GB1602728A

GB1602728A - Hydraulic systems

Info

Publication number: GB1602728A
Application number: GB3691677A
Authority: GB
Original assignee: Sperry Ltd
Current assignee: Unisys Group Services Ltd
Priority date: 1977-09-05
Filing date: 1977-09-05
Publication date: 1981-11-18
Also published as: DE2838696A1

Description

(54) IMPROVEMENTS IN OR RELATING TO HYDRAULIC SYSTEMS (71) We, SPERRY LIMITED formerly known as SPERRY RAND LIMITED, a British Company of Sperry House, 78 Portsmouth Road, Cobham, Surrey KT11 lJZ, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The present invention relates to a method of operating a hydraulic valve for moving a load at a constant (and preferably slow) speed independent of load variations.

The present invention makes use of a property of hydraulic spool valves which, in the ordinary applications of such valves, is disadvantageous. When a spool valve is cracked slightly open so as to provide a narrow metering orifice at the edge of the spool lands, a hydrodynamic force (known as the Bernoulli force) is generated by the passage of fluid though the metering orifice and this force acts in such a way as to oppose further opening of the valve and tends to draw the spool back towards the closed position. As a result, manually-operated valves require considerable force for their operation, and steady fine adjustment of a valve handling any considerable amount of power is difficult to achieve, often necessitating the use of a pilot-operated valve.

It has now been found that this normally undesirable effect can be made use of to provide a simple and reliable method of operating a constant flow valve which can be employed to feed a load such as a ram or motor when a relative slow but constant movement substantially independent of load variation is required.

The invention turns on the discovery that if there is applied to the spool of a hydraulic spool valve a constant force insufficient to overcome the Bernoulli force opposing opening of the valve and insufficient to open the valve fully, the flow through the valve is substantially independent of valve pressure drop over a considerable range of operating pressures.

According to the present invention a method of operating a hydraulic spool valve includes the step of applying to the spool of the valve a constant force which is insufficient to open the valve fully against the Bernoulli force generated over a predetermined operating range of the valve, and results in hydraulic flow through the valve being substantially constant irrespective of variations in load, and hence valve pressure drop.

The constant force may be applied to the spool of the valve by an electric solenoid or force motor to which a constant electrical current is supplied. The value of the constant current may be varied to enable a desired speed of movement of the load to be set. The solenoid or force motor may be provided with relatively stiff, resilient centring means, such as a diaphragm or a pair of springs, to increase the natural frequency of the valve and hence increase the dynamic response thereof. With the use of a solenoid, the dynamic response of the valve can be further increased by operating the solenoid over only part of the available gap thereof.

The invention may be applied, for example, to operations such as the sPark machining of metals, where the workpiece (which forms or is connected to said load) is requied to be advanced at a constant, but very slow speed.

Preferably a second valve is connected in parallel with the spool valve for rapidly pre-positioning the load or withdrawing it after operation.

A hydraulic system including a hydraulic spool valve operated in accordance with the invention will now be described in greater detail, by way of example, with reference to the accompanying drawings, in which: Figures I and 2 are graphs showing in different ways the relationship between pressure drop and flow through a solenoid-operated spool valve at different values of solenoid current insufficient to open the valve fully against the Bernoulli force, Figure 3 is a circuit diagram of the hydraulic system, and Figure 4 is a detail of a solenoid-operated spool valve of Figure 1 as seen in cross section.

Before describing the illustrated embodiment of the present invention it will be helpful to give the theory upon which the invention is based. As already mentioned, there exists a Bernoulli force (fun) which acts in direction such as to oppose the opening of a spool valve.

The Bernoulli force is related to flow rate (Q) through the valve and the pressure drop (Pv) across the valve in accordance with the equation:

where KB is a constant (1) The flow rate Q is related to the displacement X of the spool and the pressure drop Pv in accordance with the equation:

where KQ is a constant (2) (3) The force which has to be exerted on the spool to open the valve is given by the equation: KMI = SX + Fn (4) Where KM is a constant I is the current through the force-exerting means such as a solenoid or force motor S is the effective spring rate acting on the spool.

