FIELD OF THE INVENTION

This invention relates in general to screw type linear actuators and in particular to an improved structure for a linear actuator which is capable of handling relatively heavy undesirable loads.

BACKGROUND OF INVENTION

A linear actuator assembly, comprising a screw drive assembly which converts rotation of a motor shaft into linear motion of a piston rod for transmitting axial force to a workpiece which is to be moved or positioned is well-known in the prior art.

A screw driven rod-style linear actuator typically comprises an electric motor whose output shaft is connected through transmitting means to a screw drive assembly, consisting of lead or ball screw shaft and a drive nut disposed thereon and retained from rotation in such a way that the rotation of the screw shaft by the electric motor causes linear displacement of the drive nut along the screw shaft.

The assembly includes a piston rod that is attached to the drive nut, and therefore they both displace axially along the screw shaft, when the screw shaft is rotated by the motor. The direction of such axial movement of the drive nut (and the piston rod connected thereto) is dependent upon a direction of rotation of the screw shaft.

The linear actuator housing usually made in the form of a hollow cylinder or a profile tube, within which housed a screw drive assembly and a hollow piston rod concentrically disposed around part of screw shaft along longitudinal axis of the housing.

The free end of the piston rod typically include a mounting element (a clevis or other coupling) to which an actuated workpiece may be connected.

A linear actuator can be have the motor attached to the housing, either parallel or perpendicular, or coaxially to the screw shaft axis, depending on type of means transmitting rotary motion from motor shaft to the screw shaft by the use of coupling, gear drive, worm drive, driving belt, ets. The force transmitted through the piston rod of the linear actuator is directed along its extending/retracting axis, coaxially with the axis of the linear actuator assembly arrangement. Accordingly, direction of load acting upon the piston rod must be placed coaxially with the axis of piston rod, which is usually achieved by using external guides to align, stabilize and support an actuated workpiece.

If the axis of load direction does not coincide with the piston rod axis, the linear actuator assembly will be subjected to undesirable load, in particular, cantilever load, which causes torque around the axis of the piston rod extension, and radial (side) load, which causes a bending moment.

Exceeding the allowable values of undesirable loads hampers the operation of the linear actuator and leads to accelerating wear and failure of its units. It limits the use of linear actuators in various operations with unsupported load, insufficiently fixed load, offset load, external loads pushing against the actuator and other undesirable loads, which can be generate in particular a cantilever and a side load and combinations thereof.

U.S. patent No 6,145,395 Nov.14, 2000 describes a linear actuator with an internal side load compensation device comprises at least one cam beam affixed to the actuator housing approximately parallel to the axial movement direction of the linear actuator's driver mechanism, at least one cam roller affixed to the driver mechanism wherein the cam roller engages with the cam beam to effect compensation for side load, and at least one pre-load bar extending within the housing of the linear actuator essentially perpendicular to the cam beam and the axial movement direction of the driver mechanism for effecting a pre-load on the cam beam.

In the preferred embodiment disclosed in the patent description, an internal side load compensation device comprises two pairs of cam beam disposed on both sides of the actuating member along the lead screw axis, and the drive nut adapter is provided with several pairs of rollers mounted by shafts along the axial direction of movement of the drive nut in such a way that each roller is disposed between one of the pairs of cam beams. Additionally, a journal bearing or bushing is fixed in the front end cap of the actuator to handle the summation of the load on the end of the actuating assembly and compensating load provided by pre- loaded cam beams.

The effectiveness of this structure is limited by dependence on the direction of the side load. The internal side load compensation device operates with maximum efficiency when the side load is directed perpendicularly to the axes of the roller shafts. As the direction of the side load changes, the efficiency of the compensation device decreases. When the side load is directed parallelly to the axes of the roller shafts, it acts upon only a part of the device— journal bearing or bushing, what lead to binding or excessive friction in it.

Patent CN105051424, April 6, 2018 describes a servo cylinder which converts a rotational motion of a motor into a linear motion of a piston rod by using a TM screw or a ball screw and combines a linear ball bushing to the piston rod inside a housing so as to enable an infinite linear motion. A linear bushing is arranged in a housing for guiding the piston rod and comprise a ball retainer is provided for guiding a row of balls capable of infinite linear motion inside the housing.

The servo cylinder of this design can provide side load capability, but cannot handle some others undesirable loads, in particular cantilever load, which causes torque around the axis of the piston rod extension.

