You will often hear the following 5 terms introduced when discussing hydraulic seal kits. If you are unsure what they mean or even have no idea, read on to find out.
Abrasion Resistance is measured as a loss percentage based on original weight, and is the resistance of a material to wearing away by contact with a moving abrasive surface (such as a dust covered rod). It's important to note that abrasive rubbing is a progressive phenomenon that develops over time whereas the nicking of a sealing lip is an instantaneous event. Most susceptible to abrasion are seals in motion, which is why hard compounds are often used in the production of seal kits. However hard compounds can increase friction, which then generates seal-degrading heat. That is why it is so important to get the optimum mix between hard and soft compounds in the production of seal kits.
A Compression Set of a seal kit is the steady decline in sealing force that results when an elastomer is compressed over a period of time. Within a compression set is stress relaxation and permanent compression. Stress relaxation is the slow decline, whereas permanent compression means the seal has lost all effectiveness and continuing to use the seal will result in long term damage to the machine.
When a cut or nick is formed in a seal kit, it often grows when tension is applied. Tear resistance, or tear strength is the ability of a compound to resist this growth. It is important to consider tear resistance both in the production stage and as it performs in service. Low tear resistance compounds are most at risk when they are being installed, especially in designs featuring non-smooth and sharp areas and corners.
Hardness is the resistance to to indentation under specific conditions. When judging the performance of seal kits, hardness is one of the most common criteria. Though most standard seals start to fail in the 70 - 95 Shore A range, the application of the seal kit will always govern the necessary hardness. For low pressure seals, softer compounds with less Shore Hardness are fine, but under high pressure conditions the seals will need to have a much higher Shore Hardness strength.
Resilience or Rebound is the ability of the seal kit to regain its original size and shape following temporary deformation. Therefore resilience is closely linked to flexibility. Resilience is the most important factor in dynamic seals because when the seal's lip is flexed (distorted) to follow a shaft imperfection and then returns to its original resting position this causes a lot of additional stress for the seals.