Wedge type grip

A wedge type grip has a pair of wedge-shaped jaws and corresponding inclined faces of a yoke, the inclined faces of the jaws being in slidable contact with the corresponding faces of the yoke. The improved grip is provided with a mechanism for effecting relative movement between the yoke and the jaws along the axis of loading, a motor for applying a driving force to the mechanism to operate the same, and means for reducing the output speed of the motor and transferring the reduced output to the mechanism.

BACKGROUND OF THE INVENTION 
This invention relates to a wedge type grip for use in material testing 
machines, which is provided with a pair of wedge-shaped gripping members 
or jaws for holding a specimen to be tested. 
FIG. 3 diagramatically shows a known wedge type grip comprising a grip 
frame or yoke 10 and a pair of wedge-shaped gripping members or jaws 20A 
and 20B which will be designated by only the numeral 20 without the suffix 
A or B when it is not necessary to discriminate between the two members 
20A and 20B. The yoke 10 is formed with an inner space 3 which gradually 
expands from its entrance 1 toward its inner end 2, with a pair of 
inclined faces 11 formed opposite to each other and symmetrically with 
respect to an axis X of loading. Each of the jaws 20A and 20B is formed 
with an inclined outer lateral face 201 matching with the corresponding 
one of the inclined faces 11 of the yoke 10, and the jaws 20 are placed in 
the space 3 with their faces 201 in slidable contact with the 
corresponding inclined faces 11 of the yoke 10. When the yoke 10 is moved 
upwardly in the figure relative to the jaws 20, the jaws are tightened to 
grip a specimen S, and when the yoke 10 is moved in the opposite 
direction, that is, downwardly in the figure, the jaws are loosened to 
release the specimen. The relative movement of the yoke 10 and the jaws is 
conducted by hand or a hydraulic cylinder. 
The grips of the above-mentioned manual and hydraulic types have the 
following disadvantages. 
In the manual type, since a handle is manually rotated to move the yoke 
relative to the jaws to grip or release a specimen, the operation is 
troublesome and it is difficult to have a sufficient gripping force. 
In the hydraulic type, it is easy to operate the grip since mere handling 
of a selector valve suffices to supply or discharge pressure oil or air 
into or from the cylinder thereby to effect relative movement of the yoke 
and jaws of the grip. With pressurized air, however, it is impossible to 
obtain a sufficient gripping force. With pressure oil it is possible to 
obtain a large gripping force. To obtain a large gripping force, however, 
it is necessary to provide a source of pressure oil capable of producing a 
high pressure and a hydraulic cylinder of a large diameter resistive to 
high pressure, so that the whole device becomes bulky with a high 
manufacturing cost. In addition, since the pressure oil in the cylinder is 
blocked by means of a selector valve while the grip is holding a specimen, 
leakage of pessure oil will cause the gripping force to decrease. 
Accordingly, the primary object of the invention is to provide a wedge type 
grip which is compact in size and capable of providing a large gripping 
force and has good operability. 
SUMMARY OF THE INVENTION 
Briefly stated, the device of the invention comprises: 
a yoke having a pair of spaced-apart, opposed faces so inclined 
symmetrically with respect to the axis of loading as to define 
therebetween an inner space gradually expanding from the entrance of the 
space toward the inner end thereof; 
a pair of wedge-shaped jaws each having an inclined outer lateral face 
matching with the corresponding one of the inclined faces of the yoke, the 
jaws being disposed in the inner space of the yoke so that the inclined 
faces of the jaws are in slidable contact with the corresponding inclined 
faces of the yoke; 
a mechanism for effecting relative movement between the yoke and the jaws 
along the axis of loading; 
a motor for applying a driving force to the mechanism to operate it; and 
means for reducing the output speed of the motor and transferring the 
reduced output to the mechanism. 
In operation, the output speed of the motor is reduced and applied to the 
mechanism for effecting relative movement between the yoke and the jaws, 
so that the mechanism operates to move the yoke relative to the jaws 
thereby to cause the jaws to be closed by wedge action and hold a specimen 
therebetween with a predetermined gripping force. Since the output speed 
of the motor is reduced to move the yoke, a large gripping force is 
obtained with a compact device. 
The invention will be described below in detail with reference to the 
accompanying drawing.

