Workpiece clamping device

A workpiece clamping device with which workpieces of different sizes can be accommodated without having to make extensive system modifications each time the workpiece is changed and with which clamping operations can be performed with a high efficiency. An operating fluid flow-rate detector is provided in a hydraulic circuit between a hydraulic fluid source and a plurality of operating cylinders which drive clamping pawls to detect the flow rate and amount of the hydraulic fluid therethrough. The clamping position is sensed in accordance with the detected flow rate and amount.

BACKGROUND OF THE INVENTION 
The present invention relates to a workpiece clamping device employed, for 
instance, at a station in a transfer machine. More particularly, the 
invention relates to an improved workpiece clamping device which controls 
operating cylinders used to clamp and support workpieces of different 
sizes on a workpiece mounting jig. 
In a conventional workpiece clamping device of the same general type to 
which the invention pertains, in order to clamp, for instance, workpieces 
of different heights or to change the number of operating cylinders, it is 
necessary to change the pivot points of the workpiece clamping arms 
according to the heights of the workpieces. Furthermore, it is necessary 
to change the coupling points of the operating cylinders which are coupled 
to the workpiece clamping arms. In addition, the work system must be 
reconfigured; for instance, the positions and the number of detectors 
constituting limit switches and dogs for controlling the advancement, 
clamp and over-stroke positions of the operating cylinders must be 
changed. 
In the case of a conventional detector used for detecting the operative 
positions of operating cylinders, it is necessary to provide at least two 
limit switches and two dogs, and addition of operations cannot be made 
without increasing the numbers of limit switches and dogs. As a result, 
the conventional workpiece clamping device suffers from the difficulties 
that it occupies a relatively large installation space, it has a high 
manufacturing cost, and is difficult to maintain. 
In the case where a workpiece clamping device in a transfer machine or the 
like of a type which has a mounting device for inclining or turning a 
secured workpiece has limit switches and dogs for each operating clyinder, 
the workpiece clamping device requires intricate electrical wiring. In the 
case where limit switches and dogs are provided on the stationary bed, the 
positions thereof are fixed. Furthermore, if a workpiece clamping device 
detects clamping conditions from the operating pressure of the operating 
cylinders, the positions of the operating cylinders cannot be detected 
simultaneously therewith. 
As described above, in the case where, for instance, workpieces of 
different heights are machined while clamping them with the same workpiece 
clamping device, the reconfiguration required in the work system, such as 
changing the pivot points of the workpiece clamping arms and changing the 
control device for the operating cylinders according to the heights of the 
workpiece to be machined, are considerably intricate, and as a result the 
work efficiency with such a system is considerably low. 
SUMMARY OF THE INVENTION 
In view of the foregoing, an object of the invention is to provide a 
workpiece clamping device in which the operations of the operating 
cylinders can be readily changed even in clamping workpieces of different 
sizes, the work system can be readily reconfigured, and workpiece clamping 
operations can be achieved with high efficiency. 
The foregoing and other objects of the invention have been achieved by the 
provision of a workpiece clamping device having a fluid circuit arranged 
such that the operations of the operating cylinders are controlled by a 
single fluid flow-rate detecting means. In such an arrangement, operating 
cylinders for driving clamping pawls used to clamp a workpiece are 
provided on a workpiece mounting jig, and, according to the invention, 
operating fluid flow-rate detecting means for detecting the flow rate of 
the operating fluid of the operating cylinders is provided in the fluid 
circuit through which the operating cylinders are connected to a fluid 
supply source. Accordingly, the inventive workpiece clamping device is 
made compact, is simplified in construction, and is simple to maintain.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Preferred embodiments of the invention will be described with reference to 
the accompanying drawings. 
FIGS. 1 and 2 are, respectively, a front view and a plan view of a 
workpiece clamping device, provided at one station of a transfer machine, 
to which the technical concept of the invention is applied. As shown in 
FIGS. 1 and 2, the workpiece clamping device has a workpiece mounting jig 
1. The jig 1 includes a workpiece placing stand 3 set on a base 2, and a 
supporting plate 4 adapted to support and lift a workpiece W, the 
supporting plate 4 being operated by a lifting mechanism 5. 
A plurality of operating cylinders 6 (six operating cylinders in the 
embodiment shown in FIGS. 1 and 2) are disposed around the workpiece 
placing stand 3. The operating cylinders 6 have rods 7, the ends of which 
are coupled to the base parts of clamping pawls 8 adapted to clamp the 
workpiece W. 
The clamping pawls 8 are mounted on pins 9 and rockably supported by the 
workpiece placing stand 3. A pair of locator pins 10, used to position the 
workpiece W, are mounted on the workpiece placing stand 3 in such a manner 
that it is movable in and out by a lifting mechanism (not shown). 
