US3643500A

US3643500A - Gripping device

Info

Publication number: US3643500A
Application number: US805476A
Authority: US
Inventors: Philip J Anderson
Original assignee: Institute of Gas Technology
Current assignee: Gas Technology Institute
Priority date: 1969-03-10
Filing date: 1969-03-10
Publication date: 1972-02-22
Anticipated expiration: 1989-02-22

Abstract

A device for gripping test specimens or workpieces. The device includes a body having an aperture therein for receiving one end of the specimen or workpiece. Means are mounted around the one end for translating axial forces into radial inward gripping forces. Means are mounted around the specimen or workpiece, adjacent the translating means and removably secured to the body for applying the axial forces to said translating means so that the inward radial forces are transmitted from the translating means to the specimen or workpiece for positively gripping the specimen or workpiece about its periphery.

Description

, H 0 u 1 fl h l J i United States Wat-tent [is] anson Anderson [45] their, 22, 1972 [54] GWHPIPHNG DIEVlI C E 3,427,036 2/1969 Siebelhoff ..279/4 [72] inventor: Philip .ll. Anderson, Deerfield, llll. 432 [73] Assignee: institute oi Gas Technology 3,016,250 1/1962 Franck .285/342 [22] Filed: 1969 Primary Examiner-Richard C. Queisser [2]] APPL myg -m Assistant Examiner-Jlohn Whalen Altt0rneyBa1r, Freeman 6: Molmare [52] lU..=Cl 733/103, 279/47 57 mgmmr 'r [51] llnt. Cll. 60hr 3/00 [58] man flilfiemmlil ..73/103;279/46,47,48,4; A i F gYIPPIg F speclmens f T 285/245 device includes a body having an aperture therein for receiving one end of the specimen or workpiece. Means are [56] mlgfiwmms monnted around tl e one end for translating axial forces into radial inward gripping forces. Means are mounted around the UNITED STATES PATENTS specimen or workpiece, adjacent the translating means and removably secured to the body for applying the axial forces to 2,755,094 7/1956 Ben amm "279/51 i translating means so that the inward radial forces are 2,956,826 10/1960 transmitted from the translating means to the specimen or 2,982,557 5/ 1961 Anscliutz.... ..279/57 workpiece f i i l gripping the specimen or workpiece 3,246,903 4/1966 Sattler .279/47 aboutits periphery 3,335,603 8/1967 Gram .73/103 3,365,204 1/ 1968 Benjamin et a1. ..279/47 3 Claims, i Drawing Figures PATENTEUFEB22 m2 WIN 1 [IF 2 ENTOR PHILIP J. ANDEQSON INV ELM-35Gb GRIUPPING DEVICE BACKGROUND OF THE INVENTION Field of the Invention and Description of the Prior Art This invention relates to a device for gripping various specimens and workpieces and it particularly relates to a device for gripping cored and cast specimens for tensile testing purposes.

There is presently no satisfactory device or apparatus for holding or gripping the ends of cylindrical samples of concrete, rock, mortar, etc., for subjecting them to tensile tests. In fact, there is no standard tension test for concrete, rock, or mortar which is presently accepted by The American Society for Testing Materials since all the known tensile procedures have certain drawbacks.

In one tensile testing procedure, cast briquette specimens are used and special grips are required for holding them during testing. One disadvantage of this method is that no testing of cored rock or concrete is possible because a specific shape is required for the briquettes. A further significant drawback of this test procedure is that tests have shown that it is not possible to guarantee a true axial pull. Thus, the breaks which occur do not represent pure tensile stress but also involve shearing stress so the tensile strength cannot be accurately ascertained.

In another tensile test procedure, cylindrical specimens are used and threaded bolts are embedded in the ends of the specimens. The threaded bolts are cast into the specimen and the exposed ends are gripped by the self-centering chucks ofa tensile testing machine. A disadvantage of this procedure is that failures commonly occur by splitting, rather than by tension, as a result of high stress concentration at the interface between the bolt threads and the specimen. Also, this procedure can be used only with cast specimens and cannot be used with cored specimens.

Still another tensile test procedure involves the use of steel end plates which are bonded to cylindrical test specimens by use of an epoxy resin or other suitable adhesive. The end plates are drilled and tapped and threaded bolts are screwed into the tapped holes. The exposed ends of the bolts are then gripped by the chucks of the tensile testing machine. This technique has problems in sample preparation. Although this technique is useful with both case and cored specimens, the ends of the specimens must be flat and parallel with each other. Then, the end plates must be centered on the specimen so that the threaded bolts are coaxial with the specimen. These requirements result from the fact that, if the bolts are not coaxial with the specimen, the applied stress is not a pure tensile stress and the values obtained in testing do not represent the true and accurate tensile strength of the test piece. Preparation of the ends of the samples to obtain the required coaxial alignment of the end plates with the specimen, construction of jigs to obtain alignment, and the time required for setting the epoxy cement are some of the disadvantages of this test procedure.

