GB2194589A - Gear pump in a hydraulic actuator control - Google Patents

Abstract

A control for a hydraulic actuator (10, Fig. 1) forming part of a hydraulic circuit, includes a gear pump 19. In order to cause a cyclic pressure variation in the fluid delivered from the pump, at least two neighbouring teeth 39, 40 at a point of the circumference of one of the gear wheels 19b are cut to form a passage 41, which at least at the gap between the two teeth has a depth substantially corresponding to the height of a tooth, whereby a short-circuit connection between the pressure side 20a and its supply side 21a is formed when the said teeth engage teeth in the juxtaposed wheel 19a. In an alternative arrangement passages are cut to about half tooth- depth in neighbouring teeth on each gear wheel and the wheels are arranged so that teeth with the passages cut therein on one wheel co-operate with the corresponding teeth on the other wheel (Fig. 3). <IMAGE>

Description

SPECIFICATION A device for controlling a hydraulic actuator The present invention refers to a device for controlling a hydraulic actuator forming part of a hydraulic circuit including a pump having two intermeshing gear wheels.

A known problem when governing piston/cylinder units actuated by hydraulic fluid is that the ordinary friction resistance in sealings, servo valve slides and other pressure governing devices will cause pressure jolts, which are difficult to control. The phenomenom is called hysterisis, and usually makes complicated constructional features An object of the present invention is to propose a simple and cheap way to avoid hydraulic hysterisis in a hydraulic circuit.

The invention is characterized in that at least two neighbouring teeth at one point of the circumference of at least one of the wheels are cut to form parts of a passage, which at least at the gap between the two teeth has a depth substantially corresponding to the height of one tooth, whereby said teeth, when they engage teeth in the juxtaposed wheel, will form a short-circuit connection between the pressure side of the pump and its supply side, with the object of causing a cyclic pressure variation in the delivered fluid.

The amount of fluid trapped at the bottom of a gap between two teeth can cause noise and vibrations, when a tooth from the opposite wheel enters the gap. In order to remove such drawbacks it has been proposed to provide the teeth with bores or grooves, which remove fluid from the actual point.

This will influence the delivery capacity of the pump uniformly. In the present instance it is desirable to bring about a cyclic pressure variation by short-cirpuiting the pressure side to the inlet side, at least once during each revolution.

With an actuator of double-acting type the pump is advantageously driven by a reversible D.C. motor governed by speed controlling means.

The passage may be formed in one of the wheels only, and extends then at the gap between the associated teeth substantially to the base circle. Alternatively teeth at both gear wheels may contain parts of the passage, each part having a depth about corresponding to one half of the height of a teeth, the wheels being mounted upon their carrying shafts in such a manner that one passageprovided tooth at one wheel will engage between two passage-provided teeth at the other wheel.

The invention will below be described with reference to the attached drawings, in which Figure 1 shows a device according to the invention for controlling a piston actuator unit, and Figure 2 and 3 show alternative locations of the passage in the teeth.

The cylinder 10 of the actuator has two chambers 11 and 12, respectively, separated by a piston 13, and includes outer scraper rings 14, piston rod sealings 15, and a piston ring 16. in spite of high quality of material and surface finish these sealings will cause a noticeable internal friction. This friction usually makes comparatively expensive pressure control means necessary in order to ensure that the piston rod 17 and thus a tool connected thereto, or some other connected device is displaced without jolts.

In the drawing the cylinder 10 is connected to a rather simple hydralic circuit 18. This includes a conventional hydraulic pump 19, the inlet and outlet of which communicate with the chambers of the cylinder by way of conduits 20, 21. Each conduit 20, 21 furthermore communicates with a fluid reservoir 22 by way of conduits 23 and 24 having non-return valves 25 and 26, respectiveLy.

The pump 19 is driven by a reversible electric motor 27. When pressure fluid is supplied to either chamber 11 or 12 the piston rod 17 will be displaced towards one end of the cylinder. When the direction of rotation of the pumps is reversed, pressure fluid will instead be supplied to the other chamber, and the piston rod will be displaced in the opposite direction. Possible leakage from the hydraulic circuit will be compensated from the reservoir 22 by way of the non-return valves.

The electric motor 27 is operated by direct current governed by a controL device 28, which in the drawing is shown in its neutral position. The control device includes two resistances 29, 30, which each is connected in parallel with two contacts 31 and 32, respectively, by way of leads 33, 34 to each of the contacts of the motor. When the lever 28 is turned, for instance clockwise in the drawing, the plus terminal of a D.C. source will be connected to the lead 34 through a selected part of the resistance 30. Simultaneously the lever 28 will close the left hand contact 31, whereby the minus terminal is connected with lead 33. In this manner the direction of rotation of the pump 19, as well as its speed, can be controlled.

