GB2065790A - Improvements in or Relating to a Pump - Google Patents

Abstract

A pressure-controlled multicylinder reciprocating pump whose pistons are driven by a common drive element (8), wherein a preloaded hydropneumatic spring is interposed between each individual piston (10) and the drive element (8), such spring comprising piston-and- cylinder units (14, 15, 16) which are in series with the various pump pistons each piston and cylinder having a pressure chamber (21) all the pressure chambers being connected to a common hydropneumatic accumulator (24) and, by way of a check valve (28), to the output of an auxiliary pump (26) there being an adjustable pressure limiting valve (25), also connected to the output of the auxiliary pump. <IMAGE>

Description

SPECIFICATION Improvements in or Relating to a Pump This invention relates to a pump and more particularly to a pressure-controlled multicylinder reciprocating pump whose pistons are driven by a common drive element. The pump can be of either the radial piston or axial piston kind.

Constant-flow pumps driven by a motor at a constant speed are often used in hydraulic circuits. They are relatively cheap but suffer from various disadvantages.

Such pumps must be designed with regard to the maximum delivery pressure and the maximum required delivery. However, in many operations, for instance, in servohydraulics, the quantity of hydraulic fluid required varies rapidly and widely, although a constant pressure must be maintained.

Consequently, since the pump must be designed for maximum pressure and delivery, at belowmaximum loads the surplus oil must be discharged through a relief or pressure-limiting valve. The pump must therefore operate continuously at full power, with the result that the overall efficiency of hydraulic fluid supply is low.

The pump motor must always draw maximum power from its source to drive the pump, so that in many cases energy is wasted.

Pressure-controlled pumps have been used to overcome this disadvantage and among the axial piston kind of pump skew disc (or swash plate) pumps have become established for this purpose.

Radial piston pumps are known which have two concentric eccentrics as the means for driving the pump pistons, the eccentrics being adjustable by being moved angularly relatively to one another Such pumps may be adjusted right down to zero delivery. Internally supplied radial piston pumps are known in which the outer ring is pivotable. For adaptation to variations in oil consumption in the load circuit of all pumps of this kind, a deliverypressure-related variable must be fed back to react on the adjustable pump drive in order to control or adjust the pump.

Pressure-controlled pumps of this kind, while having the advantage of requiring their drive to provide no more than the energy actually required at any time, have serious disadvantages. Because of the relatively compiex mechanism, pumps of this kind are very expensive. Furthermore, their adjustment times are long because of the heavy masses involved in the mechanical systems, with the further result that a large hydropneumatic bridge accumulator usually has to be connected to the delivery side, with all the disadvantages this entails.

It is the object of this invention to provide a pressure-controlled multicylinder reciprocating pump in which the above described disadvantages are obviated or reduced.

According to this invention there is provided a pressure-controlled multicylinder reciprocating pump whose pistons are driven by a common drive element, a preloaded hydropneumatic spring is interposed between each individual piston and the drive element, such spring comprising pistonand-cylinder units which are in series with the various pump pistons each piston and cylinder having a pressure chamber all the pressure chambers being connected to a common hydropneumatic accumulator and, by way of a check valve, to the output of an auxiliary pump there being an adjustable pressure-limiting valve, also connected to the output of the auxiliary pump.

Preferably each piston and cylinder unit comprises a cylinder containing a first piston, said first piston having a piston rod with an axial bore which communicates with said pressure chamber which is partly defined by said first piston, said piston rod terminating in a chamber formed within a second piston located within said cylinder, said bore opening at said termination of the piston rod.

Conveniently the auxiliary pump is a reciprocating pump which is driven by the element driving the main pump and which is integral therewith.

In order that the invention may be more readily understood, and so that further features thereof may be appreciated the invention will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 is a transverse sectional view of a conventional three-cylinder radial piston pump, and Figure 2 is a corresponding partial view, on an enlarged scale of a pump in accordance with the invention, in which there is a pre-loaded hydropneumatic spring disposed between each individual piston of the three-cylinder or main pump and the driving element.

Referring to Figure 1 a pump casing 1 is closed at the rear by a pressure plate 2 which, in a manner not shown, carries a bearing for a pump drive shaft 3. Towards the viewer of the drawing, the pump is closed by a front pressure plate 4 which also has a bearing for the shaft 3.

Pump cylinders 6 are clamped between the plates 2 and 4 by means of continuous tension bolts or the like 5. Accordingly, the pump cylinders are rectangular in external cross-section.

