarse and fine threads for advancing the mechanism at respective high and low rates; and

a thread lock, which locks the first screw responsive to a force on the mechanism, whereby the mechanism is advanced by the second screw.

Preferably, the thread lock locks the first screw when the force on the mechanism exceeds a predetermined threshold.

Alternatively, the mechanism includes a mechanical resister which locks the second screw when the force on the mechanism is below the threshold, whereby the mechanism advances by the first screw.

Preferably, the thread lock tilts against the first screw in response to the force on the mechanism.

Alternatively, the thread lock slides against the first screw in response to the force on the mechanism.

Preferably, the thread lock includes a tooth, and the first screw includes a plurality of grooves, whereby the tooth and grooves engage in response to the force on the mechanism.

Alternatively, the thread lock includes an anti-tilting support, which limits tilting of the second screw.

Preferably, the first screw has a central cavity, in which the second screw is received.

Alternatively, at least a portion of the central cavity of the first screw is threaded so as to engage the second screw.

Further alternatively, the mechanism includes first and second jaws, which are linearly advanced relative to one another by rotation of the first and second screws.

Preferably, the jaws engage at least one external object, through which the force on the mechanism is exerted.

Alternatively, the first jaw is coupled to the first screw, and the second jaw is coupled to the thread lock.

Preferably, the first jaw is coupled to the first screw by a spring.

Further preferably, the jaws grip the at least one external object.

Alternatively, the jaws separate the at least one external object from another external object.

There is further provided, in accordance with a preferred embodiment of the present invention, a method for advancing a mechanism at a variable rate, including: providing first and second screws having respective first and second coarse and fine threads for advancing the mechanism at respective high and low rates; and

locking the first screw in response to a force on the mechanism, whereby the mechanism is advanced by the second screw.

Alternatively, locking the first screw includes locking the screw when the force on the mechanism exceeds a predetermined threshold.

Preferably, the method includes locking the second screw when the force is below the threshold, whereby the mechanism is advanced by the first screw.

Preferably, locking the second screw includes urging the first screw against the second screw by means of a spring.

Further preferably, locking the first screw includes shifting a thread axis of a threaded element that receives the first screw.

Alternatively, shifting the thread axis includes tilting the threaded element relative to the axis of the first screw.

Further alternatively, shifting the thread axis includes moving the threaded element such that its axis remains parallel to the axis of the first screw.

BRIEF DESCRIPTION OF THE DRAWINGS The present invention will be better understood from the following detailed description of preferred embodiments of the invention, taken in conjunction with the following drawings in which: Figs. 1A and 1B are schematic, side view, partly sectional, illustrations of two, respective, modes of operation of a dual advance rate vise in accordance with one preferred embodiment of the present invention; Fig. 2A is a schematic, front view, cross-sectional, illustration of a portion of the vise of Figs. 1A and 1B, taken across section lines II-II, showing a vise bolt support in accordance with one preferred embodiment of the present invention; Fig. 2B is a schematic, front view, cross-sectional, illustration of a portion of a vise using a vise bolt support in accordance with an alternative, preferred, embodiment of the present invention; Figs. 3A and 3B are schematic, side view, partly sectional, illustrations of two, respective, modes of operation of a dual advance rate vise in accordance with another preferred embodiment of the present invention; Figs. 4A and 4B are schematic, side view, partly sectional, illustrations of two, respective, modes of operation of a dual advance rate vise in accordance with yet another preferred embodiment of the present invention; Figs. 5A and 5B are schematic, side view, partly sectional, illustrations of two, respective, modes of operation of a dual advance rate vise in accordance with still another preferred embodiment of the present invention; Fig. 5C is a schematic, front view, cross-sectional, illustration of a portion of the vise of Figs. 5A and 5B, taken across section lines V-V; Figs. 6A and 6B are schematic, side view, partly sectional, illustrations of two, respective, modes of operation of a dual advance rate vise in accordance with a further, preferred, embodiment of the present invention; Fig. 6C is a schematic, front view, cross-sectional, illustration of a portion of the vise of Figs. 6A and 6B, taken across section lines VI-VI; Figs. 7A and 7B are schematic, side view, partly sectional, illustrations of two, respective, modes of operation of a dual advance rate vise in accordance with an additional, preferred, embodiment of the present invention;

Figs. 8A and 8B are schematic, side view, partly sectional, illustrations of two, respective, modes of operation of a dual advance rate vise in accordance with another further preferred embodiment of the present invention; Fig. 8C is a schematic, front view, cross-sectional, illustration of a portion of the vise of Figs. 8A and 8B, taken across section lines VIII-VIII ; Figs. 9A and 9B are schematic, side view, partly sectional, illustrations of two, respective, modes of operation of a dual advance rate jack, constructed and operative in accordance with a preferred embodiment of the present invention; Figs. 10A and 10B are schematic, side view, partly sectional, illustrations of two, respective, modes of operation of a dual advance rate jack, constructed and operative in accordance with another preferred embodiment of the present invention; Figs. 11A and 11B are schematic, side view,, partly sectional, illustrations of two, respective, modes of operation of a dual advance rate vise in accordance with another preferred embodiment of the present invention; Fig. 11C is a schematic, cross-sectional illustration, of a portion of the vise of Figs.

11 A and 11 B, taken across section lines XI-XI, showing a housing in accordance with a preferred embodiment of the present invention; Fig. 12A is a schematic, front view, cross-sectional, illustration of a portion of the vise of Figs. 11A and 11B, taken across section lines XII-XII, showing a vise bolt support in accordance with one preferred embodiment of the present invention; Fig. 12B is a schematic, front view, cross-sectional, illustration of a portion of a vise using a vise bolt support in accordance with an alternative, preferred, embodiment of the present invention; and Figs. 13A and 13B are schematic, partly sectional illustrations of two, respective, modes of operation of a dual advance rate jack, constructed and operative in accordance with a preferred embodiment of the present invention.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT Reference is now made to Figs. 1A and 1B which schematically illustrate a dual advance rate vise 10 in accordance with one, preferred, embodiment of the present invention.

