US3487680A - Hydraulic grip - Google Patents

Description

Jan. 6, 1970 F. F. EICHENBRENNER ETAL 3,437, 30

HYDRAULIC GRIP 2 Sheets-Sheet 1 Filed April 26, 1968 FIG. 2

POWER CYLINDER FIG. 6

POWER CYLINDER FIG. 5

INVENTORS FRED F. EICHENBRENNER WALER ILLG POWER CYLINDER TTORNEYS Jan. 6, 1970 F. F. EICHENBRENNER E'I'AL 3,487,680

HYDRAULIC GRIP Filed April 26, 1968 2 Sheets-Sheet 2 POWER POWER CYLINDER CYLINDER FIG. 4 FIG 3 INVENTORS FRED F. EICHENBRENNER WALTER ILLG TORNEYS United States Patent 3,487,680 HYDRAULIC GRIP Fred F. Eichenbrenner, Hampton, and Walter Illg, Yorktown, Va., assignors to the United States of America as represented by the Administrator of the National Aeronautics and Space Administration Filed Apr. 26,1968, Ser. No. 724,551

Int. Cl. G01n 3/36 U.S. 01. 73-103 s 14 Claims ABSTRACT OF THE DISCLOSURE The invention described herein was made by employees of the United States Government and may be manufactured and used by or for the Government for governmental purposes without the payment of any royalties thereon or therefor. v

This invention relates to clamping arrangements for clamping sheet stock specimens into mechanical testing machines and also to specific hydraulic clamps especially suited for this purpose. I

Prior art devices for fastening the ends of sheet metal specimens into the grips of a testing machine have usually consisted of a pair of jaws having a plurality of screws passing through them and arranged to draw the jaws together and grip the specimens as they are tightened down. These prior art devices, while not wholly unsatisfactory, suffered from several disadvantages, among these being the rotary strain induced in the specimen as the screws are tightened, which in turn leads to upsetting of the stress patterns desired in axial loading fatigue tests. A second disadvantage was the nonrepeatability of the gripping pressure since galling on the screw ends, deterioration of the threads, and variation in lubrication would vary the effect of given torques applied, to. the individual screws. In addition, these devices were rather slow and laborious to use. l

Therefore, it is an object of the present invention to provide a means of clamping sheetmetalspecimens into mechanical test machines withoutinducingrotary strains in the specimen. c l

' It is a further object to provide a device capable of clamping sheet specimens witha highly repeatable clamping pressure. 7

It is yet another rapidly and easily clamp. or release suchaspe cimen into atestmachine. Y

Another object of the presentlinvention is to provide novel hydraulic clamps which are particularly. suited to the purpose of clamping such specimens into mechanical testing machines. T l I These and other objects which will become more apparent upon an inspection and'reading of the drawings and specification are accomplished by the usefof hydraulic clamping arrangements wherein a uniform clamping pressure is applied with a minimum distortion of parts to the specimen to be fastened. A

In the drawings, FIG. 1 is a side elevation in partial section of a test machine with a sheet, specimen clamped therein.

object to provide a device; which will I,

FIG. 2 is a partial front elevation of the same assembly;

FIG. 3 is a partial view of a side elevation in partial section of a test machine with a specimen clamped therein with another form of hydraulic clamp;

FIG. 4 is a front elevation of the same assembly;

FIG. 5 is a partial view of a side elevation of another form of hydraulic clamp used in a test machine;

FIG. 6 is a partial view of a side elevation of another form of hydraulic clamp used in a test machine.

Referring now to the drawings, FIG. 1 shows the basic layout of the test setup for mechanical test of a sheet metal specimen. The specimen 10 is clamped at both its upper and lower ends with a hydraulic clamp assembly 12, including a piston 14 disposed in a cylinder block 16. The piston 14 has an elongated rod 18 secured thereto and passing through the cylinder end wall through the bore 20. Fluid seals 22 are provided in order to provide a fluid-tight hydraulic chamber 24. Passageway 26 through the piston is provided and is in communication with fluid line 28 and a source of fluid pressure 30, so that the chamber 24 maybe pressurized with a fluid pressure. Cylinder block 16 is formed with a flat end wall 17, which acts as a reaction surface for the clamping action as to be more fully described herein.

