Power operated chuck

A power operated chuck in which an actuator, such as a fluid pressure actuator, causes clamping or unclamping movement of the jaws in dependence on the position of a control member, in which the power for operation of the actuator is derived from rotation of the chuck or one of two chuck body parts on a shaft of a machine tool, the power source being housed within the chuck which is thus self contained and does not require a separate power source. The power source is in the form of a plurality of fluid pressure pumps worked by a swash plate carried by the rotating chuck or a rotating chuck body part. Fluid pressure built up by the pumps is stored in a fluid pressure reservoir and used to drive the actuator. As fluid pressure is built up the swash plate is displaced to a position where, when maximum pressure has been generated, it lies perpendicular to the axis of rotation of the chuck or the rotatable chuck body part so that no further pressure is generated as the chuck continues to rotate.

BACKGROUND OF THE INVENTION 
The present invention relates to a work holding chuck for a machine tool 
such as a lathe, capstan lathe milling machine or the like. In particular, 
the present invention relates to a power operated chuck capable of 
clamping to hold a workpiece, or unclamping to release a workpiece, in a 
single, power driven stroke which can be controlled by displacement of a 
control member. 
Conventional power chucks are either pneumatically or hydraulically 
operated and they rely for their power on power supplies separate from the 
driving motor of the machine tool; this represents a considerable 
disadvantage not only because of the expense of the chuck and its 
operating and control mechanism, but also because of the difficulty of 
mounting and setting up the chuck and its operating and control mechanism 
for proper operation. 
The power source for known pneumatically operated chucks is normally an air 
compressor which operates to provide a supply of compressed air, via a 
control valve or a plurality of control valves, to energise at least one 
pneumatic actuator which acts, via a suitable linkage, on a draw bar or 
tube which passes through a hollow spindle of the machine. The draw bar or 
tube links the actuator to the operating mechanism of a chuck the jaws of 
which are caused to open or close by the action of the actuator on the 
draw bar or tube. Known hydraulic chucks operate in a similar manner. One 
of the disadvantages encountered with such known power operated chucks is 
that of transmitting the power to close or open the jaws when controlled 
to do so while nevertheless allowing the chuck to rotate with the jaws 
clamped, at perhaps a very high speed. In some known hydraulically 
operated chucks there are various mechanical parts linked to an hydraulic 
actuator mounted at the spindle end of the machine with a chuck being 
attached to the spindle via an hydraulic actuator. Even in this case, 
however, the source of hydraulic pressure must be separate from the 
machine. 
Since the attachment, assembly and setting up of these known types of power 
chucks takes considerable time and labour they become effectively a part 
of the machine tool once fitted; although they are nominally 
interchangeable it has been found, in practice, that it is not worth doing 
so due to the amount of down time and labour involved. 
OBJECTS OF THE INVENTION 
One object of the present invention is to provide a power operated chuck 
which does not suffer from the above mentioned disadvantages of 
conventional separately driven power chucks, or of the known types of self 
contained power chucks described above. 
Another object of the present invention is to provide a chuck which is 
readily interchangeable either with other power operated chucks of the 
same type, or with manually operated chucks so that different sizes of 
work may readily be accommodated on a single machine by interchanging 
chucks of different size. 
A further object of the invention is to provide a power operated chuck 
which is robust, and which does not require a power supply separate from 
the motor driving the main spindle of the machine tool to which it is 
fitted. 
