Patent Application
20200166068
Bolted joints are used commonly in construction and machine design. A simple form of bolted joint has a bolt that captures and joins other parts, and the combination is secured with a nut by mating the threads. To secure the joint, a certain amount of clamp force must be developed at the joint by tightening by turning/wrenching the nut. The clamp force is generated by the tension generated in the bolt slightly elongated in the direction vertical to the part's surface. The tension strength and the applied torque strength are generally proportional to each other. Thus, theoretically, if you know the diameter of the bolt, the torque needed to tighten the bolt so as to reach the right amount of clamp force/tension can be predetermined. However, the proportional constant (nut factor) depends on the type and material of the bolt and nut; the presence and type of any plating, coating or lubrication; the pitch or angle of the threads; and corrosion and wear, thereby varying even from one time to the next. Too much clamp force may cause warping of the joint or breaking the bolt, and too little may risk a loose, unsecured joint. These joint failures may possibly result in expensive downtime or even accidents. It is thus a challenge for a field operator to determine when enough torque has been applied for generating a right amount of clamp force/tension. Conventionally, a field operator may rely on his/her experience or intuition while turning/wrenching a nut around a bolt, or may use a torque chart which is often inaccurate and cumbersome.
In view of the above problems and ambiguities associated with every-day nuts-and-bolts operations, this document describes a new type of nut, which eliminates the uncertainty as to deciding when enough turning/wrenching has been applied for creating a secured bolted joint.
Each of the plurality of posts 106 is formed longitudinally, having a top end portion contiguously connected to the circular bottom portion 208 of the head 102 and a bottom end portion contiguously connected to the circular top portion 308 of the flange-shaped body 104. The plurality of posts 106 are disposed with an equal distance between adjacent posts. The number of the posts is four in this example; however, the number may be any number of two or more depending on design considerations for target applications.
Some of the corners, edges and end portions of the present torque-limiting nut may be tapered or rounded. Some examples are explained with reference to
As illustrated in
Here, the wrenching action is applied only to the first hex external side surface 202, not to the second hex external side surface 302, because D1>D2. The plurality of posts 106 are configured to break away from the body 104 when the applied torque. i.e., F1×D1, exceeds a predetermined amount of torque, where F1 is the applied force by the wrenching action and D1 is the first hex dimension D1, which is the distance between the two opposite flat faces of the first hex external side surface 202. The predetermined amount of torque is the amount predetermined to be sufficient to fasten the bolt 502, the workpiece 506 and the present torque-limiting nut, in order to form a secured bolted joint. Beyond that point, the plurality of post 106 break away, leaving only the body 104 with the bolted joint. Fine adjustments, such as further tightening or loosening of the remaining nut, i.e., the body 104, can be carried out by wrenching the second hex external side surface 302 that has the second hex dimension D2 which is smaller than D1.
As known to those skilled in the art, to secure a bolted joint, a certain amount of clamp force must be developed at the joint across the workpiece by tightening by turning/wrenching a nut around the bolt. The clamp force is generated by the tension generated in the bolt slightly elongated vertically to the workpiece surface. The tension strength is generally proportional to the applied torque strength. Thus, theoretically, if you know the diameter of the bolt, the torque needed to tighten the bolt so as to reach the right amount of clamp force/tension can be predetermined. However, the proportional constant (nut factor) depends on the type and material of the bolt and nut; the presence and type of any plating, coating or lubrication; the pitch or angle of the threads; and corrosion and wear, thereby varying even from one time to the next. Too much clamp force/tension may cause warping of the joint or breaking the bolt, and too little may risk a loose, unsecured joint. These joint failures may possibly result in expensive downtime or even accidents. It is thus a challenge for a field operator to determine when enough torque has been applied for generating a right amount of clamp force/tension.
Referring to the present torque-limiting nut with the dimensions of D1>D2, while the torque is applied, the relationship F1×D1=F2×D2 holds due to the continuity of the head 102 and the body 104 connected by the plurality of posts 106. Thus, F1<F2 because D1>D2. This means: as F1 is increasingly applied, a threshold torque T(th)=F1(th)×D1=F2(th)×D2 can be reached by providing F1(th) which is less than F2(th). The threshold torque T(th) can be predetermined, on a case-by-case basis, to be the right amount of torque sufficient to tighten the body 104 of the nut to the bolted joint. By interrupting the continuity between the head 102 and the body 104, i.e., by breaking away the head 102 from the body 104, to have F1×D1≠F2×D2, the body 104 can retain the right amount of clamp force/tension, even when the applied force F1 exceeds the threshold force F1(th), i.e., even when the applied torque F1×D1 exceeds the threshold torque T(th)=F1(th)×D1.
In the present torque-limiting nut, the shapes and dimensions of the head 102, the body 104 and the plurality of post 106 are configured such that, when the applied torque F1×D1 exceeds the threshold torque T(th), the posts 106 will shear off, leaving only the body 104 with the bolted joint with the right amount of clamp force/tension. As mentioned earlier with reference to
The broken-away head 102 attached with the plurality of posts 106 can be thereafter discarded. Fine adjustments, such as further tightening or loosening of the body 104 of the present torque-limiting nut can be carried out by wrenching the second hex external side surface 302 that has the second hex dimension D2 which is smaller than D1.
The present torque-limiting nut may be made of any durable metal such as stainless steel, aluminum, alloy, etc., and may be formed by use of a Computer Numerical Control (CNC) milling machine that allows for machining parts to precise sizes and shapes, within certain tight tolerances. Based on various calculations and experiments, the present inventors have obtained optimal shapes and dimensions that resulted in proper breaking away of the plurality of posts 106 from the body 104 when the applied torque exceeds the predetermined amount of torque, i.e., the threshold torque T(th). The following are two exemplary dimensions of the present torque-limiting nut. As known to those skilled in the art, these are approximate values and/or within instrumental tolerances or resolutions.
Case 1: the material is low carbon steel with DACROMET® coating, d1 and d2 per ⅝″-11 thread (basic major diameter of ⅝″, 11 threads per inch), D1=1″, D2=⅞″, height=0.77″, base diameter of the circular collar 304=1.3″, the number of posts 106=4, for T(th)=30 ft-lbs.
Case 2: the material is low carbon steel with DACROMET® coating, d1 and d2 per ¾″-10 thread (basic major diameter of ¾″, 10 threads per inch), D1=1″, D2= 15/16″, height=0.77″, base diameter of the circular collar 304=1.38″, the number of posts 106=4, for T(th)=40 ft-lbs.
While this document contains many specifics, these should not be construed as limitations on the scope of an invention or of what may be claimed, but rather as descriptions of features specific to particular embodiments of the invention. Certain features that are described in this document in the context of separate embodiments can also be implemented in combination in a single embodiment. Conversely, various features that are described in the context of a single embodiment can also be implemented in multiple embodiments separately or in any suitable subcombination. Moreover, although features may be described above as acting in certain combinations and even initially claimed as such, one or more features from a claimed combination can in some cases be exercised from the combination, and the claimed combination may be directed to a subcombination or a variation of a subcombination.
