Known lead screw and nut assemblies are inefficient due to the fact that the sliding screw thread surfaces are subject to output loads which can result in large friction.

In general, known devices are made up of a threaded lead screw and a nut unit. Typically, the nut unit is made up of an assembly in which an outer case serves to house anti-friction components which engage the threaded lead screw. Typically, the anti-friction elements surround the threaded screw for load distribution purposes.

British Patent Specification No. 1578476 relates to a device made up of nut assembly, a threaded lead screw having a number of threads which extend

through the nut assembly, a housing means and a number of roller means equal to the number of threads on the lead screw disposed between the nut assembly and the lead thread. Each roller means has a number of annular ribs defining teeth which are conjugate with the threads of the lead screw and annular grooves on the housing. Rotation of the lead screw effects a planetary motion of the roller means thereby effecting a linear translation of the ring.

The device of British Patent Specification No. 1578476 therefore converts torque to a linear force due to the planetary motion of the rollers.

However, the nut moves at a slower rate than the lead screw while it is difficult to replicate positions following a number of cycles of motion and reversal of load which undermines the accuracy of the device.

European Patent Specification No. 0168942 attempts to overcome the problem of the device of British Patent Specification No. 1578476 by providing a device in which the axially moveable element is advanced or returned a predetermined amount per revolution of the rotatable element so that accuracy and repetitiveness of position are attained. The device of European Patent Specification No. 0168942 is provided with a rotatable ring disposed between the rollers and the nut so that the teeth on the rollers engage the screw and annular grooves on the ring to distribute the thrust force throughout the

entire roller means. Nevertheless, the nut of European Patent Specification No. 0168942 does move at the same rate in direct relationship with the lead of the screw due to engagement of the roller ends with the housing. The ring results in increased size and production costs in the manufacture of the device.

An object of the invention is to overcome non-linear and size problems of the prior art.

According to the present invention there is provided a device for converting a rotary input to a linear output comprising a threaded lead screw, a housing member, and roller means disposed between the threaded lead screw and the housing member, each roller means having a plurality of annular coupling means conjugate with the threads of the lead screw; and a gearing mechanism for engaging and retaining the roller means in radial engagement between the housing member and the lead screw so that torque is effectively transmitted between the housing member and the lead screw through the roller means and the gearing mechanism.

Preferably, the number of roller means is equal to the number of threads on the lead screw.

Advantageously, the coupling means may comprise annular grooves on the roller means, where the annular grooves are preferably leadless.

Preferably, the gearing mechanism comprises at least one gear connected to the roller means, wherein the at least one gear is typically in a geared coupling with a gear form connected to the housing member.

Preferably, the at least one gear is secured to the roller means such that it rotates therewith and the gear form is typically secured to the housing member such that it rotates therewith.

Preferably, the gear mechanism comprises two gears, the two gears typically being in a geared coupling to respective gear forms.

Typically, one gear being provided at or toward each end of the roller means.

Preferably, the gearing mechanism further comprises at least one plate member coupled to the roller means wherein the plate member typically secures the plurality of roller means in a circumferentially spaced apart relationship with respect to one another.

Preferably, the at least one plate member is rotatable with respect to the housing member.

Preferably, the at least one plate member is coupled to the housing member by a bearing mechanism.

Preferably, the housing member is coupled to a load and the lead screw is coupled to a rotatable power means.

Preferably rotation of the lead screw results in axial movement of the housing member.

Alternatively, axial movement of the housing member results in rotational movement of the lead screw.

In a preferred embodiment of the invention, the lead screw comprises three threads.

Various embodiments of the invention will now be described by way of example only, having regard to the following drawing in which: Figure 1 is a transverse cross-sectional view of a device comprising a nut assembly in accordance with the invention.

As shown in the drawings a nut assembly in accordance with the invention is generally indicated by the reference numeral 1. The nut assembly 1 is made up of a screw 2 having a longitudinal axis 5.

The screw 2 is typically an involute-helicoidal screw 2. The screw 2 is inserted in a nut member 3.

The screw 2 is supported in the nut member 3 by rollers 4 disposed between the nut member 3 and the screw 2.

The nut member 3 is provided'with an outer cylindrical wall 6, a first end 7 and a second end 8 between which the screw 2 extends. The rollers 4 are each provided with a roller longitudinal axis 9, which in use are parallel to the longitudinal axis 5 of the screw.

The screw 2 is typically rotatably driven by a reversible power unit while the nut member 3 in use is typically connected to a workload or the like to be urged along the longitudinal axis 5 of the screw 2.

The screw 2 has a plurality of lead threads e. g. the screw 2 typically has a right hand type three lead thread as shown in Figure 1. However, as will be appreciated by those skilled in the art, the screw 2 can be a left or right handed screw and can have more than three lead threads.

