PRELOADING CLAMP

Abstract: A clamp (50) for use in testing machines to prestress a joint between a driving shaft (30) and a driven shaft (20), both of which carry portions (22, 32) which are externally threaded with each shaft having a flat surface (36, 38) abutting the other in a plane substantially perpendicular to the driving axis (16). The clamp (50) is internally threaded (52) to cooperate with the external threads (22, 32) and has bolts (54) which, when tightened, produce a force on the abutting surfaces (36, 38) to compressively preload the joint from forces on the flanks of the external threads (22, 32).

Description: PRELOADING CLAMP BACKGROUND OF THE INVENTION 1. Field of the Invention. This invention relates to a clamp for use in, for example, tension and compression testing machines and more particularly to a clamp useful to prestress the joint between a grip which holds the wor piece being tested and the piston rod connected to the actuator which applies the compression and tension forces. 2. Description of the Prior Art. Tests involving repeated applications of stress, called fatigue tests, are usually performed by cyclic compression and tension. Examples of machines used in fatigue tests may be seen in the M.M. Gram U. S. Patent 3,320,798 assigned to the assignee of the present invention. As can be seen in this patent, a reciprocating actuator operates to cyclically move an actuator rod or shaft back and forth. The actuator rod is connected to a grip device which is operable to hold the speciman being tested. The grip device is connected to the actuator rod or shaft by means of a threaded connection between the grip and the actuator rod to hold the two together. Other methods of joining the grip to the actuator rod are also usable. For example, a V-band coupling manufactured by Voss Industries, Inc., of Cleveland, Ohio, could encompass a pair of flanges extending radially out from the joining ends of the grip and actuator rod which would then be tightened to secure the two members together in a rigid manner. SUMMARY OF THE INVENTION The present invention relates to a split clamp to prestress a joint between two members to be linked. The two members are elongated and have central axes. The parts are piloted to retain the axes coincident, and the members have abutting surfaces that are perpendicular to the axis of the assembly. As shown, one member can be associated with a test specimen and the other may be an actuator rod, or a shaft from a load cell used in a test system. The pilot generally is a protruding end of one member which fits within an aperture in the other member so as to assure lateral alignment. The ends of the abutting members are machined so as to be substantially perpendicular to the axis of the cyclical force application, and the apparatus is assembled by abutting the two perpendicular surfaces together with the piloting members engaged, and placing the split clamp loosely around the members in such a manner that internal threads on the split clamp cooperate with external threads arranged to load the two members axially as the split clamp is tightened. The external threads may be formed directly on an actuator rod and on the shaft of a specimen grip, for example. Collet members can be used if they have external threads loaded by the split clamp and are arranged to compress the two members forming the joint together as the clamp is tightened. In one preferred embodiment, the threads on an actuator rod are cut with opposite inclination (right hand) to the threads on a specimen grip (left hand) so that when the split clamp is fitted onto the external threads and then turned, the abutting surfaces if the members forming the joint are loaded together to produce a slight compressional force, and the flanks or sides of the mating threads which carry tensile loads are engaged. The clamp can be built with both clamping members, such as the actuator and the grip, having right-hand or left-hand threads. The parts then must be rotated relative to each other to tighten the two parts together in order to produce the desired loading on the thread flanks. The clamp bolts on the split clamp are then tightened, which causes a force on only the tensile flank of the threads and causes an increase in compressional force at the joint between the perpendicular ends of the parts or members being loaded, such as the actuator rod and the grip shaft. In a second form of the invention, a test specimen having a button end that forms an annular shoulder around the specimen is piloted into a recess formed in an actuator shaft coupling and compressional loads between the specimen and coupling are provided by a collet that has exterior threads engaging the split clamp interior threads and which bears on the shoulder formed by the button end of the specimen. The clamp is also threaded to the actuator shaft coupling and the parts can be adjusted to seat the button end firmly in the pilot recess so the abutting surfaces are engaging. Tightening the split clamp results in compressing the two parts together. In both forms of the invention, the clamp bolts are tightened sufficiently to insure that the compression force at the joint (seating the surfaces) is greater than the tension force which will be encountered during operations, such as testing, and accordingly the joint is preloaded to a value such that the abutting surfaces always are under compression during use. The actuator, when operating at rated loadings, will not increase the load on the threads. By prestressing in this manner, the fatigue load on the coupling threads is significantly reduced, while alignment of the two shafts is accurately achieved through known and simple machining techniques. