Urethane Coefficient of Friction
The coefficient of friction between polyurethane elastomers and non-lubricated surfaces decreases with increasing hardness. Since harder elastomers have the lowest coefficient of friction, these materials perform the best where sliding abrasion resistance is necessary. The softer compounds are ideal where friction is required, such as a urethane drive wheel.
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A coefficient of friction represents the relationship between the force of friction between two objects and the normal force between the objects.
It is shown by f = μFn. In this equation, f is the frictional force, μ is the coefficient of friction, and Fn is the normal force.
μ can be either μs or μk.
μs is the coefficient of static friction — the friction force between two objects when neither of the objects is moving — while μk is the coefficient of kinetic friction — the force between two objects when one object is moving, or are moving against each other.
The coefficient of friction is dimensionless — it does not have any units. It is a scalar, meaning the direction of the force does not change its magnitude. it will also depend on the objects that are causing friction. The value is usually between 0 and 1 but can be greater than 1.
A value of 0 means there is no friction at all between the objects which is only theoretically possible. All objects will have some friction when they touch each other.
A value of 1 means the frictional force is equal to the normal force. A coefficient of friction that is greater than 1 indicates that friction is stronger than the normal force. It can also be changed by the mass and speed of the moving object. As it is the resistance to sliding or rolling of the surfaces of two bodies in contact with each other, it has been found that the softer the material, the higher the coefficient of friction.
The coefficient of friction of polyurethanes is similar to that of rubbers. The softer the material, the higher the coefficient of friction — varying from 0.2 for the harder grades to approximately 3.0 for the softer grades.
This is thought to be due to the higher actual area of contact between the elastomer and the second surface. A hard material under moderate loads will not deform and follow the surface profile of the second material. The coefficient of friction reaches a maximum at approximately 140°F (60°C).
Testing will assist in providing a general guide regarding coefficient of friction. However, in practice, surface cleanliness and lubrication by dust, moisture, and oil traces will greatly affect the actual friction properties.
The frictional force can be expressed as:
Ff = μ N
Ff = frictional force (N, lb)
μ = static (μs) or kinetic (μk) frictional coefficient
N = normal force (N, lb)
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