Seal material needs to be capable of resisting the contact media, including fluids and chemical vapors. Material should be determined by taking any potential contact medium into consideration, not just the oil or the hydraulic fluid in which the seal is expected to operate. Please click to see elastomeric materials used in seals and their resistance to various chemicals.
Rotary seals under pressure
Rotary shaft seals are not expected to resist high pressure. A regular oil seal at 1000 rpm can hold up to 0.5 bar. Capability to seal in pressure decreases with increasing shaft diameter and rotational speed. Design alterations like shortened sealing lip, extended metal insert or plastic back-up rings may positively contribute to the pressure holding capacity of the seal. It is very important to determine the existing pressure in the environment prior to operation. The more pressurized the medium the higher the force applied by the sealing lip on the contact surface, resulting in increased friction and heat generation. Consequently higher levels of wear and temperature would shorten the useful life of the seal.
Rotary seals at high speed
Increasing shaft diameter and rotational speed demand higher mechanical properties from the seal. Put simply, the higher the circumferential speed of the shaft the higher the heat generated as result of friction. High speed applications therefore require seal material with good heat and wear resistance. Please click to see performances of different elastomers as a function of shaft diameter and speed.
Hydrodynamic pressure for sealing purposes can be described as the additional pressure component that develops as a result of incapability of quickly discharging the hydraulic fluid that flows in the gap between the guide ring and the rod seal during the backward movement of the piston. As a result pressure would build up with every forward charge, increasing gradually along the rod axis towards the rod seal. Hydraulic pressure can reach harmful levels, destroying the seal or even damaging the cylinder parts. Hydraulic pressure increases with radial speed, gap length and viscosity of the hydraulic fluid. It decreases with gap width. Spiral grooves are fabricated on the bore to allow reverse-flow or guide rings with return slots are used to overcome the effects of hydraulic pressure. This simple formula can be used to calculate the hydraulic pressure.