Substituting equations (1) and (3) in equation (4) gives:

It will be seen from equation (2) that for a given spool displacement X, flow rate Q is dependent on the pressure drop Pv across the valve and since Pv varies with load, it will be appreciated that in normal circumstances constant flow is rarely achieved. Equation (5) shows that the flow rate Q is also dependent on the current used in the device for effecting movement of the valve spool and normally, the level of current is chosen to produce a desired flow rate which, as already explained, is only constant if Pv, and hence the load, is constant. This is unacceptable in some applications, such as spark machining of metals, in which the workpiece (load) has to be advanced at a constant, but slow, speed which means that flow rate must not vary.

The present invention turns on the discovery that if there is supplied to the spool of a hydraulic spool valve a constant force insufficient to overcome the Bernoulli force, then flow through the valve is substantially independent of the pressure drop across the valve over a considerable range of operating pressures. Looking again at equation (5), if a constant current I is employed and the term S/KMKQ is assumed constant, then the equation can be rewritten as:

where K1 = I K2 = S/KMKQ K3 = KB/KM Now if variations in load occur, which in turn affect Pv, Q will remain substantially constant because there is a term dependent on Pv in the numerator as well as in part of the denominator leaving Q substantially constant, as hereinafter confirmed by the graphs of Figures 1 and 2.

Figure 1 is a graph of flow against current for a solenoid-operated spool valve for different pressure drops across the valve with the solenoid applying a force to the valve spool insufficient to open the valve fully against the Bernoulli force. It is seen that the flow rate through the valve varies very little with pressure drop and is remarkably constant, particularly if the lowest pressure curve (500 p.s.i.) is disregarded. This graph therefore confirms the above theory and is based on Equation 6. Another way of confirming Equation 6 is to plot flow against load pressure for a constant current and see whether flow is constant for different load pressures.Such a graph is shown by the solid line in Figure 2 and again, it is seen that flow is remarkably constant over a range of load pressures from 1,000 p.s.i. to 4,000 p.s.i. (for a constant current of 1,300 mA) in contrast to a normally operated solenoid valve which results in flow being highly dependent on load pressure as shown by the broken line in Figure 2.

Turning now to Figure 3, this shows a hydraulic system in which a load such as a work piece (not shown) undergoing a spark machining operation is moved by a double-acting hydraulic ram 1. The flow of actuating hydraulic fluid to the ram 1 is controlled by one or other of two solenoid-operated control valves 2 and 3 connected in parallel. The control valve 2 is a spool valve and is shown in greater detail in Figure 4. The valve has a body 4 with a central longitudinal bore 5 in which a spool 6 is a sliding fit. The spool 6 has three lands 7,8,9 alternating with four waist portions 11,12,13,14 and in the closed position of the valve shown in Figure 4, the lands 7,8 and 9 close, respectively, an outlet 15, an inlet 16 and a further outlet 17 provided in the body 4. Two sets of tank or reservoir ports 18 and 19 are also formed in the body 4.

Each end of the valve body 4 has a cap 21 to which is fitted a pull solenoid 22 having a coil 23 and an armature 24 connected to the associated end of the spool 6 by a rod 25. A relatively stiff compression spring 26 acts between the armature 24 and the end cap 21, the two springs 26 serving as a pair of centring springs for the armature of each solenoid 22, and hence for the spool 6. The springs 26 confine the operation of each solenoid to only a portion of the available air gap so as to increase the natural frequency of the armature and thereby increase the dynamic response of the valve. The solenoids 22 are connected to a speed control device 27 via an amplifier 28.

The valve 3 is of the so-called "bang-bang" type, which is either closed or fully open, and is used to effect rapid pre-positioning or withdrawal of the workpiece. Pull solenoids 29 are associated with respective ends of a spool 31 of the valve, the spool having two lands 32,33 and an intervening waist 34. In Figure 3 all connections are hydraulic except for those between the speed control device 27, the amplifier 28 and the solenoids 22, and between a switch 55 and the two solenoids 29. In the closed position shown in Figure 3, the lands 32,33 close outlet ports 35 and 36, respectively, there also being provided an inlet port 37 and two tank or reservoir ports 38 and 39. A pair of centring springs 41 for the spool 31 are provided, the springs acting between respective ends of a body 42 of the valve and the spool.

The inlet ports 16 and 37 of the valves 2 and 3 are connected to a supply pump via lines 43 and 44, respectively, the left-hand outlet ports 15 and 35 to one end of the ram 1 via lines 45 and 46, and the right-hand outlet ports 17 and 36 connected to the other end of the ram via lines 47 and 48. The tank ports 18, 19, 38 and 39 of the two valves 2 and 3 are connected to a tank 49 via respective lines 51, 52, 53 and 54.