SUMMARY OF THE INVENTION

According to the present invention, in order to achieve the above-mentioned object, there is provided an undesirable loads compensation device in a screw type linear actuator, comprising at least two longitudinal ball guide grooves arranged on the inner surface of the housing, at least two longitudinal ball guide counter-grooves arranged on the outer surface of the piston rod, and a ball cage with at least two rows of cells between these oppositely disposed co-responding (corresponding) grooves, in which balls held for rotation, roll between surfaces of these grooves.

The undesirable loads compensation device can be provided with at least one spring prevents the ball cage from slipping under its own weight when the linear actuator is inclined to a vertical plane. In the proposed structure of the linear actuator, the piston rod during extending/retracting and while being stopped is continuously supported by many balls in the guide grooves so that moments created by variable undesirable loads applied to the mounting element of the piston rod can be transferred to the fixed housing which is able to compensate them.

The undesirable loads compensation device prevents shifting of the piston rod relative to the axis of the linear actuator assembly arrangement and holds the piston rod against axial rotation relative to the housing without impeding the efficiency of the axial force transmission. Thereby, the linear actuator's screw drive and transmitting means (such as gearing) are isolated from undesirable loads and moments created by them, wherein effectiveness of the proposed compensation device does not depend on the direction of undesirable loads and moments created by them.

In the proposed structure of the linear actuator there is no sliding mating surfaces of parts accepting and transmitting the undesirable loads which eliminates the possibility of intense wear of such contacting surfaces, the rolling balls and guide grooves forms low-friction linear contacts with high load resistance capability.

These and other objects, advantages, and features of the present invention will become more readily apparent from the following detailed description when taken together with the accompanying drawings, wherein:

BRIEF DESCRIPTION OF THE DRAWING FIGURES

FIG.1 is a perspective view of the exterior of a linear actuator in accordance with this invention;

FIG.2 is a partially exploded perspective view of the linear actuator of

FIG.1 ;

FIG.3 is a perspective view of the first embodiment of the linear drive mechanism of the invention, showing the housing and the ball cage partially broken-away for ease of illustration;

FIG.4 is a cross-sectional view taken along the line A-A of FIG.3; FIG.5 is a longitudinal section of the linear drive mechanism shown in FIG.3, taken along the line B-B shown in FIG.4;

FIG.6 is a longitudinal section of the linear drive mechanism shown in FIG.3, taken along the line C-C shown in FIG.4;

FIG.7 is an exploded perspective view of the linear drive mechanism of

FIG.3;

FIG.8 is a perspective view of the screw drive assembly extracted from the housing;

FIG.9 is a partially exploded perspective view of the screw drive assembly, showing the piston rod partially broken-away for ease of illustration;

FIG.10 is a partially exploded perspective view of another embodiment of a screw drive assembly;

FIG.11 is an exploded perspective view of the screw shaft with the guide bushing;

FIG.12 is a schematic sectional view of the linear drive mechanism, shown in both the fully retracted and fully extended positions;

FIG.13 is a schematic sectional view of the linear drive mechanism inclined to a vertical plane, with front end of the piston rod being higher than the rear end;

FIG.14 is a schematic sectional view of the linear drive mechanism inclined to a vertical plane, with front end of the piston rod being lower than the rear end;

FIG.15 is a schematic perspective view of the arrangement of the spring supporting the ball cage;

FIG. 16 is a cross-sectional view of the second embodiment of the linear drive mechanism of the present invention;

FIG.17 is a perspective view of the grooved rail of the embodiment shown in FIG. 16; and

FIG.18 is a perspective view of the housing of the embodiment shown in FIG. 16.

DETAILED DESCRIPTION OF THE INVENTION

With reference to FIGS. 1 and 2 initially, there is illustrated an electromechanical linear actuator having a reversible electric motor 16 with a drive shaft, coupling 15, coupling housing 14, and a linear drive mechanism, generally indicated at 10. The linear drive mechanism 10 includes a screw drive assembly and an undesirable loads compensation device, are enclosed within the housing 1 1 having rear end portion close to the electric motor 16 and an opposite front end portion remote therefrom.

The electric motor 16, being a source of rotational power, transmits torque to the screw drive assembly, for which the output shaft of the electric motor 16 is connected by a coupling 15 to the end part 22a of the screw shaft 22.

The connection of electric motor to the screw shaft of linear actuator can be accomplished by the use of couplings, belts, gears, worms, with or without a gearbox and with various positions of the electric motor relative to the housing of a linear actuator.

The housing 1 1 made in the form of a profile tube with inner surface that, in the first embodiment disclosed herein, is of circular sectional configuration. According to a first embodiment of the present invention on the inner surface of the housing 1 1 are formed a longitudinal ball guide grooves (hereinafter referred to as the "grooves") which are an element of the undesirable loads compensation device (hereinafter referred to as the "compensation device").

