What are the options available to you with friction materials when selecting your caliper disc brake? The main three attributes to consider are the material's wear rate, the coefficient of friction (COF), and its energy capacity. W.C. Branham VP of Engineering and General Manager Chad Randleman explains the COF in this edition of Solutions in Motion.
Coefficient Calculation
The coefficient of friction (𝛍) is a number that is the ratio of the resistive force of friction (Fr) divided by the normal or perpendicular force (N) pushing the objects together. It is represented by the equation: 𝛍= Fr/N.
Basically the coefficient of friction is telling you how two surfaces will interact with each other when one is moving and a second surface is pressed against it. Here is a working example. If a block of ice were to be placed on steel, the ice would slide around freely as there is very little friction between the two materials. Rubber on pavement, on the opposite side of the spectrum, will not slide easily due to the resistance met by the two materials. The ice is an example of a low COF, while the rubber and pavement an example of a high COF.
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Range of Coefficients of Friction
Before diving into numbers, it should be noted that materials react differently with lubrication. Most materials in dry conditions have COF that range between .21 up to .55. There are instances where certain values go outside this range, but it’s pretty rare.
A COF of .21 (on the low end) is suitable for tensioning and constant drag applications, where a low tangential force is required. High coefficient materials are often suitable for static holding applications where you want to maximize the braking force.
The different materials available to use for brake pads include organic, semi-metallic, or sintered metal. All have different energy capacity limits. Organic metal comes in at the lowest, semi-metallic in the middle, and sintered metal in the highest range. You always want to consider the energy dissipation required for your application before purchasing materials. Download our guide or speak with a representative to see what materials are right for your project.
