The Sunray, Inc Blog

The Impact of Coefficient of Friction on Polyurethane Drive Wheels

There are always forces at work around us, whether working for us or against us. One of the predominant forces in the world of polyurethane drive wheels is the co-efficient of friction (COF).

In layman’s terms, the co-efficient of friction (COF) is the amount of resistance an object encounters when moving over another. For example, when a polyurethane drive wheel is used to turn a conveyor belt, the wheel must overcome the friction exerted on it by the belt to spin. Most of the wear and tear experienced by the belt and the polyurethane drive wheel will be the result of this friction.

Another example of co-efficient of friction is when a polyurethane wheel rolls over a surface, such as a concrete floor. If the wheel must roll over contaminants, such as debris or pits in the floor, it will require a greater coefficient of friction to keep the wheel moving. Likewise, if the floor is finished and polished, the co-efficient of friction will be less.

Companies that specialize in providing skid- or slip-resistant surfaces will often measure the coefficient of friction (COF) to determine the degree of slip resistance of a surface. The higher the coefficient of friction, the greater the slip resistance, and the harder an object will have to work to pass over it.

The coefficient of friction is always measured as a value between 0 and 1. When measured from a resting position, for example the friction between a conveyor belt and polyurethane drive wheel when the belt is not in motion, it is a measurement of “static coefficient of friction” (SCOF). When measured in motion, for example while the conveyor is running and the wheel is turning, it is a measurement of “dynamic coefficient of friction”. The COF between two objects can only be measured by experimentation. To achieve accurate results for the dynamic COF between two objects, a controlled laboratory setting is required.

Knowing the coefficient of friction of polyurethane is critical to selecting the appropriate drive wheel. Engineers can calculate the amount of drive force needed to propel a polyurethane wheel over a surface by multiplying the COF of the polyurethane by the load weight on the wheel. The higher the coefficient of friction between the wheel and the contact surface, the lighter the maximum load will need to be to maintain the same driving force as a wheel with a lower coefficient of friction.

Generally, the harder a polyurethane wheel is (the greater the durometer), the lower the coefficient of friction tends to be. Wheels with a higher durometer are typically able to withstand a greater load weight. If too heavy of a load is placed on wheels that are too small or too soft for the job, the polyurethane will begin to dig into the surface, resulting in a greater coefficient of friction and requiring additional drive force. Likewise, if too hard of a wheel is utilized with too light of a load weight, the wheel may simply slide over the surface instead of rolling; the result of too little coefficient of friction and a driving force that is too great.

For more information about polyurethane wheels and the impact that coefficient of friction has upon them, please contact Sunray, Inc. at


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