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How About Reciprocating Seals For Hydraulic Cylinders
The most common fluids used in hydraulic systems are mineral oil based. In recent years, there has been a lot of attention paid to environmentally acceptable hydraulic fluids, but their use, so far, is limited, mainly because of their relatively high-cost compared with mineral oil. In many cases, standard reciprocating seals can be used in these fluids. Fire resistant fluids, such as high water based fluids, have applications in particular areas such as long-wall mining and have led to the evolution of specialized seals, but the vast majority of applications use hydraulic mineral oil. Two main factors affecting mineral oil based systems are cost and leakage.
This article looks at two key subjects:
• The effect of tube finish on seal performance – the lowest cost tube may not give the desired performance
• The influence of the wiper on the leakage from the gland
The Effect Of Tube Finish on Seal Performance
A piston seal for a hydraulic cylinder is only part of the sealing system. Its life will depend on a number of factors, a major one being the nature of the surface against which it is running. Steel hydraulic cylinder tubes generally have a surface finish which is produced in one of three ways: -
• honing
• roller-burnishing
• drawing over a mandrel (DOM)
Hallite Seals have carried out major studies on the performance of double acting seals, used in both a double acting and single acting manner examining the affect of the method of tube manufacture and, hence, tube surface texture, on the performance of a typical double acting piston seal.
Cylinder Tube Manufacture
Hydraulic cylinder tubes are generally made from steel which is drawn over a mandrel (DOM). This process work hardens the material and gives it desirable mechanical properties. The final tube surface is obtained in one of three ways. In the honing process the tube is machined to gain the required roundness and bore tolerance and to remove surface imperfections, then finished by internally grinding or honing. In skived and roller-burnished tube the tube is again internally machined (skived) and flattened by rollers on the machining head. More recently drawing techniques have been developed such that the tube is considered ready to use in the as drawn state. This is known as special smooth inside diameter (SSID) or 'smooth bore' tube.
In terms of finished tube cost the ranking starting with the cheapest is SSID DOM, roller burnished and honed. SSID DOM tube can be 30% cheaper than roller-burnished, so its attractions to the cylinder builder are obvious. However, the quality of such tubes can be an issue which could lead to premature seal failure.
The Limitations of As-Drawn (SSID) Tube
SSID tube has been found to have limitations in application, particularly where the double acting piston seal is being driven into pressure. This can happen, for instance, when the lowering of a load is being controlled by the release of fluid from the full bore. The process of manufacture of as-drawn tube results in axial scoring of the tube; in roller-burnished and honed tube, such defects are eliminated by the finishing of the tube in a circumferential direction. These axial scores can provide sites for the fluid to jet over the seal eroding the contact face, particularly when the seal is being driven into pressure. A further difficulty is that such defects are not detected by the usual methods of surface finish measurement.
The Influence of the Wiper on Gland Leakage
The gland sealing system of a hydraulic cylinder consists of a number of elements, including the seal housings, the rod seal, the rod wiper, the rod bearing, the hydraulic fluid and the rod itself. Rods are generally hard chrome plated and relatively consistent in finish. The rod wiper has the primary function of preventing external contamination reaching the rod seal and gland bearings, and for this reason is fitted to the vast majority of hydraulic cylinders. However, this component has often been omitted in the laboratory testing of reciprocating seals.
Extensive studies have been carried out on the factors affecting reciprocating seal performance including the counter-face surface finish and the effects of contamination.
Hallite Seals carried out a series of tests where the leakage performance of five different wipers working with a common polyurethane rod seal was assessed. It is shown that he wiper is an important factor in the control of leakage from the hydraulic cylinder gland. Single lip wipers can give a much greater leakage than the seal, on its own, without a wiper. This is the result of the wiping lip collecting the oil on the rod, which provides the boundary lubrication to the seal. The use of double lip wipers reduces the leakage to the level of that of a seal on its own, but there is a danger of pressure trapping between the seal and the wiper. A double lip wiper profile has been developed, which minimizes the ring of oil on the rod collected by the wiping lip, and also has a venting feature to release the oil in the case of a pressure build up between the seal and wiper.