Telescopic cylinders are designed to work in a way which results in a very large wiped off surface. The amount of leakage is, as mentioned before, proportional to the size of the surface wiped off (the circumference of the rod multiplied by the stroke length). This means that a necessary lubrication film can result in a leakage if the surface is large enough. The demands on both seals and installation therefore increase in a telescopic cylinder.
The space in most telescopic cylinders is very limited, especially for the radial section dimension. Seal types with normal or standardized cross section dimensions can therefore seldom be used. Telescopic cylinders also work under very tough operating conditions. Due to the heavy transverse loads the fitting clearances are added for every step and the total deflection at fully expelled cylinder can be very large. The telescopic cylinders’ way of working also often results in rapid and strong pressure changes. Even vacuum may occur in double-acting telescopic cylinders due to the difficulties of reaching a sufficient oil supply. Telescopic cylinders are often used in vehicles. Due to the location close to the roadway, these cylinders are continually exposed to contamination particles.
The tough operating conditions of telescopic cylinders often result in temporary faulty eccentric running in the wiper and seal housing grooves at maximum load. This allows water to pass the wiper seals and cause corrosion in the grooves, which can disturb the sealing function. Type SIL is designed for these conditions (fig 1).
In order to achieve satisfactory results, high demands are also raised for effective wiper seals, guides with small fitting clearances, good surface roughness value in the groove and a minimum of space for the seal to move axially.
Type of seal, wiper and guide is preferably chosen before the telescopic cylinder is designed in detail. It is extremely difficult or impossible to find an optimal sealing solution when the design of the cylinder is already completed or even worse, if the cylinder is already in use and the problems start to occur.