SUMMARY OF THE INVENTION It is therefore an important object of this invention to provide an improved device for gripping test specimens or work pieces and particularly for substantially avoiding the disadvantages of prior devices for gripping specimens for tensile testing purposes.

It is a further object of this invention to provide an improved device for gripping objects for tensile testing wherein a true axial pull of the test specimen is accomplished.

It is still a further object of this invention to provide an improved device for gripping a test specimen or workpiece wherein rapid and easy attachment is provided between the specimen and the gripping device.

It is another object of this invention to provide an improved tensile testing device which is useful with both cast and cored samples.

It is yet another object of this invention to provide an improved gripping device wherein the grips may easily be constructed to accommodate samples or workpieces having various sizes and shapes.

It is still another object of this invention to provide improved devices for gripping test specimens or workpieces wherein the grips are characterized by their simplicity and economy of construction and use.

Further purposes and objects of this invention will appear as the specification proceeds.

The foregoing objects are accomplished by providing a device for gripping test specimens or workpieces wherein the device includes a body having an aperture therein for receiving one end of the specimen or workpiece, means carried by one end of the specimen or workpiece for translating axial forces into radial inward gripping forces which act against the specimen or workpiece, and means are mounted around the test specimen or workpiece and being adjacent said translating means and removably secured to said body, said last means being for applying axial forces to said translating means whereby the inward radial forces are transmitted from the translating means to the specimen or workpiece for positively gripping the specimen or workpiece about its periphery.

BRIEF DESCRIPTION OF THE DRAWINGS Particular embodiments of the present invention are illus trated in the accompanying drawings wherein:

FIG. l is a side elevational view of a test specimen having the improved gripping devices positioned on its opposite ends for tensile test purposes;

FIG. 2 is a fragmentary, enlarged partially sectioned view of the end of a test specimen locked into positive engagement with one of the gripping devices made in accordance with the invention;

FIG. 3 is an exploded view of the components of the improved grip for holding test specimens or workpieces wherein the loading gland is mechanically and manually operated; and

FIG. 4 is an exploded view of an alternate embodiment of a gripping device of my invention, wherein the loading gland is hydraulically operated.

DESCRIPTION OF THE PREFERRED EMBODIMENT In the description of the invention, reference will be made to my improved gripping device for tensile testing of various specimens, particularly concrete, mortar, or rock specimens. It is to be understood, however, that the device may be used for a variety of purposes including use in gripping a workpiece in various operations such as machining, drawing, elongating, twisting, and the like. My improved gripping device finds a particularly useful function as a gripping device for tensile testing purposes. For this reason, the detailed description of the invention is specifically directed to tensile testing.

Referring to FIGS. 11-3, my improved gripping device, generally It), for receiving the opposite ends of a test specimen 12 has four main parts. These four parts include a body 114, afrustoconical deformable ferrule 16, a rigid ring I8, and a rigid loading gland 20.

The body I4 is provided at one end with a cylindrical well 22 for receiving one end portion 2d of the test specimen l2. Although the test specimen is cylindrical in shape and circular in cross section, it is to be understood that the particular shape and cross section of the specimen may be varied, provided that the parts of the device It) in intimate contact with the test specimen have the same cross-sectional shape to provide the desired intimate contact. The outer end portion of the well 22 defines a frustoconical recess 26. The opposite end of the body Id has a reduced shank 28 for receiving the self-centering chuck of a standard tensile test machine (not shown). The central portion 30 of the body M is desirably hexagonal or the like in external cross section for receiving a wrench or the like so that the loading gland 2t may be manually tightened on the threaded outer end 32 of the elongated body M.

A deformable plastic ferrule 16 having a frustoconical outer periphery is received on the periphery of the outer end portion 24 of the test specimen 12 and is received within the frustoconical recess 26 at the outer end of the well 22 in surface to surface relationship. The ferrule 16 surrounds the end 24 of the specimen 12 as does the adjacent rigid backup ferrule 18. The inner surfaces of both the deformable ferrule 16 and the rigid ferrule 18 are cylindrical in shape so as to be snugly received by the outer periphery of the cylindrical specimen 12. The outer end (or end opposite the body 14) of the plastic ferrule 16 is provided with a chamfered or tapered recess 38 which receives an adjacent chamfered or tapered outer periphery on the rigid ferrule 18. It is important in the construction of our device that the ferrule 16 be constructed of suitable deformable plastic material.

