Grips For Holding Specimens

Abstract

Grips for tensile testing of a specimen, such as a strip of metal, arranged in pairs in longitudinal aligned relation to grip opposite end portions of a specimen strip, the tensile strength of which is to be determined in a suitable testing machine such, for example, as one attached to a dynamometer. Each of the grips comprises a frame including a pair of side plates in clamping relation with compression blocks therebetween, forming an elongated throat. A pair of gripping members, such as wedges, are slidably mounted in the throat and have bearing relation against the compression blocks. The inner opposed faces of the wedges are adapted to receive an end of the specimen therebetween. The throat is open at opposite ends for access to the wedges to move these bodily with respect to the compression blocks or relative to the bearing means thereof to cause frictional engagement between the wedges and the specimen or to release the wedges therefrom.

Description

SUMMARY OF THE INVENTION

This invention relates to improvements in grips for holding specimens such, for example, as a flat strip of material during tensile testing thereof.

In the usual tensile testing machine, two grips are used in longitudinal spaced relation, to be connected to opposite end portions of a flat strip of material. One of the grips is attached to a dynamometer and the other attached to a drawbar for producing force on the strip. Various tensile properties of the specimen can be calculated from readings of the dynamometer, the extension of the specimen under known loads and the original dimensions of the specimen. It is quite important that the specimen should not slip during the testing of the specimen.

Grips which have been used heretofore in such testing machines have been constructed with a frame having opposed flat surfaces at an angle to each other. Between these surfaces are placed two steel wedges for holding the specimen. As tensile force is applied to the specimen, this force is multiplied by the coefficient of friction between the wedges and the specimen which produces forces tending to draw the wedges more tightly into the tapered portion of the frame and thereby into wedging engagement with the specimen. However, the forces just described are opposed by frictional forces between the wedges and the frame. If the coefficients of friction at the specimen and at the frame are approximately equal, the gripping performance of the device becomes unreliable and the specimen may slip in the grip.

Attempts have been made heretofore to improve the gripping reliability of conventional grips either by providing a rough surface on the inner face of each grip or by providing forcible means for tightening the grip, such as a lever, cam, screw, worm, etc. These attempts have not been satisfactory and have presented disadvantages in service.

The rough surfaces of the gripping wedges tend to wear because of specimen slippage and thereby becomes less effective. The rough surfaces also make insertion of thin specimens more difficult. Moreover, they tend to tear particles from the specimens which, in turn, foul the rough surfaces and reduce their effectiveness. The loose particles also tend to increase the frictional forces between the wedges and the frame.

Mechanical devices for tightening and loosening the wedges require time, attention and force from the operator. Moreover, they tend to slow down the tensile testing operation and require a device which is complex and expensive to construct and unsatisfactory in service.

The basic wedging effect utilizing wedges and a supporting frame is unreliable due to the coefficient of friction between the wedges and the frame on the one hand and between the wedges and the specimen on the other, which are of about the same order. No suitable way has been found heretofore for correcting these defects, although various attempts have been made to do so.

One object of this invention is to improve the construction of grips for the purpose described and to overcome the disadvantages in prior grips, some of which have been described above.

Another object of the invention is to eliminate the necessity for rough surfaces on the gripping wedges between the latter and the specimen, as well as reducing the coefficient of friction between the wedges and the frame.

Still another object of the invention is to provide a frame in which the wedges are mounted for freedom of movement and yet these wedges are sufficiently accessible at both ends so they may be opened or tightened by finger pressure where desired.

Yet another object of the invention is to mount the wedges in a supporting frame for freedom of movement with respect to a specimen and to provide accessibility of the wedges for inspection of the specimen therebetween or the removal of any particles that may be loosened or break off.

These objects may be accomplished, according to one embodiment of the invention, by providing a pair of grips arranged in end-to-end aligned relation to receives a specimen therebetween. Each of the grips has a supporting frame, including a pair of side plates, spaced apart, with compression blocks interposed between the plates in converging relation to define an open throat. The blocks are clamped between the plates and effectively back up the wedges which are interposed in the throat between the blocks and which are in embracing relation with an end portion of the specimen. An opening or openings may be provided in one or both opposite sides of the frame for inspection of the wedges and the specimen and to remove particles therefrom.

