Abstract:
A rod joint in which the entire threaded length connection between two substantially straight rods, without shoulders, is pre-stressed, including a split, sectioned ring which fits between preformed seats on each of the rod elements.

Description:
This is a continuation of application Ser. No. 649,978 filed Jan. 19, 1976. 
    
    
     BACKGROUND OF THE INVENTION 
     In most cases of moving machinery, mechanical joints formed by various structural elements are subjected to cyclic stress. In the greater majority of cases, the cyclic stresses are relatively small and no substantial fatigue or failure results. In some instances, however, the cyclic stresses can be so large, or can be imposed upon the materials for so long, that fatigue will result. This is especially true if the parts are so constructed as to have a notch, thread, or any abrupt change in cross-section. The maximum stress will occur at that location and will be greater than the nominal stress in the part. In fact, it has been found that threads are often the location at which fatigue failure first occurs as a result of the stress concentration factor. 
     It has been known for sometime that cyclic stress fatigue could be substantially eliminated by pre-stressing the parts to an extent that the pre-stress imposed is greater than any stress which might be imposed by cyclic operation of the machinery. For example, the parts may be threaded together using a torque wrench to insure that the maximum cyclic stress is exceeded by the imposed pre-stress. 
     In pre-stressing relatively threadable members, there are two major problems which have not been solved by the prior art devices. In nearly all such devices, it has been necessary to provide a shoulder on either or both of the members in order to produce a sufficient reaction force to generate the pre-stress. Of course, if it is feasible to employ such a shoulder--for example, the head of a bolt--imposing a pre-stressing force becomes a rather simple matter. 
     On the other hand, it is often necessary to join straight, threaded members in a pre-stressed joint without being able to employ a shoulder against which the joint may be pre-stressed. For example, the joined rods may have to be movably installed in a fairly close fit within another structural element. Unfortunately, if no shoulder can be used to impose pre-stress forces, the last threads of the joint can rarely, if ever, be subjected to the entire pre-stress force. Consequently, the cyclic stresses applied in the vicinity of the last thread may equal or exceed any pre-stress, increasing the chances of failure. 
     BACKGROUND OF THE INVENTION 
     Thus, it is desirable to provide a tool or rod joint which can be pre-stressed throughout the entire length of the joint so that the pre-stress forces are larger than any cyclic force which will be imposed upon the structure. Further, it is desirable to form such a device so that there is no abrupt change in cross-section. Similarly, it is preferred to form the joint in such a manner that every thread in the joint is subjected to the pre-stress force, thus substantially eliminating the possibility of fatigue failure in the joint. Such a joint may be employed, for example, in the structure of the hydraulic motor and pump disclosed in U.S. Pat. No. 3,502,020, assigned to the assignee hereof. 
     On its presently preferred embodiment, the invention utilizes a small recessed or beveled section adjacent one end of the thread on one of the elements to be joined, and a similar structure at the opposite end of the threads on the second element to be joined. For the sake of convenience, these recesses or beveled sections shall be referred to as a seat. Between the seating means on the two elements, sections of split ring can be installed which cooperate to exert reaction forces on the elements to be joined. Surprisingly, it has been determined that a joint formed in accordance with the present invention may be stronger than the elements themselves. In other words, failure is more likely elsewhere in the structure than it is at the joint. Further, the present invention allows the joint to be utilized, as stated previously, without requiring a shoulder to be formed on either of the devices, thereby reducing the size and mass of the overall machine. 
     Those skilled in the art will realize that the present invention can be employed in a wide variety of structures. For example, either or both of the elements to be joined may be solid or hollow. Similarly, the invention could be employed with structure in the nature of a stud which is threaded into a machine body. In any event, these and many alternative uses of the invention as defined in the claims will quickly become apparent to those skilled in the art without exceeding the scope of the invention as thus defined. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 comprises a sectional illustration of a joint formed in accordance with the present invention. 
     FIG. 2 comprises a greatly enlarged sectional view of a portion of the device illustrated in FIG. 1; and 
     FIG. 3 comprises an alternate embodiment of a rod joint formed in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION 
     As shown in FIG. 1, a first preferred embodiment of the present invention may be employed for operatively connecting a pair of tubular elements 11 and 13 having cooperable threads 15 and 17, respectively. 
