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 impedes the flow of fluid thus causing a build-up of fluid pressure 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 workpiece is securely gripped, means (22) are provided to relieve pressure within the ram when the ram reaches an end of its working stroke.

BACKGROUND TO THE INVENTION 
The invention relates to chucks for use in holding tools or workpieces, and 
in particular to chucks which are controlled by a fluid-operated ram. 
DESCRIPTION OF THE PRIOR ART 
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. 
In one known form of chuck, proximity probes are provided on the cylinder 
of the ram to detect when the piston of the ram is at each end of its 
stroke. 
With the chuck described in our co-pending U.S. patent application Ser. No. 
525,729, now U.S. Pat. No. 4,616,838 problems arise from mounting 
proximity probes on the cylinder, since the cylinder is effectively 
integral with the chuck and rotates with it, making it difficult or 
impossible to make electrical connections to the probes. 
SUMMARY OF THE INVENTION 
Accordingly we have devised an alternative non-electrical solution to the 
problem. Although the solution has been devised specifically for use with 
the chuck described in U.S. Ser. No. 525,729, it is of general 
application. 
The invention provides a fluid-operated ram for use in actuating a device 
such as a chuck, the device having at least one limiting position which 
will impede the flow of fluid hence resulting in a build-up of fluid 
pressure within the ram, and means to relieve the pressure within the ram 
when the ram reaches at least one end of its working stroke. 
Since the pressure is relieved when the ram reaches at least one end of its 
working stroke, so that there is a continued flow of fluid to the ram, 
there can only be a build-up of pressure when the device (e.g. a chuck) 
reaches its limiting position. 
Preferably the ram incorporates a pressure relief valve which is actuated 
when the piston of the ram reaches the said one end of its stroke. 
The pressure relief valve may act to put the two sides of the piston into 
communication with one another. 
Where the ram is double-acting, there may also be a non-return valve to 
ensure that even if the pressure relief valve is open, the direction of 
movement of the piston can be reversed. 
Where it is desired to detect when the ram reaches each end of its working 
stroke, there may be a pair of pressure relief valves. 
Each pressure relief valve may act as a non-return valve when the other 
pressure relief valve is open. 
Each pressure relief valve may comprise a valve member spring urged on to a 
valve seat and a plunger arranged to move the valve member off its valve 
seat when the piston reaches one end of its working stroke. 
The ram may be associated with detection means arranged to detect when the 
said device reaches its limiting position. 
The detection means may comprise a pressure switch arranged to detect a 
build-up of pressure within the ram. 
Alternatively, the detection means may comprise means to detect that fluid 
flow to the ram has ceased. 
Other objects and advantages of the invention will become apparent from the 
following description of an embodiment of the invention, given by way of 
example.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The chuck shown in FIG. 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 port 17. Thus 
far the operation of the chuck is identical to that described in U.S. Ser. 
No. 525,729 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 FIG. 3, and in more detail in FIG. 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 20 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 aperture 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. 
FIG. 6 illustrates diagrammatically the cylinder 10 and piston 12. There is 
a pressure supply line 28 extending to a changeover valve 20 and there is 
a fluid return line 3O. 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 FIG. 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 FIG. 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.