GB2031519A - Hydraulic actuator with relief valve actuators - Google Patents

Abstract

This invention relates to piston and cylinder actuators, in particular hydraulically operated actuators. Its object is to provide a relief valve for such an actuator which is not easily damaged or adjusted and which is not prone to failure. This object is achieved by mounting a relief valve (21) inside a passageway (22, 31) in the piston (3) and/or piston rod (2) of piston (3) and cylinder (5) arrangement. When the relief valve (21) is open the passageway (22, 31) operatively connects an inlet (6) in the cylinder (5) on one side of the piston (3) to an outlet in the cylinder (5) on the other side of the piston (3) to relieve the pressure in the cylinder (5). The invention will find use, for instance, in hydraulically operated cutting tools, advancing rams in mining systems and slewing and elevating rams for mine roadway- forming machines. <IMAGE>

Description

SPECIFICATION Piston and cylinder arrangement This invention relates to fluid operated piston and cylinder arrangements, and in particular, relates to such arrangements having relief valves.

Many devices used in the mining industry are operated by piston and cylinder arrangements, which may be supplied with pressure fluid at up to for example about 10,000 p.s.i.

The arrangements are normally hydraulically rather then pneumatically operated. Frequently the devices have operating pressure less than the maximum supplied pressure, and therefore relief valves which open when the maximum operating pressure is exceeded are provided. The relief valves are fitted in the pressure fluid supply line to the device and may therefore be accidentally damaged, for instance by falling rock or by an operative.

Also, since they must withstand high pressures, the relief valves must be encased in bulky housings. Even so the relief valves are prone to failure especially when damaged, causing danger to operatives in the vicinity and loss of working time while repairs are effected. Moreover, the relief valves are easily accessible and therefore can be adjusted to vary the relief pressure setting to one which is higher than the maximum operating pressure, thereby increasing the likelihood of damage to the device or the relief valve.

It is an object of the present invention to provide a piston and cylinder arrangement which at least to a certain extent overcomes the above mentioned disadvantages.

According to the present invention there is provided a piston and cylinder arrangement having in the cylinder a fluid inlet on one side of the piston and a fluid outlet on the other side of the piston, the arrangement including a relief valve mounted in the piston such that, in use when the fluid pressure in the cylinder exceeds a preset limit, the relief valve opens operatively to connect the inlet to the outlet to relieve the pressure.

The piston and cylinder arrangement may be single of double acting. In the latter case the roles of the inlet and outlet are reversed on alternate working and return strokes. Preferably, in either case, a non-return or check valve is mounted on the outlet side (during a working stroke) of the relief valve to prevent pressure during the return stroke from affecting the relief valve. The check valve ensures that the seating of the relief valve is protected, and therefore that the relief valve operates at the preset pressure. If the seating is damaged there may be a build up of excessive pressure which may lead to the failure or the arrangement. The check valve is for instance a spring biased ball valve.

The fluid may be a gas such as compressed air, although pneumatically operated arrangements do not normally operate at high pressures. It is envisaged that in most cases this arrangement will be used in hydraulically operated arrangements, in which case the fluid may be water, oil, or an oil/water emulsion.

Preferably a piston rod is mounted on the piston and the relief valve is mounted coaxially with the piston rod and, conveniently, partly in the piston rod and partly in the piston. Conveniently the check valve is mounted in a passage in the piston rod.

Since the relief valve is located coaxially with the piston rod in the piston and/or piston rod, the piston and/or piston rod will provide a robust housing for the relief valve.

Moreover, since the relief valve is mounted internally of the piston and cylinder arrangement, it cannot be easily damaged, nor can it be adjusted.

It is envisaged that the piston and cylinder arrangement will be of particular, but not exclusive, use in a hydraulically operated cutting tool, such as is used in mining operations for the recovery of wrecked or obsolete material which is embedded in the mine. The relief valve would be set to relieve at a pressure which is the designed maximum operating pressure of the device, thus preventing any damage to the device. The arrangement may also be used in other piston and cylinder actuated devices, such as slewing and elevating rams of a roadheading mining machine or in self-advancing roof supports.

The invention will now be described, by way of example only, with reference to the accompanying drawing, in which; Figure 1 shows a part sectional plan view of a hydraulically operated cutting tool including an arrangement according to the invention; and Figure 2 shows a detail of Fig. 1.

Referring to Fig. 1, the cutting tool is shown and includes a single-acting hydraulic ram 1, comprising a piston rod 2 mounted on a piston 3, a spring 4 and cylinder 5. The ram 1 is adapted for connection with a pump (not shown) through coupling 6 at the end of the ram 1, and has an outlet 7 at the end of the cylinder away from the coupling 6 for venting pressure fluid to tank (not shown). A blade 8 is fixed onto the free end of the piston rod 2. The blade 8 has at its marginal ends rectangular flanges 9.Forks 10 are attached to the bottom of the ram 1 and mate with lugs 11 on the ends of the arms of a Ushaped yoke assembly 1 2. The yoke assembly 1 2 comprises two arm units 13, which are each provided at each end with a lug 11 or 14, and a base unit 15, which is provided at both ends with forks 1 6. The forks 10 on the cylinder 5 and the lugs 11 of the arm units 1 3 are held together by pins 17. The inside of each arm unit 1 3 is shaped into a blade guide 1 8 to receive in sliding fashion the flanges 9 on the marginal ends of the moveable blade 8.The lugs 1 4 on the arm units 1 3 mate with the forks 1 6 of the base unit 1 5. The forks 1 6 and lugs 14 are held together by pins 19. At least one of the pins 1 7 and 1 9 is removeable. The three remaining pins may be removeable or may be fixed in relation to the lugs 11 or 14 and forks 10 or 16. A fixed blade 20 is mounted inside the base unit 1 5 parallel to and facing the moveable blade 8.

