&null;0001&null; The invention relates to a control device for positive displacement pumps, in particular for vane cell pumps, according to the preamble of claim 1. In this device, a pressure space is connected via a throttle device to an outlet connected to a consumer. A flow control piston is also provided, which is displaceable in a housing bore and the first end face of which has a connection to the pressure space. A second end face of the flow control piston projects into a chamber which is connected to the outlet downstream of the throttle device. Depending on a differential pressure acting on the two end faces, the flow control piston opens a connection from the pressure space to a pump inlet duct. The throttle device is located in the pressure output of the pump and comprises a control pin, the control contour of which is designed in such a manner that, together with a throttling bore, a modifiable passage cross section is brought about. In this connection, one of the two elements, control pin and throttling bore, is connected to the flow control piston, and the other of the two elements is fixed in relation to the housing. &null;0002&null; Such a control device interacting with a throttle device according to the preamble of claim 1 is known from DE 22 30 306 B2. The pump is given a sloping flow characteristic by means of a stud of the control pin, which stud is conical at its free end. In such control devices, there is a risk of an excessive increase in delivery rate occurring when the pump undergoes a speed increase from the proportional range (for example no-load range) to the control range. This can be felt as a distinct drop and subsequent equally distinct rise in steering torque. If an additional pressure-limiting valve is installed in such a control device, there is then moreover an excessive increase in maximum pressure when this pressure-limiting valve responds. &null;0003&null; The object of the invention is to produce a control device, with which such an overshoot, that is to say a brief excessive increase in the delivery capacity or the delivery pressure, on rapid acceleration from very low speeds is prevented. &null;0004&null; This object is achieved by the control device characterized in claim 1 by virtue of the fact that, in an axial displacement region which, at the flow control piston, is not yet used for volume flow control, the control pin has a thickened portion, so that a minimal aperture cross section exists in this displacement region. This results in the pressure difference across the thickened region being greater and the flow control piston moving earlier, or more rapidly in the case of dynamic operations. &null;0005&null; Advantageous and expedient embodiments of the invention are indicated in the subclaims. The control pin with its thickened portion can be manufactured especially simply if the control pin is connected to the flow control piston and the throttling bore is fixed in relation to the housing, and the thickened portion is of cylindrical design. For better fine-adjustment of the control characteristic, the thickened portion can have a frustoconical contour. In this connection, it is advantageous if the larger diameter of the cone frustum is arranged on the first end face of the flow control piston. In this way, the characteristic of the control operation, the control speed, can be adjusted very finely.
&null;0006&null; The invention is explained in greater detail below with reference to two illustrative embodiments shown in the drawing. The vane cell pump illustrated and described represents only an example of application of the control device according to the invention. Instead of this, a roller cell pump or another positive displacement pump can equally be provided with a control device according to the invention. &null;0007&null; FIG. 1 shows a control device for a vane cell pump in longitudinal section according to a first illustrative embodiment, and &null;0008&null; FIG. 2 shows a flow control piston of the control device according to a second illustrative embodiment.
&null;0009&null; A drive shaft 3 is mounted in a housing 2 closed by a cover 1. In a conventional manner, the drive shaft 3 bears a rotor 4 on splining. &null;0010&null; Guided in radial slots of the rotor 4 are radially movable vanes 5 which slide sealingly along a cam ring 6. A pressure plate 7 bears sealingly against the pump assembly consisting of the rotor 4, the vanes 5 and the cam ring 6. A further pressure plate 8 bears against the pump assembly on the other side by virtue of the force of a spring 10. The vanes 5 enclose delivery chambers (not shown) between them, which are connected to a suction connection 11. The pressure oil delivered emerges from the delivery chambers via pressure openings (likewise not shown) of the pressure plate 8 into a pressure chamber 12. The pressure chamber 12 has a connection to undervane spaces 15 and 16 via part-ring-shaped ducts 13 and 14. In this way, it is possible to press the vanes 5 passing through the pressure zone at any one time outwards into the cam ring 6. &null;0011&null; In a housing bore 17 in&null;in the drawing&null;the lower part of the pump, a flow control valve 18 and a throttle device 20 are installed coaxially with one another. A flow control piston 21 of the flow control valve 18 controls an inlet duct 