What Is Breakout Torque?
Breakout torque (also known as break out torque or break-out torque) refers to the amount of torque required to initiate relative rotation between a rotary shaft and a seal or bearing, and plays a key role in systems that involve oscillatory movement. Breakout torque is a function of both the inertia of the mechanisms involved and static friction. In ball bearings, you will often see it referred to as the starting torque, and there it refers specifically to the amount of torque required to start the rotation of one ring of the bearing.
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What Causes Breakout Torque?
Needless to say, static friction between the contacting surfaces is a major element of breakout torque: the greater the friction, the greater the breakout torque. One of the interesting factors when working with a polymer is that their coefficient of dynamic friction is actually greater than their coefficient of static friction because polymers are not as stiff as metals. Note that in angular contact bearings, the starting torque is primarily due to slippage between contact surfaces.
The roughness of the surface finish of the shaft also greatly influences breakout torque. Common sense says that the rougher the shaft surface, the higher the breakout torque. If the shaft has been at rest for an extended period of time, one phenomena that increases the breakout torque is adhesion between contacting surfaces. This is especially obvious when the contacting surfaces are hard and highly lapped, such as silicon carbide running against silicon carbide [1]. Also, corrosion can sometimes cause bonding to occur between surfaces in contact, and this bond must be broken before rotation can start.
Another one of the major factors in breakout torque is the viscosity of the lubricant used. In order to achieve motion, enough torque must be generated to shear through the lubricant to achieve motion. Obviously, the more viscous the lubricant the more breakout torque is required. Also, if the equipment has been out of use for an extended period of time, it is possible for the lubricant to be forced out from between the seal and the shaft, the break-out torque may be much greater.
Estimating Breakout Torque
A key element in estimating breakout torque is the frictional moment, which can be estimated using the equation below. Note that this equation is only calculating the breakout torque based on frictional resistance.
M = 0.5µPd
The variable µ is the coefficient of static friction for the bearing, P is the equivalent dynamic bearing load, and d is the bearing bore diameter. Note that if you are working with radial needle roller bearings, instead of the bearing bore diameter you should use either the inner ring raceway diameter or the diameter under the rollers.
The coefficient of friction in ball bearings will usually be in the range of 0.001 to 0.0015. For open bearings, it varies from 0.001 (self-aligning ball bearings) to 0.005 (cylindrical roller thrust bearings and needle roller thrust bearings).
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