GB2211918A

GB2211918A - Torque-controlled hydraulic coupling

Info

Publication number: GB2211918A
Application number: GB8825600A
Authority: GB
Inventors: Joseph Gardner
Original assignee: Individual
Current assignee: Individual
Priority date: 1987-11-04
Filing date: 1988-11-02
Publication date: 1989-07-12
Also published as: GB8725861D0; GB8825600D0

Abstract

A torque limiter comprising an input member to which an input torque is to be applied, and an output member to which torque is to be transmitted from the input member, one of the two members comprising a first component, such as a housing 2, of a rotary hydraulic pump and the other of the two members comprises a second component, such as the rotor 4, of the hydraulic pump. The pump is arranged such that hydraulic fluid is pumped from a pump inlet port 15 to a pump outlet port 14 as a result of relative rotation between the two pump components 2, 4. The pump inlet and output ports are connected by a bypass passageway, and a pressure relief valve 16 is positioned in the bypass passageway so as to prevent fluid flowing from the outlet to the inlet port unless the pressure difference between the ports exceeds the relief pressure required to open the valve. The maximum torque which can be transmitted through the torque limiter is thus limited to the torque required to raise the pressure difference to the pressure required to open the valve 16. A second pressure relief valve may be arranged in parallel with the valve 16. <IMAGE>

Description

TORQUE LIMITER The present invention relates to a torque limiter.

Torque limiters are used in a variety of applications to limit the maximum torque which can be transmitted from a power source to a machine driven by that power source. For example a conventional agricultural tractor is provided with a power take-off output shaft driven by the tractor engine.

The power take-off shaft is used to transmit power to an agricultural implement towed by or simply connected to 'the tractor. The maximum torque which can be delivered by the power take-off shaft is often many times greater than the maximum power required to drive a particular piece of agricultural equipment and is generally greater than the torque required to damage the agricultural implement if it becomes jammed or otherwise inoperative. For this purpose mechanical torque limiters have been provided which comprise an input shaft which is directly coupled to a power take-off shaft, an output shaft which is directly coupled to an implement to be driven, and friction pads which are interposed between the input and output shafts. The pressure applied to the friction pads can be adjusted by tightening or slackening adjustment bolts.The slacker the bolts the easier it is for the friction pads to slip and accordingly the maximum torque which can be transmitted from the input to the output shaft can be controlled.

It is å well know characteristic of the known torque limiters that the maximum torque limit of the device can be set reasonably accurately when the pressure applied to the friction pads is first applied but that the maximum torque limit increases substantially over a period of time if no slippage occurs between the friction pads and the other components of the device. It is therefore standard practice for manufacturers of torque limiters to guarantee the performance of their products only if the pressure applied to the friction pads is regularly released and re-applied.Therefore, for a user of such a torque limiter to be sure that it will function properly, it is necessary before each use of the device to first release the pressure on the friction pads, then to apply a torque such that the friction pads slide relative to the other components of the device, and then to apply the required pressure to set the maximum torque limit at a desired level. Operators are instructed that this procedure must be repeated at least on a daily basis. As the procedure takes a considerable amount of time, sometimes as much as one hour, it is quite common for the procedure simply not to be followed. If an agricultural instrument being driven through the torque limiter subsequently jams there is a high probability that the torque limiter will deliver so much power that the implement will be seriously damaged.The user of the torque limiter will however have no basis for complaint against the torque limiter manufacturer as correct operating procedures have not been followed.

It is known to provide a shear pin in transmission systems incorporating conventional torque limiters, the shear pin being arranged to fracture as soon as short term loads exceed acceptable limits. It has been found that shear pins are more effective than conventional torque limiters in dealing with very rapid increases in loadings resulting for example from driven equipment becoming jammed. It is also known to fit free-wheel devices with conventional torque limiters so that if the speed of the driven equipment exceeds that of the power source torque is not applied to the power source by the driven equipment. Finally, it is known to use spring couplings to smooth out the fluctuations in torque applied through a transmission connecting a power source to driven equipment.Thus in the case of conventional torque limiters it is possible to have in effect four separate components connected in series, that is a torque limiter, a shear pin, a spring coupling and a free-wheel assembly. Such a series combination of units is complex and costly.

It is an object of the present invention to provide a torque limiter which obviates or mitigates the problems outlined above.