SUMMARY OF THE INVENTION 
According to one aspect of the present invention there is provided a power 
operated work holding chuck of the type having: a chuck body, radially 
movable jaws on the chuck body, fluid pressure operated actuator means, 
means linking said pressure operated actuator means to said jaws on said 
chuck body, whereby to transmit movement of said pressure operated 
actuator means to said jaws, the improvement wherein: said chuck body is 
formed in two parts which are secured together and rotatable with respect 
to one another, one of said chuck body parts housing said fluid pressure 
operated actuator means, means for mounting said one of said chuck body 
parts on a spindle of a machine tool, means defining fluid pressure 
reservoir in said one of said two chuck body parts, a power source 
comprising means within said two chuck body parts for generating, as said 
two chuck body parts rotate with respect to one another, fluid pressure 
for driving said fluid pressure operated actuator means, said power source 
acting to pump fluid from one said reservoir to the other, said fluid 
pressure generating means comprising a plurality of pumps operated by 
relative rotation of said two relatively rotatable parts of said chuck, 
said plurality of pumps being mounted in a circle on said one of said two 
chuck body parts and having operating plungers associated therewith 
extending axially of the chuck body, a swash plate carried by said one of 
said two chuck body parts in engagement with the free ends of said 
plungers of said pumps, and resilient biasing means on said other of said 
two chuck body parts, in engagement with said swash plate and resiliently 
biasing it to a position inclined with respect to the plane defined by the 
free ends of said plungers of said pumps when they are not depressed. 
By generating fluid pressure as a result of rotation of the chuck it is 
possible to dispense with the complicated and expensive separate power 
source and power feeding transmissions for transferring the power from the 
source to the chuck itself. This allows the chuck to be constructed 
integrally as a self contained unit and this in turn permits the ready 
interchangeability mentioned above. 
Derivation of power from the rotation of the chuck when mounted on a 
spindle of a machine tool requires the cooperation of a suitable fixed 
part of the machine tool. If no such suitable fixed part is available, 
such may be provided for attachment to the machine tool when a chuck 
formed as an embodiment of the invention is initially fitted. Thereafter, 
the same cooperating part may serve for any chuck which is interchanged 
with the one initially fitted, for the same purpose. 
Further features and advantages of the invention will become more apparent 
from a study of the following description of the invention, which is 
provided purely by way of non-restrictive example, and in which reference 
is made to the accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring now to the drawings, and in particular to FIG. 1 there is shown a 
chuck, generally indicated 11 comprising a substantially cyclindrical body 
12 at the front face 16 of which there are carried three jaws 17a, 17b, 
17c which are radially movable with respect to the axis of rotation of the 
chuck body 12 in the manner of a normal work holding chuck. The jaws 17 
are housed in slots 20 (better seen from FIG. 3) and, in operation of the 
chuck, make limited radial movements either radially inwardly or radially 
outwardly depending on the control movement of the chuck. 
Extending around the chuck body, spaced some distance from the front face 
16, there is a control ring 13 which, in operation of the chuck, is used 
to control the opening or closing of the jaws 17: turning the control ring 
13 in one direction to one angular end position with respect to the chuck 
body causes a powered clamping stroke, and turning the control ring 13 in 
the opposite direction to an opposite angular end position causes a 
powered opening stroke. Since the movement of the jaws 17 in both radially 
inward and radially outward directions is effected under a power stroke 
the chuck may be used for gripping either the outside of bar stock or the 
inside of tubular stock should this be desired. 
Carried on a mounting part 18 of the machine is a cooperating projection 14 
which extends towards a rear face 15 of the chuck body to cooperate with a 
swash plate 24 (see FIG. 2) in order to generate the pressure for 
operation of the chuck. 
Referring particularly to FIG. 2 it will be seen that the projection 14 
comprises a cylindrical mounting sleeve 20 within which slides a plunger 
21 resiliently biased by a spring 23 toward the rear face of the chuck 
body 12 and carrying at the outer end thereof a roller 22 which rolls over 
the swash plate 24 located within an inwardly directed flange 12a of the 
chuck body 12. 
The swash plate 24 is urged toward the rear flange 12a by the action of 
plunger arms 25e, 25f, 25g, 25h of respective fluid pumps 25a, 25b, 25c, 
25d which, although shown in line in FIG. 2, are in practice equally 
spaced in a circle around the axis of the chuck body 12. The pumps 25a, 
25b, 25c 25d have respective outlet valves 26a, 26b, 26c, 26d joined by a 
common high pressure line 28 to two high pressure reservoirs which are 
formed as two cylinders 32, 33 housing respective pistons 36, 37 biased by 
respective springs 40, 41. The reservoirs 32, 33 are connected by a line 
29 to two valves 30, 31 which are housed within the chuck body 12 and have 
respective actuating plungers 30a, 31a which extend radially outwardly and 
are positioned for radial movement. 