Fig. 1A illustrates vise 10 in a first, high advance rate, mode of operation thereof, while Fig.

1B illustrates vise 10 in a second, low advance rate, mode of operation thereof. Vise 10 includes a stationary base element 12, having a first grip jaw 14, and a movable element 16 having a second grip jaw 18, as in conventional vises. As shown in Fig. 1B, jaws 14 and 18 are designed for gripping and/or compressing an object 20 placed therebetween. As known in the art, when object 20 is forcefully gripped by jaws 14 and 18, the gripping force acting between the jaws and the gripped object urges elements 12 and 16 away from each other. In accordance with a preferred embodiment of the present invention, as described in detail below, this reaction of vise 10 to the gripping of object 20 is utilized by the vise in the transition from the first mode of operation of Fig. 1 A to the second mode of operation of Fig.

1B.

In accordance with a preferred embodiment of the present invention, vise 10 includes a first threaded member 22, having a first unthreaded portion 42, a second unthreaded portion 31, and a first threaded portion 24. Portion 42 of member 22 is rotatably mounted in an opening 41 of element 16 and secured by a widened appendage 43 on the external side of element 16. A handle 44 which extends perpendicularly through appendage 43, as in conventional vises, allows convenient, forceful, rotation of member 22.

Threaded portion 24 of member 22 preferably has an external threading which engages an inner threading 26 of a second threaded member 27. Backward motion of member 22 relative to threading 26, i. e. from left to right in Figs. 1A and 1B, is limited by a stopper pin 38 which extends through portion 24 of member 22. An outer threading 28 of threaded member 27 engages a second threaded portion 30 which is mounted to stationary element 12.

Portion 30 is preferably tiltably mounted on an axle 46 which allows tilting of portion 30 from the untilted position of Fig. 1A to the tilted position of Fig. 1B and vice versa. In the untilted position of Fig. 1A, portion 30 is preferably supported by a spacer 48 which is fixedly mounted on stationary element 12. Spacer 48 provides smooth motion of second member 27 relative to threaded portion 30, particularly during reverse motion of member 27 as described below. Second threaded member 27 is preferably urged away from element 16 using a spring 34, which resiliently supports member 27 against threaded portion 30 in analogy to the resilient support provided to the advance bolts of conventional vises.

In the absence of gripping forces, a counteracting device, most preferably a counter- tilting spring 40, maintains portion 30 at the position shown in Fig. 1A, in which a thread axis 33 of the inner threading on portion 30 substantially coincides with a thread axis 29 of external threading 28 of member 27. The untilted position of portion 30 shown in Fig. 1A corresponds to a non-grip mode of operation in which second threaded member 27 rotates with respect to threaded portion 30, advancing linearly therethrough. In this mode of operation, the rate of change of the position of jaw 18 relative to jaw 14 is controlled by the helix angle of threading 28, which is a relatively large helix angle, for example, on the order of 20 degrees. This allows quick adjustment of jaws 14 and 18 to the dimensions of given objects to be gripped by vise 10.

Rotation of first threaded portion 24 relative to inner threading 26 of member 27 is preferably prevented, in the non-grip mode of operation, by a pressure spring 32 which is mounted, preferably slightly compressed, between member 22 and member 27. Spring 32, which is preferably a very powerful spring, exerts a predetermined axial force between member 22 and member 27, controlling the static friction force acting between threading 26 and threaded portion 24. Spring 32 acts as a mechanical resister, to prevent relative rotation between portion 24 and threading 26. Thus, in the non-grip mode of operation of Fig. 1A, the advance rate of movable element 16 relative to stationary element 12 is controlled by the helix angle of threading 28. For example, if diameter of second member 27 is approximately 31 millimeters, a helix angle of approximately 20 degrees on threading 28 will provides a linear advance rate of approximately 32 millimeters per rotation of handle 44.

When object 20 is gripped between jaws 14 and 18, as shown in Fig. 1B, the reaction force exerted by object 20 urges elements 16, together with threaded members 22 and 27, away from element 12 which accommodates threaded portion 30. When the grip force exerted between jaws 14 and 18 exceeds a predetermined threshold, overcoming the counter-tilting force exerted by spring 40, threaded portion 30 is tilted to the position of Fig. 1B. At this position, thread axis 33 is tilted with respect to thread axis 29. It should be appreciated that this tilt of the thread axis results in locking engagement between threading 28 and portion 30 due to the friction force acting therebetween. Tilting of threaded member 27 together with portion 30 is preferably prevented, even when the grip force is extremely large, by a support member 36 which is fixedly mounted on a base portion of member 12 underneath threaded member 27. Thus, relative motion between threading 28 and portion 30 is prevented as long as the grip force acting on jaws 14 and 18 remains above the predetermined threshold. It

should be noted that friction between support member 36 and second threading 28 contributes to the locking of member 27. It should be further noted that member 36 takes part of the load involved in the tilting of portion 30, thereby reducing the load on other elements of the vise such as threaded portion 24 and threading 26.

