CHUCKS

Abstract: A fluid-operated ram, for example comprising a cylinder 10 and piston 12, is provided for actuating a device such as a chuck having jaws 15. When the jaws are moved by the ram to grip a workpiece this constitutes a limiting position which im­ pedes the flow of fluid thus causing a build-up of fluid pres­ sure within the ram. This build-up can be detected to give an indication as to when a workpiece has been securely and safely gripped. To ensure that a false indication is not given if the ram reaches an end of its working stroke before the work­ piece is securely gripped, means (22) are provided to relieve pressure within the ram when the ram reaches an end of its working stroke.

Description: The invention relates to a fluid operated ram on a chuck for use in holding tools or workpieces, and in particular to chucks which are controlled by a fluid-operated ram. When such a chuck is used to grip a tool or workpiece, there is a risk that the ram may reach the end of its working stroke before jaws of the chuck have gripped the tool or workpiece sufficiently tightly. Frequently such chucks use a pressure sensing device to indicate when a tool or workpiece has been gripped sufficiently tightly, since there is then an increase of pressure in the operating fluid, but there will also be a build-up of pressure when the ram reaches the end of its working stroke, and so to avoid a dangerous condition in which a chuck is rotated while a tool or workpiece is inadequately gripped, means must be provided to differentiate between the two reasons for a build-up of pressure. FR-A 2 200 451 discloses a fluid operated ram in which there is means to relieve pressure within the ram when the piston of the ram reaches each end of its working stroke. However there is no means to detect when this has happened, and the means for relieving pressure appears to be provided merely to avoid any risk of a dangerous build-up of pressure within the ram when the piston reaches each end of its working stroke. A known form of chuck, which does have detection means is described in EP-A-0 033 886. In this prior art device, proximity probes are provided to detect when the piston of the ram is at each end of its stroke. It can readily be seen that the device disclosed in EP-A-0 033 886 is quite complicated, and requires a number of special items of equipment. Furthermore, two adjustable sensors have to be carefully pre-set at particular positions, before the device will operate satisfactorily. We have now devised an alternative non-electrical solution to the problem which is not only much simpler than the arrangement shown in EP-A-0 033 886, but is also particularly suitable for use with the chuck described in our co-pending U.K. Patent application number 8224769, although our solution can be used on other chucks. The invention provides a fluid operated ram for use in actuating a device such as a chuck, said ram comprising: a piston moveable along a working stroke, said stroke having a first end and a second end, said device having at least one limiting position which generates a first condition in which flow of fluid is impeded when movement of said piston is halted at a position intermediate said first and second ends of said working stroke, thereby resulting in a build-up of fluid pressure within said ram; and means for relieving pressure within the ram upon generation of a second condition in which movement of said piston is halted upon reaching at least one of said first and second ends and flow of fluid continues, thereby avoiding a build-up of pressure within said ram, characterised in that pressure sensitive means is provided to detect said first or second condition. The pressure sensitive means can be arranged to detect the pressure build up of said first condition, thus indicating that the piston has reached the limiting position. However it is preferred that the pressure sensitive means is arranged to detect the continue flow of fluid of the second condition, thus indicating a fault condition. It is preferred that the pressure sensitive means comprises an orifice and a pressure actuated switch. By way of example, a specific embodiment of the invention will now be described, with reference to the accompanying drawings, in which: Figure 1 is an axial cross-section through a chuck generally similar to that described in U.K. Patent Application No. 8224769 but incorporating an embodiment of fluid operated ram according to the invention; Figure 2 is a partial cross-sectional view on line II-II of Figure 1; Figure 3 is a cross-section on line III-III of Figure 2; Figure 4 is a view of part of Figure 3 to an enlarged scale; Figure 5 is a perspective exploded view of some of the components shown in Figure 4; and Figure 6 is a schematic pressure circuit diagram showing the connections to the chuck incorporating the embodiment of the invention. The chuck shown in Figure 1 comprises a cylinder 10 defining an annular chamber 11 in which moves a piston 12. The piston has, uniformly spaced around it, three apertures 13 through which pass matingly shaped plugs 14. The apertures 13 and plugs 14 are so shaped that as the piston 12 moves back and forth, the plugs 13 move radially inwardly and outwardly, to operate the jaws 15 of the chuck. Fluid can be applied to one side of the piston 12 through a port 16 and to the other side through a portion 17. Thus far the operation of the chuck is identical to that described in U.K. Patent application No. 8224769 and no more detailed description will be given here. In order to bolt the chuck to the nose of a machine tool, passages 18 for bolts 19 are provided. To enable the bolts to pass through the piston and cylinder without