A relief valve 21 is mounted in the piston rod 2 and piston 3 as shown in Fig. 2, to which reference is now also made. A well 22 is sunk into the end of the piston rod 2 which fits into the piston 3. The end of the rod 2 is stepped and externally screw-threaded on the narrower portion of the step. A spacer ring 23 slip fits over part of the stepped portion of the rod 2 and is held in position by the piston 3 which screws onto the piston rod 2. The piston 3 is also stepped so that the piston spacer ring 23 define between them a space in which seal 24 is accommodated.

The well 22 is internally threaded at both its ends. The pressure relief valve 21 (a Dowty Yield Valve C 8240M-C, set to relieve at 10,000 p.s.i.) fits into the well 22 with its head 25 screwed into the end of the well 22 and with its body located in the well 22 so that there is an annular space 26 around it.

The valve has on its free end a sealing ring 27 located in a stepped portion of the valve 21. A valve seating sleeve 28 is screwed into the well 22. The sleeve 28 is internally stepped, so that the step locates the sealing ring 27, and also locates a second sealing ring 29 in a groove 30 in the well 22 between the two screw-threaded portions. A passage 31 is formed in the piston rod 2 connecting the annular space 26 to the low pressure side of the ram 1. A spring 32 biased check valve 33 is fitted in the passage 31, the action of the spring 32 being to move a ball onto a seat towards the annular space 26.

The tool is used in the following manner to cut metal arch section beams and chains. One or more of the removable pins 1 7 or 1 9 is removed and the yoke 1 2 is placed around a workpiece in the form of an arch-section beam 34, such that the fixed blade 20 is perpendicular to the plane of the web of the beam 34.

The forks 10 on the cylinder 5 are mated with the lugs 11 on the yoke 1 2 and the pin or pins 1 7 or 1 9 are replaced. Hydraulic power is supplied to the ram 1, either mechanically or manually. The spring 32 causes the check valve 33 to close, thus protecting the seating of the valve 21. As the moveable blade 8 is forced towards the fixed blade 20, forces are exerted through the blades 8 and 20 of the beam 34. When sufficient force is being exerted on the beam 34, it will fracture. After the beam 34 has been fractured and the pressure in the ram 1 released, the spring 4 returns the piston rod 2 to its original position.

When forces are applied along the top and bottom of a metal arch section beam such that the forces are opposed to one another and perpendicular to the plane of the web of the beam, the metal is first plastically deformed, and, as the forces increase, eventually fractured. The same fracturing can be induced in chain-links if the forces are applied perpendicular to the cylindrical axis of the link.

However, if the forces are applied parallel to either the plane of the web of the beam or the cylindrical axis of the link, only plastic deformation of the metal occurs. In the case of very strong materials, the tool may not be able to exert sufficient force to fracture the member.

In these cases, the pressure from the ram 1 will build up until it reaches the pre-set limit of 10,000 p.s.i. at which pressure the relief valve 21 is set to open. When the relief valve 21 opens, the pressure of the fluid will also overcome the action of the spring 32 of the check valve 33, thus allowing the pressure fluid to be vented to tank via outlet 7. If this happens, an operative using the tool can turn the power supply off and either reposition the tool to enable it to cut the beam 34 or remove the tool and use a more powerful or different means which is able to cut the member.

The relief valve can neither burst, nor be damaged because it is contained in a robust piston and piston rod arrangement, and cannot be adjusted in situ inside the piston and cylinder arrangement. Moreover the presence of the check valve ensures that the seating of the relief valve is not damaged, thereby preventing the relief valve from operating inefficiently.

Claims (11)

1. A piston and cylinder arrangement having in the cylinder a fluid inlet on one side of the piston and a fluid outlet on the other side of the piston, the arrangement including a relief valve mounted in the piston such that, in use, when the fluid pressure in the cylinder exceeds a preset limit, the relief valve opens operatively to connect the inlet to the outlet relieve the pressure.

2. An arrangement according to claim 1, which is single acting.

3. An arrangement according to claim 1, which is double acting.

4. An arrangement according to any one of the preceding claims, and including a check valve mounted on the outlet side during a working stroke of the relief valve.

5. An arrangement according to claim 4, in which the check valve is a spring biased ball valve.

6. An arrangement according to any one of the preceding claims, which is hydraulically operated.

7. An arrangement according to any one of the preceding claims, and including a piston rod mounted on the piston, the relief valve being mounted coaxially with the piston rod partly in the piston and partly in the piston rod.

8. An arrangement according to claim 7 when dependent on claim 4, in which the check valve is mounted in a passage in the piston rod.

9. A piston and cylinder arrangement substantially as hereinbefore described with reference to the accompanying drawings.

10. A hydraulically operated cutting tool, comprising a piston and cylinder arrangement according to claim 7, having a moveable blade mounted on the piston rod and a fixed blade mounted in a yoke which is attachable to the cylinder such that, in use, the fixed and moveable blades are located parallel and opposition to each other.

11. A hydraulically operated cutting tool substantially as hereinbefore described with reference to the accompanying drawings.