An eccentric 7 is secured in any desired manner to the shaft 3 and serves as a drive element. The eccentric 7 acts by way of a ball race 8 and piston heads 9 on the pistons 10 of three discrete cylinders. The pump pistons are biassed in a known manner by return springs 11.

The pump cylinders have conventional inlet valves 12 which will not be described in detail.

The radial piston pump of Figure 1 is known. A radial piston pump in accordance with the invention is however provided by a preloaded hydropneumatic spring comprising piston-andcylinder units disposed between ball bearing 8 and piston heads 9. An arrow 13 in Figure 1 shows where the hydropneumatic spring is disposed.

Referring to Figure 2, the piston and cylinder units comprise a square external cross-section cylinder 14 clamped between the end plates 2 and 4. The cylinder 14 containing pistons 15, 16.

The cylinder 14 is secured by pins 1 7. Piston 1 6 has a piston rod 18 formed with a continuous bore 19 open adjacent the end face of piston 16, and having an end stop 20 which defines the other opening of the bore. The cylinder 14 defines a compression chamber 21 which communicates via bore 19 with a chamber 22 formed in piston 15.

The chambers 21 of all the cylinders 14 of the pump are commoned by way of ducts 23 to a single hydropneumatic accumulator 24, there being an adjustable pressure-limiting or relief valve 25 associated with an auxiliary pump 26 adapted to intake hydraulic fluid from a reservoir 27. The output of the pump is connected to the accumulator 24 by an ordinary check valve 28.

A description will now be given of how the pump in accordance with the invention operates.

Figure 2 shows the normal state of a piston and cylinder unit 14 between the bearing 8 of the eccentric 7 and the piston head 9. As will be apparent, the piston 10 is in its extended position relatively to the cylinder 6.

Depending upon the setting of the relief valve 25, the pump 20 delivers hydraulic fluid through the opening check valve 28 both to the accumulator 24 and to the chamber 21, 22.

Consequently, the chamber 21,22 and the accumulator 24 experience a hydraulic pressure preloading which is adjustable by means of adjustable valve 25. Chambers 21, 22 and accumulator 24 therefore form a preloaded hydropneumatic spring. Clearly, the power which the drive 8 applies via pistons 1 5, 1 6 to the main pump piston 10 is greater in proportion as the pressure in the chamber 21,22 is higher.

Should the pistons 15, 16 increase the pressure in the chamber 21,22, the back pressure of the accumulator 24 would be overcome, the two pistons 15, 16 would move towards one another and hydraulic fluid would flow from chamber 21 through port 23 into accumulator 24.

This effect is the basis underlying the operation of a pump in accordance with the invention in a hydraulic circuit. It will be assumed that the pump in accordance with the invention forms part of a servo-circuit for supplying oil to the discrete elements of such circuit. The pump should be dimensioned conventionally so as to be able to deliver the maximum delivery required by the hydraulic circuit. Valve 25 should be adjusted to a pressure such that the pressure in chamber 21, 22 is high enough for the main pump pistons 10 to provide the maximum delivery at the required pressure of e.g. 300 bars. An abrupt drop in the requirements of the hydraulic circuit as a result of a control event would lead, if a constant-flow pump was supplying such a circuit, to an increase in delivery pressure and the increase would have to be reduced by a relief valve.In the case of a pump in accordance with the invention, however, the pressure increase in the load circuit would increase the pressure in the chambers 21,22.

Since, however, the latter pressure is higher than that to which the hydropneumatic spring 21, 24 has been adjusted, the pistons 1 5, 1 6 move towards one another and displace hydraulic fluid from chamber 21 into spring 24, with the result that transmission of the drive by the eccentric 8 to the piston 10 is reduced or ceases. A slight increase in pressure on the delivery side would occur only fairly near the end of the delivery strokes of the pistons 10. However, in the event of the pressure on the delivery side of the pump increasing abruptly, for instance, because of a number of servo elements are switched off abruptly, the pistons 1 5, 16 would move towards one another much earlier during the stroke of the eccentric 8 and displace oil into the hydropneumatic accumulator 24.In the extreme case, corresponding to an abrupt closure of the pump outlet, the piston 10 would cease to make any stroke at all and the entire operative movement of the drive element 8 would be received solely by the two pistons 1 5, 16 so that a correspondingly large amount of hydraulic fluid would be displaced into the accumulator 24.

