GB2211908A - Linear actuator - Google Patents

Abstract

A linear actuator has a screw threaded shaft 11 mounted so that it is restrained from rotational movement, a housing 13 is mounted for rotation on the shaft 11, a threaded member 20 is rotationally mounted eccentrically of the housing 13, the diameter of the threadform of the threaded member 20 being different from that of the shaft 11 and being arranged to engage the threadform of the shaft 11 on one side thereof, and drive means being provided to rotate the housing 13 so that relative movement between the threads of the shaft 11 and internally threaded member 20 will produce relative axial movement between the shaft 11 and housing 13. <IMAGE>

Description

LINEAR ACTUATOR The present invention relates to linear actuators and in particular to linear actuators of the screw thread type.

Hitherto linear actuators have been provided in which an externally threaded shaft engages an internally threaded member, the velocity ratio, that is the ratio of the number of turns of the driven component to the number of pitch lengths of linear travel of the other component of such systems being 1. It is possible by increasing the diameter of the internally threaded member, so that the threads thereon engage the threads on the shaft member at only one point, to provide a linear actuator with an increased velocity ratio.

Drive is conventionally applied to such actuators by rotation of the shaft. The shaft must consequently be supported in bearings which will run under full load at the drive speed and there will consequently be relatively high frictional losses at these bearings.

According to one aspect of the present invention a linear actuator comprises a screw threaded shaft mounted so that it is restrained from rotational movement, a housing mounted for rotation on said shaft, a threaded member rotationally mounted eccentrically of the housing, the diameter of the threadform of the threaded member being different than that of the shaft and being arranged to engage the threadform of the shaft, and drive means being provided to rotate the housing.

With this arrangement, as the housing is driven so that it rotates relative to the shaft, the threaded member will describe an orbital motion relative to the shaft. As the diameter of the threaded member is different than that of the shaft, the orbital motion will cause the threaded member to rotate relative to the shaft, thus causing relative axial movement. The speed of relative rotation will depend upon the eccentricity of the threaded member, but will generally be much slower than the drive speed.

Consequently while there will still be frictional losses between the threaded member and means for transmitting axial loads to or from the threaded member, these will be at relatively slow speed and will consequently be very much reduced as compared to the losses at the bearings of the shaft in the hitherto known devices.

An embodiment is now described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a sectional elevation of a linear actuator in accordance with the present invention; and Figure 2 is an enlarged view of the threadform of the shaft and internally threaded member, shown in Figure 1.

As illustrated in Figure 1 the linear actuator comprises shaft 11 which is non-rotatably mounted in suitable manner. The shaft 11 is provided with an acme type thread 12, which as illustrat-ed in Figure 2 has a profile angle of 290 and flat crests.

A housing 13 is mounted on the shaft 11 by means of a roller bearing 14, the internal race 15 of the roller bearing 14 slidably engaging the flat crests of thread 12.

The bearing 14 is maintained in housing 13 between a shoulder portion 16 and a circlip 17.

An internally threaded member 20 having an acme type thread 21 similar to that of shaft 11 but of larger diameter, is mounted in an eccentric bore 22 in the housing 13 by means of roller bearing 23, so that at one point, the thread 21 thereof engages the thread 12 of the shaft 11.

A load supporting member 25 is slidingly located on the shaft 11 by means of a cylindrical bearing 26 which engages the flat crests of the thread 12. A series of radially inwardly directed angularly spaced pins 27 are provided on the support plate 25 and engage in a circumferential groove 28 in the internally threaded member 20. These pins 27 transfer the thrust between the support member 25 and internally threaded member 20 while permitting relative rotational and eccentric movements therebetween.

A balance weight 30 is provided about the housing 13 to compensate for centrifugal imbalance due to the eccentricity of the internally threaded member 20, bearing 23 and the bore 22 in housing 13. Means (not shown) is also provided to drive the housing 13. Drive of the housing 13 may be achieved, for example, by means of a belt drive from a remote motor or the housing itself may be defined by the rotor of the motor.