The plungers 30a and 31a engage the inside face of the control ring 13 
which is formed with two cam faces defined by notches 13a and 13b which 
are so positioned that the valves 30 and 31 are moved in opposition with 
respect to one another, with a slight phase displacement, when the control 
ring 30 is turned between angular end positions. The line 29 is connected 
to the valves 30, 31 by respective ports 30b, 31b which will be termed 
hereinafter the high pressure or inlet ports of the valves 30, 31 which 
also have respective outlet or low pressure ports 30c, 31c connected 
together by a low pressure line 44 which communicates with two low 
pressure reservoirs constituted by cylinders 34, 35 having respective 
pistons 38, 39 biased by respective springs 42, 43. The low pressure 
reservoirs 34, 35 are connected by a return line 27 to the low pressure 
inlet ends of the pumps 25 to complete the circuit. 
The valves 30, 31 also have a further port 30d, 31d respectively which will 
be referred to hereinafter as the controlled port. The valves 30, 31 
operate to connect either the high pressure ports 30b, 31b to the control 
ports 30d, 31d respectively or, alternatively, to connect the controlled 
ports 30d, 31d to the outlet or low pressure ports 30c, 31c respectively. 
Each valve 30, 31 also has an intermediate position where all ports are 
closed. The controlled port 30d of the valve 30 is connected by a line 45 
to an hydraulic actuator 47 and to an hydrualic actuator 48, and the 
controlled port 31d of the valve 31 is connected by a line 46 to an 
hydraulic actuator 50 and to an hydraulic actuator 48. The actuators 47 
and 48 have respective actuating plungers 47a, 48a and are arranged in 
opposition with a roller 51 trapped between the two plungers. Similarly, 
the actuators 50 and 49 have respective actuating plungers 50a and 49a and 
are positioned in opposition with a roller 52 between them. The rollers 
51, 52 are carried on respective radial arms 53, 54 of a control disc 55 
which is housed within the chuck body 12 for relative rotation with 
respect thereto whereas the actuators 47, 48, 49 and 50 are fixedly housed 
on the chuck body 12. 
The valves 30, 31 and the cam faces formed by the notches 13a, 13b of the 
control ring 13 are so positioned that when, as shown in FIG. 2, the valve 
plunger 30a is extended, and the valve plunger 31a is depressed, the valve 
30 connects the high pressure port 30b with the control port 30d so that 
the high pressure reservoirs 32, 33 communicate via the lines 29 and 45 
with the actuators 47 and 49. At the same time the valve 31 is positioned 
such that the control port 31d communicates with the low pressure port 31c 
so that the actuators 50 and 48 communicate via the lines 46 and 44 with 
the low pressure reservoirs 34 and 35. In this position, therefore, the 
actuators 47, 49 will be driven to extend, turning the plate 55 in a 
clockwise direction (as viewed in FIG. 2), and the actuators 48 and 50 
will permit this movement by virtue of the fact that fluid contained 
therein can flow through the lines 46, the valve 31, and the lines 44, to 
the low pressure reservoirs 34 and 35. 
The disc 55 carries three inclined slots 56 which engage with 
correspondingly inclined projections or ridges (not shown) on the rear 
faces of the jaws 17, which latter are constrained to move radially 
inwardly or outwardly by their engagement with the chuck body 12. Rotation 
of the disc 55 in a clockwise direction will therefore cause the jaws 17 
to move radially inwardly by virtue of the engagement of the slots 56 with 
the projections or ridges (not shown) in the rear face of the jaws 17. It 
will be appreciated that any suitable alternative engagement between the 
disc 55 and the jaws 17 could be utilised, in particular a known form of 
continuous spiral or scroll groove in the disc 55 with a plurality of 
cooperating teeth in the rear face of the jaws 17 may be used. 