Clamping plate 32 is slidably disposed on a portion of the rod 18, and is retained thereon by the nut 34 which threadedly engages the end of the rod 18. The nut 34 provides a degree of adjustment to the travel of the clamping plate 32 to allow for specimens of varying thicknesses. The rod end may be formed with a hex head 40 thereon in order to loosen either the nut 34, or the locking ring 38 in case some binding in the threads is encountered.

The end of the piston 14 is threaded at 36 in order to receive a locking ring 38 threaded on its interior to engage the piston 14.

The upper portion of the cylinder isformed with a flat plate 42 which is bolted to a load measuring device such as a weigh-bar 44,.which is in turn secured to the test machine frame 46. The lower hydraulic assembly 12 has the plate .member 42 secured to the power cylinder assembly for inducing loads on the specimen 10. In use, the specimen to be tested is slipped over the projecting upper and lower rods 18, from which the clamping plates and force nuts have been removed. These elements are then replaced and fluid pressure is introduced to line 28 pressurizing the chamber 24 and causing the pistons 14 to move away. This produces a clamping action between the cylinder end walls 17 and the clamping plates 32 with a force magnitude directly related to the piston area and the pressure level maintained in the chamber 24. It should be noted that the cylinder end wall 17 acting as reaction surface is held against .movement by theclamping plate 32 by the force of the fluid pressure in the chamber 24 directed in the opposite direction.

After the desired clamping force is attained, locking rings 38 are hand-rotated until they engage the cylinder 16. Thefluid pressure is then relieved to produce a solid mechanical lock at both ends of the specimen, to allow the desired loadings to be induced.

' It should be pointed out that this lock has been accomplished without inducing rotary strain on the specimen since the hydraulic clamp 12 operates with a straight pull, and the locking ring 38 is only rotated by hand and no significant torque is applied thereto. Secondly, this clamping pressure is highly repeatable since it varies only with. the pressure applied to chamber 24, and this parameter is rather easily and precisely monitored and controlled. Thirdly, the clamping pressure applied to the specimen is evenly distributed due to the low distortion design of the clamp. This is a result in part to the arrangement whereby clamping forces are transmitted by a rod under tension rather than a compression, resulting in elimination of the usual substantial buckling deflections encountered in a piston and cylinder combination, and in part to the countering of the reaction forces in the system to produce a zero net force on the hydraulic assembly. Thus forces acting on cylinder 16 to the left due to the clamping action of plate 32 are just balanced by the force to the right induced either by the fluid pressure on the end wall or the force ring 38. Hence, distortion and misalinement of the parts is minimized, resulting in an even pressure distribution across the face of the clamping surfaces and elimination of racking loads induced by such an uneven pressure distribution. In addition, lateral loads on the weigh-bar 44 are similarly eliminated.

It is also seen that the process of installing a specimen is rendered much more rapid and is more easily accomplished than that utilizing the manually operated clamps.

A variation of this assembly is shown in FIGS. 3 and 4. The general arrangement is the same, in which a specimen is fastened at its ends by hydraulic clamp assemblies 48. However, each hydraulic clamp is provided with a plurality of piston and rod subassemblies slidably received in right and left cylinder block plates 50 and 52. Each subassembly consists of a piston 54 and a rod 56 secured thereto, the piston received in a bore in the plate 50 to form a chamber 58. The rod 56 passes through both plates through openings 60, and threadedly engages a force nut 62, which may be received in a counterbore 64 formed in plate 52.

Plates 52 and 50 are both secured to a plate 66 by means of cap screw and nut assemblies 68 passing through elongated holes 70, while this plate is in turn secured to a load cell 72 which is secured to the test machine frame.

Fluid pressure is supplied to each cell by means of a pressure source 74, line 76, and interconnecting passages 78, 80, 82, 84 and 86 which communicate with each chamber 58.