In a preferred embodiment of the invention, the threshold for locking of portion 30 to threading 28 is slightly lower than the threshold force required to overcome the mechanical resistance between first threaded portion 24 and threading 26. Thus, once portion 30 is tilted to the locked position of Fig. 1B, a slight additional force is required in order to commence rotation of portion 24 within threading 26. When additional gripping force is applied to object 20 by jaws 14 and 18, in response to rotation of handle 44, threaded member 22 advances forward in threaded member 27, i. e. from right to left in Fig. 1B, until a desired grip pressure is reached. In this mode of operation, hereinafter referred to as the grip mode, the advance rate of jaw 18 towards jaw 14 is controlled by the helix angle of threading 26. For example, if the diameter of threaded member 24 is approximately 15 millimeters, a helix angle of approximately 8.5 degrees on threading 26 will provide a linear advance rate of approximately 6 millimeters per rotation of handle 44.

Prior to the forward advance of member 22 in threading 26, only slight friction forces are exerted by spring 34 on element 16 and member 27. However, as member 22 advances with respect to member 27, spring 34 is compressed and, thus, the friction forces exerted by the spring on element 16 and member 27 increase rapidly. To prevent forward advance of member 27 in portion 30, the mechanical resistance between member 27 and element 16 does not overcome the locking engagement between member 27 and portion 30.

In a preferred embodiment of the invention, threaded portion 24 is designed to be sufficiently long so as to account for the advance of member 22 in portion 30 during pressure- gripping or compression of object 20. It should be appreciated that if vise 10 is used for compressing objects at a high compression ratio, the length of portion 24 can be extended accordingly to any suitable length.

When the grip on object 20 is released, by rotating handle 44 in a counter-grip direction, the sequence of actions described above is reversed. First, when threading 28 is still locked by second threaded portion 30, threaded portion 24 of member 22 moves in a reverse direction in threading 26, i. e. from left to right in Fig. 1B. This reverse motion, at the low linear advance rate, continues until stopper pin 38 comes into contact with member 27. At this point, reverse motion of member 22 is stopped and additional counter-grip rotation of handle

44 releases the locking engagement between portion 30 and threading 28 and enables counter- tilting of portion 30, by counter-tilting spring 40, back to the position of Fig. 1A. Once portion 30 returns to the untilted position of Fig. 1A, threading 28 of member 27 moves in a reverse direction in portion-30, at the high linear advance rate, and jaw 18 can be quickly adjusted to a new position vis-a-vis jaw 14.

In a preferred embodiment of the invention, the mechanical resistance exerted by spring 34, between element 16 and member 27, prevents reverse motion of member 27 relative to portion 30, after being released therefrom, thereby to ensure reverse motion of member 22 in threading 26 until pin 38 reaches member 27.

Reference is now made to Fig. 2A which is a schematic, front view, cross-sectional, illustration of a portion of vise 10, taken across section lines II-II in Fig. 1A. Fig. 2A shows, inter alia, the cross-section of support member 36. Reference is also made to Fig. 2B which schematically illustrates a cross-section of a support member 37 in accordance with an alternative, preferred, embodiment of the present invention, wherein member 37 is intended to take the place of member 36 in Figs. 1A and 1B. The bottom, anti-tilting, support provided to threaded member 27 by support member 36 of Fig. 2A is generally sufficient for most purposes of the present invention. Nevertheless, for some preferred embodiments of the present invention, particularly when threading 28 has a very large helix angle, a more embracing support such as support 37 of Fig. 2B may be required, to prevent self release of threading 28 from portion 30.

Reference is now made to Figs. 3A and 3B which are schematic, side view, partly sectional, illustrations of two, respective, modes of operation of a dual advance rate vise 50, in accordance with another preferred embodiment of the present invention. Fig. 3A illustrates vise 50 in a first, high advance rate, mode of operation thereof, while Fig. 3B illustrates vise 50 in a second, low advance rate, mode of operation thereof. Apart from the differences described below, the operation of vise 50 is generally similar to that of vise 10 (Figs. 1A and 1B), whereby elements indicated by the same reference numerals in both vises 50 and 10 are generally identical in construction and in operation.

In place of first member 22 of Figs. 1A and 1B, vise 50 includes a first threaded member 72, having an unthreaded portion 92 and a first threaded portion 84. Portion 92 of member 72 is rotatably mounted in an opening 91 of element 16 and secured by widened appendage 43 on the external side of element 16, as described with reference to Figs. 1A and 1B. In the embodiment of Figs. 3A and 3B, relative motion between first threaded portion 84

and inner threading 26 is mechanically resisted by a twist-spring 82 which twistably connects between first member 72 and second member 27. One end of spring 82 is fixedly connected to member 72, at point 81, while the other end of spring 82 is fixedly connected to second threaded member 27, at point 83. A predetermined counter-twisting torsional force imposed by spring 82 provides a mechanical resistance substantially equivalent to that imposed by pressure spring 32 in the embodiment of Figs. 1A and 1B.

It will be appreciated that when member 72 advances forward relative to member 27, during gripping and/or compression of object 20, the counter-twisting torsional force exerted by spring 82 increases rapidly. When handle 44 is rotated in a counter-grip direction to release object 20, the increased counter-twisting force ensures continued reverse motion of member 22 in threading 26 until pin 38 reaches member 27. Thus, object 20 is released only after pin 38 is in contact with member 27.

In place of support spring 34 of Figs. 1A and 1B, vise 50 includes a slightly compressed support spring 80 which is placed between element 16 and a separator disc 94. A stopper pin 95 prevents separator disc 94 from moving along portion 92 of member 72 towards member 27. Other aspects of vise 50, such as the operation of second threaded portion 30, are generally as described above with reference to Figs. 1A, 1B, 2A and 2B.