According to the present invention there is provided a torque limiter comprising an input member to which an input torque is to be applied, and an output member to which torque is to be transmitted from the input member, wherein one of the said members comprises a first component of a rotary hydraulic pump and the other of the said members comprises a second component of the hydraulic pump, the pump being arranged such that hydraulic fluid is pumped from a pump inlet port to a pump outlet port as a result of relative rotation between the two said pump components, the pump inlet and outlet ports are connected by a bypass passageway, and a pressure relief valve is positioned in the bypass passageway so as to prevent fluid flowing from the outlet to the inlet port unless the pressure difference between the ports exceeds the relief pressure required to open the said valve, whereby the maximum torque which can be transmitted through the torque limiter is limited to the torque required to raise the said pressure difference to the said required pressure.

Preferably, the hydraulic pump is a vane pump, the first component supporting a plurality of spring loaded vanes which sweep out a chamber defined by the second component as the two components rotate relative to each other, the vanes thereby pumping fluid through the chamber from the inlet port to the outlet port.

Preferably, the pressure relief valve is of a type which recloses as soon as the pressure difference between the inlet and outlet ports drops below the said required pressure. Two identical pressure relief valves could be arranged in parallel in the passageway so as to enable the torque limiter to operate in an identical manner in both forward and reverse directions.

A further valve may be provided in parallel with the or each pressure relief valve, the further valve being arranged to open rapidly in response to the pressure across it exceeding the pressure at which the associated said pressure relief valve opens. The further valve may be of a type which is maintained open until the pressure across it drops significantly below that which was required to open it. The further valve thus provides a response which is in mechanical terms analogous to that provided by a conventional shear bolt as described above.

Where only a single pressure relief valve is provided, a further valve may be connected in parallel therewith, the further valve being a one-way valve connected in opposition to the pressure relief valve. Thus the one-way valve opens in response to even a small excess pressure in the pump inlet relative to the pump outlet.

A spring expansion device may be connected to the hydraulic circuit of the pump so as to smooth out large short term fluctuations in the pressure applied across the pressure relief valve.

An embodiment of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which: Fig. 1 is a schematic sectional view perpendicular to the axis of an embodiment of the present invention; and Fig. 2 is a sectional view on the line 2-2 of Fig. 1 assuming rotation of components shown in Fig.

1 to the position shown in broken lines.

Referring to the drawings, the illustrated torque limiter comprises an input member 1 in the form of a splined shaft of a conventional type for coupling the input member to for example a tractor power take off. An output member in the form of a housing 2 is connected by a conventional universal joint to a propellor shaft 3 to which torque is to be delivered from the splined input shaft 1.

The housing 2 supports the input shaft 1 and a rotor 4 mounted on the input shaft in radial thrust bearings 5 and 6 and a side thrust bearing 7. The rotor 4 supports two spring loaded vanes 8, the spring loading biasing the vanes 8 radially outwards from the rotor and into contact with the wall 9 of a chamber defined within the housing.

As best seen from Fig. 1, the chamber defined by the housing is of eccentric shape such that the chamber is divided by the vanes 8 into three sub-chambers 10, 11 and 12. If the rotor 4 rotates in the direction of arrow 13 relative to the housing 2 the chamber 10 increases in volume whereas the chamber 12 decreases in volume. If there is no communication between chambers 10 and 12 the differential pressure which is the result of any relative rotation between the housing and rotor opposes the relative rotation between the housing and rotor and effectively jams the two components subject to any minor leakage of hydraulic fluid which might occur.

In accordance with the present invention a bypass passageway is provided which connects chambers 10 and 12 together, the passageway extending from a port 14 to a port 15. The assembly effectively acts as a pump having an outlet port 14 connected to an inlet port 15 and providing fluid can flow between the two ports there is little resistance to the relative rotation of the rotor and housing. A pressure relief valve 16 is positioned in the bypass passageway however and this pressure relief valve prevents flow of fluid from port 14 to port 15 unless the pressure in chamber 12 substantially exceeds that in chamber 10. Thus, providing the pressure difference across the pressure relief valve is insufficient to open that valve the rotor 4 and hence the input shaft 1 rotates at substantially the same speed as the housing 2 and hence the propellor shaft 3.As soon as the torque applied through the device is sufficiently high for the pressure difference across the pressure relief valve 16 to open, the input shaft 1 slips relative to the propellor shaft 3 and thus the maximum torque applied to the propellor shaft 3 is limited.