However, the number of rollers 4 present in the nut assembly 1 of the invention will typically be either the same as or a multiple of the number of leads present on the screw 2. Accordingly, in a preferred embodiment of the invention, the nut assembly 1 is provided with three rollers 4 circumferentially equi-spaced about the screw 2.

Each roller 4 is provided with annular grooves or ribs 14 which form a coupling means and which are arranged perpendicular to the longitudinal axis 9

and which are conjugate to the threads of the screw 2.

Each roller 4 is also supported in the nut assembly 1 at each end by a stub shaft 10 supported in a locator plate 11. Each locator plate 11 is coupled to the cylindrical wall 6 by sliding bearings (not shown) such that the locator plate 11 can rotate about the longitudinal axis of the screw 6 and thus each locator plate 11 can rotate relative to the rest of the nut member 3.

Internally, a middle shoulder portion 20 of the cylindrical wall 6 of the nut member 3 projects inwardly and is provided with a plurality of annular grooves 15 between the first end 7 and the second end 8 of the nut member 3. The annular grooves 15 of the nut member 3 have a shape complimentary to the shape of the ribs 14 of the rollers 4 and are typically of substantially equal pitch to the grooves 14 of the roller 4. In addition, the nut member 3 and the rollers 4 typically have equivalent numbers of grooves 15, and ribs 14 respectively.

As shown in the drawing, each roller 4 is provided with a first gear 12 and a second gear 13 at each end thereof adjacent the stub shaft 10. The first and second gears 12,13 are preferably integral with the rollers 4. The locator plate 11 houses the stub shaft 10 and serves to ensure that each gear 12,13 and roller 4 assembly remains equi-spaced throughout its planetary motion around the screw 2. A gear

ring insert 16 is located either side of the middle shoulder 20 and each gear ring insert 16 is held in place by at least a pin 17 to prevent relative rotation of the insert 16 with respect to the cylindrical wall 6. Each gear insert 16 is provided with an internal gear form 22 which engages with the respective gear 12,13. A longitudinal axis securing means such as a circlip 24 secures each of the locator plates 11 within the inner bore of the nut member 3 but the skilled person will realise that other embodiments of securing means could be utilised instead of the circlip 24, such as a screw threaded cap (not shown) or the like. The outer surface of the cylindrical wall 6 is coupled to the load by any suitable coupling means such as a keying mechanism 26 as shown in Figure 1, although the skilled person will realise that other coupling means could be utilised such as flanges (not shown) or the like.

Operation of the nut assembly 1 will now be described. Rotation of the screw 2 causes each of the rollers 4 to rotate about the screw 2 in a planetary motion since they follow the path of the threads of the screw 2. The planetary motion of the rollers 4 causes each of the locator plates 11 to rotate about the longitudinal axis of the screw 2.

However, the cylindrical wall 6, which is attached to the load, does not itself rotate but is moved axially along the length of the longitudinal axis of the screw 2, and the gearing action between the gears 12,13 and respective gear ring inserts 16

ensure that the cylindrical wall 6 does not rotate and also ensure that each roller 4 is driven in its planetary motion.

In normal use, rotation of the screw 2 results in axial movements, but not rotational movement, of the nut member 3 along the longitudinal axis of the screw 2. Thus, the internal gear form 22 acts in conjunction with the gears 12,13 and the nut member 3 to effect control of the relative rates of movement of the nut member 3 and the screw 2. The effect of the gearing mechanism provided by gears 12,13 and the gear ring insert 16 is that if one or more of the rollers 4 tends towards or becomes rotatably jammed then the gearing mechanism will continue to axially translate the nut member 3 due to the driving effect of the gear ring insert 16 acting upon the gears 12,13 of the jammed roller 4 as long as at least one of the rollers 4 is not jammed. In other words, the one or more rollers 4 tending to jam will be forced to rotate, not by the action of the screw 2 but by the action of the gearing mechanism. A further purpose of the first and second gears 12,13, the stub shaft 10 and the gear ring insert is to collectively ensure that each roller 4 is maintained in permanent contact between the nut member 3 and the screw 2 to effect accurate and repeatable conversion of torque to linear force.

Accordingly, all torque is effectively transmitted through the first and second gears 12,13 on each roller 4 to cause the rollers 4 to move at the correct speed.

The device of the invention can be manufactured simply at low cost due to the reduction of the number of parts and simplicity of assembly.

The device of the invention is not bulky due to the absence of a ring between the screw 2 and the nut member 3. More importantly the device of the invention results in the comparatively frictionless, repeatable and efficient conversion of a rotary input to a linear output.

The invention is not limited to the embodiments herein described which may be varied in construction and detail. For instance, the skilled person will also realise that each roller 4 may be provided with only one gear 12 or 13 to co-operate with a respective gear ring insert 16.