Following the closure of the jaws 17 upon extension of the actuators 47 and 
49 the system will reach a terminal position with the actuators 47 and 49 
fully extended and the actuators 48 and 50 fully retracted. Assuming that 
a workpiece has been satisfactorily clamped within the jaws 17 the spindle 
on which the chuck is mounted would then be rotated in order to effect the 
desired operations on the workpiece and this causes the swashplate 24 
housed in the chuck body 12 to rotate with respect to the fixed projection 
14, the roller 22 rolling over the rear face of the swashplate 24. 
The spring 23 in the cylinder 20 of the projection 14 is stronger than the 
springs of the pumps 25 and thus the spring 23 will be partly extended 
causing each plunger 25e, 25f, 25g, 25h to be depressed once per 
revolution of the chuck as it passes the contact point with the roller 22 
of the projection 14. This causes the pumps to operate to draw fluid from 
the low pressure reservoirs 34, 35 and to pass it to the high pressure 
reservoirs 32, 33. When this reaches a predetermined pressure, determined 
by the strength of the spring 23, the back pressure on the ball valves 26 
at the outlet of the pumps 25 will cause these to remain closed so that 
the swashplate 24 is held in a position perpendicular to the axis of 
rotation of the chuck body 12 with the roller 22 running smoothly over it. 
This condition represents a fully charged condition of the chuck ready for 
subsequent opening or closing operations. Continued rotation of the chuck 
body 12 will not result in over-pressure being generated since, when the 
swash plate 24 is in the perpendicular position described above the pumps 
26 are not operated as they pass the position of the projection 14 so that 
no over-pressure relief valve or the like is required. The mechanism 
reaches, effectively, a balanced condition when fully charged. This 
balanced condition can be varied by making the spring 23 in the cylinder 
20 adjustable, for example by means of a screw threaded adjuster locating 
the end of the spring 23 remote from the plunger 21. 
When the chuck is stationary and it is desired to move the jaws 17 the 
control ring 13 is merely moved to the other end of its range of travel 
and fluid pressure from the reservoirs 32 and 33 will be transferred 
through the valves 30 and 31 and the actuators 47-50 to the low pressure 
reservoirs 34, 35 causing a partial rotation of the disc 55 in so doing. 
This, in turn, causes the desired radial movement of the jaws 17. The 
reservoirs 32, 33 should be made sufficiently large to house enough fluid 
pressure for several adjusting movements, opening and closing the jaws 17, 
in order to ensure that the chuck will remain operative even if the 
workpiece is not properly positioned after two or three attempts. Even 
though utilising the mechanism several times will cause the pressure in 
the reservoirs 32, 33 to decrease, and therefore cause a corresponding 
reduction in the force with which the jaws 17 grip the workpiece, the 
pressure within the reservoirs 32, 33 is restored after several rotations 
when it again reaches the fully charged or balanced position referred to 
above, so that a full clamping force is applied by the chuck before any 
force is applied to the workpiece by the operating tool. 
The weakest pressure applied by the chuck, when the pressures in the 
reservoirs 32, 33 and the reservoirs 34, 35 are very nearly the same, will 
be determined by the strength of the biasing springs 40, 41, 42, 43. The 
strength of these springs must, in turn, be balanced with the strength of 
the spring 23 in order to determine the maximum clamping force since the 
relative strengths of these springs determines the pressure within the 
high pressure reservoirs 32, 33 when they are fully charged. It is 
envisaged that an adjusting arrangement for the springs 23 and 40-43 would 
be provided to adjust the clamping force to suit different workpieces. For 
example, if a relatively delicate workpiece such as a thin walled tube was 
to be clamped by the chuck it would be necessary to reduce the strength of 
the springs 40-43, and correspondingly the spring 23, in order to ensure 
that the maximum pressure within the high pressure reservoirs 32, 33 was 
not so great as to apply a force to the jaws 17 to deform the walls of the 
workpiece. 
Referring now to FIGS. 3 to 5, the practical embodiment is shown in greater 
detail. In this embodiment the chuck body 12 is formed with a plurality of 
drillings which form the cylinders for the actuators 47-50 and for the 
reservoirs 32-35. Similarly, two radial stepped bores are provided in the 
body to act as housings for the valves 30, 31 and the pumps 25 are housed 
in axial bores in the body 12. 