Each of the plates 50 and 52 are mounted so as to be slidable in track assemblies 88, fastened to the upper plates 66. A pair of bars 90 fastened to each of the plates 52 and 50 fitting into the track assemblies may be engaged with bearing assemblies 92 so that each of the plates may be freely slid back from the position shown in FIG. 3 when cap screw and nut assemblies 68 are not in place.

In use, the plates 50 and 52 are Withdrawn to a point where the rod 56 does not pass into plate 52, cap screw and nut assemblies and force nuts 62 having been removed. The specimen 10 having been predrilled for this purpose is then slipped over the rods 56 and plates 50 and 52 are advanced into engagement. Cap screws and nut assemblies 68 and the force nuts 62 are then installed to finger tightness. The introduction of fluid pressure into each clamp assembly 48 causes a pressurizing of all of the chambers 58 to produce a desired clamping force on the specimen 10.

This apparatus while incorporating the advantages ant. functioning of the single cell hydraulic clamp, possesses the additional advantage of producing a uniform clamping pressure simultaneously over a relatively great area, 7

This apparatus is similar to that shown in FIGS. 1 and 2 except that instead of a single clamping plate 32, a forked member 94 is used, and the clamping action occurs between the tine members 96 and 98 instead of between the clamping plate 32 and the cylinder. Thus, the fork '94 may be secured to the power cylinder or load cell rather than the cylinder while keeping the specimen centered along the line of action of the power cylinder, resulting in a somewhat smaller and simpler structure, while retaining the advantages of the basic hydraulic clamping arrangement. The time 98 acts as the reaction member and is maintained against movement by the action of time 96 by the effect of the nullifying force applied by the cylinder block 16 produced either by the internal fluid force or the locking ring 38.

A final variation is shown in FIG. 6. Here, a separate reaction plate 102 is secured to the power cylinder, and the clamping plate is not connected thereto, as in the forked arrangement of FIG. 5, allowing freer clamping movement of the plate 100.

Therefore, it can be seen that an arrangement has been provided for clamping specimens into a test machine with a minimum of distortion of the specimens induced by the clamping process, with a highly controllable and repeatable clamping pressure, and which is applicable to specimens of considerable proportions. The process is also fast and relatively effortless.

In addition, it can be seen that a novel hydraulic clamp has been provided in which distortion of the components is minimized, allowing great clamping pressures to be exerted without significant deflection of the parts which would lead to uneven pressure distribution across the clamping surfaces.

It should be understood the invention is not to be limited to the particular embodiments disclosed, as these are intended to be illustrative of the invention. It also should be noted that some of the inventions advantages may be gained by the use of a single hydraulic clamp in a testing machine. The specimen may be first clamped at one end with a prior art device, and since the other end is free, rotary strains are largely avoided. Then the opposite end may be clamped with a hydraulic clamp according to the present invention, thus eliminating the inducement of rotary strain in the specimen.

Therefore, the invention is to be limited only by the following claims:

1. A hydraulic grip comprising:

block means having a passage formed therein;

a piston movably fitted into said passage and having a portion which protrudes out of said passage in a portion of its range of movement;

means for selectively producing a net fluid force on said piston and disposed opposite said one direction;

, connector means drivingly connecting said piston and said clamping element;

a reaction element interposed between said clamping element and said piston; and

means securing said reaction element against move ment by said clamping element in response to said fluid force;

an enlarged element fitted over said piston; and

means for positioning said element against movement along said piston at selective points along said protruding portion of said piston.

2. The grip of claim 1 wherein said connector means includes a rod extending through said passage, and also includes engagement means drivingly connecting said rod to .said clamping element.

3. The grip of claim 1 wherein said positioning means comprises a threadedconnection between said piston and said enlarged element.

4-. The grip of claim 2 wherein said engagement means includes a threaded portion on said rod extending through said clamping element and a 9116? nut threadedly engaging said portion.