As seen in FIG. 2, when the device 10 and specimen 12 are ready for testing, the frustoconical deformable ferrule 16 is received within the frustoconical recess 26 of the well 22 of the body 14. Also, the rigid ferrule 18 is positioned adjacent the outer side of the deformable plastic ferrule 16.

The loading gland 20 is received on the threaded outer end 32 of the body 14. A central opening 40 in the outer end ofthe loading gland permits the passage of the cylindrical test specimen 12 therethrough. An annular inwardly extending ridge or annular lip 42 bears against the rigid ring or ferrule 18 with an axial force when the loading gland 20 is secured on the body 14.

Although the operation of the device 10 shown in FIGS. 1-3 is believed to be clear from the foregoing, a brief description of the manner of using the device 10 will be provided. The plastic ferrule 16 is slid over the end 24 of the test specimen 12 so that the narrow end thereof points inwardly to be received in the recess 26 of the body 14. The rigid ferrule 18 is positioned around the end 26 of the specimen 12 and on the side of the ferrule 16 that is opposite the narrow end thereof. The loading gland 20 is inserted around the specimen 12 so that its threaded inner periphery is received by the threaded end 32 of the body 14. The annular lip 42 bears against the annular outer end of the rigid ferrule 18. When the

ferrules

16 and 18 and the loading gland 20 are in proper position on the specimen 12, the end 24 of the specimen 12 is inserted into the well 22. The loading gland 20 is then threaded onto the body 14. As the gland 20 is tightened, the annular lip 42 presses or axially bears against the annular end of the rigid ferrule 18 with an axial force and the rigid ferrule 18 in turn provides an axial force acting on the deformable plastic ferrule 16, so as to force it into the annular recess 26 of the well 22.

Due to the combined effect of the conical exterior periphery of the plastic ferrule 16, the interior well 22 of the body 14, and the cylindrical shape of the inner surface of the plastic ferrule 16, the applied load from the loading gland 20 upon the body 14 centers the test specimen 12 so that the axes of the specimen 12 and the body 14 are substantially coaxial. The loading of the loading gland 20 applies an axial force on the annular end of the rigid ferrule 18 or 43 which in turn transmits an axial force to the deformable plastic ferrule 16. As a result of the frustoconical outer shape of the deformable ferrule 16, the axial force is translated into an inward radial force which acts equally against the specimen 12 and against the inner surface of the well 22. This ultimately causes deformation of the plastic ferrule 16 so as to provide a uniform peripheral gripping load on the sample. When two of the gripping devices 10 are placed on opposite ends of a specimen 12, as shown in FIG. 1, and when the shanks 28 of the gripping devices 10 are placed in the test machine, tensile stress is applied to the specimen 14. While the tensile force is being applied to the specimen 12, the axial deformation of the plastic ferrule 16 is restrained by the rigid backup ferrule 18 so that the radial pressure exerted against the specimen by the plastic ferrule 16 increases.

Referring to the alternate embodiment shown in FIG. 4, the gripping device 100 again includes four basic parts-a body 102, a deformable ferrule 104, a rigid ferrule 106, and a loading gland 108. The gripping device differs from the gripping device 10 in the provision of hydraulic pressure for applying axial force to the deformable ferrule 104. The use of hydraulic pressure provides certain advantages. First, greater axial forces can generally be provided by hydraulic means than by mechanical force applied to a loading gland from a wrench. Secondly, and of even greater importance, the use of hydraulic pressure enables a constant gripping force to be provided, regardless of deformation of the plastic ferrule 104 or of the specimen itself.

The body 102 is of substantially the same construction as the body 12 in the embodiment shown in FIGS. 1-3. The body 102 includes a central well for receiving the end of a test specimen. The well 110 has a frustoconical outer end portion 112, the outermost portion having the enlarge diameter. A threaded outer periphery 114 is provided on the end of the body 102. The plastic deformable ferrule 104 has a frustoconical outer periphery 116 which is designed to move into surface to surface contact with the frustoconical end portion 112 of the body 102. The deformable ferrule 104 has a central aperture 118 aligned with the well 110 and the body 102. The aperture 118 has a frustoconical outer end portion 120.

The rigid ferrule 104 is different from the rigid ferrule in the embodiment of FIGS. 1-3. The rigid ferrule 104 is construced as a movable piston and cooperates with the loading gland 108 which has an annular cylindrical pressure chamber 122 formed therein. The rigid ferrule 106 has a cylindrical outer periphery 124 which is received within the annular pressure chamber 122. The outer periphery 124 includes an O-ring hydraulic seal which slides and seals against the outer wall of the chamber 122. The inner cylindrical periphery 128 of the right ferrule 106 slides against the inner wall 130 defining the pressure chamber 122 and includes an O-ring seal 129. The lower or inner end portion 132 includes a frustoconical outer periphery 134 which moves into surface to surface mating contact with the frustoconical outer periphery 124 on the plastic ferrule 104. The central aperture 136 is coextensive and coaxial with the well 110, the aperture 118 and the deformable ferrule 104.