interfering with the operation of the chuck, the piston 12 has passages therein, provided with sealed liners 20. The significance of this embodiment of the invention lies in the way in which means are provided to detect when the piston has reached each end of its working stroke. As shown in Figure 3, and in more detail in Figure 4, a further passage 21 is provided through the piston 12 and in this passage two non-return valves are provided back to back. Each non-return valve comprises a valve member 22 which engages a valve seat 23. The valve members are urged on to the valve seats by a compression spring 24 acting between the two valve members. Each valve member has an integral plunger 25 which projects to a point which is proud of the surface of the piston 12. Each plunger 25 is provided with flutes 26 to guide the movement of the valve member whilst permitting fluid flow past the plunger 25 when the valve member is moved off its associated seat. Each seat 23 is provided by the end of a collar 27 which is screwed into a threaded portion of the passage 21. If the position of the jaws 15 in the chuck are adjusted correctly, and the chuck is only used to grip workpieces of a stipulated size, the jaws will always come into firm engagement with a tool or workpiece before the piston 12 reaches the end of its working stroke, and so there will be a build-up of pressure within the chuck and flow of fluid to the cylinder will cease, apart from relatively small leakage flow. If however the jaws 15 should be incorrectly set, or the chuck be used to grip a tool or workpiece of inappropriate size, such that the piston 12 approaches either end of its working stroke, one of the plungers 26 will abut against one end wall of the cylinder and the associated valve member 22 will be lifted off its valve seat 23. The oppositely acting valve member 22 will act as a non-return valve and pressurised fluid will be able to flow through the piston from one side to the other. Thus fluid will continue to flow to the cylinder and there will be no build-up of pressure. This continuous flow, or lack of build-up of pressure, can be detected as described below and used to indicate a fault condition. Because the oppositely acting valve member 22 acts as a non-return valve, the piston can still be moved away from the end of its stroke when desired by reversing the flow of fluid to the cylinder. The fluid will flow past the open valve member 22 but as it cannot flow in the reverse direction past the oppositely acting non-return valve, the piston will start to move again, and the open valve member 22 will in due course return to its seat 23. The above sequence of events will take place regardless of the direction in which the piston is moving, since the two valves are mirror images of one another. If the piston moves to the right as viewed in the Figures and reaches the end of its working stroke, the right hand valve member 22 will be lifted off its seat and the left hand valve member 22 will operate as a non-return valve. If however the piston is moving to the left as viewed in the Figure and reaches the end of its working stroke, the left hand valve member 22 will be lifted off its seat 23 and the right hand valve member will operate as a non-return valve. Because the valve members 22 relieve the pressure within the cylinder when the piston reaches the end of its working stroke, the only condition which will bring about-a build-up of pressure within the cylinder is when the chuck jaws 15 securely grip a tool or workpiece. It would clearly be possible to provide an indication of this, for use in chuck control purposes, by fitting a pressure actuated switch in the supply line to the piston and cylinder. However it is common practice to provide pressure reducing valves in the supply line so that the supply pressure can be adjusted to cope with, for example, a delicate thin walled workpiece. This would mean that each time the supply pressure was adjusted, the level of operation of the pressure actuated switch would also have to be adjusted, which would be inconvenient. Accordingly, in this embodiment, means are provided to detect whether or not fluid is flowing to the piston and cylinder. Clearly, when the jaw securely grips a workpiece, flow to the piston and cylinder substantially ceases. If however the piston reaches the end of its working stroke, flow to the piston and cylinder continues, because flow can take place through the piston itself. Figure 6 illustrates diagrammatically the cylinder 10 and piston 12. There is a pressure supply line 28 extending to a changeover valve 29 and there is a fluid return line 30. The supply line 28 is fed from a pump 31, via a filter 32, pressure gauge 33, and pressure activated switch 34. The filter 32 filters the supply from the pump, the pressure gauge 33 gives a visual indication of the actual pressure in the supply line, and the pressure activated switch 34 provides a signal, for example for use with control circuitry, to indicate that the pump is providing an adequate pressure. There is also a pressure relief valve 35 to protect the supply line and its components in the event that the pressure should exceed a predetermined limit. If it is desired to move the piston 12 to the right as viewed in Figure 6, then the valve 29 is positioned as shown so that pressurised fluid is supplied to the port 16. This causes fluid to exhaust through port 17 to the return line 30. If it is desired to move the piston 12 to the left as viewed in Figure 6, then the valve 29 is changed over, for example by means of a solenoid, to reverse the connections to the ports 16 and 17. The connections are made through a conventional rotating distributor 36 to