BRIEF DESCRIPTION OF THE DRAWINGS

This embodiment of the invention is illustrated in the accompanying drawings, in which:

FIG. 1 is a side elevation of a portion of a tensile testing machine illustrating the grips used therein and showing the application of the grips to a specimen;

FIG. 2 is a cross section through the lower grip taken substantially on the line 2--2 in FIG. 3 and illustrating parts thereof in elevation;

FIG. 3 is a top plan view of the lower grip;

FIG. 4 is a top plan view of the upper grip;

FIG. 5 is a side elevation of the upper grip;

FIG. 6 is a plan view of an end seal;

FIG. 7 is a side elevation thereof;

FIG. 8 is a side elevation of one of the gripping wedges of the top grip;

FIG. 9 is a similar view at right angles thereto;

FIG. 10 is a top plan view of the tension block of the lower grip;

FIG. 11 is a side elevation thereof;

FIG. 12 is a bottom plan view of the tension block of the upper grip;

FIG. 13 is a side elevation thereof;

FIG. 14 is a face view of one of the compression blocks for the lower grip;

FIG. 15 is a similar view at right angles thereto;

FIG. 16 is a side elevation of one of the compression blocks for the upper grip; and

FIG. 17 is a top plan view thereof.

DETAILED DESCRIPTION OF DISCLOSURE

The invention is adapted for use in holding specimens such as elongated stips of flat material. Some of these specimens may be very thin as, for example, of the order of 0.001 inch, for testing the tensile strength thereof. The grips are arranged usually in vertically aligned relation, as shown in FIG. 1, where the upper and lower grips are designated, respectively, at 1 and 2 and have a flat strip S therebetween for determining the tensile strength of this strip. The lower grip is connected to a drawbar 3 which, in turn, is connected to suitable means for producing force. The upper grip 1 has a bar 4 extending therefrom to suitable means for measuring the force, such as a dynamometer 5.

The grips 1 and 2 have their major parts similar in construction except that one is inverted with respect to the other. These parts will be described by reference to the same numerals except as hereinafter explained.

Each of the grips 1 and 2 is constructed of a suitable frame including a pair of side plates 10 extending in parallel spaced relation and having shoulders 11 on the inner faces of the respective plates adjacent their lateral opposite edges, as indicated particularly in FIG. 2. These shoulders 11 extend lengthwise of the plates in converging relation, as will be apparent.

Compression blocks 12 are interposed between the spaced side plates 10 and hold the side plates spaced apart. These compression blocks 12 are confined by the shoulders 11 and are clamped in place by bolts 13 which extend through the side plates and through the compression blocks 12, as will be apparent from FIGS. 2 and 5. Thus, the blocks are confined in secure relation, converging toward one end of the grip at which the specimen S is received.

Within the frame of each grip, a throat is formed between the opposing surfaces of the compression blocks 12. A pair of gripping wedges 14 are fitted loosely in this throat for lengthwise displacement with respect thereto. The opposing surfaces of the pair of wedges 14 are adapted to embrace opposite sides of an end of the specimen S. The gripping surfaces of these wedges may be smooth or roughened as found desirable, but a roughened surface is not required according to this invention, although it may be used, especially for testing extremely hard and/or smooth material. An aligning pin 15 is inserted loosely in holes provided in the opposed gripping surfaces of the wedges 14 to allow freedom of expanding movement relative to each other, but to keep the pair of wedges properly disposed in side-by-side relation.

When the pair of wedges 14 are centered within the grip, the outer faces of these wedges are spaced from the opposed faces of the compression blocks 12 with a channel 16' between each block and the adjacent wedge 14, which channel is closed at the lateral edges thereof by the side plates 10. These spaces are filled with stacks of rollers 16 disposed substantially throughout the height of the grip along the length of the respective blocks, not only for keeping the wedges 14 in proper positions to grip the specimen, but also to insure of freedom of lengthwise movement with respect thereto.

