Reciprocating Seals for Hydraulic Cylinders

The most common fluids used in hydraulic cylinder 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 specialised seals, but the vast majority of applications use hydraulic mineral oil. Two main factors affecting mineral oil based systems are cost and leakage.
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.

Pneumatic Cylinder, Pneumatic Valve, Air Cylinder, Air Treatment Units, Pneumatic Accessories, Hydraulic Components, Pneumatic Cylinder Manufacturer, Outdoor SPA, Steam Shower Room, Electric Chain Hoist.

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 pneumatic 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.
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.