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a cross-sectional side view of the joint between an actuator shaft and a specimen grip taken along lines 1-1 of Figure 2; Figure 2 is a top view of the split clamp shown in position to encompass the shaft and grip; Figure 3 is a side part sectional view of a modified form of the invention showing the split clamp used for directly compressing a specimen into a pilot receptacle on an actuator rod coupling; and Figure .4 is a sectional view taken as on line 4 — 4 in Figure 3. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Figure 1 shows a specimen grip 10 with hydraulic connections 12 used in actuation thereof to produce a clamping force for holding a specimen 15 on which tension and compression forces will be applied along an axis 16. A grip shaft 20 is shown having an exter¬ nally threaded portion 22 which, in the preferred embodiment, may be inclined as a left-handed thread of predetermined pitch. An actuator rod or shaft 30 connected to a conventional hydraulic actuator 31 is shown having an externally threaded portion 32 which, in the preferred embodiment, is inclined as a right-handed thread having a pitch which may be the same as (or different from) threads 22. The preloading arrangement of the present invention is useful where reciprocating or torsional loads are being applied to a specimen held in the grip. The actuator shaft 30 thus is reciprocated along axis 16 or is rotated on axis 16 (or both) with the hydraulic actuator 31. A lower end surface 36 of shaft 20 and an upper end surface 38 of shaft 30 are precisely machined to be substantially perpendicular to axis 16. End surface 36 has an aperture 42 therein and end surface 38 has a pilot pin 44 thereon sized to fit within the aperture 42 for alignment purposes when end surfaces 36 and 38 are butted together to form a joint therebetween. Because of the pilot pin 44 and the machined end surfaces 36 and 38, the lateral and axial angular alignment will be substantially exact. A split clamp 50 is shown having internal threads as at 52 of substantially the same pitch as threads 22 and 32, and having a left-handed section and a right-handed section so as to cooperate with the threads 22 and 32, respectively. Split clamp 50 has a plurality of clamp bolts 54, best seen in Figure 2, which, when tightened, operate to pull the two halves of split clamp 50 together. In order to provide a preload condition at the joint between end surfaces 36 and 38, the split clamp 50 is placed loosely around the shafts 20 and 30. The clamp 50 is rotated manually to an orientation with respect to the shafts 20 and 30 until the internal threads 52 of split clamp 50 mesh with both the upper and lower threads 22 and 32 of shafts 20 and 30, respectively. The bolts 54 are placed loosely in the clamp 50. Thereafter, clamp 50 may be lightly rotated so as to produce a small compression force on the abutting surfaces 36 and 38 between shafts 20 and 30. With the shafts 20 and 30 now being laterally well aligned with the axes of the two shafts 20 and 30 coincident, the bolts 54 can be tightened to produce the desired prestressed condition. It will be noted that tightening bolts 54 causes the effective thread pitch diameter of the clamp to be reduced causing additional compressional forces to be exerted at the end surfaces of the shafts 20 and 30 by virtue of the flank angle of the threads which produces a force on only the tensile flank of the threads. The "tensile flank" means the thread flank that produces tensile forces in the clamp 50 and compressive forces on the ends of the shafts 20 and 30. For example, it can be seen in Figure 1 that a tensile flank 60 of the threads 22 on shaft 20 is being forced toward a tensile flank 62 of the threads 52 on clamp 50. The other flank 64 of the threads 22 on shaft 20 is shown to receive no force from the other flank 66 of the threads on split clamp 50. This is shown in Figure 1 by the slight space 68 therebetween (shown exaggerated for clarity). The same situation is true with respect to the tensile flank of the threads 32 on shaft 30 and accordingly it can be seen that a compressional force on the end surfaces 36 and 38 can be applied which is greater than the tensile force applied on the actuator to preload the joint so it will not move or misalign during use. The threads 22 and 32 are of sufficient number so that the stress is low and their fatigue life considerably extended. In the first embodiment, the threads