The jaws 17 located on the front face 16 of the chuck body 12 are formed, 
in a known way, comprising a main jaw body 57 having three shouldered 
steps and secured by screws 58 to a rear jaw body 59 which is T-shaped in 
cross section and housed in a T-section slot in a carrier 62 mounted in 
the chuck body 12 and constrained to move radially along a slot 63 within 
which it is housed. The rear face of the main jaw body 57 has a plurality 
of serrations 61 which correspond with serrations on the front face of the 
carrier 62 whereby the main jaw body 57 can be adjusted radially with 
respect to the carrier 62 by slackening the screws 58 and sliding the main 
jaw body 57, together with the rear jaw body 59 along the slot in the 
carrier 62, retightening the screws 58 to clamp the parts of the jaw 
together in the selected position. The carrier 62 has a plurality of 
rectangular teeth 64 which cooperate with a scroll 65 on the front face of 
the disc 55, and which corresponds to the arcuate slots 56 described with 
reference to FIG. 2. Angular movement of the disc 55 about the axis of the 
chuck body therefore causes radial movement of the jaws 57 as these are 
constrained to move radially within the slot 63. 
The reservoirs 32-35 are formed as drillings axially of the body 12 and are 
located, as will be seen from FIG. 4, in pairs diametrically opposed and 
in quadrature with respect to one another. The low pressure reservoirs 34, 
35 are somewhat shorter than the high pressure reservoirs 32, 33 so that 
the two valves 30, 31 can be located circumferentially in correspondence 
with the ends of the low pressure reservoirs 34, 35 and between the ends 
of the high pressure reservoirs 32, 33. Intercommunication between the 
reservoirs, the valves and the pumps is formed by suitable drillings in 
the body 12, and also by means of a distributor sleeve which cooperates 
with a plurality of annular grooves formed in the inner curved surface of 
the central bore 19 in the chuck body 12. In FIG. 3 there can be seen a 
radial drilling 67 which corresponds to the high pressure line 28 and 29 
of FIG. 2, interconnecting the two high pressure reservoirs 32, 33 and the 
outlets of the pumps 25. An annular groove 66 in the bore 19 of the chuck 
body 12 communicates with the radial bore 67 and with an axial bore 
communicating with the high pressure ports 30b, 31b of the valves 30, 31. 
The valves 30 and 31 are formed as radial drillings in the body 12 which 
house stepped valve bodies 68, 69, respectively. Within the body 68 of the 
valve body 30 there is a valve plunger 30a which is a spring biased 
radially outwardly by a spring 70 and carries two sealing rings 71, 72 
spaced along its length. At the radially inner end of the plunger 30a 
there is a spigot 73 which cooperates with a ball 74 which is biased by a 
spring 76 into engagement with the opening of the high pressure port 30b 
to close this in reinforcement with the action of the high pressure fluid. 
The bore in the valve body 68 within which the valve plunger 30a slides has 
two annular grooves 77, 78 machined therein, and these grooves communicate 
via respective radial drillings 79, 80 with the low pressure ports 30c and 
the controlled ports 30d respectively. In the position of the valve shown 
in FIG. 4, the plunger 30a is fully extended into the recess 13a of the 
control ring 13, the ball 74 is biased to close the high pressure port 
30b, and the control port 30d communicates with the low pressure port 30c 
via the valve chamber formed by the bore within the valve body 68. In this 
position, therefore, the actuators 47 and 49 are connected to the low 
pressure reservoirs 34 and 35 and the chuck will be in the open position 
as illustrated in FIG. 2. If the control ring 13 is then turned clockwise 
(as seen in FIG. 4) the cam face within the recess 13a engages the 
radially outer end of the plunger 30a and causes this to travel radially 
inwards. As the plunger 30a moves inwards the sealing ring 72 first 
engages an intermediate part of the valve chamber 80 closing the 
communication between the port 30c and 30d. On further movement of the 
plunger 30a the spigot 73 at the radially inner end thereof engages the 
ball 74, which closes the high pressure port 30b, and lifts this from its 
seat permitting communication between the high pressure port 30b and the 
controlled port 30d. The high pressure reservoirs 32, 33 are now in 
communication with the actuators 47 and 49 via the valve 30. As the 
control ring 13 is moved to depress the plunger 30a a corresponding 
release of the plunger 31a of the valve 31 will commute the valve 31 so 
that the controlled port 31d thereof is placed in communication with the 
low pressure port 31c while the high pressure port 31b is first allowed to 
close. The actuators 47 and 49 are thus extended while the actuators 48 
and 50 are permitted to retract. This causes a rotation of the disc 55 and 
thus closure of the jaws 17. Turning the control ring 13 in the 
counter-clockwise direction resets the valves 30 and 31 to the position 
shown in FIG. 4 connecting the circuit for extension of the actuators 48 
and 50 and retraction of the actuators 47 and 49 to return to the jaw open 
position illustrated in FIG. 2. 