Each operating cylinder 6 is divided into an extension pressure chamber 11a 
and a retraction pressure chamber 11b by a piston 6a. The chambers 11a and 
11b are connected to a fluid circuit X, which is connected to an 
operating-fluid supplying source Q (a hydraulic pressure supply source, 
for instance,) as shown in FIG. 3. In the fluid circuit X, an 
operating-fluid flow-rate decoder 20 is provided to detect the flow rate 
of the operating fluid in the circuit X or in the operating cylinder 6. 
As shown in FIG. 4, an adjusting screw 27 for determining the retraction 
limit position of a piston 21b is disposed at the left end of a return 
cylinder 21. The retraction limit position of the piston 21b is adjustable 
by advancing or retracting the adjusting screw 27. The adjusting screw 27 
is used to set the quantity of fluid required for operating the operating 
cylinders 6. 
An opening-closing rod 28a of a valve 28, which is a component of a leakage 
compensator 24, is projected into the flow rate measuring cylinder 22 at 
the right end of the latter. The rod 28a is normally urged in the leftward 
direction of FIG. 4, and therefore the valve 28 is normally closed. 
Specifically, when oil leakage occurs in the fluid circuit X or in the 
operating cylinders 6 for some reason, the hydraulic pressure inside of 
the pressure chamber 22A of the flow rate measuring cylinder 22 is 
decreased. The piston rod 22a is pushed by the hydraulic pressure of the 
pressure chamber 22B to the stroke end, that is the right end of the 
cylinder 22. 
Thereupon, the piston 22b pushes the opening-closing rod 28a in the 
righward direction against the force of the spring 28b so that the valve 
28 is opened, as shown in FIG. 4. The hydraulic pressure from the supply 
source Q is applied through the leakage compensator 24, the inlet 29, the 
chamber 22A and the outlet 30 to the pressure chambers 11a of the 
operating cylinders 6 to maintain the clamping pressure of the cylinders 
6. 
As shown in FIG. 3, a dog 26 provided on the end of the piston rod 22a of 
the flow rate measuring cylinder 22 is contactable with limit switches 
LS1, LS2, LS3, etc. The positions of the limit switches LS1, LS2, LS3, 
etc., are adjusted so as to correspond to the retraction position, the 
clamping position, the over-stroke position, etc., respectively, of the 
operating cylinders 6, and are used to detect the respective positions. 
The limit switches LS1, LS2, LS3, etc., form the detector 23. 
In the construction described above, while a workpiece W is clamped, 
hydraulic pressure from the supply source Q is applied through the port 31 
to the pressure chamber 22B of the operating fluid flow rate measuring 
cylinder 22 so that the piston 22b is pushed in the rightward direction, 
as shown in FIG. 3. The hydraulic fluid in the pressure chamber 22A is 
discharged through the port 30 and flows through the fluid circuit X into 
the pressure chambers 11a of the operating cylinders 6. As the volume of 
the hydraulic fluid in the chamber 22B is increased, the piston rod 22a is 
moved in the rightward direction and the piston rods 7 of the cylinders 6 
are moved to the left. When the operating ends of the piston rods 7 are 
brought into contact with the workpiece W, as shown by the dotted lines in 
FIG. 3, the movement of the piston rod 22a to the right is stopped at the 
position indicated by the dotted lines. At this time, the dog 26 of the 
piston rod 22a contacts the limit switch LS2 to detect the workpiece 
clamping condition. 
In order to retract the pistons of the operating cylinders 6, a solenoid 
valve 25 is set so as to apply hydraulic pressure from the supply source Q 
through the return cylinder 21 to the pressure chamber 11b. The hydraulic 
fluid in the pressure chamber 11b pushes the pistons 6a in the rightward 
direction to discharge the hydraulic fluid in the chambers 11a. The 
hydraulic fluid from the chamber 11a flows through the port into the 
pressure chamber 22A. The hydraulic fluid in the pressure chamber 22B 
flows through the port 31 and the solenoid valve 25 to an oil tank T. 
Simultaneously the hydraulic fluid in the chamber 22B flows through a 
pilot operated check valve 32 (which is opened by the hydraulic pressure 
from the return cylinder 21) and the port 30 into the chamber 22A, too. In 
this operation, the piston 21b is moved to the left by the hydraulic 
pressure in the return cylinder 21, and the piston rod 22a is also moved 
to the left. When the operating cylinders 6 are moved to the retraction 
position, the movement of the piston rod 22a in the leftward direction is 
stopped by the adjustment screw 27 and the dog 26 is brought into contact 
with the limit switch LS1, as shown by the solid lines in FIG. 3, so that 
the retraction position of the pistons of the operating cylinders 6 is 
detected. 
