What Modern Refinement Owes to Bonded Rubber
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SINCE the bonding of rubber to metal became a practicable proposition, the number of cases in which the process has, with advantage, been applied to automobile construction is extensive. While at the works of Metalastik, Ltd., Evington Valley Road, Leicester, recently, we examined numerous units incorporating bonded rubber, which have been developed by this company.
Readers will remember that we announced and described in our issue dated August 4 the Metalastik Spring Aid, a rubber suspension device, and many will be already familiar with certain of the other products of this maker. However.
briefly to enumerate some specific examples, will, we are sure, prove informative to the majority.
Torsional vibration dampers, in which an elastic medium replaces contacting friction surfaces, enjoy considerable popularity. Amongst other attractions are the reduction of damper weight and the fact that the action of the rubber conforms to a known characteristic, whereas, in the case of the friction type, at a given torque slip occurs, and no torque impulse of greater magnitude can be returned to the shaft.
The results that can be obtained, with the Metalastik dampers, are most satisfactory. Moreover, it is possible to design to within very fine limits of the requirements, as indicated by torsiograph readings. The main variables are the hardness and thickness of the rubber and the moment of inertia of the damper mass.
The Metalastik concern can determine precisely what the actual frequency of the damper will be from its knowledge of the rubber characteristics and the proportions of the inertia member. However, it is the customary practice to experiment by fitting the damper indicated by the theoretically correct design and then to carry out a series of torsiograph tests. From the readings taken it can be deduced whether slight alterations will be necessary in either the rubber quality or the inertia member. A fitment is then obtained which approaches to perfection as near as is practicable. This process may be well described as " tuning." We inspected dampers for both transmission members and crankshafts Somewhat similar are the Metalastik clutch centres, but, in this case, the outer periphery is constructed in a series of dogs formed on the driving and driven members, to serve the purpose of a complete safety factor. These units are remarkably light and are designed to suit standard proprietary clutches.
This company also possesses the patent rights in this country for the" Floating Power" system of engine mounting. This unique system arranges for the engine to be flexibly mounted on its principal axis, i.e., on a line passing from the universal joint through the centre of gravity of the complete engine block. The engine is then free to float on a line of perfect balance, and excessive movement is restricted by the introduction of a flexible torque-reaction mounting.
Bonded-rubber mountings for bodies and cabs are made in a variety of forms; two examples are the flat safety type and the bonded bush. Metalastik units of the latter type are used in the Diatlex cab mounting of the new Commer Superpoise range, to quote one instance. In the case of Metalastik bushes, it should be noted that a feature here, in addition to the fact that the rubber is bonded to the inner and outer metal parts, is that the rubber is normally in a state of very low stress, because it is introduced in a plastic condition. Furthermore, in diametric loading, all the rubber takes a share in withstanding the force to which the bush is subjected—part in compression, part in tension and part in shear.
A variety of couplings and universal joints is available. They do not wear, are noiseless, and, of course, afford a cushioned drive whilst permitting slight misalignment and considerable angular deflections of the shafts. The elastic medium can be arranged to function in pure shear, or alternatively in tension and compression between radial vanes integral with the driving and driven members. Another interesting suspension development is the Metalastik torsion spring. Here, to some extent, the unit resembles a coupling, one side being fixed and the other attached to a torque arm. Between the radially disposed faces of the metal portions there is, however, a considerably large volume of rubber to permit the requisite degree of angular deflection. Turning now to the actual operation of bonding, this is effected briefly, by first plating the metal parts, by dectro deposition, with an alloy of secret specification. It is claimed that one of its properties is that the rubber, when treated in a vulcanizing press, " fuses homogeneously" with it. Destruction tests for tension and shear invariably result in the rubber_tearing, the actual joint remaining intact. A most important point that should be mentioned in conclusion is that the Metalastik company is not limited purely to the bonding of rubber to steel. It can bond with equal success to ferrous and non-ferrous metals, and the light alloys. In cases where excessive heat, oil and oily vapours cannot he avoided, synthetic rubber is used.