Reference is now made to Figs. 4A and 4B which are schematic, side view, partly sectional, illustrations of two, respective, modes of operation of a dual advance rate vise 100 in accordance with another preferred embodiment of the present invention. Fig. 4A illustrates vise 100 in a first, high advance rate, mode of operation thereof, while Fig. 4B illustrates vise 100 in a second, low advance rate, mode of operation thereof. Apart from the differences described below, the operation of vise 100 is generally similar to that of vise 10 (Figs. 1A and 1B), whereby elements indicated by the same reference numerals in both vises 100 and 10 are generally identical in construction and in operation.

In place of first member 22 of Figs. 1A and 1B, vise 100 includes a first threaded member 122, having an unthreaded portion 142 and a first threaded portion 124. Portion 142 of member 122 is rotatably mounted in an opening 141 of element 16 and secured by widened appendage 43 on the external side of element 16, as described with reference to Figs. 1A and 1B. In the embodiment of Figs. 4A and 4B, relative motion between first threaded portion 124 and inner threading 26 is mechanically resisted by a pressure spring 132, similar to spring 32 of Figs. 1A and 1B, which is slightly compressed between second member 27 and a bearing 135 on portion 142 of first member 122. The provision of bearing 135, which is preferably a

pressure-resistant ball bearing, prevents undesired mechanical resistance between member 27 and member 142 at a stopper pin 145 therein, thereby allowing easier rotation of portion 124 in threading 26. One end of spring 132 is in contact with bearing 135, which may be a ball bearing, while the other end of spring 132 is in contact with an inner portion of member 27.

The static friction force imposed between threading 26 and first threaded portion 124 provides the necessary mechanical resistance between members 122 and 27, as described above with reference to the embodiment of Figs. 1A and 1B.

In place of support spring 34 of Figs. 1A and 1B, vise 100 includes a slightly compressed support spring 134 which is placed between element 16 and a separator disc 144.

Stopper pin 145 prevents separator disc 144 from moving along portion 142 of member 122 towards member 27. Other aspects of vise 100, such as the operation of second threaded portion 30, are generally as described above with reference to Figs. 1A, 1B, 2A and 2B.

Reference is now made to Figs. SA and SB, which are schematic, side view, partly sectional, illustrations of two, respective, modes of operation of a dual advance rate vise 150 in accordance with another preferred embodiment of the present invention. Fig. 5A illustrates vise 150 in a first, high advance rate, mode of operation thereof, while Fig. 5B illustrates vise 150 in a second, low advance rate, mode of operation thereof. Reference is also made to Fig.

5C which is a front view, cross-sectional, illustration of a portion of the vise of Figs. SA and SB, taken across section lines V-V in Fig. SA. Apart from the differences described below, the operation of vise 150 is generally similar to that of vise 50 (Figs. 3A and 3B), whereby elements indicated by the same reference numerals in both vises 50 and 150 are generally identical in construction and in operation.

In place of second threaded portion 30 of Figs. 3A and 3B, vise 150 includes a second threaded portion 180 which engages threading 28 only along a portion of the circumference of second threaded member 27, e. g. only from the bottom, as shown particularly in Fig. 5C.

Portion 180 is preferably tiltably mounted on an axle 196 which allows tilting of portion 180 from the untilted position of Fig. SA to the tilted position of Fig. 5B and vice versa. In the untilted position of Fig. SA, portion 180 is preferably supported by a spacer 182 which is fixedly mounted on stationary element 12. The remaining circumference of threaded member 27, not engaged by portion 180, is preferably supported by an unthreaded brace 160 which prevents motion of member 72 perpendicular to its thread axis. In the locked position of Fig.

5B, member 27 is pressed between portion 180 and brace 160 which assists in the locking of member 27. The operation of portion 180, e. g. locking of member 27 to produce the low

advance rate mode of operation, is generally the same as described above with reference to second portion 30. Other aspects of vise 150 are generally as described above with reference to Figs. 1A-3B.

Reference is now made to Figs. 6A and 6B which are schematic, side view, partly cross-sectional, illustrations of two, respective, modes of operation of a dual advance rate vise 200 in accordance with another preferred embodiment of the present invention. Fig. 6A illustrates vise 200 in a first, high advance rate, mode of operation thereof, while Fig. 6B illustrates vise 200 in a second, low advance rate, mode of operation thereof. Reference is also made to Fig. 6C which is a front view, cross-sectional, illustration of a portion of the vise of Figs. 6A and 6B, taken across section lines VI-VI in Fig. 6B. Apart from the differences described below, the operation of vise 200 is generally similar to that of vise 150 (Figs. 5A- SC), whereby elements indicated by the same reference numerals in both vises 150 and 200 are generally identical in construction and operation.

In analogy to threaded portion 180 of Figs. 5A-5C, vise 200 includes a second threaded portion 230 having a threaded region 231 which engages threading-28 along a portion of the circumference thereof, as shown particularly in Figs. 6C. However, in contrast to portion 180, portion 230 further includes a locking tooth 233, shown particularly in Figs.

6A and 6B, adapted to engage one of a plurality of locking grooves 240 which are formed in a second threaded member 227. Second threaded member 227, which is generally similar to member 27 of Figs. 1A-5B, includes an inner threading 226, engaging first threaded portion 84, and an outer threading 228 engaging region 231 of portion 230. Portion 230 is preferably tiltably mounted on an axle 246 which allows tilting of portion 230 from the untilted position of Fig. 6A to the tilted position of Fig. 6B and vice versa. In the untilted position of Fig. 6A, portion 180 is preferably supported by a spacer 232 which is fixedly mounted on stationary element 12. The remaining circumference of threaded member 227, not engaged by portion 230, is preferably supported by an unthreaded brace 210 which prevents motion of member 72 perpendicular to its thread axis.

