GB2092702A - Improvements in or relating to screw and nut actuators - Google Patents

Abstract

A screw and nut actuator has an actuating rod 15 carried by a nut 14 non-rotatably engaged with a lead screw 7. Rotation of the lead screw 7 causes relative axial movement of the nut 14 and hence of the actuating rod 16. The lead screw 7 is so mounted that it permits tensile loading but prevents compressive loading. This may be accomplished by holding opposite ends of the lead screw 7 against movement together but permitting them to move apart. Alternatively or additionally the lead screw 7 may be pretensioned to avoid compressive loading. The nut 14 may be mounted on four lead screws 7 equiangularly arranged about the axis of a motor drive shaft 23. <IMAGE>

Description

SPECIFICATION Improvements in or relating to screw and nut actuators This invention relates to mechanical actuators of the kind known as screw and nut actuators in which an actuating rod is carried by a nut nonrotatably engaged with a lead screw which is rotatable to cause relative axial movement of the nut and hence of the actuating rod.

Conventional screw and nut actuators use a single screw which is axially immobilised and is either in compression or tension depending on the load direction. When in compression the screw behaves as a strut which causes radial deflections resulting in attempted misalignments and hence increased loads on bearing surfaces. When in tension there are no radial deflections because all the forces tend towards a straight line.

It is an object of the present invention to provide a screw and nut actuator in which compressive loading of the lead screw is avoided.

According to the present invention there is provided a screw and nut actuator of the kind defined, in which the nut is carried by at least one lead screw so mounted that its opposite ends are held against movement together but are free to move apart and/or the screw is pretensioned thereby permitting tensile loading of the screw but preventing compressive loading thereof.

The invention will now be further described by way of example only, with reference to the accompanying drawings, in which: Fig. 1 is a longitudinai section of one embodiment of actuator according to the invention, and Fig. 2 is a cross-section on line Il-Il of Fig. 1.

Referring now to the drawings, the actuator comprises a two-part cylindrical casing 1 a, 1 b with square end walls 2, 3 interconnected at corresponding corners by four tie-rods 4 engaging end nuts 5. An intermediate wall 6 separates the two parts of the casing 1 a, 1 b and has four clearance holes 6a for respective lead screws 7 extending axially of the casing through corresponding clearance holes 3a in the end wall 3. Thrust bearings 8, 9 are provided at each end of each lead screw 7. As shown in Fig. 2 the four lead screws 7 are equiangularly spaced about the longitudinal axis of the cylindrical casing 1 a, 1 b.

The left-hand (LH) end of each screw 7 is rotationally fast with a gear 10 and the right-hand (RH) end is provided with a hexagonal nut 11.

Each gear 10 is held in position by a grub screw 12 and a locking nut 13. Bodily shifting of the screws 7 in the axial direction is thus prevented by the gears 10, as regards movement towards the right, and by the hexagonal nut 11 as regards movement towards the left. Small clearances between the screws 7 on the one hand and the walls 3, 6 and the bearings 8, 9 on the other hand do however permit movement of either end of each screw 7 away from the other end thereof.

The possibility of such relative movement is important for reasons explained below.

The four lead screws 7 are in threaded engagement with a generally square driven nut 14 fixed to one end of an actuating rod 1 5 which extends along the axis of the casing 1 a, 1 b to emerge through a bush 1 6 in the end wall 3 and an end cap 1 7. Rotation of the screws 7 causes movement of the driven nut 14 axially of the casing part 1 b. The maximum extent of such movement is defined by LH and RH limit switches (not shown) which control the operation of a drive motor 18.

The drive motor 1 8 is mounted in the casing part 1 a. The motor 1 8 is an a.c. induction motor with a cylindrical stator 1 9 held in the casing part 1 a by retaining rings 20. Stator windings are shown at 21. A rotor 22 is mounted within the stator 19 on a motor drive shaft 23. The LH end of the drive shaft 23 is rotatably mounted in a bearing 24 held in the end wall 2 and the reduced diameter RH end of the drive shaft 23 is carried in a needle roller bearing 25 fixed in the intermediate wall 6. A gear 26 is secured to the RH end of the drive shaft 23 by a grub screw 27 and meshes with the four gears 10 at the LH ends of the lead screws 7.