When the housing 13 is rotated relative to the shaft 11, if the load applied to the support member 25 is sufficiently low, friction between the threads 12 and 21 will be lower than that of the bearing 23 and the internally threaded member 20 will rotate with the housing 13 giving a drive ratio of 1:1. If however the load applied to support member 25 is such that friction between the threads 12 and 21 is greater than that of the bearing 23, the housing will rotate relative to the internally threaded member 20 and will cause the internally threaded member 20 to describe an orbital motion about the shaft 11. Because of frictional engagement between the threads 12 and 21 and the difference in their diameters, this orbital motion will produce rotation of the internally threaded member 20 with resulting relative axial movement between the internally threaded member 20 and shaft 11.For example, if the diameter ratio of the thread 12 to thread 21 is 3:4, for every orbit of the internally threaded member 20 about the shaft 11, the internally threaded member 20 will rotate by one quarter of a turn and consequently, for four revolutions of the housing 13 the internally threaded member 20 will move one pitch of the screw thread, axially relative to the shaft 11, that is a drive ratio of 1:4.

The device described above will consequently give a linear actuator the drive ratio of which will vary with the load applied.

Various modifications may be made without departing from the invention. For example, rather than having shaft 11 fixed axially and support member 25 and housing 13 etc.

movable axially thereon, the housing 13 and support member 25 may be fixed axially and the shaft 11 movable axially.

Also, while frictional losses occurring between the internally threaded member 20 and support member 25 are very much reduced due to the relatively slow speed of rotation of the internally threaded member 20 when a load is applied, these losses may be reduced even more, by the use of low friction elements. For example, the pins 27 may be replaced by rollers or alternatively roller bearings which are arranged to slide laterally in order to permit eccentric movement of the internally threaded member 20.

For a given nominal diameter of threads 12 and 21, the diameter ratio and hence the drive ratio may be varied, within the limitations of the thread form, by varying the radial overlap of the threads. This may be achieved by providing means for varying the eccentricity of bearing 23 with respect to the housing 13, for example by the interposition of an annular member, the internal diameter of which is eccentric to the external diameter, between the outer face of the bearing 23 and the housing whereby eccentricity of the internally threaded member 20 may be adjusted by rotation of the annular member. Furthermore, while in the embodiment described above, the drive ratio will change from 1:1 to a reduced ratio as a result of the load applied to the support member 25, means may alternatively be provided for locking the bearing 23 so that change in the ratio may be effected as desired.

While in the above embodiment the shaft 11 has an external thread and member 20 an internal thread of larger diameter, the shaft 11 may be tubular with an internal thread and member 20 be mounted within the shaft and have an external thread of smaller diameter.

Claims (13)

1. A linear actuator comprising a screw threaded shaft mounted so that it is restrained from rotational movement, a housing mounted for rotation on said shaft, a threaded member rotationally mounted eccentrically of the housing, the diameter of the threadform of the threaded member being different than that of the shaft and being arranged to engage the threadform of the shaft, and drive means being provided to rotate the housing.

2. A linear actuator according to claim 1 in which the threadforms of the shaft and threaded member have flat crests.

3. A linear actuator according to claim 1 or 2 in which the housing is mounted on the shaft by means of a low friction element which will permit rotation and axial movement of the housing with respect to the shaft.

4. A linear actuator according to any one of claims 1 to 3 in which the threaded member is located in and eccentric bore in the housing by means of a low friction element.

5. A linear actuator according to any one of the preceding claims in which a load supporting member is slidingly mounted on the shaft and is coupled to the threaded member by means which will transmit axial loads while permitting relative rotation and eccentric movement of the threaded member with respect to the support member.

6. A linear actuator according to claim 5 in which a series of angularly spaced radially inwardly extending members are provided on the load supporting member, these members engaging in a circumferential groove in the threaded member.

7. A linear actuator according to any one of the preceding claims in which the housing and support member are movable axially along the shaft.

8. A linear actuator according to any one of claims 1 to 6 in which the housing and support member are fixed axially and the shaft is movable axially with respect thereto.

9. A linear actuator according to any one of the preceding claims in which the drive means comprises a belt or like means by which the housing is driven from a remote motor.

10. A linear actuator according to any one of claims 1 to 8 in which the housing is defined by the rotor of a motor.

11. A linear actuator according to any one of the preceding claims in which the threaded member is mounted with respect to the housing in a manner which will permit adjustment of its eccentricity within the extremes of overlap of the threads of the shaft and threaded member.

12. A linear actuator according to any one of the preceding claims in which means is provided for selectively locking the threaded member rotationally relative to the housing.

13. A linear actuator substantially as described herein with reference to, and as shown in, figures 1 and 2 of the accompanying drawings.