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Cheap Dampers for Small Oil Engines Needed

15th November 1957
Page 64
Page 64, 15th November 1957 — Cheap Dampers for Small Oil Engines Needed
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QCOPE exists for the development of cheap and effective shock-absorbing dampers to be fitted in conjunction with the mountings of oil engines with a small number of cylinders, so that shock loads and resonance are adequately damped, and small vibrational movements are insulated to the highest degree without damping.

This was one of the points which emerged from a paper, " The Suspension of Internal-Combustion Engines in Vehicles," read by Mr. M. Horovitz, executive engineer (Sales), Metalastik, Ltd., to the Institution of Mechanical Engineers in London on Tuesday.

A recent development of an improved form of rubber and steel spring had, apart from engine mountings, also found application as a vehicle suspension unit, he said. [This appears to consist of a thick-section convoluted rubber sleeve with a coil spring moulded in near its internal diameter.] The helical steel spring stabilized the rubber compression sleeve, so that large deflections could be permitted. Buffers for rebound and excessive sideways movement were incorporated.

Hydraulic Dampers Help

On a number of installations since the 1939-45 war, hydraulic dampers had been fitted to engine suspension systems in order to limit movement caused by sudden torque-reaction forces and shock loads, and to damp engine movement when passing through resonant speeds. Dampers of this kind were beneficial in some installations, despite their effect on vibration transmission at speeds well above resonance.

Rubber should be introduced in the damper fitting to prevent the transmission of noise and vibration through metalto-metal contact between engine and chassis.

Apart from vibration insulation, the design of an engine suspension system had to take into account several factors. These included control of the engine longitudinally where accelerating and braking forces, as well as clutch-control forces might cause excessive movement c28 Moreover, engine movement had to be regulated under road shocks—shock loads of up to three times static had been observed on vehicles traversing rough ground—whilst torque-reaction forces in low gear had not to cause excessive movement. Ease of fitting and removal for engine overhaul had to be catered for, whilst to reduce costs mounting points ought to be few and be close to main chassis members.

Clutch judder might be accentuated by the engine-mounting arrangement, as a sudden torque impulse would tend to move the engine about the axes of vibration. This could often be overcome by flexible clutch centres, which reduced the shock-torque load, but a change in engine suspension might also be necessary. The external force applied by mechanical clutch operation should not cause clutch judder if a correct linkage were used, together with allowance for this force in the mounting ,design.

Crankshaft torsional vibrations induced torque-reaction vibrations in the power unit. In view of their high frequency, even a small amount transmitted through the flexible mountings might cause vibration " periods " in the vehicle by magnification, because of resonance of a part of the vehicle structure. The correct method of eliminating trouble from this source lay in fitting a crankshaft torsional vibration damper.

Pronounced vibration, at a fairly low speed in top gear, in cars had been a major source of worry in recent years. It was popularly referred to as " boom " and was found almost exclusively on cars with four-cylindered engines and chassisless construction.

Although there were other contributory factors. Mr. Horovitz thought that the final deciding factor was still a resonance of part of the vehicle structure with the secondary out-of-balance force transmitted from the engine.

Harmonic torque components could pass right through the transmission system and even through the axle and springs back to the chassis. This did not normally cause any difficulty, but resonance of part of this system could also create vibrations and " boom."

For example, the amount of rubber used in spring bushes had been found to make a difference in the noise in a vehicle, whilst in another case the addition of small weights to the springs had reduced the level of vibration.

The propeller shaft and other transmission elements could also transmit other vibrations if a metal-to-metal path were provided. By inserting rubber in couplings and/or mountings for propellershaft centre bearings, differentials and so on, it was not usually difficult to eliminate this source of vibration trouble.


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