5. FIXED HANDLES
Various kinds of couplings are used for securing fixed handles to the shaft:
- handles with brass boss or nutscrew moulded into the plastic material for screwed assembly on a threaded shaft;
- handles with built-in self-locking boss made of special technopolymer (original ELESA design) for push-fi t assembly on a plain shaft (unthreaded) made from a normal drawn rod (ISO tolerance h9). This solution prevents spontaneous unscrewing in time due to the vibrations to which the lever is subjected or the rotary forces exerted inadvertently by the operator’s hand while handling the lever itself;
- handles with threaded hole obtained from moulded plastic material.
For executions with threaded holes obtained from moulded plastic material, the measure of keeping the thread undersized with respect to the specifications laid down in the standards has been taken.
This enables the threads of the nut screw to adapt slightly to the screw, when tightening at ambient temperature, thus creating a coupling with an elastic reaction that gives an effective locking effect.
Even better results may be obtained by hot assembly: the handle is heated to 80÷90°C before being screwed onto the threaded pin. This method of assembly initially facilitates the rewing operation in that the thread of the nut screw is expanded when screwed in and subsequently enables an extremely efficient locking effect to be obtained from shrinkage on cooling, due to the slight roughness of the surface of the thread on the shaft.
The solution with a self-locking bushing made of special technopolymer (Fig.1) is, in any case the most effective against spontaneous unscrewing in that the elastic coupling is not susceptible to any vibrations or rotary forces exerted by the operator’s hand.
The lock is also such as to ensure that the handle does not come out even when subjected to a normal pulling action along its axis. In relation to this, the results of the research work and tests carried out at the ELESA laboratories are provided and they confirm the technical validity of the coupling with self-locking bushings made of special technopolymer (Fig.2 and 3).
The graph in Fig.2 shows the variations in axial translation effort expressed in [N] as a function of the variations in diameter of the shaft (mm), dry and degreased with trichloroethylene. The two curves represent the minimum and maximum values in hundreds of tests conducted on a type of self-locking handle with a hole having a O 12 mm. The area A contains the values that refer to shaft with a commercial diameter of 12 mm (tol. h9).
The graph in Fig.3 shows the variations in axial translation effort (mean values) as a function of the surface area of the shaft. As may well be imagined, the presence of lubricating or emulsifying oil on the surface of the shaft lowers the handle removal effort. It may however be readily noted that, even in this unfavourable condition, the axial effort required to slide the handle out is always such as to ensure that this cannot actually happen in practice.
The use of this kind of handle ensures a considerable saving in that it does not entail machining thread on the end of the shaft. The self-locking bushing made of special technopolymer enables an elastic coupling to be obtained and the handle itself maintains all its surface hardness and wear resistance typical of thermosetting materials.
Assembly instructions: fit the handle onto slight chamfered shaft end and push as far as possible by hand or by means of a small press. Alternatively it is possible to tap the handle with a plastic or wooden mallet until firmly in place. In this case we strongly recommend to use a cloth or other suitable soft material over the product to avoid any surface damage.