As shown in FIGS. 3-6, screw drive assembly disposed within the housing 1 1 includes drive nut 23 threadedly engaged with screw shaft 22 and piston rod 21 attached to the drive nut 23 for axial movement with it.

In one embodiment of the present invention, as best seen in FIG. 4 the inner surface of the housing 1 1 provided with four grooves 31 , and the outer surface of the piston rod 21 provided with four counter-grooves 32.

The grooves 31 are disposed at equal distances around the circumference of the cross section of the inner surface of the housing 1 1 and the countergrooves 32 are disposed at equal distances around the circumference of the cross section of the outer surface of piston rod 21 so that the grooves 31 and counter-grooves 32 can be disposed in pairs of opposing corresponding grooves. The surfaces of the grooves 31 and counter-grooves 32 are concave in cross section and in each pair the surfaces of oppositely disposed corresponding grooves 31/32 are facing each other. The piston rod 21 made in the form of a profile tube on the outer surface of which are formed a counter-grooves 32.

The outer cross sectional outline of the drive nut 23 corresponds with inner cross sectional outline of the piston rod 21 and as shown in FIGS. 7-8, drive nut

23 screw-engaged with the screw shaft 22, engages with piston rod 21 via mating surfaces and fastens with set screws 28 to its rear end part. The front end of the piston rod 21 may include a mounting element (not shown) to which an actuated workpiece may be connected to perform a desired operation with extension and retraction of the piston rod 21.

Front and rear openings of the housing 1 1 are closed and blocked with front end cap 13 and rear end cap 12.

The screw shaft 22 is supported rotatably by the double row angular contact ball bearing 17 to carry the axial and radial loads. The ball bearing 17 is inserted into the hollow of rear end cap 12 and held in place by a lock nut 19.

The rear end 22a of the screw shaft 22 is mounted in the ball bearing 17 and is fixed on the screw shaft 22 by a lock nut 18.

The guide bushing 24 is mounted rotatably on the front end 22b of the screw shaft 22 by means of a lock washer 25 to maintain proper concentric alignment between the screw shaft 22 and the piston rod 21 (see FIGS. 5, 6, 9, 11).

The guide bushing 24 is sized to fit closely within the piston rod 21 but without resistance to the axial movement of the piston rod 21 relative to the screw shaft 22. The guide bushing 24 is made from a material having a low coefficient of friction or with sliding coating on the inner and outer surfaces. Thus, the screw shaft 22 rotates inside the piston rod 21 , which together with the drive nut 23 moves along the longitudinal axis of the housing 1 1.

In addition, the guide bushing 24 can function as a stopper, which is confronted by the drive nut 23 at full extension of the piston rod 21 to limit outward movement of the piston rod 21 . To prevent damage, the guide bushing

24 have stop surface 26 made from an elastic material, for example, rubber or the like. With the same purpose, the damping washer 27 is fitted between the rear cover 12 and the drive nut 23.

Embodiments of the present invention may be carried out with many varieties of screw drive assembly constructions and in particular with various types of drive mechanisms such as lead screws, ball screws, and roller screws.

FIG. 10 illustrates another embodiment of the screw drive assembly consisting of cylindrical ball nut 43 with screw shaft 42, and piston rod 41. The rear end of the piston rod 41 is made with internal thread into which the ball nut 43 is screwed with its external thread, and it is fixed in position with set screws 44. The screw shaft 44 of a ball screw drive assembly is made with a ball running grooves (ball screw raceway).

In addition to the four grooves 31 are formed on the inner surface of the housing 1 1 and four counter-grooves 32 are formed on the outer surface of the piston rod 21 , the compensation device includes a ball cage 33 concentrically disposed between the inner surface of the housing 1 1 and the outer surface of the piston rod 21 , displaceably in an axial direction of movement of the piston rod 21 (see FIGS. 4-9).

The ball cage 33 with four rows of cells (one row for each pair of opposing corresponding grooves 31/32) is positioned in such a way that each ball 34 is enclosed between groove 31 and counter-groove 32 in contact with the concave spherical surface of each of them.

The balls 34 are held by the ball cage 33 such that each ball 34 can rotate freely in the cell of the ball cage 33, and thus as the piston rod 21 moves relative to the housing 1 1 , the balls 34 are rolled between surfaces of the grooves 31 and counter-grooves 32, moving along with the ball cage 33 in the axial direction of movement of the piston rod 21. The movement of the ball cage 33 is limited by front end cap 13 to one side, and by protruding heads of the set screws 28 fastening the drive nut 23 to the rear end of the piston rod 21 to the other side (see FIGS. 6-8).

