DIRECT DRIVE VALVE

Abstract: A tandem spool have has left and right spools (16, 18) mounted for longitudinal movement within a valve housing (10, 12, 14). An electric torque motor (20, 22), including a rotor and a stator are arranged to drive the spools via an eccentric coupling arrangement (30). A rotor housing (60, 62) contains hydraulic fluid within which the rotor (52, 54) is immersed, the rotor housing having afluid-retaining wall which passes between the rotor and the stator. Accordingly, the electrical connections to the stator coils do not have to pass through a hydraulic seal, neither is the stator subject to the stress of hydraulic pressure. A linear force motor could be used instead of a torque motor.

Description: similarly non-magnetic material. The wall or membrane is desirably of a material which can be thinly worked so that degradation of the motor performance is minimised. Conveniently, the moving member housing may be bolted onto the valve housing. Reduced susceptibility to contamina ion may be provided by ensuring that there is a substantially constant gap between the moving member and stator throughout the working range of travel of the motor. 10 The coupling may comprise a flexible driving member extending through a longitudinal bore in the spool and having one end secured to the spool. The motor may be arranged to drive the flexible driving member via an eccentric on the drive shaft of _■ t - the motor which moves against a camming surface of the driving member, so converting rotational motion of a drive shaft to a longitudinal motion of the driving member. If the flexible drive member is fle <xible only in bending, but rigid in compression and tension, then 0 the spool can be driven in both directions by the flexible member. Furthermore, where the bore within the spool is of a larger internal size than the size of the flexible member, then slight flexing of the member can take place which will take the rise and fall out of 5 the eccentric drive, while at the same time still providing for rigid linear motion. The flexible member may be adjustably secured to one end of the spool, for example by means of a thread and nut arrangement, so that small adjustments can be Q made in the relative positions of spool and flexible member. This enables the spool to be easily phased relative to fixed flow ports within the valve housing. In addition, this type of secure ent is substantially DIRECT DRIVE VALVE The present invention relates to direct drive valves, and particularly though not exclusively to tandem spool valves. According to a first aspect of the present invention there is provided a spool valve having a spool mounted for longitudinal movement within a valve housing; an electric motor, including a moving member and a stator, arranged to drive the spool via a coupling; and a moving member housing containing hydraulic fluid within which the moving member is immersed, the moving member housing having a fluid- retaining wall which passes between the moving member and the stator. This arrangement has the advantage that the moving parts of the valve may be entirely immersed in fluid and therefore do not have to pass through a 1 seal to atmosphere; this eliminates a significant source of frictional loss. A further advantage that the electrical connections to the stator coils do not have to pass through a hydraulic seal. Also, the stator is not subject to the stress of hydraulic pressure which it would experience if it were to be immersed in the fluid. Preferably, the moving member is immersed in fluid from a return line of the valve. The moving member may be the rotor of a rotary torque motor; alternatively it may be part of a linear force motor. The fluid-retaining wall or membrane may be of titanium, aluminium, a plastics material, or another free from backlash. In one specific embodiment, the spool valve is a tandem spool valve having a second spool mounted for longitudinal movement within the housing, both of the spools being driven by means of a common flexible driving member. The easiest way of achieving this is to position the spools one over each end of the flexible member, and to drive the spools by an actuating means arranged to act upon the centre of the flexible member. Where the actuating means comprises an eccentric on a drive shaft of the motor, the drive shaft may extend within an annular centre portion of the driving member, the inner surface of the annular portion comprising the camming surface. If desired, this annular centre portion could be integral with the two ends of the member or alternatively it could be separate with the respective ends (or flexible quills) being attached ' thereto. This type of tandem valve, which uses two separate spools for the individual valves, has the advantage that distortions arising from manufacture of the spools is not likely to be as detrimental to the operation of the valve as it would be if the two spools were to be manufactured in a single piece. In addition, the two separate spools may be adjustable in position relative to each other. The invention may be carried into practice in various ways and one specific direct drive valve will now be described, by way of example, with reference to the drawings, in which: Figure 1 is a perspective cutaway view of a valve embodying the present