PREFERRED EMBODIMENT OF THE INVENTION 
Referring to FIG. 1, there is shown a yoke generally designated by 10 and 
having a pair of spaced-apart, opposed faces 11A and 11B so inclined 
symmetrically with respect to an axis X of loading as to define 
therebetween a space 3 gradually expanding from the entrance opening 1 of 
the space 3 toward the inner end 2 thereof. The inclined faces 11A and 11B 
will be designated by only the numeral 11 without the suffix A or B when 
it is not necessary to discriminate between the two faces 11A and 11B. The 
yoke 10 is provided with an externally threaded hollow cylindrical member 
12 projecting from the upper surface of the yoke 10 and an inlet port 10a 
extending from the outer surface of the yoke 10 to the inner space 3 
thereof. 
In the inner space 3 of the yoke 10 there are provided a pair of 
wedge-shaped gripping members or jaws 30A and 30B, which will be 
designated by only the numeral 30 without the suffix A or B when it is not 
necessary to discriminate between the two members 30A and 30B. 
A rod 13 extends through the hollow cylindrical member 12 and is provided 
at the inner end thereof with a pair of recesses 13A and 13B, in which the 
jaws 30A and 30B engage, respectively, so that the position of the jaws 
along the axis X of loading is kept stationary. The rod 13 is fixed to a 
frame, not shown, so as not to be rotatable. 
A cap nut 14 is rotatably supported by the rod 13 and threaded on the 
hollow cylindrical member 12 of the yoke 10, so that rotation of the cap 
nut 14 causes the yoke 10 to move axially relative to the jaws 30. 
The cap nut 14 is formed with a through hole 14a for mounting a manual 
handle 42 on the cap nut. A large gear 21 is fixed to the cap nut 14 for 
rotation therewith. As shown in FIG. 2, a bracket 22 extends 
perpendicularly from the rod 13 at one side thereof and supports an air 
pressure motor 23. A small gear 24 is fixed to the output shaft 23d of the 
motor 23 and meshes with the large gear 21 fixed to the cap nut 14. The 
number of the teeth of the small gear 24 is smaller than that of the large 
gear 21. The motor 23 is provided with a pair of inlet ports 23a and 23b, 
through one of which, say, 23a, pressurized air is fed into the motor 23 
to rotate it in one direction, and pressurized air supplied to the motor 
23 through the other port, say, 23b rotates the motor in the opposite 
direction. The motor 23 is provided with a discharge port 23c which 
communicates with the supply port 10a formed in the yoke 10 through a hose 
41. 
In the illustrated embodiment, the hollow cylindrical member 12 and the cap 
nut 14 threaded thereon constitute a mechanism for moving the yoke 10 
relative to the jaws 30, and the small and large gears 24 and 21 meshing 
with each other constitute a reduction gear. 
In operation, when pressurized air is supplied through the inlet port 23b 
to the motor 23, the motor is rotated to rotate the cap nut 14 through the 
small and large gears 24 and 21, so that the yoke 10 is lowered relative 
to the jaws 30 held stationary in vertical direction thereby to open the 
jaws. One end of a specimen S is inserted between the open jaws, and 
pressurized air is supplied through the other port 23a to the motor, 
whereupon the cap nut 14 is rotated in the opposite direction, so that the 
yoke 10 is raised thereby to close the jaws, which grip the specimen with 
a force of a predetermined strength. Since the output speed of the motor 
23 is reduced by the combination of the small and large gears 24 and 21, a 
sufficiently large clamping torque of the cap nut 14 is obtained. Thus a 
required gripping force can be obtained with a small motor. 
When a tension test is conducted on a specimen with its opposite ends held 
by a pair of grips of the above-mentioned design in the above-mentioned 
manner, the jaws 30 of the upper grip are pulled downwardly so that the 
yoke 10 is raised relative to the jaws 30 thereby to increase the gripping 
force. Since the yoke 10 is connected to the output of the motor 23 
through the reduction gear, it seldom happens that a pulling counterforce 
causes the motor to rotate in the reverse direction thereby to decrease 
the gripping force. 
When the test has been finished, pressurized air is supplied to the motor 
23 through the port 23b thereby to open the jaws 30, whereupon the 
specimen S is taken out of the grip. 
When the jaws are opened or closed, the air discharged from the motor 23 
through the discharge port 23c is led through the hose 41 and the inlet 
port 10a into the space 3 in the yoke 10 so as to blow any dust out of the 
space 3. However, this feature is not essential for the present invention. 
Since the object of the invention is to provide a grip in which the driving 
speed of a motor is reduced to move the yoke relative to the gripping 
members thereby to obtain a large gripping force with a small device, the 
motor may not only be of air pressure type but also of any other type such 
as a hydraulic or an electrical type. In the illustrated embodiment, the 
reduction gear comprises a pair of spur gears, it may also be a worm gear 
type. In FIG. 4, motor 23 drives worm gear 24' which, in turn, meshes with 
and drives worm wheel 21'. The mechanism for effecting relative movement 
between the yoke and the jaws comprises an external and an internal thread 
meshing with each other. It may also be a rack-and-pinion mechanism. 
In accordance with the invention, since the output speed of the motor is 
reduced to move the yoke relative to the jaws, it is possible to provide a 
large gripping force with a compact device at a low cost without using a 
large source of pressure oil or a large cylinder. Since the pulling 
counterforce of the yoke is prevented by the reduction gear from being 
transferred onto the motor, it seldom happens that a counterforce produced 
upon pulling of a specimen causes the motor to rotate in the reverse 
direction thereby to reduce the gripping force of the jaws.