23 of the pump, in a known manner, with a control collar 22. In the initial position depicted, the control collar 22, loaded by a spring 24, bears against the throttle device 20. In this connection, the inlet duct 23 is closed by the control collar 22. In a bore portion 17A lying to the right of the control collar 22, a closing element 25, for example a pipe union, is located, which is connected to a consumer, for example servo power steering. A supply duct 26 joins the pressure chamber 12 to the bore portion 17A. &null;0012&null; The throttle device 20 comprises a control pin 27 which is connected firmly to the flow control piston 21, and a throttling bore 28 which is arranged on the closing element 25 and is thus connected firmly to the housing 2. The two elements, control pin 27 and throttling bore 28, could also each be connected to the other elements, housing 2 and flow control piston 21, with the same effect. &null;0013&null; At its free end, the control pin 27 has a contour which allows the delivery flow conducted to the consumer to be influenced in a speed-dependent manner. In the illustrative embodiment, this contour comprises an essentially cylindrical end portion 30 which is followed by a conical portion 31 and a portion 32 of smallest cross section. According to the invention, a thickened portion 34 of the control pin 27 is located between this portion 32 and a first end face 33 of the flow control piston 21, which end face faces the throttle device 20. The thickened portion 34 extends over such an axial length that, when the flow control piston 21 is displaced by a travel corresponding to this length, no volume flow control takes place yet. This means that, after such displacement, no connection yet exists between the supply duct 26 coming from the pressure chamber 12 and the inlet duct 23. &null;0014&null; In the illustrative embodiment according to FIG. 1, the thickened portion 34 has a cylindrical contour. In the illustrative embodiment according to FIG. 2, the thickened portion 35 has a frustoconical contour. In this connection, the larger diameter of the cone frustum is arranged on the first end face 33 of the flow control piston 21. This is especially advantageous because the characteristic of the control operation, the control speed, can thus be adjusted very finely. &null;0015&null; A space 36 accommodating the spring 24 of the flow control piston 21 is connected, via a control line 37 indicated by a broken line, an annular groove 38 and a throttling location 40, to an outlet 41 which is arranged on the closing element 25. In certain embodiments, the annular groove 38 and the throttling location 40 can be omitted. The space 36 is delimited on one of its sides by a second end face 42 of the flow control piston 21. &null;0016&null; The control device works in the following manner. The entire delivery flow of the pump flows firstly via the supply duct 26 into the bore portion 17A. Up to the limit point at a pump speed of, for example, 1000/mm, the delivery flow flows past the end face 33 of the control collar 22 to the outlet 41. The delivery flow flows through the passage cross section depicted, between the control pin 27 and the throttling bore 28. In this connection, owing to the pressure drop which arises, the flow control piston 21 is displaced slightly to the left, the control collar 22 nevertheless not yet commencing the opening of the inlet duct 23. Owing to the throttling cross section between the thickened portion 34 and the throttling bore 28, which is kept very small, a great pressure difference arises across the throttle device 20 and thus across the flow control piston 21. This great pressure difference forces an accelerated movement of the flow control piston 21. This in turn leads to a more rapid reaction of the control mechanism, so that an overshoot and an excessive increase in the delivery capacity is prevented. &null;0017&null; Reference Numbers &null;0018&null; 1 cover &null;0019&null; 2 housing &null;0020&null; 3 drive shaft &null;0021&null; 4 rotor &null;0022&null; 5 vane &null;0023&null; 6 cam ring &null;0024&null; 7 pressure plate &null;0025&null; 8 pressure plate &null;0026&null; 9 - &null;0027&null; 10 spring &null;0028&null; 11 suction connection &null;0029&null; 12 pressure chamber &null;0030&null; 13 part-ring-shaped duct &null;0031&null; 14 part-ring-shaped duct &null;0032&null; 15 undervane space &null;0033&null; 16 undervane space &null;0034&null; 17 housing bore &null;0035&null; 17A bore portion &null;0036&null; 18 flow control valve &null;0037&null; 19 - &null;0038&null; 20 throttle device &null;0039&null; 21 flow control piston &null;0040&null; 22 control collar &null;0041&null; 23 inlet duct &null;0042&null; 24 spring &null;0043&null; 25 closing element &null;0044&null; 26 supply duct &null;0045&null; 27 control pin &null;0046&null; 28 throttling bore &null;0047&null; 29 - &null;0048&null; 30 end portion &null;0049&null; 31 conical portion &null;0050&null; 32 portion of smallest diameter &null;0051&null; 33 first end face &null;0052&null; 34 thickened portion &null;0053&null; 35 thickened portion (conical) &null;0054&null; 36 space &null;0055&null; 37 control line &null;0056&null; 38 annular groove &null;0057&null; 39 - &null;0058&null; 40 throttling location &null;0059&null; 41 outlet &null;0060&null; 42 second end face |
