GB2290662A - Torque limiter device - Google Patents

Abstract

A torque limiter device for the protection of rotatable parts such as gear boxes having associated driving means comprises first and second elements (8, 18) and biasing means (23) acting between them. A switch (40) for the driving means is associated with one of the elements (18) and an actuator (36) for the switch (40) is associated with the other element (8) and arranged spaced from the switch (40). The limiter is securable to the rotatable part in such a way that the first and second elements (8, 18) are capable of being brought together, against the action of the biasing means (23) once torque in the rotating part and thereby torque reaction in the device reach a predetermined level. In this way the actuator (36) is capable of contacting the switch (40) to cut out the driving means. The device is thus capable of protecting machinery by preventing safe torque values from being exceeded. <IMAGE>

Description

PROTECTION SYSTEM The present invention concerns a system for limiting damage to machinery. The invention is particularly directed to systems for protecting machinery by preventing safe torque values from being exceeded.

The invention finds particular, but not exclusive, use in relation to torque protection for shaft mounted gear boxes. In this respect, it is noted that problems arise in the prevention of damage to machinery, particularly machinery including shaft mounted gear boxes and motor gear units and/or motors.

Often, bearing in mind the limited range of motors readily available, it is generally not possible to obtain a motor which has an output closely matching desired input of selected machinery. Thus, it is often necessary to use a motor which is in fact much more powerful than that required to drive the machinery. This can be most disadvantageous, particularly in the event of an overload which can result in serious damage to the machinery.

In the case of gear boxes, there are various cut out systems available which are designed to disengage the motor from the machinery in the event of an overload.

Examples include mechanical trips on slipping clutches.

However, they often provide unsatisfactory results in that they cannot be guaranteed to slip at the appropriate time i.e. they may slip too often or not readily enough.

In this field, electrical cut out systems are also of little benefit. In particular, systems designed to detect changes in motor current or motor speed are unsatisfactory because, for a considerable increase in output torque on a gear box, for example, there may be a negligible change in motor speed or current.

The present invention seeks to alleviate at least some of the aforementioned problems.

Accordingly, with this object in mind, the present invention provides a torque limiter device for protection of rotatable parts having associated driving means, the limiter device comprising first and second elements and biasing means acting between them, a switch for the driving means being associated with one said element and an actuator for the switch being associated with the other said element and arranged spaced from the switch, the limiter being securable to the rotatable part and arranged such that the first and second elements are capable of being brought together, against the action of the biasing means, once torque in the rotating part and thereby torque reaction in the device reach a predetermined level so that the actuator is capable of contacting the switch to cut out the driving means.

Preferably, the torque limiter device is adapted for use with rotatable parts such as gear boxes, particularly shaft mounted gear boxes. Suitably, in use, the limiter device may be used in place of a torque arm supplied by a gear box manufacturer. Example driving means are motors, particularly electric motors. The switch for the driving means would be arranged, in use, to communicate with the motor electrical supply.

The biasing means are suitably pre-loaded so that effectively no movement of the first and second elements takes place until overload occurs. In preferred embodiments, means are provided for adjusting the preload of the biasing means. Preferably, such adjustment means may be actuable when the torque limiter is in use, secured to a rotating part, so that that biasing means may be set to match the desired working load of the driving means. Preferred biasing means take the form of springs, especially disc springs.

In some embodiments one element of the device is at least partially slidably accommodated in the other element. In preferred embodiments, one element takes the form of a piston cylinder and the other element the form of a piston rod. Preferably, any such piston rod has an enlarged head and the biasing means are arranged to act between the enlarged head of the rod and an internal surface of the piston cylinder. Suitably the enlarged head provides a shoulder and the biasing means are arranged to act between the shoulder and the piston cylinder. The biasing means may be arranged to bias the piston rod inwardly or outwardly of the cylinder, as appropriate.

Suitably, means are provided for securing the first element, such as the piston rod, to the rotatable part and means are provided for securing the second element, such as the piston cylinder, against rotation with respect to the rotatable part. Example securing means is a clevis.

An embodiment of the present invention will now be described further, by way of example, with reference to the accompanying drawings in which: Fig. 1 illustrates the location of a limiter device, in use; and Fig. 2 shows a cross section of an example device.

Turning firstly to Fig. 1, this shows a gear box 1 including an output shaft 3 which, in use, is driven by a motor. The motor rotates the shaft 3 in a clockwise direction. Conventionally, to prevent counter clockwise rotation of the gear box 1, a torque arm is fixedly secured to the box and to a bracket (or similar) alongside the gear box. In the present invention, the torque arm is modified to include an overload protector or torque limiter device, indicated generally at 5.

Fig. 2 shows the torque limiter 5 in more detail. This comprises a first member generally in the form of a piston rod 8 and a second member in the form of a piston cylinder 18. The piston rod 8 has a body 9 and an enlarged head 12 which defines a shoulder 13. The piston cylinder 18 has a reduced diameter part 20 which defines an opening 21 of a size matching that of the body 9 of rod 8. The rod 8 is also provided with a screw thread along part of its length. The cylinder 18 has a matching screw thread at the opening 20 to allow interengagement of the rod 8 and cylinder 18. At its end opposite opening 21 the cylinder is screwthreadedly connected to a cylinder end plug 19.

Biasing means 23 in the form of disc springs are located around the body 9 of rod 8 in the cylinder 18 and are arranged to act between the head 12 of the rod 8 and reduced diameter part 20 of the cylinder 18. A nut 27 is screwthreadedly connected to piston rod 8, externally of the opening 21 of cylinder 18. The nut 27 provides a means for adjusting pretensioning of the disc springs 23.

