Seal Selection Based on Hardness -- A Brief Discussion on NBR and Filled PTFE Seals

NBR and PTFE are the most commonly used sealing materials for DMS Seals. This article discusses seal selection from the perspective of material hardness.

1. NBR is an elastic rubber, measured in Shore A hardness. For general hydraulic applications, the typical hardness range is 60~90 Shore A. 70A is the standard general-purpose grade. It offers good elasticity and compression recovery, and can adapt to minor irregularities on sealing surfaces. 60A is softer with excellent conformability, yet it is prone to extrusion and nibbling damage in high-pressure conditions. High-hardness NBR of 85~90A delivers better extrusion resistance and suits high-pressure static sealing. Its downside is reduced elasticity, which demands higher machining precision for grooves and piston rods; minor surface defects can easily lead to leakage.

NBR seals rely on the elastic preload of rubber. Their main failure modes include permanent compression set, thermal-oxidative aging, fluid swelling and fatigue cracking. After long-term compression, elasticity cannot recover; oil temperature and chemical media attack break the rubber cross‑linking structure. The rubber turns hard, brittle and cracks, causing a steady drop in sealing contact pressure and eventual leakage.

Seal hardness directly affects conformability, extrusion resistance and failure mode. NBR nitrile rubber and filled PTFE are two widely used materials in hydraulics, with completely different hardness scales and sealing mechanisms.

2. Filled PTFE is not an elastomer. It is generally measured in Shore D, and is much harder than rubber. It has nearly no inherent resilience and relies on springs and back-up rings to provide preload for sealing. Virgin PTFE is susceptible to cold flow, so fillers such as glass fiber and carbon powder are commonly added to improve dimensional stability.

Its typical failure modes are creep / cold flow, wear and plastic flow. Under sustained pressure, the material slowly undergoes plastic slippage and the sealing surface gradually degrades. When surface wear occurs under high-speed reciprocation, there is no elastic compensation like rubber. Once clearance increases, leakage rises rapidly. Its advantages include an ultra-low friction coefficient and broad chemical compatibility, making it suitable for high-speed, corrosive and wide-temperature-range applications.

Fillers such as carbon fiber, bronze powder and glass fiber are added into PTFE to improve creep resistance, extrusion resistance and impact performance. Different fillers are selected for different working conditions.

3. Selection should be based on operating conditions: For general medium-to-low pressure hydraulics, low reciprocating speed and oil-based media, 70A NBR is preferred for its low cost and good tolerance. For applications requiring high pressure, high-speed sliding or chemical resistance, filled PTFE seals shall be adopted, with dedicated grooves mandatory.

Hardness is not better when higher. Over-hard rubber loses elasticity, and insufficient preload on PTFE will result in premature failure. During design, pressure, clearance, sliding speed, media and temperature should all be evaluated to match the material and hardness, balancing service life and cost.