As is apparent from the above description, in the above-described 
embodiment, one flow rate measuring cylinder 22 is provided for all of the 
operating cylinders 6, and the operating fluid discharged from the flow 
rate measuring cylinder 22 is detected from the stroke of the piston rod 
22a, whereby the retraction position, the clamping position, the 
over-stroke position, etc., of the operating cylinders 6 are detected by a 
single operating fluid flow-rate detector 20. 
FIG. 5 shows a second embodiment of the invention. In this embodiment, in 
correspondence to three different sizes of workpieces W, three different 
clamping positions of the operating cylinders 6 are detected by means of 
limit switches LS2, LS3 and LS4. 
In this embodiment, leakage compensation for the cylinders 6 is carried out 
using a solenoid valve 34. When the dog 26 contacts the one of the limit 
switches LS2, LS3 and LS4 corresponding to the particular workpiece to be 
clamped, the solenoid valve 34 is opened and hydraulic pressure from the 
supply source Q is applied through the port 35 and the cylinder 22 to the 
pressure chambers 11a of the operating cylinders 6 to maintain the 
clamping pressure of the cylinders 6. 
The other arrangement of the second embodiment is the same as in the 
above-described first embodiment, and hence a further detailed description 
thereof is omitted. Elements corresponding functionally to those in the 
first embodiment are designated by like reference numerals or characters. 
In this construction, while the particular workpiece W is clamped, 
hydraulic fluid from the supply source Q is supplied through the port 36 
into the pressure chamber 22B of the flow rate measuring cylinder 22. The 
piston 22b is pushed in the rightward direction and the hydraulic fluid in 
the pressure chamber 22A is discharged through the port 37 and flows into 
the pressure chambers 11a of the operating cylinders 6. The movement of 
the pistons 22b in the rightward direction is stopped when the piston rods 
7 of the operating cylinders 6 contact the particular workpiece W, that 
is, when the piston rods 7 are moved to the one of the clamping position 
P2, P3 and P4 corresponding to the size of the workpiece W. At this time, 
the dog 26 contacts the one of the limit switches LS2, LS3 and LS4 
corresponding to the clamping position to detect the clamping position of 
the cylinders 6. 
As described above, when the dog 26 contacts the limit switch corresponding 
to the clamping position of the particular workpiece W, the solenoid valve 
34 is opened and hydraulic pressure is applied through the pressure 
chamber 22A of the cylinder 22 to the pressure chambers 11a of the 
operating cylinders 6 to compensate for leakage which may occur after the 
workpiece is clamped. 
In order to retract the pistons of the cylinders 6, the solenoid valve 25 
is changed to apply hydraulic pressure from the supply source Q through 
the return cylinder 21 to the pressure chambers 11b so that the pistons 6a 
are pushed to the right. At this time, the solenoid valve 34 is discharged 
and the hydraulic pressure of the supply source Q through the port 35 into 
the chamber 22A is shutted. The hydraulic fluid in the pressure chamber 
11a is discharged and flows through the port 37 into the pressure chamber 
22A of the cylinder 22. The hydraulic fluid in the pressure chamber 22B 
flows through the port 36 to an oil tank T. The piston rod 22a is moved to 
the left by the pressure of the hydraulic fluid in the pressure chamber 
22A and the return cylinder 21, and stopped when the dog 26 contacts the 
limit switch LS1 corresponding to the retraction position of the pistons 
of the cylinders 6. 
FIG. 6 shows a third embodiment of the invention. In this embodiment, a 
small flow rate measuring cylinder 22 is employed as a measuring unit for 
sensing the flow rate of the operating fluid. With this arrangement, a 
pulse is outputted every predetermined unit flow of hydraulic fluid to the 
plurality of operating cylinders 6, and these pulses are counted to detect 
the flow rate of the operating fluid. 
In this third embodiment, the operating fluid flow rate detecting means 
comprises a fluid flow-rate cylinder 22, a detector 23 having a limit 
switch LS which contacts a dog 26 disposed on a piston rod 22a, the 
detector 23 detecting a predetermined stroke of the piston rod 22a and 
outputting pulses, a counter 38 for counting the pulses from the detector 
23 (limit switch LS), and a mechanical operated changing valve 39 which is 
mechanically changed in position every predetermined stroke of the piston 
rod 22a. 