In the untilted position of Fig. 6A, locking tooth 233 does not engage grooves 240 and, thus, member 227 is free to rotate on threaded region 231. When the grip force exerted on jaws 14 and 18 exceeds a predetermined threshold, as described above with reference to Figs. 1A and 1B, portion 230 is tilted from the position of Fig. 6A to the position of Fig. 6B.

This tilting pushes locking tooth 233 into one of grooves 240, providing locking engagement between portion 230 and member 227. Once member 227 is locked by tooth 233, vise 200 is

switched from the high advance rate mode to the low advance rate mode, as described in detail above with reference to Figs. 1A-5B. Fig. 6C clearly illustrates the locked position of vise 200, in which tooth 233 is accommodated by one of grooves 240. Other aspects of vise 200 are generally as described above with reference to Figs. lA-SB.

Reference is now made to Figs. 7A and 7B which are schematic, side view, partly sectional, illustrations of two, respective, modes of operation of a dual advance rate vise 250 in accordance with another preferred embodiment of the present invention. Fig. 7A illustrates vise 250 in a first, high advance rate, mode of operation thereof, while Fig. 7B illustrates vise 250 in a second, low advance rate, mode of operation thereof. Apart from the differences described below, the operation of vise 250 is generally similar to that of vise 10 (Figs. 1A and 1B), whereby elements indicated by the same reference numerals in both vises 250 and 10 are generally identical in construction and operation.

In place of first member 22 of Figs. 1A and 1B, vise 100 includes a first threaded member 272 having a slightly widened mounting portion 292, an unthreaded portion 281 and a first threaded portion 274. The difference in diameter between portions 292 and 281 defines a preferably slanted, circumferential step 285. Portion 292 of member 272 is rotatably mounted in an opening 291 of element 16 and secured by widened appendage 43 on the external side of element 16, as described with reference to Figs. 1A and 1B. In the embodiment of Figs. 7A and 7B, relative motion between first threaded portion 274 and inner threading 26 is mechanically resisted by a pressure spring 282, preferably more resilient than spring 32 of Figs. 1A and 1B, which is wrapped around member 272 between second member 27 and step 285 of mounting portion 292. In the non-grip mode of operation shown in Fig, 7A, one end of spring 282 is in contact with step 285 while the other end of spring 282 is in contact with an inner portion of member 27. The static friction force imposed between threading 26 and first threaded portion 274 provides the necessary mechanical resistance between member 272 and member 27, as described above with reference to the embodiment of Figs. lAand 1B.

When the grip force exerted on jaws 14 and 18 exceeds a predetermined threshold, as described above with reference to Figs. 1A and 1B, portion 30 is tilted from the unlocked position of Fig. 7A to the locked position of Fig. 7B. Once member 27 is locked by portion 30, vise 200 is switched from the high advance rate mode of operation to the low advance rate mode of operation. Continued rotation of handle 44 advances threaded member 272 linearly into threaded member 27. This linear advance of member 272 results in radial

expansion of spring 282 to the position shown in Fig. 7B, due to compression of the spring.

At the expanded position of Fig. 7B, spring 282 is preferably no longer in urging contact with portion 292 of member 272 and, thus, the mechanical resistance imposed by spring 282 on member 272 is reduced. The reduction in mechanical resistance allows for easier rotation of member 272 by handle 44 at the low advance rate, e. g. gripping, mode of operation. Other aspects of vise 250 are generally as described above with reference to Figs. 1A, 1B, 2A and 2B.

Reference is now made to Figs. 8A and 8B which are schematic, side view, partly sectional, illustrations of two, respective, modes of operation of a dual advance rate vise 300 in accordance with another preferred embodiment of the present invention. Fig. 8A illustrates vise 300 in a first, high advance rate, mode of operation thereof, while Fig. 8B illustrates vise 300 in a second, low advance rate, mode of operation thereof. Apart from the differences described below, the operation of vise 300 is generally similar to that of vise 10 (Figs. 1A and 1B) or vise 100 (Figs. 4A and 4B), whereby elements indicated by the same reference numerals in both vises 300 and 100 and/or 10 are generally identical in construction and in operation. Reference is also made to Fig. 8C which is a front view, cross-sectional, illustration of a portion of vise 300, taken across section lines VIII-VIII in Fig. 8A.

In place of first member 22 of Figs. 1A and 1B, vise 300 includes a first threaded member 322, having an unthreaded portion 342 and a first threaded portion 324. Portion 342 of member 322 is rotatably mounted in an opening 341 of element 16 and secured by widened appendage 43 on the external side of element 16, as described with reference to Figs. 1A and 1B. In the embodiment of Figs. 8A and 8B, relative motion between first threaded portion 324 and inner threading 26 is mechanically resisted by a pressure spring 332, similar to spring 32 of Figs. 1A and 1B, which is wrapped around member 322 between second member 27 and a clutch 350. One end of spring 332 is in contact with a housing 352 of clutch 350, urging housing 352 against element 16, while the other end of spring 332 is in contact with an inner portion of member 27. In contrast to the embodiment of Figs. 1A-2B and 4A and 4B, spring 282 imposes a mechanical resistance directly only on rotation of member 27, while rotation of member 322 is limited by clutch 350 as described below. The combined effect of spring 322 and clutch 350 provides the necessary mechanical resistance between first threaded portion 324 and threading 26.

Reference is now made particularly to Fig. 8C. Situated inside housing 352, clutch 350 preferably includes a leaf spring 354 which extends through portion 342 of member 322 and

contacts the interior surface of housing 352. When member 342 is rotated in a grip direction, as indicated by an arrow 377, the friction between leaf spring 354 and housing 352 provides the desired mechanical resistance between member 322 and member 27 via spring 332.