The mode of operation will now be described starting with the actuating rod 1 5 in the retracted position as shown in Fig. 1. The drive motor 1 8 is started to cause rotation of the lead screws 7 by way of the motor drive shaft 23, gear 26 and the four gears 10. The driven nut 14 is thus moved towards the right in Fig. 1 and the actuating rod 1 5 fixed thereto is extended from the casing 1 a, 1 b by a corresponding distance. As the driven nut 14 approaches the vicinity of the end wall 3 the RH limit switch is operated and the motor 1 8 is stopped.When the motor 1 8 is now restarted to rotate the drive shaft 23 in the opposite direction the driven nut 14 and the actuating rod 1 5 are moved towards the left in Fig. 1 until the LH limit switch is operated as the nut 1 4 approaches the vicinity of the intermediate wall 6.

It will be appreciated that during extension of the actuating rod 1 5 under load those (RH) sections of the lead screws 7 between the driven nut 14 and the hexagonal nuts 11 at the RH end will be subject to tensile forces. By contrast, when the actuating rod 1 5 is being retracted under load those (LH) sections of the lead screws 7 between the driven nut 14 and the gears 10 at the LH end will be subject to tensile forces. In each case, the other sections of the lead screws 7 would be expected to be subjected to undesirable compressive stresses were it not for the fact that the ends of said other sections, namely the LH ends of the lead screws 7 in the case of extension and the RH ends in the case of retraction, are free to move in the direction away from the driven nut 14.The actual distances involved in such floating movement are extremely small and easily accommodated by the clearances between the ends of the lead screws 7 and the adjacent components, i.e. the drive motor 1 8 and the end cap 17.

Preferably, the lead screws 7 are pretensioned by tightening the hexaginal nuts 11 at the RH ends during assembly. Such pretensioning is useful to ensure straightness of the lead screws 7 but it also has the effect of reducing the floating movement of the screw ends that is required to compensate the compressive stresses. Thus, both the RH and LH sections of the lead screws will have an assembly tension which, depending upon whether the actuating rod is being extended or retracted, will be increased in the one sections of the lead screws and reduced in the other sections. Only when the tensile prestress in said other sections has reduced to zero is it necessary to compensate the compressive forces by floating movement of the corresponding ends of the screws.If the tensile prestress is sufficiently high any compressive load can be accommodated by partially or wholly relieving the tensile forces in the screw sections and no floating movement of the screw ends need occur.

Numerous modifications may be made in the actuator described above by reference to the drawings, without departing from the scope of the invention. For example the number of lead screws may be reduced or increased although four does seem to be an optimal number in achieving a wellbalanced design making the best use of the space available within the casing. It is possible to provide only a single lead screw positioned on the axis of the casing but multiple screws have the advantage that there is no resultant torque reaction on the actuator rod during operation.

The actuator may be driven by an alternative kind of motor, e.g. pneumatic or hydraulic, as convenient. The helix angle of the engaging screw threads of the lead screws and the driven nut is such that an external load acting through the actuating rod cannot move the nut along the screw even when the motor is switched off.

Claims (5)

1. A screw and nut actuator of the kind defined, in which the nut is carried by at least one lead screw so mounted that it permits tensile loading but prevents compressive loading thereof.

2. An actuator as claimed in claim 1, wherein opposite ends of the or each lead screw are held against movement together but are free to move apart.

3. An actuator as claimed in claim 1 or 2, wherein the or each lead screw is pretensioned.

4. An actuator as claimed in any one of the preceding claims, wherein four lead screws are equiangularly arranged about the axis of a motor drive shaft and the one ends of said screws are provided with respective driven gears engaged by a common driving gear on the motor drive shaft.

5. A screw and nut actuator substantially as herein described with reference to and as illustrated in the accompanying drawings.