It is envisaged that an interlock system will be provided to prevent 
opening of the jaws while the chuck is turning, and to prevent starting of 
the machine tool motor while the jaws are open. Such an interlock may be 
formed electrically by means of a microswitch in circuit with the machine 
tool motor and mechanically linked, for example, to the valve plungers 
30a, 31a so that, for clamping in one direction the switch associated with 
the plunger 30a would be operative while for clamping in the opposite 
direction the switch associated with the plunger 31a would be operative. 
Alternatively, for fully automatic operation of the chuck, there may be 
provided an associated actuating mechanism for mounting on the machine 
tool and for engagement with the control ring 13 which operates to turn 
the control ring in order to open or close the jaws of the chuck without 
requiring manual intervention. Such an operating mechanism could, of 
course, be associated with electrical switches which isolate the motor 
when the chuck jaws are opened. 
It will be appreciated, from a review of FIGS. 3 to 5 of the drawings, that 
in this embodiment the operating parts of the chuck are housed in an 
annular arrangement surrounding the central bore 19 of the chuck body so 
that for long bar work a hollow spindle machine tool may be used and the 
bore of the spindle will be entirely unobstructed by the chuck. Similarly, 
the chuck may be mounted to the machine spindle in exactly the same way as 
a conventional manually operated chuck so that embodiments of the present 
invention offer all the advantages of interchangeability and versatility 
of conventional manually operated chucks while also having the advantage 
of speed and ease of operation of power chucks. 
Moreover, because of the design of the chuck incorporating all the 
reservoirs, valves and pumps in bores in a single body, embodiments of the 
present invention may be manufactured considerably more cheaply than has 
hithertofore been possible for power operated chucks. 
Referring now to FIG. 7 there is shown, schematically, a second embodiment 
of the invention, in which the chuck body is formed in two parts 111 and 
112 which are connected together using known means (not shown) such as, 
for example, a roller bearing, which permit the two chuck body parts 111 
and 112 to rotate with respect to one another about a main axis of 
rotation indicated generally 110 in FIG. 7. When the chuck is mounted on a 
machine tool the chuck body part 111 is secured to the spindle of the 
machine tool in a known way, for example by means of three axial pins (not 
shown), and the other chuck body part 112 is located against a fixed 
abutment on the machine tool so that the rotatable chuck body part 111 can 
rotate with respect thereto. 
In the schematic diagram of FIG. 7 the chuck body part 111 is shown in two 
parts for convenience. The chuck has radially movable jaws 113 which are 
housed in slots (not shown) in the front face of the chuck body part 111. 
In the rear face of the jaws 113 are formed recesses engaged by one arm of 
a respective bellcrank lever 115 the other arm of which engages in an 
annular recess 119 in an actuator tube 117 housed in a bore in the chuck 
body part 111. 
Opening and closing movements of the jaws 113 are effected by axial 
movement of the actuator tube 117 to cause rotation of the levers 115 in a 
known way. The actuator tube 117 has a radial flange 117a adjacent the end 
remote from the recess 119 and this flange is housed sealingly within a 
first annular recess 120 in the bore in the chuck body part 111 to act as 
a piston, separating the recess 120 into two chambers which can 
respectively be fed with high and low pressure from a pressure source to 
cause axial movement of the actuator tube 117 in one direction or the 
other. 
