Coefficient of Friction (CoF) is found everywhere, from the bottom of your shoes to the wheels on your car! This unseen yet critical property can determine the type of movement between two contacting surfaces. In this post, we will discuss the importance of Coefficient of Fricition when designing polyurethane products, how CoF is measured, and things to consider in your design.
What is Coefficient of Friction
Coefficient of Friction (CoF) is the resistance to motion between two contacting surfaces. Depending on the function of your product or component you may require low CoF or high CoF. For example, low CoF is often preferred for products that need to display more slip or better part lubricity, while high CoF parts will provide more grip between components and contact surfaces.
How is Coefficient of Friction Measured?
There are two ways to measure friction by static and kinetic forces. Static CoF is calculated by the amount of force to create motion, while kinetic CoF is calculated by the amount of force needed to maintain motion. For accurate results, an ASTM D1894 standardized test is commonly used to measure the amount of force needed to pull one material across another surface. However, a quick and easy way to test CoF is to place two objects together on a tilted surface, before they begin to slip. The material that portrays more slip, will demonstrate a lower CoF compared to the other. This process can be seen demonstrated in the video in this post with our Durethane® F foam and Durethane® G graphite technologies. Both thermoset materials tested were engineered to offer unique physical properties, including CoF.
Other Materials Compared to Coefficient of Friction
Like most things, Coefficient of Friction can differ based on the type of materials used, contact surface, and environmental conditions, such as temperature and moisture. For example, low CoF parts are best achieved with harder materials, while high CoF parts are generally softer in hardness. Polyurethanes typically resemble the CoF of most plastics and rubbers, and tend to display higher CoF when compared to metals. Most metals typically have lower CoF as a result of their hardness an chemical compositions. There are however, many additives that can be used in polyurethane chemistry to increase or decrease CoF as required. If you are curious in learning more about thermoset polyurethanes compared to plastics, rubbers, and metals, check out the following links: Polyurethanes vs Plastics, Polyurethanes vs Metals, Polyurethanes vs Rubber.
How to Use Coefficient of Friction in Your Design
Coefficient of Friction is a key physical property for products that require a specific amount of friction between contact surfaces. Although plastics, rubbers, and metals offer different CoF properties, polyurethanes can be engineered to offer controlled friction in various environments. The CoF required in a product will be driven by its function in operation. For example, most recreational skate wheels rely on friction for acceleration and control. The higher the friction, the less likely skaters are to lose control, whereas, lower friction can potentially lead to the wheels giving way. CoF can be utilized in a variety of applications, including, paper media handling, medical devices, and military & defense to name a few.
Conclusion
CoF can play a key role in your product's performance that requires any type of movement or force. When deciding the appropriate material for your product design, it is important to consider environmental conditions, hardness, and contact surfaces for better wear life. For material assistance, download our Durethane® data sheets here for key information.
