Resolution of the main problems
Diesel effect
Piston seals with acetal resin guide rings used on earth-moving machines are sometimes subjected to water hammer or when the machine is working under very heavy conditions, the plastic part melts and carbon particles are deposited on the guide surface.
Seals without guides, on the other hand, show evidence of exposure to very high temperatures with scorching and charring of the sealing area in most direct contact with the fluid.
Research carried out in collaboration with cylinder manufacturers and major manufacturers of hydraulic components has made it possible to explain this phenomenon.
Oil, in a hydraulic system, contains air. This air can be released by vibrations and form pockets that naturally also contain oil vapour.
If this air-oil mixture reaches a ratio of 7.5 to 1 by weight and a temperature of 200-250 °C (which can be achieved with a pressure of 100 bar), it can ignite spontaneously, as is the case in the Diesel cycle engine.
The angle at which the cylinder operates brings the air pocket in close proximity to the seal and in the event of mixture combustion, the flame produces localised burns on the guides or on the part of the seal closest to the pocket.
Normally, combustion occurs in an area of about 15 per cent of the total circumference, and this area is often opposite the area of maximum friction where the weight of the piston rests. This indicates that combustion takes place in a small air pocket.
The air-oil mixture cannot escape except through the gasket.
Therefore, the higher the seal quality, the greater the possibility of explosion.
The explosion of oil in hydraulic cylinders has been identified as the 'Diesel Effect' in the search for causes of excessive leakage on earth-moving machines operating under very severe conditions.
Since it is impossible to produce sealing systems capable of resisting fluid explosion, the best protection is to eliminate air by providing suitable valves on hydraulic circuits. Even where it is impossible to completely eliminate the presence of air, a reduction can still be achieved with a suitable tank and circuit design.
An alternative that does not eliminate the phenomenon but reduces its consequences is to protect the seal by fitting a square-section phosphor bronze ring upstream and downstream of the guide rings. This does not prevent the occurrence of the Diesel effect but reduces the action of the flame on the seal and extends the life of the sealing system.