Friction torque, abbreviated as \(\tau_f\), is the resistance to rotation experienced by a rotating object due to frictional forces between its components. When an object rotates, there is often contact between moving parts, such as bearings or gears, which results in friction. This friction opposes the motion and generates a torque that acts to slow down or resist the rotation of the object.
Friction Torque Formula |
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\( \tau_f \;=\; F_n \cdot \mu \cdot r \) (Friction Force) \( F_n \;=\; \dfrac{ \tau_f }{ \mu \cdot r }\) \( \mu \;=\; \dfrac{\tau_f }{ F_n \cdot r }\) \( r \;=\; \dfrac{ \tau_f }{ F_n\cdot \mu }\) |
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| Symbol | English | Metric |
| \( \tau_f \) (Greek symbol tau) = Friction Torque | \(lbf-ft\) | \(N-m\) |
| \( F_n \) = Normal Force Pressing the Surfaces Togeather | \( lbf \) | \( N \) |
| \( \mu \) (Greek symbol mu) = Friction Coefficient | \(dimensionless \) | \(dimensionless \) |
| \( r \) = Effective Radius at which the Friction Force Acts | \(in\) | \(mm\) |
Friction torque can be caused by various factors, including surface roughness, lubrication conditions, and the materials involved. Reducing friction torque is important in many mechanical systems to improve efficiency and performance. This can be achieved through various means, such as using lubricants to reduce friction between surfaces, optimizing surface finishes, or using bearings with low friction coefficients. It's important to note that this formula assumes that the friction force is static and does not vary with velocity. In reality, friction can be more complex, especially in dynamic situations where velocity and other factors come into play. Additionally, this formula assumes a simple situation where the coefficient of friction is constant and does not change with factors such as temperature or surface condition.
Friction Torque Examples
- Rolling element bearing friction torque occurs in ball bearings and roller bearings. Although rolling friction is much lower than sliding friction, contact between rolling elements, races, cages, and lubricant still generates resisting torque.
- Brake friction torque is produced when brake pads press against a rotating disc or when brake shoes press against a drum. The friction force creates a torque that opposes rotation and slows or stops the rotating component.
- Thread friction torque in screws and bolts occurs when a nut moves along a threaded fastener or when a screw is tightened. Friction between mating threads produces a resisting torque that must be overcome during rotation.
- Gear mesh friction torque occurs when gear teeth engage and slide against each other during meshing. Even with lubrication, tooth surface friction produces torque losses and reduces transmission efficiency.
- Wheel axle friction torque occurs when a wheel rotates on an axle or bearing system. Friction within the support mechanism creates a torque that opposes wheel rotation.
- Electric motor friction torque is generated by friction in bearings, seals, and other rotating contacts inside the motor. This friction torque must be overcome before the motor can accelerate the load.
- Machine tool spindle friction torque occurs in spindle bearings and seals. The friction produces resistance to spindle rotation and contributes to heat generation during operation.
- Pivoted mechanism friction torque occurs in devices such as valves, levers, and control linkages. Friction at the pivot points generates torque that resists angular movement.