5. The grip of claim 1 wherein said reaction element comprises an end wall formed in said block means.

6. A hydraulic grip comprising:

block means having a plurality of spaced parallel passages formed therein;

disposed therein drivingly connected to a first one of said members and movable in response to fluid pressure therein, means for producing a clamping action between said elements in response to movement of said slidable element, and supply means for first member, means for producing a clamping action between said elements in response to movement of said slidable element, and supply means for introducing fluid pressure to said chamber; and

a plurality of pistons each slidably fitted into a respec- 5 introducing fluid pressure to said chamber means for tive passage; locking said slidable element against movement in means for creating fluid chambers between one of the positions in its range of movement; and

respective piston ends of each piston and said block means for measuring loads induced on said specimen. means; 10 12. Apparatus for testing a sheet specimen comprising: means providing fluid communication between said fluid a pair of spaced elements;

chambers; power means selectively producing relative movement means for providing a fluid pressure to said chambers; between said pair of spaced elements;

a clamping element spaced from said pistons; means for clamping portions of said specimen to said means drivingly connecting each of said pistons and 1 spaced elements, including at least one hydraulic said clamping element; clamp comprising a pair of members, at least one a reaction element in juxtaposition with said clamping of which is connected to one of said spaced eleelement; and ments, and a hydraulic chamber having a slidable means for securing said reaction element against move element disposed therein drivingly connected to a ment by said clamping element, whereby a uniform first one of said members and movable in response clamping force is created by each piston and passage to fluid pressure therein, means for producing assembly by the introduction of fluid pressure to a clamping action between said elements in said chambers. response to movement of said slidable element, and

7. The grip of claim 6 wherein said clam-ping element supply means for introducing fluid pressure to said is located with said chambers between said pistons and chamber, and wherein said hydraulic clamp further said clamping element, and said reaction element is interincludes at least one other slidable element and posed between said pistons and said clamping element. hydraulic chamber, and means drivingly connecting 8. The grip of claim 7 wherein said connecting means said at least one other slidable element and said each includes a plurality of rods each connected to a refirst member, and wherein said supply means also spective piston and passing through its respective fluid introduces fluid pressure to said at least one other chamber. fluid chamber.

9. The grip of claim 8, wherein each of aid C nn g 13. The apparatus of claim 12 further including means means further includes a portion of the IOd extending providing fluid communication between said fluid through a passage in said clamping element, a nut member h b threadedly engaging Said Portion of the Tod and of a 14. Apparatus for testing a sheet specimen comprising: larger size than said passage. a pair of spaced elements;

Apparatus for testing Sheet Specimen Comprising: power means selectively producing relative movement a pair of spaced elements; between said pair of spaced elements;

power means selectively producing relative movement means f clamping portions of said specimen to said between said pair of spaced elements; spaced elements, including at least one hydraulic means for clamping portions of said specimen to said Clamp comprising a pair of ember at least one Spaced elements, uding at least one hydraulic of which is connected to one of said spaced elements, Clamp comprising a p of members, at least n and a hydraulic chamber having a slidable element Of which i Connected t One Of Said Spaced disposed therein drivingly connected to a first one ments, and a hy u i Chamber having a slidable of said members and movable in response to fluid element disposed therein drivingly connected to a pressure h i means for producing a clamping first One Of Said members said Chamber interaction between said elements in response to moveposed therebetween and movable in response to fluid ment f id lid bl d d supply means pressure therein and the Second of Said members for introducing fluid pressure to said chamber, and interposed between said slidable element and said means f mechanically locking said pair of bers against movement relative to each other; and means for measuring loads induced on sald specimen.

References Cited means for measuring loads induced on said specimen. 11. Apparatus for testing a sheet specimen comprising:

a pair of spaced elements;

power means selectively producing relative movement between said pair of spaced elements;

UNITED STATES PATENTS 2,481,684 9/1949 Rock 279-4 X 2,777,701 1/1957 Hohwart et al. 279-4 X 3,323,357 6/1967 Gloor 73-103 spaced elements, including at least one hydraulic clamp comprising a pair of members, at least one of which is connected to one of said spaced elements, and a hydraulic chamber having a slidable element JERRY W. MYRACLE, Primary Examiner US. Cl. X.R.

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