However, when member 342 is rotated in a counter-grip direction, i. e. against the direction of arrow 377, leaf spring 354 slides along the interior surface of housing 352, allowing member 342 to move in a reverse direction smoothly until pin 38 contacts member 27. Other aspects of vise 100, such as the operation of second threaded portion 30, are generally as described above with reference to Figs. 1A, 1B, 2A, 2B, 4A and 4B.

Reference is now made to Figs. 9A and 9B which are schematic, side view, partly sectional, illustrations of a dual advance rate jack 400, constructed and operative in accordance with a preferred embodiment of the present invention. Fig. 9A illustrates jack 400 in a first, high advance rate, mode of operation thereof while Fig. 9B illustrates jack 400 in a second, low advance rate, mode of operation thereof. Jack 400 includes a first element 412, having a first support jaw 414 mounted to one end thereof, and a second element 416 having a second support jaw 418 mounted on one end thereof. The unjawed end of second element 416 is preferably rotatably connected to a middle portion of element 412 using an axle 415, as in conventional jacks.

As shown in Fig. 9B, jaws 414 and 418 are designed for supporting a heavy object 420, such as a car, against a surface 410 such as the ground. As is known in the art, when object 420 is supported by jaws 414 and 418, the supporting force acting on the jaws urges the jawed ends of elements 412 and 416 towards each other. In accordance with a preferred embodiment of the present invention, as described below, this reaction of jack 400 to the support of object 420 is utilized by the jack in the transition from the first mode of operation of Fig. 9A to the second mode of operation of Fig. 9B.

In accordance with a preferred embodiment of the present invention, jack 400 includes a first threaded member 422, having an unthreaded portion 442 and a first threaded portion 424. The length of portion 424 can be extended according to specific lifting requirements.

Portion 442 of member 422 extends through a bearing 441, preferably a ball bearing, and has a widened appendage 443 on the other side of bearing 441. A handle 444 which extends perpendicularly through appendage 443, as in conventional jacks, allows convenient, forceful, rotation of member 422.

Threaded portion 424 of member 422 preferably has an external threading, which engages an inner threading 426 in element 416 near the jawed end thereof. Backward motion

of member 422 relative to threading 426, i. e. from left to right in Figs. 9A and 9B, is limited by a stopper pin 438 which extends through portion 424 of member 422. Jack 400 further includes a second threaded member 427 having an outer threading 428 with a thread axis 401, which engages a second threaded portion 430 with a thread axis 403, mounted on element 412. As shown in Figs. 9A and 9B, the helix angle of threading 428 is in a direction opposite that of threading 426 and portion 424. In a preferred embodiment of the invention, threading 428 has a relatively large helix angle, for example, a helix angle on the order of 20 degrees.

Portion 430 is preferably tiltably mounted on an axle 446 which allows tilting of portion 430 from the untilted position of Fig. 9A to the tilted position of Fig. 9B and vice versa. In the untilted position of Fig. 9A, portion 430 is preferably supported by a spacer 448 on portion 430.

In the absence of gripping forces, a counter-tilting spring 440 maintains portion 430 at the position shown in Fig. 9A, in which thread axis 403 substantially coincides with thread axis 401. The untilted position of portion 430 shown in Fig. 9A corresponds to a non-support mode of operation in which second threaded member 427 rotates with respect to threaded portion 430, advancing linearly therethrough. In this mode of operation, the rate of change of the position of jaw 418 relative to jaw 414 is controlled by the large helix angle of threading 428 in combination with the opposite helix angle of threaded portion 424. This allows quick adjustment of jaws 414 and 418 to the height of given objects to be lifted by jack 400.

Pressure spring 432 is mounted, preferably slightly compressed, between second member 427 and a separator disc 450, which is preferably mounted on portion 442 of first member 422 and supported by a stopper pin 455. Spring 432 exerts a predetermined force which controls the static friction force acting between member 422 and member 427. This imposes a predetermined mechanical resistance which prevents rotation of member 422 relative to member 427. Thus, in the non-support mode of operation shown in Fig. 9A, the advance rate of jaw 418 relative to jaw 414 is controlled by the helix angle of threading 428 in combination with the opposite helix angle of threaded portion 424. For example, if the diameter of member 427 is approximately 20 millimeters and the diameter of member 422 is approximately 10 millimeters, a helix angle of approximately 20 degrees on threading 428 in combination with a helix angle of approximately 3 degrees on portion 424, will provide a linear advance rate of approximately 25 millimeters per rotation of handle 444.

When object 420 is supported over surface 410 using jaws 414 and 418, as shown in Fig. 9B, a force due to the weight of the object urges the jawed end of element 416, together

with threaded members 422 and 427, away from the unjawed end of element 412 which accommodates threaded portion 430. When the force between elements 412 and 416 exceeds a predetermined threshold, overcoming the counter-tilting force exerted by spring 440, threaded portion 430 is tilted to the position of Fig. 9B. At this position, thread axis 403 is tilted with respect to thread axis 401. It should be appreciated that this tilt of the thread axis results in locking engagement between threading 428 and portion 430. Thus, relative motion between threading 428 and portion 430 is prevented as long as the support force acting on jaws 414 and 418 remains above a predetermined threshold.

In a preferred embodiment of the invention, the threshold for locking of portion 430 to threading 428 is slightly lower than the threshold force required to overcome the mechanical resistance between member 422 and member 427. Thus, once portion 430 is tilted to the locked position of Fig. 9B, member 427 stops rotating while portion 424 continues to rotate within threading 426. When additional support force is exerted on jaws 414 and 418, in response to rotation of handle 444, threaded member 422 advances forward in the unjawed end of element 416, i. e. from right to left in Fig. 9B, until a desired height of object 420 is reached. In this mode of operation, hereinafter referred to as the lift mode, the advance rate of jaw 418 away from jaw 414 is controlled only by the helix angle of threading 426. For example, if the diameter of portion 424 is approximately 10 millimeters, a helix angle of approximately 3 degrees on threading 426 will provide a linear advance rate of approximately 1.65 millimeters per rotation of handle 444.