There is recovery of energy in each individual pump stroke because, as the eccentric 9 recedes, the pressure of the fluid stored in the accumulator 24 causes the fluid to force the pistons 1 5 and 1 6 apart from one another and thus impart an acceleration pulse to the eccentric 8 (of Figure 1).

As will be readily apparent, the energy recovered per operative movement of the eccentric is greater in proportion as the required pump delivery is smaller and, therefore, as more hydraulic fluid is forced into the accumulator 24 during the operative stroke of the pump. The function of the check valve 28 is to ensure that the hydraulic fluid displaced from the chamber 21 actually flows into the accumulator 24 and not through the valve 25 into the reservoir 27.

Considering now a state in which only a few loads are connected in the pump load circuit and suddenly a number of extra loads are cut in, the pattern is as follows: Before the cutting-in of the extra loads, the two pistons 1 5, 1 6 force a particular amount of hydraulic fluid into the accumulator 24 at each operative movement of the ball bearing 8, the displaced fluid returning to the chambers 21,22 as the eccentric 8 recedes. In response to an abrupt cutting-in of loads to the pump load circuit, the pressure in the hydraulic circuit drops abruptly and the pistons 1 5, 1 6 displace less oil than previously into the accumulator 24 on the operative stroke. As can be gathered from Figure 2, the masses to be moved in the unit 14 are small, being just the pistons 15, 16 and the column of fluid in the chambers 21-23 and accumulator 24. Correspondingly, the response time of the pump in accordance with the invention to a variation in the output condition is very short.

The auxiliary pump 26 is required to deliver only a very small quantity of hydraulic fluid-i.e., sufficient to top up leakage losses in the unit 14.

The pump 26 can therefore be very small and therefore cheap, such as a cheap gear pump.

Similar considerations apply to the hydropneumatic accumulator 24, which is required to receive only the maximum quantity of fluid reciprocated by the pistons 1 5, 1 6 in the extreme case. Also, as can be gathered from Figure 2, the pistons 15, 1 6 are of much larger cross-section than the piston 10 of the main pump cylinder, and so, despite a relatively high delivery pressure of the pump, a relatively low pressure is sufficient in the chamber 21, 22 and therefore in the accumulator 24. Consequently, the accumulator can be a small device which does not require official approval and which is available commercially at low cost. Similar considerations apply to the adjustable pressure-limiting or relief value 25.Also, since the components of the unit 14 are relatively simple items which can be mass produced very cheaply, the additional cost incurred by utilising the invention as compared with a conventional radial piston pump is very slight, more particularly when the bulky and complicated mechanisms of many prior kinds of adjusting pumps are considered. The invention also has the special advantage over the known adjustable pumps that all moving parts run in hydraulic fluid and all movements are minimal and are rectilinear. There are therefore no lubrication problems such are as found in conventional adjustable pumps.

As will be apparent from the foregoing, the pump in accordance with the invention has such a rapid response time that it is unnecessary to provide a bulky high-pressure bridging accumulator on the delivery side, nor is there any need for the pressure-limiting valve which would otherwise be necessary as a safety feature.

Since as previously explained the valve 25 can be used to adjust the pump delivery pressure the means in accordance with the invention make it possible for one particular kind of pump to be used for different delivery pressures without modification.

The auxiliary pump 26 can take the form of a very small single-piston pump which forms an integral part of a multicylinder piston pump in accordance with the invention, such auxiliary pump being driven by the eccentric 8. This feature reduces costs considerably.

Claims (5)

Claims

1. A pressure-controlled multicylinder reciprocating pump whose pistons are driven by a common drive element, wherein a preloaded hydropneumatic spring is interposed between each individual piston and the drive element such spring comprising piston-and-cylinder units which are in series with the various pump pistons each piston and cylinder having a pressure chamber all the pressure chambers being connected to a common hydropneumatic accumulator and, by way of a check valve, to the output of an auxiliary pump there being an adjustable pressure-limiting valve, also connected to the output of the auxiliary pump.

2. A pump according to claim 1 wherein each piston and cylinder unit comprises a cylinder containing a first piston, said first piston having piston rod with an axial bore which communicates with said pressure chamber which is partly defined by said first piston, said piston rod terminating in a chamber formed within a second piston located within said cylinder, said bore opening at said termination of the piston rod.

3. A pump according to claim 1 or 2 wherein the auxiliary pump is a reciprocating pump which is driven by the element driving the main pump and which is integral therewith.

4. A pump substantially as herein described with reference to, and as shown, in Figure 1 as modified by Figure 2 of the accompanying drawings.

5. Any novel feature or combination of features described herein.