Friction Clutch: Torque Capacity and Mean Radius | FIRGELLI

Clamping the driving and driven surfaces

The sectional illustration shows alternating plates connected to an outer input drum and an inner output hub. An axial application force presses their friction surfaces together. With two active interfaces, one output disc works between two input plates. Springs can supply the clamp force, while mechanical, hydraulic, pneumatic and electromagnetic arrangements can control application or release.

Relative motion during engagement generates heat. After the parts synchronize, friction can transmit torque without relative sliding, up to the applicable static threshold. Whether synchronization occurs, how long it takes and how much heat is produced depend on the two sides' speeds, inertias, external torques and actuation history.

The calculator treats each active interface as having the same normal load, coefficient and effective radius. It does not calculate pressure distribution, release travel, plate distortion or losses elsewhere in the assembly.

The animation changes the plate count to show the selected number of active interfaces. Input plates connect to the outer drum and output plates to the inner hub. An annular thrust bearing transfers the axial application load without being counted as a driving friction interface. The plates open with exaggerated equal gaps and close into contact. The displayed torque threshold always uses the entered full clamp force; the release sequence does not calculate engagement time or slip speed. Being within the estimate means only that entered demand does not exceed the calculated static threshold.

Controlled connection of rotating machinery

Friction clutches are used to connect rotating drives in industrial machinery, vehicles and handling equipment. Designs can be dry or lubricated and may use one plate or a stack of plates. The friction combination, actuation and cooling must suit the torque and engagement duty.

Some friction devices are specifically designed as slipping torque limiters. That does not make every engagement clutch a suitable continuous-slip device. The selected product's permissible slip energy, power and control behavior determine how it can be used.

Torque threshold and effective radius

The model uses T = μF_nR_mz. F_n is normal load at each active interface, R_m is the effective friction radius and z is the number of active interfaces. Enter radius in millimeters; the calculator divides it by 1,000 before computing torque in N·m. A single disc working on both sides has two active interfaces, not one.

The displayed Friction Force is μF_nz, the sum of tangential friction-force magnitudes over the interfaces. It is not a net lateral force on the shaft. Capacity / Demand is T/T_d, and Torque Difference is T − T_d. A positive difference alone does not verify pressure, thermal, fatigue or operating margins.

For an annulus with inner radius r_i and outer radius r_o, the uniform-wear model gives R_m = (r_o + r_i)/2. Uniform pressure instead gives R_m = (2/3)(r_o³ − r_i³)/(r_o² − r_i²). The tool takes the effective radius directly; it does not select either distribution or calculate it from diameters.

Static and sliding coefficients may differ. Use the coefficient consistent with the torque being estimated. No temperature, speed or coefficient change is predicted, and the calculation does not establish a recommended capacity ratio.

Example: two active interfaces

At the defaults F_n = 5,000 N, μ = 0.35, R_m = 100 mm and z = 2, the estimated threshold is 350 N·m. The summed friction-force magnitude is 3,500 N. Compared with 300 N·m demand, the ratio is 1.17 and the difference is 50 N·m.

If demand rises to 400 N·m, the same threshold gives a ratio of 0.875 and a difference of −50 N·m. The model cannot support that torque as static friction, but it does not calculate the subsequent slip rate.

For a 240 mm outer diameter and 160 mm inner diameter annulus, the radii are 120 and 80 mm. Uniform wear gives a 100 mm effective radius. Uniform pressure gives approximately 101.33 mm. At 5,500 N, μ = 0.35 and two interfaces, these assumptions yield 385 and about 390.13 N·m respectively. Those are alternative pressure-distribution assumptions, not verified product ratings.

The lowest capacity setting, 1,000 N, μ = 0.15, radius 40 mm and one interface, gives 6 N·m. The highest, 15,000 N, μ = 0.45, radius 140 mm and four interfaces, gives 3,780 N·m. No plate-pressure or thermal check accompanies those independently entered extremes.

Torque transmission and heat duty

A friction clutch can transmit torque during relative motion and then synchronize. A positive dog clutch relies on tooth engagement and needs suitable engagement conditions. A fluid coupling transmits torque through fluid motion and has its own slip and cooling characteristics. These are different design approaches, not interchangeable torque ratings.

Slip power is torque multiplied by relative angular speed: P_slip = T_slip |ω_in − ω_out|. Energy is the integral of that power over time. Neither speed nor time is entered here, so even a large capacity-to-demand ratio cannot demonstrate adequate heat capacity or an acceptable engagement.

Use actual clutch data for pressure, speed, thermal duty, material compatibility and service requirements. A generic temperature threshold, lining thickness or spring replacement interval is not supplied by this model.

Frequently asked questions

What counts as a friction face?

Count active interfaces transmitting torque. A single disc with both sides clamped has two. Multiple plates may provide more interfaces depending on how they connect to the input and output.

Is clamp force divided among the faces?

In the ideal axially loaded stack modeled here, the same axial normal load acts at each interface. Entering a total summed across interfaces and multiplying by the face count again would overcount it.

Is Mean Radius the outer plate radius?

No. It is an effective friction radius based on the contact distribution. Uniform wear and uniform pressure give different values for an annular face.

Does Capacity / Demand certify a safe design?

No. It is a torque ratio. Pressure, heat, speed, fatigue, transient loads and other design requirements are not assessed.

Does being within the torque estimate mean the clutch has synchronized?

No. The indicator only compares two torque values. Speeds and inertias are not modeled.

Can a friction clutch be used as a torque limiter?

Some products are designed for that duty. Use their permitted slip energy, power and operating conditions; do not assume every engagement clutch can slip continuously.

Why might startup need more torque than steady operation?

Acceleration requires inertia times angular acceleration in addition to external load torque. Neither inertia nor an acceleration ramp is entered here.

Can the tool diagnose glazing or recommend a lining treatment?

No. It uses an assumed constant coefficient and has no inspection or thermal model. Maintenance and material choices depend on the exact clutch.