Rigorous Tests for
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a Vibration Damper THE study of shock-absorber design' over a period of years has had as its chief object the elimination of discomfort to the passenger, but the value of this fitment in extending tyre life and reducing the liability of the chassis to damage by vibration fatigue is becoming increasingly recognized. For that reason, many users have devices of this type on practical test.
The Woodhead-Monroe damper, which is now being tried out on passenger and goods vehicles by a number of corporations and other operators, has been manufactured by the Monroe organization in America for some 15 years, and it. has been in use on commercial vehicles throughout this period.
Comprehensive Tests
Before Jonas Woodhead and Sons, Ltd., began to make it at Leeds, tests were carried out. to confirm versatility and durability, as well as to ascertain its 'suitability for the road conditions of Britain.
Bench tests continue to •be made at the shock-absorber experimental department of the Woodhead organization, special rigs having been built to conduct long-life breakdown trials. For this purpose, the upper dirt shield is removed and a water jacket built on to the outside, so that the damper may run at a constant temperature, and the device is then worked through a 4-in, stroke at a speed of 120 r.p.m. for 5,000,000 cycles:
This test occupies approximately 50 days and the 5,000,000 cycles imply 5,000,000 strokes of the piston in each direction,' giving a total of 10,000,000 movements. At the end Of each million cycles the damper is removed from the breakdown rig and a power curve is taken to ensure that no control is being lost. At the end of 5,000,000 cycles the average control loss is under 6 per cent.
The Woodhead-Monroe damper is designed in such a manner that maintenance, in its accepted form as applied to the damper, is not required. As there is no evidence of leakage, no toppingup is required, so that the maintenance crew is saved an unpopular and often neglected duty.
The 2-in, bore fitting, when used in the orthodox arrangement of four dampers to a vehicle, is capable of controlling units weighing up to 25,000 lb. unladen. The valves and working parts are designed so that the :stresses imposed have a high safety margin, and it is stated that there has never been any case reported of stress failure inside the device.
The only items which may need attention from time to time are the rubber grommets, which normally will last until a major overhaul becomes necessary, when, if replacements be required, they can be made at a low cost.
Although of prime interest to the passenger operator, the damper is also of value to the user of goods vehicles, for, apart from the advantages already dealt with, risk of damage to fragile goods can be greatly reduced.
The Woodhead Monroe damper has no secondary movement, the travel of the piston being the same as the movement of the axle. The attachment ring on the top of the dirt shield is fixed to the chassis member, the ring con taming inserts of synthetic rubber, so that when the fixing nuts have been tightened, the mounting is cushioned to a limited degree. The lower ring, which s attached to the base cup, is similarly fitted to the axle.
When the vehicle wheel rises, the base cup, together with the inner cylinder, is forced upwards and the piston rod, which is attached to the chassis frame, moves downwards through the cylinder, which is entirely filled with fluid. To make room for more of the piston rod to enter the cylinder, the fluid has to be forced into a reservoir jacket.
Variable Action
The fluid' travels into the reservoir through the valve which is seen at the base of the cylinder. This contains a metered bleed disc and pre-loaded blow-off discs, by means of which it is possible. to vary the amount of absorber action.
When the piston rises, the fluid returns through this valve and at the same time the fluid which is being dispersed by the upward movement of the piston passes through holes drilled in the piston itself. The flow is checked by a further metered bleed disc and blow-off disc, which is set so as to control the movement of the piston. As the piston rises from the cylinder, any surplus fluid is removed from the rod by means of a dual sealing washer which is located at the head of the' pressure cylinder.