The changing valve 39 may be constructed as shown in FIGS. 8a and 8b, for 
example. A changing lever 61 is disposed at the end of an opening-closing 
rod 59 of the changing valve 39. The changing lever 61 engages a recess 60 
of the piston rod 22a of the flow rate measuring cylinder 22 at the end 
portion of the former and is held at one of two settled positions by means 
of a spring 62. FIG. 8a shows the same state of a position of the piston 
22b as that shown in FIG. 6. In the state shown in FIG. 8a, the hydraulic 
fluid from the supply source Q flows through a port 40 into the pressure 
chamber 22B and the hydraulic fluid in the pressure chamber 22A is 
discharged through a port 41. The piston 22b is moved to the right, and 
when the changing lever 61 is pushed by the left end of the recess 60, the 
changing lever 61 is rapidly changed by the spring 62 to move the 
opening-closing rod 59 to the left, as shown in FIG. 8b. At this time, the 
changing valve 39 is set to the state shown in FIG. 7, the hydraulic fluid 
from the supply source Q flows through the port 41 into the pressure 
chamber 22A, and the hydraulic fluid in the pressure chamber 22B is 
discharged out through the port 40. The above-described operation is 
repeated. The other arrangement of the third embodiment is the same as in 
the above-described first embodiment, and hence a further detailed 
description thereof is omitted. Elements corresponding functionally to 
those in the first embodiment are designed by the like reference numerals 
or characters. 
In this arrangement, as shown in FIG. 6, when a workpiece W is clamped, the 
hydraulic fluid from the supply source Q is supplied through the changing 
valve 39 and the port 40 into the pressure chamber 22B of the flow rate 
measuring cylinder 22. The piston 22b is pushed in the rightward direction 
so that the hydraulic fluid in the pressure chamber 22A is discharged 
through the port 41. The hydraulic fluid discharged from the pressure 
chamber 22A flows through the changing valve 39 into the pressure chambers 
11a of the operating cylinders 6. 
As shown in FIG. 7, the piston rod 22a is moved to the right, and when the 
dog 26 contacts the limit switch LS, the changing valve 39 is changed. The 
hydraulic fluid from the supply source Q flows through the changing valve 
39 and the port 41 into the pressure chamber 22A of the cylinder 22. The 
piston 22b is pushed to the left and the hydraulic fluid in the pressure 
chamber 22B is discharged through the port 40. The discharged hydraulic 
fluid from the pressure chamber 22B flows through the changing valve 39 
into the pressure chambers 11a of the cylinders 6. When the piston 22b is 
moved to the position shown in FIG. 6, that is, in FIG. 8a, the changing 
valve 39 is changed and the direction of the hydraulic fluid flowing into 
and out of the cylinder 22 is changed. 
The above-described operations are repeated. Every time the dog 26 contacts 
the limit switch LS, a detector 23 having the limit switch LS outputs the 
signal which is counted by means of a counter 38. The amount of the 
hydraulic fluid flowing into the pressure chambers 11a in the interval of 
two adjusted signals is a fixed value, namely, two times as large as the 
volume shown by the shaded portion of the cylinder 22 in FIG. 7. The total 
amount of the hydraulic fluid supplied into the cylinders 6 is the product 
of the above-described fixed value and the number of the signals counted. 
The total amount of the hydraulic fluid supplied into the cylinders 6 is 
established as the amount which is required for pushing the piston rods 7 
to the position for clamping the workpiece W. Therefore, the number of the 
signals to be counted until the workpiece W is clamped is accordingly 
determined. Workpiece clamping is detected when the counter 38 counts the 
above-described predetermined number of the signals. 
At this time, the movement of the piston 22b is stopped when the piston 
rods 7 of the operating cylinders 6 contact the workpiece W. Specifically, 
when oil leakage occurs in the fluid circuit X of in the operating 
cylinders 6 for some reason, the hydraulic pressure of the pressure 
chamber 22A or 22B is decreased. The piston 22b is moved by the hydraulic 
pressure to supply through the changing valve 39 into the pressure 
chambers 11a of the cylinders 6 to maintain the clamping pressure. 
In order to retract the piston rods 7 of the operating cylinders 6, the 
solenoid valve 25 is set to apply the hydraulic pressure from the supply 
source Q to the pressure chambers 11b of the cylinders 6. The pistons 6a 
are pushed to the right, and the hydraulic fluid in the pressure chambers 
11a is discharged through the check valve 42. 
In this third embodiment, the flow rate measuring cylinder 22 is small, 
making the system more compact. 