invention; figure 2 is a plan view of the valve; figure 3 is a side elevation of the valve; figure 4 is a section along the line B-B of figure 2; figure 5 is a section along the line A-A of figure 3; and figure 6 is a section along the line C-C of figure 2. The exemplary embodiment shown in the figures is a tandem valve incorporating a pair of independent rotary torque motors driving a pair of linear displacement spool valves. The two motors are driven by separate control lanes, and are electrically and magnetically independent of each other, as are the two hydraulic systems which are connected to the respective spool valves. A package of this type, incorporating a combined rotary motor and linear spool valve, results in a robust reliable construction and excellent performance characteristics. As may best be seen by the schematic view of figure 1 , the valve drive system comprises a main centre housing 10 made up of a left valve housing 12 containing a valve spool 16 of a first valve, and a right valve housing 14 containing a second valve spool 18 of a second valve. Actuation of the valve spools is by means of a valve drive system comprising two rotary torque motor packages 20, 22 assembled at opposite ends of a common rigid drive shaft 24. Each motor package is located within an individual steel housing 26, 28 (best seen in figure 2) which are bolted one on each side of the main centre housing 10. Conversion of the torque motors' rotary motion to the linear motion required at the valve spools is through an eccentric-type drive exhibiting a high mechanical efficiency. As can be seen in figure 1, the motor drive shaft 24 incorporates an eccentric 30 which moves within an annular ring 32 positioned centrally of two flexible quills 34, 36 which pass respectively through longitudinal bores within the left and right valve spools 16, 18. Each of these quills is flexible in bending (but rigid in compression) so as to provide a rigid drive to each spool, but to absorb the small vertical deflections that derive from the eccentric drive method. Each quill is threaded through the outer end of its respective valve spool into a tab washer and lock nut securing arrangement 38, 40. This enables each valve spool to be individually adjusted to achieve optimal harmonisation between the two hydraulic systems. The drive to each of the spools is free from backlash at both the attachment points to the spools themselves and also to the central eccentric drive. The use of separate valve spools and a common pair of drive quills means that distortion arising in manufacture which might otherwise occur if the two spools were manufactured as one long spool valve is avoided. Each of the two valve spools 16, 18 is matched directly into its respective valve housing 12, 14, so eliminating the complication of a main valve sleeve. This reduces the total number of static seals in the assembly. In the embodiment shown, each of the valve spools 16, 18 has two control lines and two outer return lines, although it will be appreciated that other arrangements could be envisaged. The exact shapes of the valve spools and the control ports in the valve housings are designed in a form which will minimise induced induced flow forces. Spool position feedback is achieved by means of an integrated quadruplex LVDT (linear variable defferential transformer) unit 42 mounted at the left hand end of the left valve housing 12. The physical connection between the LVDT unit and the left valve spool 16 is via the locknut arrangement 38. To avoid the use of a dynamic seal to atmosphere, the LVDT internal parts are immersed in the system fluid. When both of the drive motors are de-energised the valve spools are arranged to move to a slightly offset position by left and right lightly loaded disk-type bias springs 44, 46 (figure 4) acting at the respective ends of the valve spools. The motor packages 20, 22 are designed to provide the required force levels at the valve spools over the longest possible displacement with a symmetrical force characteristic about the spool mid position. This objective is achieved by making use of rotary torque motors each comprising respectively a stator 48, 50 carrying the motor windings and a rotor 52, 54 carrying magnetic pole pieces. The motors are designed to maintain a constant gap between the rotor and stator throughout the working range of travel. This constant gap feature means that the force capability of the motor reduces only slightly as the rotor moves from its centre position to the extremes of its angular travel. The motor is controllable over the full operating angular travel range with or without the use of additional spring centring devices. This means that the normal operating input currents are kept to a minimum, especially when the motor is connected to spool valves which incorporate flow force compensation. Nevertheless, when required the motor is capable of generating forces of up to 1000N. Finally, the constant air gap ensures good contamination tolerance. The working displacement of the motor is plus and minus twenty five degrees from the central position which, when translated to spool travel, represents plus and minus one millimetre at the main valve. This relatively large spool travel has the benefit of allowing the use of narrow control ports, which renders the valve