of shaft 20 have been inclined in a left-handed manner while the threads of shaft 30 have been inclined in a right-handed manner, which allows the shafts to be oriented and then the clamp 50 rotated to the position where the clamp fits both threads and then the clamp further rotated to load the proper thread flank before torquing the bolts. However, other arrangements are possible. For example, the threads of both the upper and lower shafts 20 and 30 could have the same direction of lead of the threads, which would allow the clamp to be oriented in any direction, but in this event the joint parts must then be rotated relative to each other to tighten the joint in preparation for torquing the clamp bolts for preloading. Also, the threads on the shafts do not have to have the same pitch nor do they have to have the same diameter as long as the internal threads of the clamp match them. A second embodiment of the present invention is shown in Figures 3 and 4, and can be utilized for axially preloading two shafts under compression, through intervening members that carry the axial force for making a joint of the two shafts or rods. In the specific form shown, instead of using a specimen grip that is separate from the coupling member, the coupling member itself forms a portion of the grip for a test specimen. The coupling member of the present invention indicated generally at 60 comprises a split clamp assembly 61 that forms two collar or clamp sections 61A and 61B, that have internal threads as described in the previous form of the invention. The internal threads on the two collar sections are shown at 61C and 61D, and can have opposite hand threads as discussed in the previous form of the invention. In this form of the invention, the rod or shaft members that are to be mounted together comprise a test specimen 64 that has a button end or collet mounting end indicated generally at 65. The specimen 64 is cylindrical, and has a cylindrical neck 66 coupling the test portion of the specimen to the button end 65, which also has a cylindrical outer surface, and which is of larger size than the member 66, so that a shoulder 67 is formed surrounding the neck 66. A collet assembly 70 is utilized for making the connection, and in the form shown four individual split collet members 70A, 70B, 70C, and 70D are provided around the button end 65. The outer surface of the collet sections formed in the collet assembly 70 (there can be less or more sections if desired) is threaded, as shown by the threads 71 in Figure 3, and these threads are on the outer periphery of the collet and are made to mate with the thread sections 61C of the split clamp assembly 61. The mating threads would have the same lead. The collet sections, as can be seen in Figure 3 as well have compressive shoulder surfaces 73 that are made to fit over the shoulder surface 67. The collet assembly 70 has a central opening that is slightly larger than the diameter of the neck 66 of the specimen so that the collet assembly will fit over the button head 65 and the shoulders 67 and 63 are engaged when the assembly 70 is properly positioned around the button head 65. The collet assembly forms an inner cylindrical surface that surrounds the button head for a substantial distance as well, as can be seen in Figure 3. The split clamp assembly 61 attaches to a second shaft assembly 75 comprising an actuator rod 74, which has a threaded coupling or adapter 76 at its end with an external thread 76A that mates in the thread section 61D of the split clamp assembly 61. As can be seen, the threaded coupling 76 forming a part of the actuator shaft 74 has a recess or bore 78 that is of size to receive and pilot the button end 65 of the specimen 64, and provide a close fit around the outer surface thereof so that the two shafts are coaxial at the central axis 80 and their separate axes are precisely aligned when the shafts are piloted together. The two split clamp sections 61A and 61B can be placed around the collet assembly 70 and around the shaft coupling or adapter 76, and then held in place with suitable cap screws 82 that pass through apertures in the split clamp section 61B and thread into apertures in the split clamp section 61A. The clamp sections can then be tightened down, by rotating it to tighten the parts as previously explained in connection with the first form of the invention. The end surface 65A of the specimen button head which is machined precisely perpendicular to the axis 80, and the bottom surface 78A of the pilot recess 78 (also precisely machined perpendicular to axis 80) are thus seated, and under slight pressure. Then the cap screws 82 can be tightened to force the split clamp sections inwardly toward each other against the tension side of the thread flanks on both the collet assembly 70 and on the threaded "~ adapter 76 to force the interfacing surfaces 65A and 78A of the joint against each other. The preload or compressive forces on the interfacing surfaces can be adjusted to reach the desired level to insure that an adequate compression preload is obtained so that the tensile loads applied