Seals are a critical item in hydraulic equipment. The problems of sealing the reciprocating parts of cylinders used in mobile plant are reviewed with particular reference to telescopic tipping (hoist) cylinders, cylinders for earth moving equipment and steering cylinders. The merits of various seal configurations for these applications are discussed.HYDRAULIC FLUID POWER SYSTEMS are widely used in mobile plant because of their compactness and cost effectiveness. Typical applications include the raising of the bodies in tipping trucks, the operation of the blades and buckets in earth moving machinery and the provision of power steering for a wide variety of equipment. One of the principal drawbacks of hydraulic equipment, however, is leaks, and some reports (1)* have placed the problem of leaking cylinder glands as second only to that of leaking pipe joints and fittings. This article examines some of the problems and solutions in sealing the reciprocating parts in hydraulic cylinders used in mobile plant, and looks specifically at telescopic tipping (hoist) cylinders, excavator cylinders and steering cylinders.