When the support of object 420 is released, by rotating handle 444 in a counter-lift direction, the sequence of actions described above is reversed. First, when threading 428 is still locked by second threaded portion 430, threaded portion 424 of member 422 moves in a reverse direction in threading 426, i. e. from left to right in Fig. 9B. This reverse motion, at the low linear advance rate, continues until stopper pin 438 comes into contact with element 416.

At this point, reverse motion of member 422 is stopped and additional counter-lift rotation of handle 444 releases the locking engagement between portion 430 and threading 428 and enables counter-tilting of portion 430, by counter-tilting spring 440, back to the position of Fig. 9A. Once portion 430 returns to the untilted position of Fig. 9A, threading 428 of member 427 moves in a reverse direction in portion 430, at the high linear advance rate, in addition to the reverse motion of member 422, and jaw 418 can be quickly adjusted to a new position vis-a-vis jaw 414.

Reference is now made to Figs. 10A and 10B which are schematic, side view, partly sectional, illustrations of two, respective, modes of operation of a dual advance rate jack 500 in accordance with another preferred embodiment of the present invention. Fig. 10A illustrates jack 500 in a first, high advance rate, mode of operation thereof, while Fig. 10B illustrates jack 500 in a second, low advance rate, mode of operation thereof. Apart from the differences described below, the operation of jack 500 is generally similar to that of jack 400 (Figs. 9A and 9B), whereby elements indicated by the same reference numerals in both jacks 500 and 400 are generally identical in construction and in operation.

In place of first member 422 of Figs. 9A and 9B, jack 500 includes a first threaded member 522, having an unthreaded portion 542 and a first threaded portion 524. Portion 542 of member 522 preferably extends through bearing 441 and is secured by widened appendage 443 on the other side of bearing 441, as described above with reference to Figs. 9A and 9B.

In the embodiment of Figs. 10A and 10B, relative motion between first threaded member 522 and second threaded member 427 is mechanically resisted by a twist-spring 532 which twistably connects between first member 522 and second member 427. One end of spring 532 is fixedly connected to member 522, at point 531, while the other end of spring 532 is fixedly connected to second threaded member 427, at point 533. A counter-twisting force imposed by spring 532 provides a mechanical resistance substantially equivalent to that imposed by friction spring 432 in the embodiment of Figs. 9A and 9B. Other aspects of jack 500, such as the operation of second threaded portion 430, are generally as described above with reference to Figs. 9A and 9B.

The above descriptions of a first group of preferred embodiments of the present invention have generally assumed that the mechanism for shifting between the operational modes of the vise or jack is a tilting mechanism, which locks the second threaded member relative to the second threaded portion that receives it. The following descriptions of a second group of preferred embodiments of the present invention describe sliding mechanisms as alternative mechanisms for locking the threaded portion and shifting between operational modes. It will be appreciated by those skilled in the art that whereas the sliding mechanisms described below utilize a threaded member sliding substantially perpendicularly to the thread axis, locking can also be effected by the threaded member sliding substantially along the thread axis, or by any other suitable locking mechanism.

Reference is now made to Figs. llA, llB, and IIC. Figs. 11A and 11B are schematic, side view, partly sectional, illustrations of two, respective, modes of operation of a

dual advance rate vise 600, in accordance with another preferred embodiment of the present invention. Fig. 11 C is a schematic cross-sectional view of vise 600 taken along line XI-XI of Fig. 1 lA. Fig. 11A illustrates vise 600 in a first, high advance rate, mode of operation thereof, while Fig. 11B illustrates vise 600 in a second, low advance rate, mode of operation thereof.

Apart from the differences described below, the operation of vise 600 is generally similar to that of vise 10 (Figs. 1A and 1B), whereby elements indicated by the same reference numerals in both vises 600 and 10 are generally identical in construction and in operation.

In the absence of gripping forces, a spring 640 maintains a threaded portion 630 at the position shown in Fig. 11A, in which a thread axis 632 of the inner threading on threaded portion 630 substantially coincides with a thread axis 634 of external threading 28 of member 27. Threaded portion 630 is supported in a housing 633, mounted on element 12, which allows threaded portion 630 to slide horizontally. Preferably threaded portion 630 slides within housing 633 along a guide having a dovetail cross-section, as illustrated schematically in Fig. 11 C, although other suitable cross-sections, known in the art, may also be used for this purpose. The position of threaded portion 630 shown in Fig. 1 lA corresponds to a non-grip mode of operation in which second threaded member 27 rotates with respect to threaded portion 630, advancing linearly therethrough. Also as shown in Fig. 11A, threaded portion 630 is maintained in position against a wall 637 of housing 633 by spring 640. As described with reference to Fig. 1A, this arrangement allows quick adjustment of jaws 14 and 18 to the dimensions of given objects to be gripped by vise 600.

