A clutch is a mechanical device that engages and disengages power transmission between a driving shaft and a driven shaft. It works by pressing friction plates together using a clamping force. When engaged, friction between the surfaces transfers torque. When disengaged, the plates separate, interrupting power flow. Clutches are used in vehicles, industrial machinery, and power tools.

Clutch torque capacity is calculated using: T = μ × F × r_mean × n, where μ is the coefficient of friction, F is the axial clamping force (N), r_mean is the mean friction radius (m), and n is the number of friction surfaces. For a single-plate clutch, n = 2 (both sides of the friction disc). For multi-plate clutches, n equals the total number of contact surfaces.

Typical friction coefficients vary by clutch type and material. Dry clutch with organic friction material: 0.25–0.45. Dry clutch with sintered metal: 0.35–0.55. Wet clutch (oil-immersed): 0.06–0.12. Ceramic clutch: 0.40–0.60. Paper-based wet clutch: 0.08–0.13. Selecting the correct μ is critical — it directly multiplies into the torque calculation.

The mean friction radius (r_mean) is the effective average radius at which friction force acts on the clutch plate. For uniform wear: r_mean = (r_outer + r_inner) / 2. For uniform pressure: r_mean = (2/3) × (r_outer³ − r_inner³) / (r_outer² − r_inner²). The uniform wear formula is the conservative standard used in most engineering design specifications.

A service factor (Ks) is a safety multiplier applied to the required torque to account for real-world operating conditions such as shock loads, vibration, start-stop cycles, and environmental factors. A service factor of 1.0 means ideal conditions. Automotive clutches typically use 1.2–1.5, industrial applications 1.5–3.0, and heavy shock-load applications up to 3.0 or higher. The design torque = required torque × service factor.

A single-plate clutch has one friction disc and two contact surfaces. It is simpler, cheaper, and easier to maintain, but provides lower torque capacity for a given diameter. A multi-plate clutch has multiple friction and drive plates stacked alternately, multiplying the number of friction surfaces and therefore the torque capacity without increasing the clutch diameter. Multi-plate clutches are used in motorcycles, performance vehicles, and compact industrial drives where high torque density is needed.

Power transmitted by a clutch is: P = T × ω = T × (2π × N / 60), where T is torque in Nm, ω is angular velocity in radians per second, and N is rotational speed in RPM. This calculator computes transmitted power automatically once you input torque capacity and shaft speed.

The uniform pressure assumption assumes constant contact pressure across the friction face, giving a slightly higher r_mean and therefore higher torque estimates. It is more accurate for new, unworn clutches. The uniform wear assumption assumes pressure varies inversely with radius, giving r_mean = (r_o + r_i)/2. This is the conservative design standard recommended for engineering calculations as it reflects the steady-state condition of a clutch in service.