A curved surface of the grooves 31 and counter-grooves 32 have a radius close to that of the balls 34 and thereby the piston rod 21 is prevented from rotation, without impeding its axial movement relative to the housing 1 1 . The linear actuator of the foregoing construction operates as follows:

When reversible electric motor 16 is actuated, screw shaft 22 rotates in one direction or another, depending upon motor connections to the power source.

Rotation of the screw shaft 22 causes linear movement of drive nut 23 and piston rod 21 joined together along the screw shaft 22 in direction corresponding to the direction of rotation of screw shaft 22.

The stroke limits of the piston rod 21 are regulated by limit switches (not shown). Limit switches function to disengage power to the electric motor 16 upon the piston rod 21 reaching the outer or inner limits of its stroke (the fully retracted position or the fully extended position).

In the process of extending / retracting the piston rod 21 , the balls 34 due to frictional engagement with the surfaces of the grooves 31 and counter-grooves 32 are rolled in the direction of movement of the piston rod 21. As is known, when a ball is rolled between a moving and fixed plate, the speed of movement of the ball center is half the speed of the moving plate relatively the fixed plate, therefore the linear distance travelled by ball cage 33 with rolling balls 34 will be half of the distance traveled by the counter-grooves 32 (hence by the piston rod 21) relatively the housing 1 1.

FIG.12 illustrates the fully retracted position (at the top) and the fully extended position (in the bottom) of the piston rod 21 and the corresponding positions of the ball cage 33.

The piston rod 21 extending from the fully retracted position to the fully extended position travels the distance S corresponding to the distance C travelled by ball cage 33. When rolling the balls 34 without slipping, each intermediate position of the piston rod 21 corresponds to a certain position of the ball cage 33.

The ball cage 33 is held in each position due to frictional engagement of the balls 34 with the surfaces of the grooves 31 and counter-grooves 32.

In many applications, a linear actuator has an inclined arrangement, or changes an angle relative to the horizontal plane in the process of extending / retracting a piston rod. The contact of the balls 34 with the surfaces of the grooves 31 and counter-grooves 32 may be weakened as the contacting surfaces wear out. In the positions of the linear actuator close to the vertical plane and when the undesirable load weakens or changes direction, the ball cage 33 with the balls 34 may slide under its own weight.

When position of the front end of the piston rod 21 is higher than its rear end, the ball cage 33 with balls 34 may slide towards the rear end of the piston rod 21 (see FIG. 13).

When position of the front end of the piston rod 21 is lower than its rear end, the ball cage 33 with balls 34 may slide towards the front end of the piston rod 21 (see FIG. 14).

The direction of sliding of the ball cage 33 with balls 34 is shown by an arrow.

In the case when the working position of the linear actuator has an inclined arrangement the compression springs 35 and 36 can be arranged between the ball cage 33 and the elements limiting its linear movement to prevent the ball cage 33 with the balls 34 from sliding under its own weight (see FIG. 15).

The compression spring 35 is contiguous with the rear end of the ball cage 33 one side and the protruding heads of the set screws 28 on the other side. The compression spring 36 is contiguous with the front end of the ball cage 33 one side and the inner wall of the front end cap 13 on the other side.

The piston rod 21 during extending/retracting is movably supported by many balls 34, rolling between the grooves 31 and counter-grooves 32 with very little rolling resistance and with little sliding. When being stopped, the piston rod 21 is continuously supported in any position.

When undesirable loads applied to the mounting element of the piston rod 21 the compensation device prevents shifting of the piston rod 21 relative to the axis of the linear actuator assembly arrangement and holds the piston rod 21 against axial rotation relative to the housing 1 1 without impeding the efficiency of the axial force transmission.

In addition, the compensation device also acts as an anti-rotation device, usually provided in many linear actuator designs, namely, retains the drive nut 23 from rotating relative to the housing 1 1 during the rotation of the screw shaft 22. FIGS. 16-18 illustrate a second embodiment of the present invention, differs from the first embodiment in that the housing 1 1 ' is provided with grooved rails 30 mounted on the inner surface of the housing 11 '. The grooves 3T are formed on the rails 30 which secured to the inner surface of the housing 1 T by screws 37.

The second embodiment of the present invention can be use lightweight materials such as extruded aluminium in the housing 1 T which significantly reduces the weight of the linear drive mechanism 10 and linear actuator as a whole.

The grooved rails 30 are made of a hard, wear-resistant alloy, and they can easily be replaced as they wear out. Besides, grooves 3T are much easier to machine, especially for long stroke linear actuator.