Frictional Clutch-box: Plate-stack Torque Calculator | FIRGELLI

Axial force and the plate stack

Alternating members connect to the driving and driven sides. An axial load presses the contact faces together. Friction across those interfaces transfers torque; releasing the load allows the members to turn relative to one another. Actual disengaged drag depends on the construction.

The illustration uses z+1 alternating members to show z active interfaces. A single driven plate contacted on both sides has two active interfaces. The same axial force passes through an ideal series stack; it is not divided by the number of interfaces.

The animation illustrates the load path. Below the calculated capacity, the two sides rotate together. Above it, the output side turns at a prescribed slower rate. That rate is illustrative, not a computed slip speed. Plate spacing and radial dimensions are schematic.

Where a clutch-box fits

A clutch can connect and disconnect a drive without stopping its input shaft. Plate clutches occur in vehicle transmissions and production machinery, with the actuation and cooling arrangements chosen for the machine.

This calculator compares an ideal contact capacity with a stated torque demand. It does not supply ratings for a particular tractor, lathe, press or marine transmission. Machine-specific engagement energy, permissible speed, cooling and release requirements need the actual clutch data.

Read the torque and force outputs

The calculator evaluates T = μFR_mz, with F in newtons and R_m converted from millimeters to meters. T is an ideal static capacity in N·m. Sum of Face Limits is μFz: the sum of the tangential-force limits across the interfaces, rather than the force at one face or the axial actuator force.

Capacity Ratio is T/T_d. Torque Margin is T − T_d. A ratio of one and zero margin put demand exactly at the modeled limit; they do not establish a practical design reserve. All selectable demand values are positive.

Mean radius must match the assumed contact-pressure distribution. For an annulus, the uniform-wear approximation gives R_m = (R_o + R_i)/2. Uniform pressure gives R_m = (2/3)(R_o³ − R_i³)/(R_o² − R_i²). The calculator accepts R_m directly and does not select either assumption.

The coefficient and clamp force are held constant. This equation does not calculate release-bearing force, engagement time, heating, wear or fatigue life. Release force depends on the actual linkage, spring or pressure-actuated arrangement.

Example: two active interfaces

At the defaults, F = 3,000 N, μ = 0.35, R_m = 100 mm and z = 2. Capacity is 0.35 × 3,000 × 0.100 × 2 = 210 N·m. The summed face limits are 2,100 N. Against the 200 N·m demand, Capacity Ratio is 1.05 and Torque Margin is 10 N·m.

For a separate annulus with outer radius 90 mm and inner radius 60 mm, the uniform-wear mean radius is 75 mm. At μ = 0.35 and z = 2, clamp forces of 3,000, 4,000 and 6,000 N give 157.5, 210 and 315 N·m respectively. These are hypothetical comparisons, not settings for a named machine.

The same annulus has a uniform-pressure mean radius of 76 mm, giving 212.8 N·m at 4,000 N. That value can be evaluated algebraically; the radius slider advances in 5 mm steps. Neither pressure assumption predicts the actual state of a particular worn disc.

Across the controls, calculated capacity ranges from 1.6 to 10,560 N·m. These bounds describe the combinations accepted by the tool, not validated clutch ratings.

Capacity is one part of clutch selection

Increasing clamp force, effective radius or the number of interfaces increases this model's static capacity in direct proportion. The resulting assembly still needs checks for contact pressure, release operation and the loads carried by its supporting components.

When the sides rotate at different speeds, friction dissipates energy. The permitted slip duration and repetition depend on the clutch's thermal limits. A static capacity above demand does not establish an acceptable engagement cycle or continuous-slip duty.

A purpose-built torque limiter and an engagement clutch may have different overload behavior. Use the specified function and duty of the actual product; lowering a clamp setting does not by itself establish dependable overload protection.

Frequently asked questions

Does Friction Faces mean the number of plates?

No. It means active interfaces. The illustrated stack uses z+1 alternating members for z interfaces; a driven plate contacted on both sides contributes two interfaces.

Is clamp force divided between the interfaces?

No. The ideal series stack has the same axial load through every interface. Their torque contributions add.

What does Sum of Face Limits mean?

It is μFz, the sum of the tangential-force limits at all active interfaces. It is not the release actuator force.

Which mean radius should I use?

Use the effective radius appropriate to the contact-pressure assumption. Uniform wear and uniform pressure give different annular mean radii; the formula section shows both.

Does the animation calculate engagement or slip speed?

No. Its rotation and plate spacing illustrate the stack and capacity comparison. Dynamic motion and engagement travel are not solved.

Can this identify the cause of a clutch slipping or juddering?

No. It holds coefficient and clamp force constant and does not diagnose temperature, contamination, linkage condition or vibration.

Can I exceed a published rating by running more slowly?

The calculation provides no basis for exceeding a product rating. Check the actual speed, torque and thermal limits together.

Can the static capacity establish continuous-slip duty?

No. Relative speed, duration and repetition must also be checked against the clutch’s thermal capability.