The first and second elements are each fixedly mounted to a clevis, shown as 15 and 25 respectively. In this regard body 9 of the piston rod 8 is screwthreadedly connected to clevis 15 and cylinder end plug 19 screwthreadedly connected to clevis 25.

The piston cylinder 18 is provided with a lateral opening 30. A plate 32 is secured to piston head 12 and arranged to extend through the opening 30. At the free end of the plate 32 there is a nut and bolt arrangement 36.

A micro-switch 40 is mounted on the external surface of piston cylinder 18 alongside the opening 30. The nut and bolt arrangement acts as an actuator for the microswitch. A nut and bolt actuator is useful in that the nut may be tightened or untightened to alter the spacing between the actuator and microswitch. This provides a means of adjusting the torque limiter.

Appropriate circuitry (not illustrated) will be provided to permit the micro-switch to actuate a motor for a gear box or other apparatus.

The apparatus may be employed as follows to protect a shaft mounted gear box.

Clevis 15 is secured to gear box 1 and clevis 25 is fixedly secured in relation to the gear box, suitably by mounting it to a bracket alongside the box. In use, a motor is actuated to drive rotation of shaft 3 of gear box 1. As the gear box 1 is driven there is a torque reaction in the limiter 5. When the motor is providing the desired output the actuator 36 is arranged spaced from the microswitch 40. The spacing is typically very small, such as a few thousandths of an inch. The actuator 36 and switch 40 are retained at this spacing by the action of disc springs 23.

Once the torque in the gear box 1 (and thus the torque reaction in limiter 5) exceeds a certain predetermined level, the head 12 of piston rod 8 is capable of moving against the bias of disc springs 23, towards reduced diameter part 20 of the cylinder. This in turn displaces plate 32 and actuator 36, to permit the actuator to contact micro-switch 40 to cut out the driving means.

It will be appreciated that the torque limiter 5 may be preset so that piston head 12 will only be displaceable once the torque reaction in the limiter 5 exceeds a specified level. This level could be determined using a tensometer, for example. Alternatively the level can be set by adjusting nut 27 and the nut in the actuator 36 when the limiter is in use, with a motor running. In this way the limiter may be set so that the switch and actuator are just spaced apart when the motor is working at the desired torque and are brought together when this is exceeded. Suitably the limiter is constructed so that the microswitch is actuated once gear box torque exceeds the desired working level by 5%.This may be arranged by securing the pretensioning nut 27 and selecting appropriate disc springs 23 so that the piston head 20 will move the desired distance only once the specified torque reaction is exceeded.

The present torque limiter has particular application to slow moving conveyor systems of the type having a guide track and an endless linkage which is advanced along the track. Typically, such linkages are driven using a motor and large reduction gear boxes. Difficulties arise in such systems in that, if a fault arises in the system to restrict free movement of the linkage, the motor may nevertheless continue to drive the gearbox and restrained linkage. This puts the linkage and motors under increasing strain which can cause damage, and possibly breakage, to the system. The present invention can alleviate such problems by providing a means for detecting excessive torque levels in a gearbox (or other apparatus) and cutting out the motor as soon as the excessive torque is detected. Thus, the present invention may be employed to detect problems at an early stage.

It will be appreciated that various modifications may be made to the illustrated embodiment. For example, the torque limiter device may employ biasing means under tension or compression. The illustrated embodiment is however preferred in terms of ease of manufacture.

Claims (13)

1. A torque limiter device for protection of rotatable parts having associated driving means, the limiter device comprising first and second elements and biasing means acting between them, a switch for the driving means being associated with one said element and an actuator for the switch being associated with the other said element and arranged spaced from the switch, the limiter being securable to the rotatable part so as to be arranged such that the first and second elements are capable of being brought together, against the action of the biasing means, once torque in the rotating part and thereby torque reaction in the device reach a predetermined level so that the actuator is capable of contacting the switch to cut out the driving means.

2. A torque limiter device according to Claim 1 wherein the biasing means are pre-loaded so that movement of the first and second elements takes place substantially only when torque in the rotating part exceeds a desired working level.

3. i A torque limiter device according to Claim 1 or Claim 2 wherein means are provided for adjusting the pre-load of the biasing means.

4. A torque limiter device according to Claim 3 wherein the adjustment means are arranged so as to be actuable when the torque limiter is in use, secured to a rotating part, so that that biasing means may be set to match the desired working load of the driving means.

5. A torque limiter device according to any preceding claim wherein the biasing means take the form of springs.

6. A torque limiter device according to Claim 5 wherein the biasing means are disc springs.

7. A torque limiter device according to any preceding claim wherein one element of the device is at least partially slidably accommodated in the other element.

8. A torque limiter device according to Claim 7 wherein one element takes the form of a piston cylinder and the other element the form of a piston rod.

9. A A torque limiter device according to Claim 8 wherein the piston rod has an enlarged head and the biasing means are arranged to act between the enlarged head of the rod and an internal surface of the piston cylinder.

10. A torque limiter device according to Claim 9 wherein the enlarged head provides a shoulder and the biasing means are arranged to act between the shoulder and the piston cylinder.

11. A torque limiter device according to any preceding claim which includes means for securing the first element to the rotatable part and means for securing the second element, against rotation with respect to the rotatable part.

12. A torque limiter device substantially as hereinbefore described with reference to, and as illustrated in, any one of the accompanying drawings.

13. A shaft mounted gear box provided with a torque limiter device as claimed in any one of the preceding claims.