GB2216942A - Connecting elements of high pressure devices - Google Patents

Abstract

A connection between elements 14, 38 of high pressure devices such as pumps and valves comprises an axially slidable cage 54 having apertures 60 for accommodating ball latches 50. The ball latches engage between a wedging surface 48 provided on the outer wall of the socket 38 and the wall 52 of the stem 14. Bias means 56 is provided for exerting an axial thrust on the cage 54 for maintaining the wedging action. The configuration of the socket 38, retaining means and stem is such as to enable the stem 14 to be axially latched within the socket irrespective of its attitude with respect to the socket. As shown the stem 14 is a valve spindle and the connection secures the stem to socket 38 at the end of a valve actuating rod. <IMAGE>

Description

HIGH PRESSURE DEVICES This invention relates to high pressure fluid-handling devices, and in particular to high pressure hydraulic and pneumatic pumps and valves.

High-pressure devices tend to suffer from erosion, particularly when used to handle abrasive or aggressive media. It is therefore desirable to use very hard materials such as tungsten carbide, for vulnerable components such as valve spindles and seats, and pump pistons. However, hard materials tend to be brittle and therefore cannot be used in circumstances such that they are subjected to high local stresses, or to bending stresses due for example to misalignment. It is therefore extremely difficult to design and manufacture reliable equipment incorporating tungsten carbide and similar materials.

We have unexpectedly found that such problems can be largely or completely eliminated, by the use of a ball latch, or a similar self-wedging retaining device, for mounting and retaining components of high-pressure fluid devices, such as valve spindles and pump pistons.

Such latches can enable the retained component to be effectively self-centering and self-aligning, so that stresses are effectively distributed, and operational wear can be taken up automatically.

In one specific embodiment of the present invention, a high-pressure valve comprises a valve seat and spindle, and an operating member such as a stem for moving the spindle towards and away from the seat, the valve spindle being located and retained in the operating member by a self-wedging ball latch and a thrust member.

In another specific embodiment of the present invention, a high-pressure pump comprises a piston which is reciprocated by an eccentric rotary cam, the piston comprising a stem located and retained in a head driven by the cam, by & self-wedging ball latch and a thrust member.

The invention will be further described by way of example only, with reference to the accompanying drawings in which: Figure 1 shows a manually operated high-pressure valve, to which the invention is applied, in axial section, Figure 2 shows, in axial section and on a much larger scale, the seating of the valve spindle in the valve stem, Figure 3 shows, in axial section, a high-pressure pump to which the invention is applied, Figure 4 shows the plunger of the pump, on a much larger scale.

Figure 1 shows a manually operated stop/metering valve, used at fluid pressures up to 60000 psi (414 MPa).

The valve comprises a stainless steel body 2, into which are screwed a valve bonnet 4 and a valve retainer 16 incorporating a high pressure flow union. The valve retainer holds in place a valve seat 26, which is engaged by the conical tip 28 of a valve spindle 14.

The valve bonnet accommodates the lower end of a valve stem 6 on which is a screw-threaded bush 10 connected to the upper end of the valve spindle 14. The screw bush 10 engages screw threads inside the valve bonnet, so that when the valve stem is rotated by means of a handle 8, the spindle tip 28 is moved towards or away from the valve seat 26. The valve spindle is sealed by a seal assembly 12, and the space above the valve seat communicates with a high pressure flow union or socket 20 provided in the valve body, by way of an internal passage 22.

Valves of this general construction are well known.

It will be seen that the contact area between the tip of the spindle and the frusto-conical valve seat, is small, and that the space between the spindle tip and the seat may act as a nozzle in relation to the flow through it.

Flow velocities will therefore be very high in this region, if the valve is part-closed. Furthermore, some leakage is inevitable when the valve is fully closed, and this will also involve high-speed fluid flow between the spindle tip and the seat.

Consequently, the spindle tip and valve seat are subject to rapid wear, and need to be renewed frequently. It is therefore very desirable that the valve spindle should be easily replaceable.

Positive positional location of the spindle is very important, particularly if the valve is to be used as a metering valve. It has already been proposed to make the valve spindle detachable from or rotatable in the valve stem, but known valves of this nature develop slackness or backlash between the valve spindle and stem when used, which impairs their usefulness as metering valves.

Valve wear could be reduced if it were possible to use very hard materials, for example tungsten carbide.

However, this has proved impossible in existing valves, because misalignment and/or high local thrust stresses between the tungsten carbide spindle and the valve stem lead to fracture of the tungsten carbide, which is inherently brittle.

In accordance with the present invention, the valve stem is provided with an axial socket to receive the end of the valve spindle as a plug-in element, the valve stem incorporates a ball latch to retain the spindle1 and a self-aligning thrust bearing is provided between the base of the socket and the end of the valve spindle.

This is illustrated in Figure 2, which shows the upper end region of the valve spindle 14, seated in the socket 38 provided in the screw bush region 10 of the valve stem 6.

The upper end, or base, of the socket 38 has a frusto-conical or otherwise dished thrust seat 40.

