The present invention relates to a system of binding or assembling by means of a double-threaded bolt. This bolt is coupled to a respective nut which incorporates concentric male and female threads. This nut, which makes installation of the dual threaded bolt easier, acts as a threaded insert once installed, facilitating final operation of said bolt.

It is therefore an improved double-threaded bolt; being significantly easier to install, to ensure that this system can be applied to a large number of mechanical joints.

STATE OF THE ART

A double-threaded bolt, is a bolt where the head has been replaced by a threaded stud having a different thread pitch to the body of the bolt. So, in this bolt's function; screwing in a first direction, the parts of the union are drawn together, whereas if screwing in the opposite direction, the parts are forced apart. The displacement we get per turn is the result of the difference between the two corresponding pitches. It is therefore applicable to any bolted union in which we want either closing and opening, or to maintain a defined distance between the parts.

The use of double-threaded bolts is also particularly appropriate to prevent fatigue cracks in pieces where conventional bolt's heads would place concentrated clamping forces on the clamped element's bolt head clamping face. So in pieces where the reduction in weight or volume, is taken to the limit, their use can be very convenient.

In mechanics, the double-threaded bolts are not well known, especially through their difficult installation, but there are several precedents as the following patents: US3876318, US4728202, US2990218 and WO2008107498; last one from the same inventor of this application.

These bolts can be mounted to join two pieces or even in one piece, giving control of the degree of closure or opening of a slot, acting as a clamp. In the first case, the difficulty of mounting lies in that the final position of the bolt must be appropriate: as the bolt is varying its position while screwed, so the installation will have to be well controlled, to ensure that at the end of the installation the bolt has been placed in the working position, which depends on the bolts' relative position to the two parts of the assembly. This is even more complicated if this union between pieces uses more than one bolt, because to achieve correct clamping in this case each individual bolt has to start threading into both receptacle threads simultaneously and be threaded in simultaneously as the receptacle threads approach one another as not to cause binding of the bolts. The complication in case of application of this system in clamps with a slot is further increased: mounting also requires elastic deformation of the receptacle object to reach the position where the bolt, coming from outside, engages both receptacle threads simultaneously. Then as the screw advances toward its working position, the slot in the receptacle object will recover its original position.

SUMMARY OF THE INVENTION To overcome the drawbacks of mounting described in prior art, it is hereby proposed the complementary use of a nut with two concentric threads, one on the inside of (female thread) and another on the outside of (male thread) the nut. First the nut fulfills the mission of facilitating the installation of the double-threaded bolt, to be then fixed in the corresponding receptacle object's threaded part, acting as an integrated threaded insert. In this way, it never behaves like a conventional nut, rather, like a heli-coil.

These male and female threads can either have the same pitch, corresponding to one of the double-threaded bolt, or two different pitches which correspond with both threads of the bolt.

In the first case, with two threads of equal pitch and corresponding to one of the two in the bolt, using this nut allows to said bolt to be screwed in first without any difficulty to approximately its final position in one of the two receptacle threads of the object or objects to be assembled. Then, this nut must be screwed into the still free end of the bolt and in the still free receptacle thread of the assembly. To perform the simultaneous threading operation with the nut, it is necessary to control the position of the bolt in order to help the nut to find the origin of the bolt's thread. After that, the bolt stays fixed while the nut screws easily to its final working position. Once installation is finalized, we see normal functionality in the double-threaded bolt: when we turn the bolt in one direction it closes the assembly and vice versa, while the nut can be fixed forever, thus forming the female thread of the second receptacle face of the assembly, thus behaving as a threaded insert or heli-coil.

In the case of use of a nut with different thread pitches, which correspond to the two thread pitches on the bolt, first, the nut is screwed on the bolt; such that the nut and bolt now present the same thread pitch on their outer perimeters. Then we screw both pieces together as one male in the consecutive receptacle threads in which they are to be inserted, which will carry both together to their final working position, where we leave the nut fixed, like a threaded insert. From that moment turning the bolt will give normal double-threaded bolt functionality.

BRIEF DESCRIPTION OF THE DRAWINGS ■ FIG.1 shows a double-threaded bolt (1) and its corresponding nut with concentric external and internal threads with same pitch (2).

■ FIG. 2 shows a cross section of the bolt-nut set.

^■ FIG. 3 shows a bicycle crank arm (3) with clamp closure, which makes use of the previous bolt-nut set.

■ FIG. 4 shows a cross section of that crank, practiced at the closure.

■ FIG. 5 shows an alternative setting for a double threaded bolt and corresponding nut assembly having been assembled: the nut has two concentric threads but different pitches from each other, being equal to the two pitches of the bolt.

■ FIG. 6 shows a cross section of the previous assembly.

^■ FIG. 7 shows a set of bolt and nut, corresponding to a second alternative configuration : access to the bolt is placed at the same end to the nut. DESCRIPTION OF A PREFERRED EMBODIMENT

We propose a closing bracket, in this case applied to a bicycle crank arm (3), for anchoring to a spindle. For closure, it makes use of a double threaded bolt (1) and a corresponding nut (2) with concentric threads with same pitches. For them we propose a effective thread pitch of 0.5 mm, equivalent to the difference between pitches of the two threads of the bolt, since that pitch is relatively small to achieve a strong clamping without applying a large torque on the screw; and relatively large for the slot opening and closing the clamp, the screw does not move too far from its position bulging through the sides of the crank. Thus we have the bolt (1) with M7xlmm threads on the stud (12) and M9xO.5mm in the threaded head (11), this head which has a hexagonal footprint for a 5mm Allen key. Since this assembly allows both the closure and the opening of the slot in the clamp, for the latter is proposed that the size of its hole is precisely the working dimension when tightening, rather than having room for multi-axial movement as given in conventional braces.

As for the mounting, the bolt is first introduced into the crank, threading down to what will be its final position, after which the nut will be threaded into the crank arm until it contacts the tip of the bolt, at which point the bolt is turned gently as to allow both the nut's outer and inner threads to engage the crank's and bolt's threads simultaneously, being then threaded effortlessly until the end of the nut's thread.

OTHER POSSIBILITIES

The above relative to clamps is fully applicable to screw joints between two different receptacle threads. In this case the advantage presented, in comparison to conventional screw mounting, is that the double threaded bolt tends to align the pieces that they are pulling, keeping them aligned and parallel, since the threads on both sides of mounting behave as two centering cones. An example of this usage includes setting the corresponding teeth in a jaw implant.

Depending on accessibility in the mounting of described bolt-nut, it may be necessary to manipulate the bolt from the other side, so that in some applications both components are accessed by a single point of the set (Fig.7), in which case we have to use tools that allow simultaneous work on both pieces, positioned concentrically.