Coefficient Of Friction (COF)

curling-cof Coefficient of friction is defined as the ratio of the force of friction to the force pushing two bodies together and is a measurement often used to determine the nonslip rating of a floor surface. It’s commonly called COF, and calculated not by weight but by force. If you have 0.52 COF, it means that for a 50 pound object sitting on the floor, you’ll need 26 pounds of force to start sliding it across the floor. Once it starts sliding, a different and less commonly used COF figure (dynamic COF) is used and the amount of force needed to keep it sliding goes down.

The most common form of COF measurement is surprisingly simple. As implied above, COF meters or “pull meters” like you might see in the field use a 50 pound weight. This is placed on top of a standard Neolite heel. The heel is put on the surface to be tested, and then pulled across it using a scale. Generally, the total force needed to get the heel-weight system moving is recorded and divided by the 50 pound weight. This produces the COF for that surface. There are more sophisticated ways of doing it, but they are outside the scope of this article.

Being that a major reason to measure COF is compliance, it’s notable that most of the laws and regulations recommend particular COF values rather than requiring them (OSHA Article #1, OSHA Article #2). It is possible to have too high a COF. Nonslip and antislip floors are all fine and good, but if the COF gets too high the surface becomes actively difficult to walk on.

A more technical explanation is to define COF between two surfaces — such as between the floor and a rubber heel, or between the floor and a wooden board. You cannot define COF just for the floor alone: the amount of friction between the floor and a rubber heel is different than the amount of friction between the floor and a wooden board. Therefore, most tests use a standard shoe heel.

There are two forms of COF, static and dynamic COF.

Static COF is the COF when the two surfaces are stationary with respect to each other. An example of static COF is the amount of force it takes to slip while standing on a floor. Let’s say you have a flat-bottomed cabinet standing on a nonslip floor in the middle of a room. It takes a certain amount of force to start it moving before you can push it into the corner. If you divide that force by the weight of the cabinet, you get the static coefficient of friction between the cabinet and the floor.

Dynamic COF is the amount of friction when two surfaces are moving with respect to each other. The amount of force needed to keep dragging a sled across the snow is an example of dynamic COF. Note that as soon as the sled stops moving, the force needed to get it going again is determined by static COF.

If you have slippery floor problems, schedule us to inspect your environment and we’ll provide you a written safety report including COF measurements with recommendations.