The rollers 16 are of sufficient diameter to roll freely in the channel formed between the spaced opposed surfaces of the blocks 12 and wedges 14. Each stack of rollers 16 is retained at its upper end by a pin 17 which extends transversely of the channel and is inserted at its opposite ends in openings formed in the spaced side plates 10 of the frame.

The stack of rollers 16 of the upper grip is supported by a pair of pins 18 extending laterally outwardly from the wedges 14. The stack of rollers of the lower grip is supported by pins 18 extending laterally from the adjacent face of the block 12.

Each of the wedges 14 has a protruding finger 19 on the converging end thereof toward the specimen to facilitate engagement and movement of the wedges by the fingers of the operator. This may be desirable especially when it is needed to open the wedges to insert or release a specimen.

Each of the side plates 10 has a hole 20 therethrough near the base of the pair of wedges for access thereto and to the specimen within the grip. This may be desirable especially for inspection of the wedges and of the specimen and to remove pieces or particles that may break off especially when very thin material is being tested.

Sealing means is provided at each opposite end of the grip, as indicated generally at 21. This sealing means is intended to keep debris out of the grip especially in the area over the stack of rollers.

The sealing means is mounted on each end of each of the blocks 12 as, for example, by means of a fastening screw 22. Each seal 21 may be constructed, for example, in the manner illustrated in FIGS. 6 and 7, FIG. 7 being somewhat exaggerated in thickness for clearness of illustration. In this embodiment, the seal 21 comprises a clamping plate 23 extending over a sheet 24 which is preferably of elastic material, such as Neoprene, and should be relatively thin, with adequate flexibility. Beneath the sheet 24 is a second clamping plate 25 and this, in turn, is superposed over a second flexible yieldable sheet 26, such as Neoprene. These sheets and plates are secured together to form a composite seal, as by adhesive connection therebetween.

This stack of elements forming the seal 21 is secured by a screw 22 to the end of the block 12 substantially to cover the area of the block and to fit between the inner faces of the side plates 10. The thin flexible sheet 24 is sufficiently larger in area than the layers below it so as to lap over the protruding finger 19 on the end of the adjacent wedge 14, as will be apparent from FIGS. 2 and 5. The layers 25 and 26 need not bear against the outer face of the adjacent wedge 14, but the structure of the seal is such as to be confined securely on the block and to keep debris from entering into the movable area of the grip.

The upper grip a shown in FIGS. 4, 5, 12 and 13 has a tension block 27 fitting between the spaced side plates 10 and confined by shoulders 28 on the upper ends of the side plates. This block 27 has holes therethrough to receive fastening bolts 29 that extend through the projecting portions of the side plates 10 in the manner illustrated in FIGS. 4 and 5, thus to anchor the tension block 27 securely in the upper end of the upper grip.

The tension bar 4 of FIG. 1 is inserted lengthwise through the upper end of the frame and is secured in a center hole 30 formed in this tension block 27 so as to anchor the upper grip effectively to the tension bar.

The lower grip 2 has a tension block 31, shown in FIGS. 2, 3, 10 and 11. The block 31 is also confined by shoulders 32 on the projecting lower ends of the side plates 10 of the lower block and is clamped in rigid relation between the side plates by bolts 33. The block 31 has a center opening 34 for receiving the end of the draw bar 3 to which the lower grip is securely connected by means of the block 31.

The larger ends of the wedges of both grips are exposed between the side plates 10 for freedom of engagement by the operator with his fingers pressing one or both of these against the specimen as to increase the gripping action thereon. This may be accomplished by inserting a finger between the projecting end portions of the side plates inwardly with respect to the block 27 or 31 and pressing upon the adjacent end of the wedge.

When the upper grip is suspended vertically from a dynamometer or other mechanism, the weight of the wedges is usually sufficient to close them on the specimen and to start the gripping action when force is applied to the specimen. Should any initial slippage occur due to irregularities in specimen flatness, the initial gripping action can be started by a light finger pressure on the back end of one of the wedges which are made accessible at these points for this purpose.