a machine tool spindle 37 which is connected to the chuck. In order to detect the return flow, a sharp-edged orifice 38 is provided in the return line 30. A pressure actuated switch 39 is provided between the orifice 38 and the valve 29. When there is any significant flow down the return line 30, the orifice 38 causes a build-up of pressure which is detected by the pressure actuated switch 39. When the chuck is activated to grip a tool or workpiece, then if the chuck is set correctly and operates correctly, there will be a flow of fluid to the chuck, and a corresponding return flow through the line 30, for a relatively short period of time, until the tool or workpiece is adequately gripped. Flow will then substantially cease, pressure in the return line will drop, and the pressure actuated switch 39 will indicate this accordingly. If however the piston reaches the end of its working stroke before the tool or workpiece is adequately gripped, flow will continue indefinitely. Thus the switch 39 can be coupled to a control circuit which will indicate a fault condition if flow continues for more than a predetermined length of time. No seal can be perfect, and so there will be a certain amount of leakage flow down the return line 30. However the pressure produced by the flow of fluid through the sharp-edged orifice 38 is proportional to the square of the flow. Thus the pressure produced by the leakage flow will be very small indeed compared to the pressure produced by the flow indicating a fault condition. There will also be a certain amount of leakage flow through paths other than the return line 30 but this leakage flow can be returned to supply via a line 40. The invention is not restricted to the details of the foregoing embodiment. For example, although the embodiment utilises two mirror image valves arranged back to back, the two valves could be arranged at separate locations on the piston 12, provided that the valves are interconnected by a fluid flow passage. Although the invention is shown applied to a chuck of the general type shown in U.K. Patent Application No. 8224769, the invention is clearly applicable to other chucks, and indeed clearly has useful application with any device where a fluid-operated ram is used to operate a device having at least one limiting position which will result in a build-up of fluid pressure within the ram. It is not essential that the valves be arranged in the piston. Valves could be aranged in the cylinder such that when the piston reaches an end of its working stroke it contacts and operates one of the valves to put the two sides of the cylinder into communication with one another.

Claims: 1. A fluid operated ram (10) for use in actuating a device such as a chuck, said ram comprising: a piston (12) moveable along a working stroke, said stroke having a first end and a second end, said device having at least one limiting position which generates a first condition in which flow of fluid is impeded when movement of said piston (12) is halted at a position intermediate said first and second ends of said working stroke, thereby resulting in a build-up of fluid pressure within said ram; and means (22) for relieving pressure within the ram upon generation of a second condition in which movement of said piston (12) is halted upon reaching at least one of said first and second ends and flow of fluid continues, thereby avoiding a build-up of pressure within said ram, characterised in that pressure sensitive means (39) is provided to detect said first or second condition. 2. A fluid operated ram as claimed in Claim 1, in which the pressure sensitive means detects the pressure build-up of said first condition, thus indicating that the piston (12) has reached the limiting position. 3. A fluid operating ram as claimed in Claim 1, in which the pressure sensitive means (38, 39) detects the continued flow of fluid of the second condition, thus indicating a fault condition. 4. A fluid operated ram as claimed in any one of Claims 1 to 3, in which the means for relieving pressure within the ram (10) comprises a pressure relief valve (22) which is actuated when the piston (12) of the ram (10) reaches an end of said working stroke. 5. A fluid-operated ram as claimed in Claim 4, in which the pressure relief valve (22) acts to put the two sides of the piston (12) into communication with one another. 6. A fluid operated ram as claimed in Claim 4 or Claim 5, the ram (10) being double-acting, and incorporating a non-return valve to ensure that even if the said pressure relief valve (22) is open, the direction of movement of the piston (12) can be reversed. 7. A fluid operated ram as claimed in any one of Claims 4 to 6, in which there is a pair of pressure relief valves (22) to detect when the ram reaches each end of its working stroke. 8. A fluid operated ram as claimed in Claim 7, in which each pressure relief valve (22) acts as a non-return valve when the other pressure relief valve is open. 9. A fluid operated ram as claimed in Claim 7 or Claim 8, in which each pressure relief valve comprises a valve member (22) spring urged on to a valve seat (23) and a plunger (25) arranged to move the valve member off its valve seat when the piston (12) reaches one end of its working stroke. 10. A fluid operated ram as claimed in Claim 2, in which the detection means comprises a pressure switch arranged to detect a build-up of pressure within the ram. 11. A fluid operated ram as claimed in Claim 3, in which the detection means comprises an orifice (38) and a pressure actuated switch(39).

Inventors: JACKSON, JOSEPH FENWICK
Applicants: PRATT BURNERD INTERNATIONAL LIMITED
Application year: 1986
Main classification: B23B31
IPC codes: B23B31