less sensitive to wear and erosion at the control edges. In practice, the limit of travel is controlled by electrical limit stops and, as a backup, by a mechanical stop comprising a dowel pin 56 (figures 1 and 5) inserted into the drive shaft 24 which abuts machined stop faces (not shown) in the centre housing 10. The stators 48, 50 are clamped in place within an alloy housing (for example of aluminium) 56, 58 which acts as to protect it, and also aids heat dissipation. The rotors 52, 54 are enclosed within respective titanium membranes 60, 62 which are bolted and sealed to either side of the centre housing. The interior of each of the two membranes is flooded with hydraulic fluid so that the rotors are immersed with the membranes providing the fluid containment. This has the advantage that the rotor is contained in clean (filtered) fluid, so that the possibility of debris becoming trapped within the mechanism and causing failure is remote. It will be appreciated that since the titanium membrane passes between the rotor and the stator, there is also a small air gap between the outer surface of the membrane and the stator. A further advantage of immersing the rotor in fluid is that dynamic seals are thereby eliminated from the motor drive/valve assembly thereby reducing operating friction to a minimum and increasing reliability. The use of titanium means that very thin wall sections can be used, so ensuring that there is only a minimal effect upon the motor drive performance. Although it would be possible to immerse the stator in fluid as well, this is not considered to be a good design solution due to the difficulty in passing the electrical leads out of the fluid, via a seal, to the external atmosphere. Also, retaining the stator outside the membrane means that it is not subject to the stress of hydrostatic pressure which it would experience if it were to be immersed in the fluid. The return line seepage fluid from each of the two valves passes past the small matched clearance of the spool return lands, and into a central cavity 64 (figure 5). The fluid in this central cavity is connected through the centre of each spool to the return fluid in the closed cavities 66, 68 created respectively by the LVDT housing and the right hand valve end cap. No differential pumping action occurs within these enclosed spaces which could cause a transfer of fluid between the two systems. In an alternative embodiment (not shown) the valve may be activated by a linear force motor, the fluid- containing membrane 60 then extending between a stator and a moving member of the motor.

Claims: CLAIMS 1. A spool valve having a spool mounted for longitudinal movement within a valve housing; an electric motor, including a moving member and a stator, arranged to drive the spool via a coupling; and a moving member housing containing hydraulic fluid within which the moving member is immersed, the moving member housing having a fluid-retaining wall which passes between the moving member and the stator. 2. A spool valve as claimed in Claim 1 in which the moving member is immersed in fluid from a return line of the valve. 3. A spool valve as claimed in Claim 1 or Claim 2 in which the fluid-retaining wall is of titanium, aluminium, a plastics material or another non-magnetic material. 4. A spool valve as claimed in any one of the preceding claims in which there is a substantially constant gap between the moving member and the stator over the working range of travel of the motor. 5. A spool valve as claimed in any one of the preceding claims in which the moving member is the rotor of a torque motor. 6. A spool valve as claimed in any one of Claims 1 to 4 in which the motor is a linear force motor. 7. A spool valve as claimed in any one of the preceding claims in which the coupling comprises a flexible driving member extending through a longitudinal bore in the spool and having one end secured to the spool. 8. A spool valve as claimed in Claim 7 when dependent upon Claim 5 in which the torque motor is arranged to drive the driving member via an eccentric on a drive shaft of the motor which moves against a camming surface of the driving member, so converting rotational motion of the drive shaft to a longitudinal motion of the driving member. 9. A spool valve as claimed in Claim 7 or Claim 8 in which the driving member is substantially rigid under longitudinal tension and compression, but is flexible in bending. 10. A spool valve as claimed in any one of Claims 7 to 9 in which the driving member is adjustably secured to the spool at the said one end so that adjustments can be made in the relative longitudinal positions of the spool and driving member. 11. A spool valve as claimed in any one of Claims 7 to 10 including a second spool mounted for longitudinal movement within the valve housing, both spools being driven by means of the same flexible driving member. 12. A spool valve as claimed in Claim 11 when dependent upon Claim 8 in which the drive shaft of the motor extends within an annular centre portion of the driving member, the inner surface of the annular portion comprising the camming surface.

Inventors: FUELL FREDERICK JAMES, FUELL, FREDERICK, JAMES
Applicants: FAIREY HYDRAULICS, FAIREY HYDRAULICS LIMITED
Application year: 1989
Main classification: F15B13
IPC codes: F15B13, F15B18, F16K31