Piston seals with acetyl resin guides used on earth moving machinery, which in the presence of shock hammering or over heating, at times, can suffer when plastic parts melt and carbon particles deposit on the surface of the guide. Seals not equipped with guides have traces of burns and charring to the area due to the exposure of high temperatures and therefore resulting in more direct contact with fluids. Research conducted in collaboration with cylinder manufacturers and leading producers of hydraulic components have helped to explain this phenomenon.
The oil in a hydraulic system contains air. The air can release as a result of vibration and naturally form air pockets and vapors of oil. If this air/oil mixture reaches the ratio of 7.5 to 1 by weight and a temperature of 200 – 250 °C, (which can also be achieved with a pressure of 100 bar), can ignite spontaneously as in the diesel engine cycle. The angle with which the cylinder operates moves the air pocket in the vicinity of the seal and in the case of the mixture combusting; it produces localized burns on the part of the guide or on the seal closest to the bag. Usually the burning occurs in an area of about 15% opposed to the maximum friction of the weight burden of the piston. This indicates that combustion occurs in a small bag of air. The air/oil mixture can not escape via the seal, therefore the higher the quality seal, the greater the possibility of ignition.
The ignition of oil in the hydraulic cylinder has been identified as “Diesel Effect” in the search for the causes of excessive losses on machinery operating in very severe conditions. Being unable to produce sealing systems capable of resisting fluid ignition, the best protection is to remove the air on provision that adequate valves are present in the hydraulic circuit. Even if it is impossible to completely eliminate the presence of air you can still get a reduction provided a suitable tank and circuit design is utilized. An alternative that does not eliminate the problem but reduces the impact, is to protect the seal mounting ring in a phosphor bronze square section utilized upstream and down of the guide rings. This does not prevent the occurrence of “Diesel effect” but reduces the activity of the flame on the seal and prolongs the life of the sealing system.