When object 20 is gripped between jaws 14 and 18, as shown in Fig. 11B, the reaction force exerted by object 20 urges elements 16, together with threaded members 22 and 27, away from element 12 which accommodates threaded portion 630. When the grip force exerted between jaws 14 and 18 exceeds a predetermined threshold, overcoming the force exerted by spring 640, threaded portion 630 moves away from wall 637 and pushes against a latch 631, which rotates about a pin 635. Latch 631 contacts external threading 28 of member 27 and forces threading 28 to move upwards in threaded portion 630, to the position shown in Fig. 11B. At this position, thread axis 632 is offset with respect to thread axis 634. It will be appreciated that this offset of thread axis 632, together with the contact between latch 631 and threading 28, results in locking engagement between threading 28 and threaded portion 630 due to the friction forces acting therebetween. Thus, relative motion between threading 28 and threaded portion 630 is prevented as long as the grip force acting on jaws 14 and 18 remains above the predetermined threshold. It should be noted that friction between support

member 36 and second threading 28 contributes to the locking of member 27. It should be further noted that member 36 takes part of the load involved in the downward sliding of portion 630, thereby reducing the load on other elements of the vise such as threaded portion 24 and threading 26.

When the grip on object 20 is released, by rotating handle 44 in a counter-grip direction, the sequence of actions described above is reversed. First, when threading 28 is still locked by second threaded portion 630, threaded portion 24 of member 22 moves in a reverse direction in threading 26, i. e. from left to right in Fig. 11B. This reverse motion, at the low linear advance rate, continues until stopper pin 38 comes into contact with member 27. At this point, reverse motion of member 22 is stopped and additional counter-grip rotation of handle 44 releases the locking engagement between portion 630 and threading 28. Spring 640 and latch 631 then force threaded member 630 to the unshifted position shown in Fig. 11A. Once portion 630 returns to the unshifted position of Fig. l lA, threading 28 of member 27 moves in a reverse direction in portion 630, at the high linear advance rate, and jaw 18 can be quickly adjusted to a new position vis-a-vis jaw 14.

Reference is now made to Fig. 12A which is a schematic, front view, cross-sectional, illustration of a portion of vise 600, taken across section lines XII-XII in Fig. l l A. Fig. 12A shows the cross-section of support member 636. Support member 636 holds pin 635, which in turn holds latch 631. Reference is also made to Fig. 12B which schematically illustrates a cross-section of a support member 637 in accordance with an alternative, preferred, embodiment of the present invention. The bottom, anti-tilting, support provided to threaded member 27 by support member 636 of Fig. 12A, is generally sufficient for most purposes of the present invention. Nevertheless, for some preferred embodiments of the present invention, particularly when threading 28 has a very large helix angle, a more embracing support such as support 637 of Fig. 12B may be required, to prevent self release of threading 28 from portion 630.

Reference is now made to Figs. 13A and 13B which are schematic, partly cross- sectional, illustrations of two, respective, modes of operation of a dual advance rate jack 700 in accordance with another preferred embodiment of the present invention. Fig. 13A is a side view illustrating jack 700 in a first, high advance rate, mode of operation thereof, while Fig.

13B is a top view illustrating the shifting mechanism of jack 700 in a second, low advance rate, mode of operation thereof. Apart from the differences described below, the operation of jack 700 is generally similar to that of jack 400 (Figs. 9A and 9B), whereby elements indicated

by the same reference numerals in both jacks 700 and 400 are generally identical in construction and in operation.

In place of element 412 of Figs. 9A and 9B, jack 700 includes an arm 712. The upper end of arm 712 is fixedly connected to first and second brackets 722 and 724 respectively.

Most preferably arm 712 and first and second brackets 722 and 724 are constructed as one piece. Bracket 724 is penetrated by a pin 704, and bracket 722 is penetrated by a pin 720.

Pins 704 and 720 are rotatably connected to a housing 702, which is penetrated by threading 428 of member 427. Housing 702 has a cylindrical inner wall 726, and a conical inner wall 728. Housing 702 houses a first half-thread 710 having a thread axis 730 and an exterior conical wall 714, and a second half-thread 718 having a thread axis 738 and an exterior conical wall 716.

In the absence of gripping forces, half-thread 710 is held in place against thread 428 by one or more, preferably two, compression springs 706 and interior wall 726 of housing 702.

Similarly, half-thread 718 is held in place against thread 428 by one or more, preferably two, compression springs 708 and interior wall 726 of housing 702. In the absence of gripping forces, thread axes 730, 738, and 401 are substantially coincident, as illustrated in Fig. 13A, and it will be appreciated that rotation of handle 444 will effect the high advance rate mode for jack 700 by screw 428 rotating in half-threads 710 and 718.

When gripping forces are created by jaws 414 and 418 contacting objects, brackets 722 and 724 experience forces towards the right in Figs. 13A and 13B, so that housing 702 is forced towards the right. These forces urge member 422 towards the left in Figs. 13A and 13B, so that half-threads 710 and 718 are also forced towards the left. The motion of housing 702 towards the right, and of half-threads 710 and 718 towards the left, results in interior conical wall 728 contacting exterior conical walls 714 and 716, respectively forcing half- thread 710 and half-thread 718 towards each other and towards thread 428. At this position, thread axis 730 and thread axis 738 are offset laterally with respect to thread axis 401. It will be appreciated that these offsets of the thread axes, as well as the increased contact of the half-threads with threading 428, result in locking engagement between threading 428 and half- threads 710 and 718. Thus, relative motion between threading 428 and half-threads 710 and 718 is prevented as long as the gripping force acting on jaws 414 and 418 remains above a predetermined threshold. It will be appreciated that rotation of handle 444 will effect the low advance rate mode for jack 700 by threaded portion 424 rotating in inner threading 426.

It will be appreciated by those skilled in the art that whereas the descriptions above of the preferred embodiments termed jacks have described the jacks as acting in a generally vertical manner, similar preferred embodiments can equally well be used at any angle, including use horizontally, as separators.

The preferred embodiments described above are cited by way of example, and the full scope of the invention is limited only by the claims.