In operation of the system, the workpiece is pre-positioned rapidly by actuating the switch 55 which energises the left-hand solenoid 29 of the control valve 3 and pulls the spool 34 to the left against the action of the left-hand compression spring 41. In this way, the valve inlet 37 is connected to the outlet 35, whereby pressure fluid from the pump flows through lines 44 and 46 to the left-hand end of the cylinder of the ram 1, the piston 56 of which is accordingly extended, i.e. moved to the right as seen in Figure 2. This movement of the piston 56 expels hydraulic fluid from the other end of the cylinder of the ram 1, which fluid is transferred to the tank 49 via the line 48, ports 36 and 39 and lines 54 and 53, ports 36 and 39 being in communication with each other as a result of movement to the left of the spool 34.When the workpiece is at the required position, the switch 55 is moved to the centre position, whereupon the left-hand solenoid 29 is de-energised and the springs 41 return the spool 34 to the central, closed position.

The workpiece now has to be advanced at a slow, constant speed in order to undergo a spark machine operation and in order to effect this movement, the control valve 2 is employed. Firstly, the speed control device is set to give the required speed of movement of the workpiece and the amplified signal therefrom is fed to the right-hand solenoid 22 which, upon energisation, pulls the spool 6 of the valve 2 to the right as seen in Figure 3, whereupon the inlet port 16 is placed in communication with the outlet port 15 so that pressure fluid continues to flow to the left-hand end of the cylinder 1, further to extend the piston 56 and hence move the workpiece in the required direction.Hydraulic fluid expelled from the cylinder is now transferred to the tank 49 via the ports 17 and 19 of the valve 2 which are in communication with each other following the movement to the right of the spool 6. The speed control device 27 is arranged to energise either of the solenoids 22 with a constant current which is such as to produce a force on the spool 6 of the valve 2 insufficient fully to open the valve against the action of the Bernoulli force so as to give a substantially constant flow of pressure fluid through the valve irrespective of any variations in the load on the system imposed by the workpiece as already explained above.

Should the workpiece have to be reversed for further spark machining, the right-hand solenoid 22 is de-energised and the left-hand solenoid energised instead. This results in the spool 6 being pulled to the left through the central position, whereupon the inlet port 16 now communicates with the outlet port 17 so as to supply pressure fluid to the right-hand end of the cylinder of the ram 1. This will cause the piston 56 to retract and hence reverse the direction of movement of the workpiece. In this case, pressure fluid will be expelled from the left-hand end of the cylinder of the ram 1 and transferred to tank through the ports 15 and 18 of valve 2 which are now in communication with each other.Once the machining operation has been completed, rapid withdrawal of the workpiece can be effected by moving the switch 55 so as to energise the right-hand solenoid 29 of the control valve 3, whereupon full supply pressure is supplied to the right-hand end of the cylinder of the ram 1, hydraulic fluid being transferred to tank via the ports 35 and 38 which are now in communication with each other. At the same time as the control valve 3 is brought into operation, the previously energised solenoid 22 of valve 2 is de-energised and the springs 26 will return the spool 6 to the central, closed position. On completion of the withdrawal of the workpiece, the right-hand solenoid 29 of the valve 3 is de-energised and the spool 34 is returned to its central, closed position by the springs 41.

Thus, the present invention provides a simple and yet effective method of operating a valve to move a load at a substantially constant speed which is highly advantageous in certain applications such as the accurate machining of a workpiece.

WHAT WE CLAIM IS: 1. A method of operating a hydraulic spool valve, including the step of applying to the spool of the valve a constant force which is insufficient to open the valve fully against the Bernoulli force generated over a predetermined operating range of the valve, and results in hydraulic flow through the valve being substantially constant irrespective of variations in load, and hence valve pressure drop.

2. A method according to claim 1, wherein the constant hydraulic flow through the valve is used to move a load at a constant speed.

3. A method according to claim 2, wherein the load is a workpiece being machined.

4. A method according to claim 3, wherein the load is being machined in spark machining apparatus.

5. A method according to any of the preceding claims, wherein the constant force is applied to the spool of the valve by a solenoid or force motor to which a constant electrical current is supplied.

6. A method according to claim 5, wherein the value of the constant current supplied to the solenoid or force motor can be varied.

7. A method according to claim 6, wherein a chosen value of current corresponding to a desired speed of movement of the load may be set.