As the tensile load on the specimen increases, the rollers 16 move slightly upward with respect to the wedges in the upper grip and away from the pins 18, which normally support them. The specimen is thus firmly clamped between the wedges 14. At the completion of the test, the specimen ends are easily removed from the wedges by opening them by light finger pressure applied to the finger portions 19.

When no specimen is inserted in the upper grip, the wedges 14 will drop and be supported by the rollers 16 and the wedging action therebetween. The lower grip, however, is provided with a pressure pad 35, shown in FIGS. 2 and 3, supported upon mounting springs 36 interposed between the pad 35 and the adjacent face of the block 31. Guide pins 37 extend through the coiled springs 36 to maintain the alignment of these springs. The yieldably supported pressure pad 35 holds the wedges 14 in place in the lower grip and thus confines these wedges in positions to receive the specimen therebetween.

The use of the grips will be apparent from the foregoing explanation. They are effective in a very simple and inexpensive construction to overcome the problems with prior grips used heretofore and to secure effective gripping of material without danger of slippage especially for use in tensile testing machines.

If the grips are to be used in other than a vertically suspended position, the weight or friction of the wedges operating in that relation may be overcome as by the insertion of a spring pressure pad against the wedges and thereby produce a normal closing force approximately equivalent to the force produced in the upper grip by the weight of the wedges.

While the invention has been illustrated and described in one embodiment, it is recognized that variations and changes may be made therein without departing from the invention as set forth in the claims.

Claims

I claim:

1. A grip for holding a specimen for testing, comprising a frame, said frame including a pair of side plates in side-by-side relation and spaced apart transversely thereof, said side plates having inwardly projecting shoulder portions thereon adjacent opposite ends of the side plates, compression blocks interposed between the side plates and abutting laterally in directions lengthwise of the side plates against the shoulder portions, clamping means for securing the side plates and compression blocks together, said blocks being spaced apart with opposed faces in converging relation forming a throat therebetween and having wedge-shaped gripping members slidably mounted in the throat between the compression blocks for securing the specimen therebetween in gripping engagement during testing, the blocks and shoulders being arranged to carry the stresses from the gripping elements to the side plates.

2. A grip for holding a specimen, according to claim 1, wherein the compression blocks are disposed in converging relation toward the end receiving the specimen, and the gripping members are wedge shaped in laterally bearing relation against the respective compression blocks and having substantially smooth gripping surfaces.

3. A grip for holding a specimen according to claim 1, including a stack of roller bearings interposed between each of the gripping members and the adjacent compression block for freedom of sliding movement relative thereto.

4. A grip for holding a specimen according to claim 3, including pins extending in bridging relation between the side plates and anchored at opposite ends thereto in approximate alignment with the stacks of roller bearings for retaining the latter.

5. A grip for holding a specimen according to claim 3, including one or more pins extending in the space between each of the compression blocks and the adjacent gripping member for supporting each stack of roller bearings in said space in one direction.

6. A grip for holding a specimen according to claim 1, wherein the opposite ends of the gripping members are open between the side plates for finger engagement to move the gripping members in either direction.

7. A grip for holding a specimen according to claim 1, wherein at least one of the side plates has an opening therein extending partway only of the length and width of the side plate over opposed surfaces of the gripping members for visual inspection of the specimen therebetween.

8. A grip for holding a specimen according to claim 1, wherein each of the gripping members has a portion at one end thereof, projecting from the side plates, and sealing means secured to an end of the compression block in overlapping sealing relation with the adjacent end of the gripping member.

9. A grip for holding a specimen according to claim 8, wherein the sealing means includes a flexible sheet spanning the end of the compression block and in wiping relation with the adjacent portion of the gripping member.

10. A device for holding a specimen according to claim 1, including a pressure pad engaging the lower ends of the wedges, pins connected with the pad and having slidable guided relation with the side plates, and coiled springs sleeved over the pins in bearing relation with the pad.