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Polyurethane Load Bearing Ability:

Moulded polyurethanes have excellent load bearing ability. When placed in compression urethanes can support higher loads than other elastomers of similar Hardness. The high load capacity allows moulded urethane components to be made smaller resulting in reduced weight and cost of raw materials. The magnitude of compressive force which can be supported by a given polyurethane compound is dependent on three main factors: hardness, loaded surface condition and shape factor.

Hardness:

One of the most common ways to classify polyurethane elastomers is by hardness. Hardness is defined as the relative resistance of a surface to indentation by an indenter of specified dimension under a specified load. The most common way to measure hardness is with the use of a durometer. The A scale durometer is widely used throughout the rubber industry. Hardness can be a useful indicator of how a polyurethane part will perform based on loads and deflections.

Fatigue Considerations:

In reviewing physical properties for urethane, fatigue is an important consideration in the design of parts for cyclic dynamic applications. Fatigue and cut growth resulting from cyclic stress-strain are related. When testing for fatigue resistance for a specific application, it’s important to test at a strain energy experienced by the part in actual service.

Strain energy is a function of both the modulus of the material and the strain cycle the material sees. Also important with polyurethanes is to test at various levels of stoichiometry (polymer to curative ratio) because we have seen dramatic improvements in flex fatigue resistance by this chemical adjustment.

This is another reason why teamwork between the design engineer, the product engineer, and our chemists is so important to problem-solving.

Polyurethane Shear

Elastomers in shear are generally used in mounting and suspension applications. Urethane elastomers deflect more easily in shear than in compression. Deflection of a urethane system in shear is a function of three things: the shear stress, the shear modulus–G–of the urethane compound, and the thickness of the urethane. For bonded urethane applications used in shear, it’s pivotal that certain design guidelines are followed. When under shear loading, the bond line is in tension. By utilizing generous fillets, stress risers are eliminated or greatly reduced at the bonded interface. In every application that would use the high shear strength of urethanes; the weakness is the dependence on a good bond between the urethane and metal.

Polyurethane Abrasion Resistance:

Unquestionably, the urethanes have outstanding abrasion resistance. They outwear metals, plastics, and other rubbers by a wide margin, often by 8 to 1 or more. Abrasion is caused by many actions, such as impingement, erosion, impact, scuffing, and sliding. Add to that the many variables which affect the abrasion rate such as pressure, temperature, velocity, and lubricity. It’s obvious, then, that abrasion is very application specific.

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