As best seen in Fig. 2, a second pressure relief valve 17 is provided connected in parallel with pressure relief valve 16. Whereas pressure relief valve 16 is structured such that it recloses as soon as the pressure across it drops below its normal opening pressure, pressure relief valve 17 has a characteristic such that it opens at a higher pressure than pressure relief valve 16 but recloses at a lower pressure than pressure relief valve 16.

Valve 17 is thus effective to respond rapidly to very high short term pressure surges resulting for example from equipment driven by the propellor shaft 3 jamming suddenly. The function of pressure relief valve 17 is thus analogous to that of a shear bolt.

The pressure relief valves 16 and 17 are of conventional design and will not be described further herein. Each pressure relief valve is connected by appropriate passageways 18 and 19 (Fig. 1) to the ports 14 and 15. Each of the ports 14 and 15 extends between the end walls of the housing 2 and terminates in a void 20 which enables fluid to flow freely into or out of the respective ports.

It will be appreciated that a completely sealed unit can be manufactured. The setting of the torque limit can be easily adjusted by adjustment of the opening pressure of the pressure relief valves and the nature of the pressure relief valves is such that there is no "drift" of the setting with time as is the case with torque limiters based on friction discs. If the drive does slip in use to relieve excess torque there is no need to subsequently reset the pressure relief valves.

As an extra feature, a one-way valve can be connected in parallel with the valves 16 and 17 between ports 14 and 15. The one-way valve would be arranged to remain shut at all times unless the pressure in chamber 10 exceeded that in chamber 12.

The one-way valve would thus have no effect except when the propellor shaft 3 was in effect trying to drive the input shaft 1 but in those circumstances would effectively -relief the linkage between the propellor shaft 3 and the input shaft 1 to enable the propellor shaft 3 to free-wheel. In an alternative arrangement the one-way valve could be replaced by a further pressure relief valve so that the torque limiter would then have a similar characteristic in both forward and reverse directions.

Claims

1. A torque limiter comprising an input member to which an input torque is to be applied, and an output member to which torque is to be transmitted from the input member, wherein one of the said members comprises a first component of a rotary hydraulic pump and the other of the said members comprises a second component of the hydraulic pump, the pump being arranged such that hydraulic fluid is pumped from a pump inlet port to a pump outlet port as a result of relative rotation between the two said pump components, the pump inlet and outlet ports are connected by a bypass passageway, and a pressure relief valve is positioned in the bypass passageway so as to prevent fluid flowing from the outlet to the inlet port unless the pressure difference between the ports exceeds the relief pressure required to open the said valve, whereby the maximum torque which can be transmitted through the torque limiter is limited to the torque required to raise the said pressure difference to the said required pressure.

2. A torque limiter according to claim 1, wherein the hydraulic pump is a vane pump, the first component supporting a plurality of spring loaded vanes which sweep out a chamber defined by the second component as the two components rotate relative to each other, the vanes thereby pumping fluid through the chamber from the inlet port to the outlet port.

3. A torque limiter according to claim 1 or 2, wherein the pressure relief valve is of a type which recloses as soon as the pressure difference between the inlet and outlet ports drops below the said required pressure.

4. A torque limiter according to claim 1, 2 or 3, wherein two identical pressure relief valves are arranged in parallel in the passageway so as to enable the torque limiter to operate in both forward and reverse directions.

5. A torque limiter according to any preceding claim, wherein a further valve is provided in parallel with the or each pressure relief valve, the further valve being arranged to open rapidly in response to the pressure across it exceeding the pressure at which the associated said pressure relief valve opens.

6. A torque limiter according to claim 5, wherein the further valve is of a type which is maintained open until the pressure across it drops significantly below that which was required to open it.

7. A torque limiter according to claim 1, 2 or 3, wherein a single pressure relief valve is provided, and a further valve is connected in parallel therewith, the further valve being a one-way valve connected in opposition to the pressure relief valve.

8. A torque limiter according to any preceding claim, comprising a -spring expansion device connected to the hydraulic circuit of the pump so as to smooth out large short term fluctuations in the pressure applied across the or each pressure relief valve.

9. A torque limiter, substantially as hereinbefore described with reference to the accompanying drawings.