by the actuator shaft assembly 75 from an actuator 84 does not exceed the compressive load on the two interfacing surfaces. In this form of the invention as well as in the first form, the parts are piloted together very precisely, because the surfaces that mate can be machined very precisely with respect to the central axis 80, and the desired compressive loads then can be applied easily. Intervening members actually provide the compressive force in a load path that forces the shafts together. The collet assembly 70 can be considered part of a shaft assembly in that the load path is from the outside of the collet sections and then through interfacing surfaces to provide the compressive force. The end surfaces at the joint, in both forms of the invention are preloaded to a level proportional to the torque on the bolts (54 or 82). The axial stiffness of the joint through the interfacing end surfaces of the shafts is much greater than the axial stiffness of the clamp. The fatigue component of the load is thus primarily transmitted in compression through the interfacing shaft end surfaces and a design can be much smaller than previous designs with the same fatigue life rating, which had not been axially preloaded. The stiffness of the joint can be made greater than the axial stiffness of the clamp by making the clamp sections relatively thin and highly stressed, which has an additional cost advantage. Torsional movements can also be transmitted with the coupling device, either separately or in combination with axial loads. The torque applied (torsional loading) is transmitted by a combination of friction on the ends of the interfacing surfaces and by friction on the clamp threads. The walls of the split clamps 50 and 61 must be made thick enough to carry the shear forces generated by the torque loading, in addition to the tensile forces caused by the axial preload. The reciprocating and torsional loading of a specimen can be accomplished using an axial-torsional load frame, for example, the 309 Series load frame made by MTS Systems Corporation of Eden Prairie, Minnesota. It should also be noted that two sets of bolts are not necessary in that the split collar clamp can have a hinge or a pivot hook on one side and the bolts on the opposite side. The feature of being able to tighten the clamp to compress the interfacing ends of the shafts together can be accomplished with adjustable fasteners on only one side of the split collar clamp. Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention. For example, the pilots may be reversed, and both parts can be held by collets. Likewise, other types of actuators may be utilized instead of a hydraulic actuator.

Claims: WHAT IS CLAIMED IS: 1. A loaded joint comprising: a first member having a longitudinal axis; a second member having a longitudinal axis; the first and second members each having means thereon forming abutting end surfaces to hold the axes of both members substantially colinear to form a joint; threadable means connected to each of said first and second members adjacent the joint for coupling the first and second members together; split clamp means having internal threads, and positioned to the exterior of the first and second members so that the threadable means of the first and second members interfit with the internal threading of the split clamp means before the internal threading on the split clamp means and the threadable means are fully seated, when the end surfaces of the first and second members are abutting; and means for tightening the split clamp means for applying a compression force on the abutting surfaces of the first and second members through the threadable means thereon to seat the members at the joint. 2. The loaded joint of Claim 1 and cooperating interfitting means between the first and second members, respectively, to form pilot means for retaining the first and second members co-linear when the abutting surfaces are placed under compression. 3. The loaded joint of Claim 1, wherein said threadable means connected to said first and second members comprises a separate externally threadable member coupled to at least one of the first or second members, said separate member having means coupling it to the said one member to permit axial loads to be applied to the said one member with respect to the other member. 4. The loaded joint as specified in Claim 3 wherein said separate member comprises split collet means, and wherein the split collet means has an internal shoulder facing toward the other member when it is in place on the one member, said one member having a shoulder engaging said internal shoulder of said split collet means to transfer compressive forces toward the other member. 5. The loaded joint of Claim 1, wherein said threadable means on the first member has a right-hand thread of predetermined pitch, and the threadable means on the second member has a left-hand thread of predetermined pitch, and said split clamp means having mating threads for said right and left hand threads. 