Between this and the lower end of the valve stem, the socket 38 has an enlarged region, which accommodates a self-centering, self-wedging ball latch 36.

The ball latch comprises a frusto-conical annular wedge surface 48 which tapers towards the outer end of the socket 38, that is, towards the lower end face of the valve stem. This wedge surface may be machined into the internal surface of the valve stem within the socket 38, but it is more convenient to provide it on a separate sleeve 58 which is seated in and suitably retained in the lower outer region of the socket 38.

Within this wedge surface is a ring of steel balls 50, located in apertured seats 60 in an axially movable cage 54, which can slide substantially without friction, within the wedge surface 48. The balls 50 roll on the wedge surface 48, and depending on the axial positions of the balls and cage relative to the wedge surface, the balls project radially inwards within the cage sleeve 54 as shown on the left of Figure 2, or can retreat radially outwards so as to lie flush with or even clear of the internal surface of the cage as shown on the right of Figure 2.

A compression coil spring 56 is seated under light pre-stress, between the inner end region of the cage and the inner end of the socket 38, so that the cage and the balls are always urged towards the outer, narrower, region of the wedge surface 48, causing the balls to project radially inwards.

If the valve spindle 14 is inserted into the ball latch in the socket 38, its upper end, which is preferably chamfered as shown at 46, will push the latch balls 50 into the socket and radially outwards, along the wedge surface 48, to the position shown on the right hand side of Figure 2, enabling the spindle to enter easily within the ring of balls. However, once the spindle has been inserted, it cannot be pulled out of the socket, because frictional contact between the balls and the spindle and wedge surface, together with the axial thrust of the spring 56, will move the balls towards the narrower end of the wedge surface 48, causing the balls to move radially inwards into tight frictional engagement with the spindle. The balls therefore become firmly wedged between the wedge surface 48 and the spindle, preventing the latter from being pulled out of the socket. This effect will arise even in the absence of the spring 56, but the spring is desirable to eliminate any looseness between the balls and the spindle, so that the described wedging action comes into play immediately the spindle is moved in an axially outwards direction, and irrespective of the attitude of the valve. The wedging of the balls provides substantially instantaneous positive retention of the spindle in the socket.

The illustrated spindle has a shallow reverse taper or wedge surface 52 machined in it adjacent to its chamfered end, to enhance the wedging action, but in most cases, it is not necessary to provide a reverse taper, as the balls will provide ample grip on a plain cylindrical surface of the spindle.

The spindle can be released from the valve stem very easily, it is only necessary for the user to insert a thin sleeve or finger into the socket to engage the outer end of the cage sleeve 54 and push it into the socket, to release the wedging action at the balls. The spindle can then easily be removed, and a fresh spindle can be plugged into the socket. Changing a worn or broken valve spindle is therefore exceptionally easy.

To transmit thrust from the valve stem to the valve spindle, a thrust member 42 is provided between the flat upper end surface 44 of the spindle1 and the thrust seat 40 provided in the valve stem. In the illustrated embodiment, this thrust member is a ball, provided with a flat lower surface resting on the flat end of the spindle. The thrust member can be made of stainless steel, bronze, PTFE, polyamide, or any other suitable material. The thrust ball can be a separate component as illustrated, or may be integral with the spindle end. Other forms of thrust bearing can be used, for example a full sphere, or a flat pad.

The described construction has numerous significant advantages. The ball latch does not require the spindle to be in any particular axial position relative to the socket, for positive locking, therefore wear does not lead to loss of positive axial location between the spindle and the valve stem. The ball latch is effectively self-centering and self-aligning, at the same time it allows slight lateral shifting of the valve spindle and it allows the spindle to rotate within the balls, so that the spindle, when closed, can take up the optimum position on the valve seat. Furthermore, because the spindle and valve stem are relatively rotatable, the spindle tip and valve seat are less subject to scoring when the valve is closed: The valve can be tightened down without the spindle rotating in contact with the seat.

The illustrated thrust bearing between the spindle and stem is self-centering and self-seating relative to both the valve stem and the spindle end, and acts as a load spreader. This, together with the self-centering ability of the spindle and latch, substantially eliminate local stress concentrations, and off-centre stresses in the spindle, and this makes it possible to manufacture the spindle from tungsten carbide or other hard brittle material, with little risk of fracturing the spindle when the valve is tightened in its closed position. It will be understood that only the spindle, which is relatively small, need be made of the expensive hard material, the rest of the valve stem is of stainless steel or other metal.

We have therefore provided, for the first time, the practical possibility of using tungsten carbide and similar materials as high-pressure valve spindles, leading to reduced wear, reduced maintenance, and reduced overall cost despite the relatively high cost of tungsten carbide.

Ball latches can be used with advantage for locating other components of high pressure fluid handling devices. Figure 3 shows, in section, a high pressure electro-hydraulic pump, generally as described in our British Patent Specification No. 1301687. A crank case 72 accommodates a drive shaft 74 on which is an eccentric cam 76, engaged by shoes 78. In opposite sides of the crank case are mounted respective high pressure pump cylinder bodies 80 each with a high pressure outlet valve 88. A pump piston stem 92 projects into each cylinder through suitable seals, and at the outer end of each piston stem is a piston head 90 which is acted on by the respective shoe 78 through a thrust tappet 84. The cylinder body also incorporates a high pressure inlet valve 86. A compression spring 82 holds the piston head tightly against the thrust tappet 84.