The loading gland 108 includes a threaded inner portion which threadably engages the threaded end 114 of the body 102. The central aperture 138 again is coextensive with the well 110, with the aperture 118 of the plastic ferrule 104, and with the aperture 136 of the rigid ferrule 106. The test specimen (not shown) passes through these apertures and is inserted into the well 1 10.

An hydraulic inlet line 140 is provided in the loading gland 108. This inlet 140 is connected by suitable pressure seal means (not shown) with a source of hydraulic fluid pressure (not shown).

In operation of the embodiment of FIG. 4, when the test specimen is received by the well 110 and the rigid ferrule 106 bears against the plastic deformable ferrule 104 by securing of the loading gland 108 to the body 102, hydraulic fluid is passed into the inlet 140. The hydraulic fluid acting in the pressure chamber 102 drives the piston-rigid ferrule 104 downwardly or applies axial pressure thereto. This axial pressure causes the rigid ferrule 106 to move against the plastic deformable ferrule 104. Because of the frustoconical outer periphery of the deformable ferrule, the axial force imparted by the piston-ferrule 106 is translated into an inward radial force which bears against the outer periphery of the test specimen.

In principle, it is seen that the operation of the embodiment of FIG. 4 is substantially the same as the operation of the embodiment of FIGS. 1-3. The difference lies in the use of hydraulic pressure for applying the necessary axial force, which force is ultimately translated into a radial gripping force. The hydraulic pressure can be controlled to applying constant pressure in the chamber 122 so that regardless of deformation of the workpiece or of the plastic ferrule, a substantially constant axial force and thereby a substantially constant radial force acts against the workpiece.

From the foregoing, it is seen that ali of the objections previously set forth have been accomplished. The device provides substantial advantages over prior art gripping devices. For tensile test purposes, both cored and cast specimens may be tested for tensile strength by the use of the device it) or the alternate embodiment Mill. The gripping device, as described, assures that only the axial force on a specimen is tested. The gripping devices are readily attached and used without any special preparation. Both the gripping device it] and the gripping device 100 may be constructed to accommodate any desired size, shape, or cross section of a test specimen or workpiece. The gripping device described is useful generally for the secure gripping of an article which is to be tested or is to be machined or is to have various operations performed on it,

While in the foregoing there has been provided a detailed description of particular embodiments of the present invention, it is to be understood that all equivalents obvious to those having skill in the art are to be included within the scope of the invention as claimed.

What I claim and desire to secure by Letters Patent is:

i. A method for tensile testing a brittle test specimen, such as concrete, mortar and rock, said specimen having a central axis, comprising the steps of providing a pair of deformable Al'n nonrigid ferrules, the entire peripheral internal shape of said ferrules being the same as the entire outer peripheral shape of said specimen, mounting one of said ferrules around each of the opposite ends of said specimen to completely encompass each of said ends with one of said ferrules, confining each of said ferrules in a space formed by said specimen and first and second relatively axially movable members and having a volume substantially equal to the volume of each of said ferrules, moving said first and second members axially to thereby apply an axial force to each one of said confined deformable ferrules, translating said axial force into an inwardly directed radial force with said deformable plastic ferrules acting against the said outer periphery of each end of said specimen for providing a radial inward gripping load of at least a preselected level on each of said ends, and applying a true tensile force to said first and second members which is transmitted through said ferrules to said test specimen along its central axis.

2. The method of claim ll wherein said radial inward gripping load increases as said tensile force increases.

3. The method of claim 1 wherein said radial gripping load is substantially constant.

Claims

1. A method for tensile testing a brittle test specimen, such as concrete, mortar and rock, said specimen having a central axis, comprising the steps of providing a pair of deformable nonrigid ferrules, the entire peripheral internal shape of said ferrules being the same as the entire outer peripheral shape of said specimen, mounting one of said ferrules around each of the opposite ends of said specimen to completely encompass each of said ends with one of said ferrules, confining each of said ferrules in a space formed by said specimen and first and second relatively axially movable members and having a volume substantially equal to the volume of each of said ferrules, moving said first and second members axially to thereby apply an axial force to each one of said confined deformable ferrules, translating said axial force into an inwardly directed radial force with said deformable plastic ferrules acting against the said outer periphery of each end of said specimen for providing a radial inward gripping load of at least a preselected level on each of said ends, and applying a true tensile force to said first and second members which is transmitted through said ferrules to said test specimen along its central axis.

2. The method of claim 1 wherein said radial inward gripping load increases as said tensile force increases.