6. The loaded joint according to Claim 1 further including actuator means connected to the first member to impart a cyclical force thereto along the colinear axes. 7. The loaded joint according to Claim 6 further including actuator means connected to the first member to impart torsional forces thereto on the colinear axes. 8. The apparatus according to Claim 1 wherein a recess is formed in the end of the first member and a pilot fixed to the end of the second member is sized to fit within the recess to assure lateral alignment. 9. The apparatus according to Claim 8 wherein the threadable means of the first and second members are of opposite inclination but of the same pitch. 10. The apparatus according to Claim 1 wherein the threadable means of the first and second members are of different pitch. 11. The apparatus according to Claim 1 wherein the split clamp means comprises two clamp sections and clamp bolt means on opposite sides of the first and second members tightenable to move the clamp sections together. 12. The apparatus of Claim 1 wherein said first member comprises a test specimen, and said second member comprises an actuator rod for loading the test specimen, the first member having threadable means connected thereto by an interfitting shoulder forming means for applying a compressive load between the specimen and the actuator rod. 13. The apparatus as specified in Claim 12 wherein said means to permit applying a compressive load comprises a button head formed integrally with said specimen, said button head forming a shoulder surrounding other portions of said specimen, and wherein said means to permit applying a compressive load to the specimen comprises a split collet having an internal shoulder facing the shoulder formed by said button head, and having an annular outer surface surrounding at least a portion of the button head of said specimen. 14. The apparatus as specified in Claim 8 wherein said first and second members directly pilot together, one of said members having the recess for receiving an outer peripheral surface of the other member for locating the members laterally relative to longitudinal axes of said first and second members. 15. A coupling for coupling a specimen grip to an actuator under a preload comprising: a first member having a central axis and a threaded portion and an end with a surface substantially perpendicular to the axis; a second member having a threaded portion and an end with a surface substantially perpendicular to the axis, the end of the first member abutting the end of the second member with the axes substantially coaxial with the first member; pilot means operable between the first and second members to retain the first and second members coaxial; and split clamp means including clamp bolt means for tightening the split clamp means, the split clamp means being internally threaded and engaging end portions of the first and second members so that the threaded portions of the first and second members cooperate and interfit with the internal threading of the split clamp means to exert a compressive load between the end surfaces of the first and second members and the clamp bolt means being tightened to load against the interfitting threads to increase compression force on the abutting surfaces of the first and second members as the clamp bolt means are tightened. 16. The method of preloading a joint between a first member having a first axis and including means forming a first threaded portion, and a second member having a second axis and including means forming a second threaded portion, comprising the steps of: forming a first end surface on said first member substantially perpendicular to the first axis; forming a second end surface on said second member substantially perpendicular to the second axis; placing the first and second members with the first and second end surfaces abutting and the first and second axes substantially colinear to form a joint; placing a split clamp with an internally threaded portion and clamp bolts around the first and second members with the internally threaded portion engaging the means for forming the first and second threaded portions; orienting the clamp to produce engagement on the thread flanks that produce compression forces on the ends of the first and second members and tensile forces in the clamps; and tightening the clamp bolts which will produce a compression force on the first and second end surfaces to preload the joint and to produce a reaction force on the tensile flank of the threads of the means forming the first and second threaded portions. 17. The method of Claim 12 in which the step of placing the split clamp around the first and second members includes the step of placing the clamp bolts loosely therein. 18. The method of Claim 13 further including the step of rotating the first member until the internally threaded portion of the split clamp meshes with both of the means forming the threaded portions of the first and second members.

Inventors: GRAM MARTIN M, GRAM, MARTIN, M.
Applicants: MTS SYSTEM CORP, MTS SYSTEMS CORPORATION
Application year: 1988
Main classification: G01N3
IPC codes: F16B2, F16B7, F16B31, F16B39, G01N3