Return flow conduits 35 and 36 are connected to supply conduits 20 and 21, and is each provided with a throttle valve 37, 38.

When the pump is driven at low speed all fluid delivered will flow out through the throttle valve, which is connected to the conduit 20 or 21, occasionally serving as a pressure conduit.

This basic-electro-hydraulic governing system would normally be subjected to severe hysterisis problems.

The pump 19 in a conventional manner includes two interengaging gear wheels 19a, 1 9b is, however, designed in such a manner, that the hydraulic pressure in conduits 20, 21 will pulsate, so the pressure cyclically sinks from an ordinary level. This pulsation means that small oscillations are developed in the hydraulic system, which varies the necessary operating pressure, so the frictional resistance- is overruled without the need for initially using an increased pressure, which easily causes the jolts. Hereby a very good governing of the movement of the piston rod is obtained, as the risk of hysterisis-causing overpressures during the initiation of a piston movement is removed.

When the pump 19 is driven at low speed the fluid will, as mentioned above, escape through either throttle vale 37 or 38. The piston 13 will not move, but will be subjected to the pulsations in the fluid conduit, which reduces the tendencies for the piston to be held by friction forces.

Figure 2 shows a first embodiment of a pump- suited to form part of the device according to the invention.

The gear wheel 19 is driven by the motor 27, and drives, in turn, the gear wheel 19b.

When a driving tooth engages a tooth in the juxtaposed wheel a sealing contact face is obtained, whereas there will be a play at the back of the driving tooth. Normally a small play is provided between the top of the tooth and the base circle of the juxtaposed wheel.

In Figure 2, two teeth 39, 40 in wheel 19b are- formed with parts of a passage 41, which in the gap between the two teeth extends substantially down two the base circle. In this manner a short-circuit connection between the pressure side of the pump and its inlet is obtained at-a certain moment during each revolution.

On the present occasion the pump 19 is reversible, and its connections to the conduits 20 and 21 are denoted 20a and 21a, respectively. Full line arrows indicate- the direction of flow for one direction of rotation, and broken line arrows indicate conditions during the opposite direction of rotation.

The passage 41 may be obtained in differ -ent ways, and be located for instance as a groove about at the middle plane of the wheel, or as a recess at either end of the tooth. The number of teeth, taking part in forming the passage will depend upon the diameter of the wheel and the spacing between the teeth. In order to cause a noticeable return flow the gap between the two teeth 39 and 40 may be cut so it reaches somewhat below the base circle.

Figure 3 shows an alternative way of forming the passage 41.

-Two teeth 39a, 40a in gear wheel 19b are formed with parts of the passage, which in each tooth extends about down to one half of the height of the tooth.

In the driving gear wheel 19a at least one tooth 40b in a corresponding manner is formed to provide part of the passage, also here extending down about to one half Qf the height of the tooth. Together the passage parts at two engaging teeth will proyide a passage having the height of one tooth.

The passage 41 will ensure the desired short-circuit connection, independently of the driving connection between the teeth 39b, 40b and teeth 39a, 40a.

In order to increase the length of the shortcircuit connection also tooth 39b is formed with part of the passage. The arrangement presupposes that the gear wheels 19a, 19b are mounted upon their carrying shafts 42, 43 in such a manner that just the teeth mentioned will always mesh. If it is desirable to obtain a strong short-circuit connection the passage parts can be extended to somewhat below one half of the height of the teeth.

The invention is not limited to the embodiments shown in the drawings, as several modifications are possible within the scope of the appended claims. The hydraulic system will conventionally include overpressure valves, and actual position indicators.

Claims (5)

1. A device for controlling a hydraulic piston/ cylinder actuator forming part of a hydraulic circuit including a gear pump having two meshing gear wheels, of which one is driven, characterized in that at least two neighbouring teeth at one point of the circumference of at least one of the wheels are cut to form parts of a passage, which at least at the gap between the two teeth has a depth substantially corresponding to the height of one tooth, whereby said teeth, when they engage teeth in the juxtaposed wheel will form a short-circuit connection between the pressure side of the pump and its supply side, with the object of causing a cyclic pressure variation in the delivered fluid.

2. A device according to claim 1, where the actuator is of a double acting type, characterized in that the pump is driven by a reversible D.C. motor governed by speed controlling means.

3. A device according to either of claims 1 or 2, characterized in that the passage is formed in one of the wheels only, and at the gap between the associated teeth extends substantially to the base circle.

4. A device according to either of claims 1 or 2, characterized in that teeth at both gear wheels contain parts of the passage, each part having a depth about corresponding to one half of the height of a tooth, the wheels being mounted upon their carrying shafts in such a manner that one passage-provided tooth at one wheel will engage between two passage-provided teeth at the other wheel.

5. A device for controlling a hydraulic piston/ cylinder actuator substantially as herein before described with reference to and as shown in the accompanying drawings.