Pumps of this nature are well known. When the drive shaft 74 rotates, the pistons are caused to reciprocate within the respective cylinder bodies, pumping fluid at high pressure from the interior of the crank case through the piston and cylinder into external pipe work.

If the medium being pumped is abrasive or aggressive, it is very desirable to use tungsten carbide or similar material as the piston. However, to maintain the correct line of thrust from the eccentric cam to the piston, a swivel tappet arrangement is required such as that illustrated in Figure 3. Since tungsten carbide is very expensive. it is undesirable that it be used to form the entire piston including the piston head, and therefore attempts have been made to construct pistons, with a tungsten carbide piston stem seated in a steel piston head also containing the inlet valve. However this has led to serious problems.If the tungsten carbide stem is fastened in the steel head by means of silver solder in conventional manner, it is impossible in practice, tQ completely eliminate axial misalignment between the piston head and piston stem, owing to the clearance which must be provided between them to receive the solder. Bending moments inevitably arise in operation of the pump, causing the brittle tungsten carbide stem to fracture, especially in the case of pumps of small size, such as 6 mm. Removing and replacing a soldered piston stem is inconvenient, and replacing the entire piston unit is costly.

We have found that these problems can be overcome if the piston stem is seated and located in the piston head, by a ball latch for example generally as shown in figure 4, in which reference numerals corresponding to those shown in fugure 2 are used. However it will be noted that in this embodiment, the balls are located between the inner end of a slidable guide bush 96, and a spring seat or cap 94, instead of in apertures in an integral cage as in figure 2.

As already described, such a latch is effectivel-y self-centering and self-aligning and self-compensating with respect to wear. The risk of fracture of the piston stem due to misalignment and bending moments is therefore reduced or eliminated. It will be understood that in the case of a pump, it may be necessary to modify the thrust bearing between the inserted piston stem and the piston head, to allow flow through from t piston head into the piston stem.

The use of a ball latch to retain a piston stem in the piston head has the further advantage that it enables piston stems of different effective sizes to be fitted in a common size of piston head, leading to greater flexibility in manufacture. This advantage arises regardless of the material used for the piston stem, that is to say, the piston stem can be made of the same material as the piston head, such as steel.

The illustrated pump provides for fluid flow axially through the piston head and stem into the cylinder, with an inlet non return valve in the piston head. Such an arrangement may not be convenient, if a separate piston stem is used, seated in the piston head, as the presence of the ball latch to retain the stem in the head may not allow sufficient space for the inlet valve. In this case, the inlet valve may alternatively be provided in the cylinder body, and the piston stem and piston head can be made solid instead of apertured.

(Include a drawing of pump design incorporating ball latch if possible.)

Claims (13)

1. A device comprising a socket, a stem and retaining means for axially latching the stem within the socket by virtue of a wedging action exerted between the stem and the socket by the retaining means, wherein the device is configured so that the axial latching can be achieved for a range of differing attitudes of the stem with respect to the socket.

2. A device according to claim 1, wherein the wedging action of the retaining means is achieved by means of a plurality of ball latches which can be wedged between the stem and a wedge surface which tapers inwardly towards the open end of the socket.

3. A device according to claim 2, wherein the wedge surface is integral with the socket wall.

4. A device according to claim 2, wherein the wedge surface is formed on a separate sleeve seated within the socket.

5. A device according to claim 2, claim 3 or claim 4, wherein the ball latches are captive within apertures provided in an axially movable cage, which when slid axially into the socket serves to release the wedging action for facilitating removal of the stem from the socket.

6. A device according to claim 2, claim 3 or claim 4, wherein the ball latches are located between the inner end of a slidable guide bush and a cap, which bush and cap when slid axially into the socket serve to release the wedging action for facilitating removal of the stem from the socket.

7. A device according to claim 5 or claim 6, comprising bias means for exerting an axial thrust on the cage or the bush for maintaining the wedging action of the retaining means.

8. A device according to any one of claims 2 to 7, wherein the stem is provided with a surface which tapers outwardly toward the end of the stem which is intended to be innermost within the socket.

9. A device according to any one of the preceding claims1 wherein a thrust member is provided on the end of the stem for engagement with a seat at the inner end of the socket.d

10. A device according to claim 9, wherein the thrust member is spherical in form.

11. A valve comprising a device according to any one of the preceding claims, wherein the valve comprises an operating member for moving the stem towards and away from a valve seat, the end of the stem remote from the valve seat being retained in the operating member by the device.

12. A pump comprising a device according to any one of claims 1 to 10, wherein the pump comprises a piston which is reciprocated by an eccentric rotary cam, the piston comprising the stem which is located and retained in a head driven by the cam by the device.

13. A device substantially as hereinbefore described with reference to Figure 2 or Figures 3 and 4 of the accompanying drawings.