A friction clutch joins an engine to its load through pressed friction faces, so the engine can run while the load is stopped and pick it up smoothly by slipping. Its torque capacity is the friction coefficient times the clamp force times the mean radius times the number of friction faces, and it must exceed the engine’s torque by a wide margin, while oil on the faces can cut the capacity to a third.
Part of Internal Combustion
How an engine clutch works, how its torque capacity and slip heat are calculated, and how linings fail.
An Engine Needs To Run While The Load Stands Still
An internal combustion engine cannot start under load, and it cannot idle if its load is locked to it. A clutch breaks the connection so that the engine starts and idles free, then re-forms it gradually. The corpus gives the principle in Friction Clutch: a friction clutch transmits torque through friction between two surfaces pressed together, with
T_max = μ × F × R_mean
for one friction interface, where μ is the friction coefficient, F the clamp force and R_mean the mean radius of the friction. The same article notes that a multi-plate clutch multiplies capacity by the number of interfaces. A single driven disc squeezed between two faces has two interfaces. It also records lining coefficients against iron of about 0.3 to 0.4 dry for leather and about 0.1 to 0.15 wet with oil, and describes cone and plate forms and linings of leather, cork, fibre and wood. Clutches and Engagement and Jaw Clutch cover positive-engagement alternatives, which have no slip and must be engaged with the speeds matched.
Two numbers govern the design: the torque the clutch can carry without slipping, and the heat it must absorb while it slips. The first must exceed the engine’s torque; the second is Clutch & Gearing’s point that a clutch must be sized for energy and not torque alone.
What You Need
- A flat driving face on the flywheel and a flat pressure plate, faced on a lathe or lapped.
- A friction disc with a lining of leather, cork or fibre, as the corpus describes.
- Springs to provide the clamp force, and a lever to compress them to release.
- A release bearing or collar that can take the thrust.
- Guarding around the whole assembly.
Method: Sizing And Setting A Clutch
Step 1: Find the engine’s torque. Torque = power ÷ angular speed.
Step 2: Choose the lining and the radius. Use the dry friction coefficient for the lining, and take it lower where oil can reach it.
Step 3: Compute the torque capacity and compare it with the engine torque; leave a wide margin.
Step 4: Estimate the heat during engagement with the corpus slip-energy relation, and make sure the faces can absorb it.
Step 5: Set the free play so the clutch fully releases and fully engages. Too little free play leaves it dragging or slipping.
Worked Example: A Small Engine Clutch
Engine torque. Assume 3 kW at 3,000 rpm. ω = 3,000 × 2π ÷ 60 = 314.16 rad/s. Torque = 3,000 ÷ 314.16 = 9.55 N·m.
Capacity. Assume a single driven disc (two interfaces) with leather linings at μ = 0.3, a mean radius of 0.1 m and a clamp force of 2,000 N. These are assumed values.
- T = μ × F × R × n = 0.3 × 2,000 × 0.1 × 2 = 120 N·m.
- Margin: 120 ÷ 9.55 = 12.6 times the engine torque.
- With oil on the faces at μ = 0.1: T = 0.1 × 2,000 × 0.1 × 2 = 40 N·m, a third of the dry capacity, but still 4.2 times the engine torque.
A clutch with ample dry margin can still be well inside its limit when oily. The margin matters for the load, which may demand much more torque than the engine alone.
Slip heat. Assume the engine and its flywheel have an inertia of 0.05 kg·m² and the load an inertia of 0.5 kg·m², starting from rest. Using the corpus relation from Clutch & Gearing, E = ½ × [I₁ × I₂ ÷ (I₁ + I₂)] × ω².
- Equivalent inertia: 0.05 × 0.5 ÷ 0.55 = 0.04545 kg·m².
- ω² = 314.16² = 98,696.
- E = ½ × 0.04545 × 98,696 = 2,243 J, about 2.2 kJ.
The result assumes that the engine adds no torque during engagement and that the engine speed does not fall, a simplification. About 2.2 kJ per start goes into the faces as heat; repeated starts in quick succession add up.
Common Mistakes
- Oil on the lining. A leaking crankshaft seal or over-greased release bearing makes the clutch slip.
- Too little free play. The clutch never fully engages, and the lining burns.
- Slipping the clutch to hold a load. The heat ruins the lining.
- Engaging a jaw clutch at a large speed difference. The shock can break shafts.
Safety
Old clutch linings may contain asbestos, and the assembly stores spring energy
Salvaged friction linings of unknown composition may contain asbestos, whose dust is a cause of serious lung disease (recalled hazard, flagged). Do not blow the dust off or sand it; wet it, wear a mask and dispose of it carefully. A clutch spring holds large force, so compress and release it with a tool. Guard the clutch: a spinning flywheel and disc take fingers and clothing.
What’s Next
- Learn next: Gearbox Basics — the ratio between engine and load
- Combine with: Crankshaft — the flywheel end of the engine