FIG. 9 shows a fourth embodiment of the invention. In this above-described 
first through third embodiments, piston-type operating-fluid flow-rate 
detectors 20 are employed. On the other hand, in the fourth embodiment, 
instead of the flow rate measuring cylinder 22, a hydraulic motor 44 is 
provided in the operating fluid flow-rate detector 20. The detector 23 
detects the rotation rate of the hydraulic motor 44 disposed in the 
hydraulic circuit X, and the detector 23, having the limit switch LS, 
outputs pulse signals with a frequency proportional to the rate of 
rotation of the hydraulic motor 44. The pulse signals outputted from the 
detector 23 are counted by a counter 45. The counted number is used to 
detect the retraction position, the clamping position, the over-stroke 
position, etc., of the operating cylinders 6. 
Instead of the counter 45, by provision of an external controller 59, as 
shown in FIG. 10, it is possible to detect a plurality (three) of 
different clamping positions in correspondence to a plurality (three) of 
different sizes of workpiece W to be clamped. The external controller 59 
includes the above-described counter 45. Furthermore, the controller 59 
has setters 46, 47 and 48. Each of the setters is used for clmaping the 
corresponding workpiece W. The counted number of pulses required for 
clamping a workpiece is determined according to the size of the workpiece 
W. Each of the setters 46, 47 and 48 is used to set a respective 
predetermined number of pulses for clamping the associated workpiece W. 
The number of pulses actually counted by the counter 45 is indicated by 
the indicator 50 disposed at the upper portion of the controller 59. A 
setter 49 is used to set the number of pulses required for unclamping a 
workpiece, that is, for retracting the pistons of the operating cylinders 
6. Terminals 51, 52, 53 and 54 are used to choose the unclamping or 
clamping positions of the pistons of the cylinders 6 to be detected. The 
terminal 51 is used for detecting the unclamping position. Each of the 
terminals 52, 53 and 54 is used for detecting the corresponding clamping 
position. When the position of the piston to be detected is selected by 
the use of one of the terminals 51 through 54, a lamp disposed on the 
corresponding setter is lighted. The terminal 55 is connected with the 
power source. The above-described external controller 59 is applicable to 
the third embodiment described above, too. 
In the operating fluid flow rate measuring means 20 of the fourth 
embodiment, a flow-rate meter, shown in FIG. 11, for example, is employed 
as a hydraulic motor 44. A terminal 56 of the flow-rate meter is connected 
to the external controller 59. The turbine blade 57 rotates according to 
the flow rate of the hydraulic fluid flowing in the flow-rate meter from 
the port 44a to the port 44b. A pickup 58 (as detector 23) detects the 
rotational rate of the turbine blade 57 and outputs pulse signals with a 
frequency proportional to the detected rotational value to the external 
controller 59. When the indicator 50 indicates the same number as the 
number set at the selected setter, the corresponding position of the rods 
7 of the cylinders 6 is detected. 
The fourth embodiment is advantageous in that, as the detection angle of 
rotation is decreased, the accuracy of detection is improved, and thus the 
operating cylinders can be more readily controlled. The other arrangement 
and functions of the fourth embodiment are the same as those of the first 
embodiment, and hence a further detailed description thereof is omitted. 
Elements corresponding functionally to those of the first embodiment are 
designated by the same reference numerals or characters. 
In the third and foruth embodiments, hydraulic pressure from the supply 
source Q is supplied to the pressure chambers 11a while the operating 
cylinders 6 are operated to clamp a workpiece, and the operating-fluid 
flow rate detector 20 is provided with leakage compensation. 
In the inventive workpiece clamping device in which the operating cylinders 
for driving the clamping pawls which clamp a workpiece are mounted on the 
workpiece mounting jig, a single operating-fluid flow-rate detector for 
detecting the flow rate of the operating fluid in the operating cylinder 
is provided in the fluid circuit through which the operating cylinders and 
the operating fluid supply source are connected to each other. Due to this 
arrangement, the invention has the following excellent effects and merits: 
(a) As described above, the operating cylinders can be controlled by a 
single operating fluid flow-rate detector, which can be mounted to be 
readily adjusted. Therefore, even a workpiece clamping device having a 
plurality of operating cylinders can be controlled with only one flow rate 
detector. Accordingly, the workpiece clamping device is simple in 
construction and high in operability. 
(b) As it is unnecessary to provide detectors, such as limit switches and 
dogs, and electrical wiring for each of the operating cylinders, the 
device is simple in arrangement. 
(c) Even if the number of operating positions of the operating cylinders is 
increased, it is unnecessary to increase the number of detectors. 
Furthermore, as the flow rate of the operating fluid of the operating 
cylinders is detected to control the latter, the operating positions can 
be detected with high accuracy. 
(d) As the workpiece clamping device of the invention is simple in 
construction, it has a low manufacturing cost and is simple to maintain.