8. A method according to any of claims 5 to 7, wherein an armature of the solenoid or force motor is centred by resilient centring means, whereby to increase the natural frequency of the valve and also the dynamic response thereof.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. inlet 37 is connected to the outlet 35, whereby pressure fluid from the pump flows through lines 44 and 46 to the left-hand end of the cylinder of the ram 1, the piston 56 of which is accordingly extended, i.e. moved to the right as seen in Figure 2. This movement of the piston 56 expels hydraulic fluid from the other end of the cylinder of the ram 1, which fluid is transferred to the tank 49 via the line 48, ports 36 and 39 and lines 54 and 53, ports 36 and 39 being in communication with each other as a result of movement to the left of the spool 34. When the workpiece is at the required position, the switch 55 is moved to the centre position, whereupon the left-hand solenoid 29 is de-energised and the springs 41 return the spool 34 to the central, closed position. The workpiece now has to be advanced at a slow, constant speed in order to undergo a spark machine operation and in order to effect this movement, the control valve 2 is employed. Firstly, the speed control device is set to give the required speed of movement of the workpiece and the amplified signal therefrom is fed to the right-hand solenoid 22 which, upon energisation, pulls the spool 6 of the valve 2 to the right as seen in Figure 3, whereupon the inlet port 16 is placed in communication with the outlet port 15 so that pressure fluid continues to flow to the left-hand end of the cylinder 1, further to extend the piston 56 and hence move the workpiece in the required direction.Hydraulic fluid expelled from the cylinder is now transferred to the tank 49 via the ports 17 and 19 of the valve 2 which are in communication with each other following the movement to the right of the spool 6. The speed control device 27 is arranged to energise either of the solenoids 22 with a constant current which is such as to produce a force on the spool 6 of the valve 2 insufficient fully to open the valve against the action of the Bernoulli force so as to give a substantially constant flow of pressure fluid through the valve irrespective of any variations in the load on the system imposed by the workpiece as already explained above. Should the workpiece have to be reversed for further spark machining, the right-hand solenoid 22 is de-energised and the left-hand solenoid energised instead. This results in the spool 6 being pulled to the left through the central position, whereupon the inlet port 16 now communicates with the outlet port 17 so as to supply pressure fluid to the right-hand end of the cylinder of the ram 1. This will cause the piston 56 to retract and hence reverse the direction of movement of the workpiece. In this case, pressure fluid will be expelled from the left-hand end of the cylinder of the ram 1 and transferred to tank through the ports 15 and 18 of valve 2 which are now in communication with each other.Once the machining operation has been completed, rapid withdrawal of the workpiece can be effected by moving the switch 55 so as to energise the right-hand solenoid 29 of the control valve 3, whereupon full supply pressure is supplied to the right-hand end of the cylinder of the ram 1, hydraulic fluid being transferred to tank via the ports 35 and 38 which are now in communication with each other. At the same time as the control valve 3 is brought into operation, the previously energised solenoid 22 of valve 2 is de-energised and the springs 26 will return the spool 6 to the central, closed position. On completion of the withdrawal of the workpiece, the right-hand solenoid 29 of the valve 3 is de-energised and the spool 34 is returned to its central, closed position by the springs 41. Thus, the present invention provides a simple and yet effective method of operating a valve to move a load at a substantially constant speed which is highly advantageous in certain applications such as the accurate machining of a workpiece. WHAT WE CLAIM IS:

1. A method of operating a hydraulic spool valve, including the step of applying to the spool of the valve a constant force which is insufficient to open the valve fully against the Bernoulli force generated over a predetermined operating range of the valve, and results in hydraulic flow through the valve being substantially constant irrespective of variations in load, and hence valve pressure drop.

9. A method according to any of the preceding claims, wherein the valve is double

acting and capable of controlling the movement of a load in either of two mutually opposite directions of movement thereof.

10. A method according to claims 5 and 9, wherein the solenoid or force motor is one of two such devices respectively operable to move the spool of the valve in the two possible directions of movement thereof, one or other solenoid being energised in dependence upon the desired direction of movement of the load.

11. A method according to any of the preceding claims, wherein a second valve is connected in parallel with the first-mentioned spool valve and is operative to effect rapid movement of the load for pre-positioning or withdrawal of the latter, the parallel arrangement of the two valves being connected to a piston and cylinder device operative to move the load.

12. A method of operating a hydraulic spool valve, substantially as herein particularly described with reference to the accompanying drawings.