| Hydraulic Seals Selection (continued)
Piston and rod seals need to have different profiles because their performance required is different:
• Rod seals must prevent external leakage and achieving this may reduce their service life but this is accepted as the seal is usually easily replaced.
• Piston seals can have a thicker lubrication film which results in more dynamic leakage but this is internal leakage and rarely affects their performance of the machine.
Hydraulic Seals Selection Criteria
Selecting the most suitable seal for a rod or piston is not easy for the design engineer because some of the many factors that must be taken into account are often overlooked. The selection criteria are generally the requirements which have to be met by a seal.
It must
• achieve the required tightness
• be made from a material with adequate elasticity and dimensional stability to compensate for variations of the cylinder inner diameter.
• be resistant to extrusion, even a large gap high pressure.
• have good compatibility with the media throughout the working temperature range. A low swell is necessary.
• have low friction, high abrasion resistance and be easy to install and replace.
• material should not abrade or wear and allow particles to enter the system.
In addition to these there are other operating conditions which affect this analysis.
Some of these are:
• System pressure - Hydraulic seals must be able to operate at high and still be tight at low pressure.
• Stress imposed by the variations of pressure when working.
• Location of the hydraulic equipment, outdoor or indoor.
• Working temperature range.
• Installation condition.
• Length of stoke and frequency.
• Speed of movement.
• Expected friction.
• Operating and stationary periods.
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