The pressure source of the chuck feeds a reservoir 122 and comprises an 
array of fluid pumps 125 which are automatically actuated by rotation of 
the chuck part 111 with respect to the chuck body part 112, when mounted 
on a machine tool spindle. There are six pumps 125a, 125b, 125c, 125d, 
125e, 125f in the array of pumps 125, arranged in a circle around the 
chuck body part 111. 
Basically, as schematically shown in FIG. 7, each pump comprises a plunger 
sliding in a bore at one end of which is a unidirectional valve. The bores 
along which the plungers of the pumps 125a-125f slide are connected 
together by a conduit 129 leading to a low pressure reservoir 123, and the 
action of the pumps as the plungers are reciprocated along their bores is 
to draw fluid from the low pressure reservoir 123 and to pump it under 
pressure via a conduit 124 to high pressure reservoir 122. Sequential 
actuation of the plungers of the pumps 125a-125f is effected by means of a 
swash plate 126 which is mounted on the rotatable chuck body part 111 and 
cooperates with a spring loaded hardened steel pin 140a carried by the 
other chuck body part 112 which is held against rotation as the array of 
pumps 125 is rotated with the chuck body part 111. The swash plate 126 is 
held under a flange 111a of the rotatable chuck body part 111 and its 
inclination to the axis of rotation of the chuck body 111 can be adjusted 
by adjusting the projection of the spring loaded pin by means of the screw 
threaded carrier 128, this thus determines the length of stroke of each 
plunger of the pumps 125. 
The high pressure reservoir 122 fed by the pump array 125 is connected by a 
conduit 124a and two branch conduits 124a.sub.1 and 124a.sub.2 to 
respective valves 131, 132 mounted within the chuck body 111, and the low 
pressure reservoir 123 is connected by a conduit 129a and respective 
branch conduits 129a.sub.1 and 129a.sub.2 to respective valves 130, 133. 
The valves 130, 131, 132, 133 are simple open/shut valves having radially 
extending plungers which are engaged by a control ring 136 mounted on the 
chuck body for limited turning movement thereabout. The valves are opened 
by depression of the plunger, and spring biased to shut. The control ring 
has four recesses 136.sub.0, 136.sub.1, 136.sub.2, 136.sub.3 forming cam 
surfaces for operation of the four valves 130, 131, 132, 133. The four 
valves 130-133 are arranged diametrically opposite one another in pairs 
and the positions of the recesses 136.sub.0 -136.sub.3 are so arranged 
that opposite pairs of valves are opened or closed together. Thus, in the 
position shown in FIG. 7, the valves 130 and 132 are open, and the valves 
131 and 133 are shut since their plungers extend into the respective 
recesses 136.sub.1, 136.sub.3. As will be seen from FIG. 7 the circuit is 
so arranged that the valves 131-133 connect one chamber of the first 
recess 120 in the actuator formed by the chuck body part 111 with the high 
pressure reservoir 122 and the other chamber with the low pressure 
reservoir 123, or vice versa in dependence on the position of the control 
ring 136. Thus, in one position of the control ring 136 the jaws are 
driven to open, and in the other position, that is the position shown in 
FIG. 7, the jaws are driven to close. 
The pressure with which the jaws close depends on the pressure to which the 
high pressure reservoir 122 is charged by the actuation of the pumps 125, 
and this in turn depends on the force with which the inclination of the 
swash plate 126 is maintained. The force applied to the swash plate is 
adjustable by the adjuster 128. As the pressure in the reservoir 122 
increases the back pressure in the pumps 125 increases and the spring 
loaded plunger 140a is forced back against the action of its biasing 
spring until the swash plate 126 is perpendicular to the axis of rotation 
110 of the chuck body. This corresponds to the fully charged or balanced 
condition discussed in relation to the embodiment of FIGS. 1 to 6 and thus 
will not be described in greater detail here.