     At one end of the member 13, adjacent the outer end of the thread 17, a beveled or sloped seating surface 19 may be provided on a suitable angle, as will be described below. Adjacent the inner end of the thread 15 and member 11, a recess 21 may be provided, having a sloped seating surface 23. 
     Intermediate the seats 19 and 23, a plurality of sections 25 may be installed in a ring. Preferably the ring sections each have sloped seating surfaces 27 and 29. As the elements 11 and 13 are threaded together, the ring sections having been installed, the seating surfaces 19 and 27 will interact, as will the seating surfaces 23 and 29, to exert a pre-stress between the parts. As illustrated in FIG. 1, this action will draw the threads together at the mating surfaces closest to the ring 25, leaving a slight clearance, or lower frictional contact, on the opposite side of the threads. In other words, as the elements are threaded onto one another and the ring interacts between the surfaces 19 and 23, reaction forces will be exerted upon the elements in the directions of the arrows illustrated in FIG. 2. Thus, the ring sections will be captured between the pieces and exert pre-stressing reaction forces thereon, without requiring in either part a shoulder above the tubular portions of the members 11 and 13. As a result, stress concentration factors will be minimized and the pre-stress force will be exerted over the entire length of the threaded joint. 
     Those skilled in the art will realize that the opening bore in the member 13 could, if desired, comprise a straight bore and a sloping surface 19 would not be required to generate the desired reaction forces since the ring sections could act against the bottom of the bore. It Will also be clear to those skilled in the art however, that a sloped surface is preferable in the interaction between the ring and the element 11, as illustrated at 23 and 29. With respect to these alternatives, the sloped seating surface 19 in the element 13 is desired because the use of such a cooperative arrangement eliminates close tolerance requirements which could be generated by the use of a straight bore having a radial surface adjacent the end of the screw 17. 
     Those skilled in the art will also realize that, if desired, the recess 21 in the element 11 could be produced so that it is radially smaller, relative to the axis of element 11, than the root of the thread 15. 
     In any event, it has been found that the specific angle selected for the cooperative relationship of surfaces 23 and 29 is unimportant, so long as the angle is selected from within a suitable range of angles. In this regard, when the tangent of the angle of the surfaces 23 and 29, relative to the axis of the element 11, approaches the coefficient of friction between element 11 and the ring 25, it will be impossible to cause relative rotation between those parts sufficient to generate the pre-stressing reaction forces. For the sake of convenience, and since the precise lower angle cannot be specifically determined for all materials and combinations thereof, in view of the different coefficients of friction, the angle of which the tangent is approximately equal to the coefficient of friction will be referred to here as the &#34;sticking angle.&#34; 
     Thus, when the angular relationship of the sloped surfaces 23 and 29 reaches the sticking angle, relative to the axes of the joint elements, sufficient relative rotation therebetween cannot be obtained to accomplish pre-stressing in accordance with the result desired from the present invention. On the other hand, if close tolerance requirements are considered to be unimportant, it will be realized that the invention could be employed under circumstances in which the surfaces 23 and 29 are radially oriented, relative to the joint elements. 
     Referring now to FIG. 3, there has been illustrated a second embodiment of the present invention, in which elements which are similar to the first embodiment have been provided with the same identification numerals, preceded by the numeral &#34;1&#34;. Thus, sloped surface 29 of the first embodiment is illustrated as sloped surface 129, etc. The second embodiment clearly illustrates that the inventive concept can be employed in a variety of structures. Thus, in the second embodiment, the ring 125 is employed on the end of the inner element 113 against a groove 121 in the outer element 111. In other words, the various sloped surfaces, etc., can be formed on either of the elements to be joined in such a way as to create the desired pre-stress force in the element which is larger than any anticipated or potential cyclic stress. Thus, both elements can be hollow or tubular, either or both can be solid, etc. Similarly, the invention can be employed with structural elements of quite different sizes, or even in place of the prior art, shouldered structures, if desired. 
     As a result of this structure, the joint itself, comprising the two elements to be joined and the ring sections, is stronger than either of the elements which are joined. Thus, the possibility of fatigue failure in the area of the joint is significantly reduced or eliminated entirely. 
     Those skilled in the art, having read the above description, will quickly be able to employ this concept, as defined by the claims, in a wide variety of structures, many of which will not even resemble those depicted in the drawing. Of course, all such structures should be considered to fall within the scope of this invention as defined by the claims.