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Clamp Load Vs Torque Capacity

Corvette Tech Tip: Clamp Load Vs Torque Capacity

Corvette Tech Tip: Clamp Load Vs Torque Capacity

Reprinted with permission from Mantic Engineering Pty Ltd

The most widely misunderstood characteristic of a clutch cover assembly is the Clamp Load. Clutch cover assemblies have been rated by clamp load since their invention. The clamp load is one of the main factors influencing how much torque can be transmitted by a clutch. But there are other factors that also affect the torque capacity and these can be overlooked when selecting a clutch for a vehicle.

The fundamental definitions of Clamp load and Torque Capacity are:

Clamp Load – The load exerted by the diaphragm to clamp the clutch disc between the pressure plate and the flywheel.

Torque Capacity – The load exerted by the diaphragm to clamp the Clutch disc between the pressure plate and the flywheel multiplied by the coefficient of friction multiplied by the mean effective radius multiplied by the number of clutch discs.

It is possible to have two cover assemblies with the same clamp load but have different torque capacities due to differences in the coefficient of friction, the inside and outside diameters of the clutch disc or the number of clutch discs.

Traditionally clutches have been sized by using comparative measures, i.e. previously the owner had a10”clutch with 650kg clamp load. Now they are looking for more capacity, for example 20% more so they assume they need a 10” clutch with approximately 650 x 1.2 = 780kg of clamp. This is a reasonable argument if the friction coefficient, the diameters and the number of discs are identical but this is not always the case.

A major reason for this method of measurement has been that without a clutch dynamometer, all calculations are just that – calculations. Mantic uses figures that mean something, because “We have the only clutch dynamometer in Australia” which allows us to accurately measure torque capacity.

In the final analysis it is far more important to know how much torque can be transmitted by a clutch than knowing the clamp load. Engines are rated in horsepower or kilowatts and torque, not kilograms.

The torque capacity of a single disc clutch:

torque capacity formula
“With this in mind, Mantic performance clutches have torque capacity ratings so you can match it accurately to your vehicles maximum engine torque capacity. Next time someone tries to tell you their cover assemblies have more clamp load, ask them what is its maximum torque capacity then do the comparison before deciding which clutch better suits your vehicle.”

Increased Torque Capacity

The advantage of increased Torque capacity is that more power/ torque can be transmitted through the clutch. Torque Capacity is calculated by multiplying the load exerted by the diaphragm to clamp the clutch disc between the pressure plate and the flywheel multiplied by the coefficient of friction multiplied by the mean effective radius multiplied by the number of clutch discs. i.e. Torque Capacity = Clamp Load x Coef. of Friction x Mean effective radius x No. of Clutch Discs.

A smaller clutch has a smaller mean effective radius and a smaller diaphragm which usually also means a lower clamping force, so these two features will have the effect of reducing the Torque Capacity. However these two features do not reduce the capacity by the same amount as the extra clutch disc increases the Torque Capacity. The net result is an increased Torque Capacity.

What are these advantages and how is this achieved?

The Torque Capacity is affected by 4 factors:

  • By decreasing the diameter the torque capacity is reduced.
  • By decreasing the clamp load the torque capacity is reduced.
  • By adding a second clutch disc the torque capacity is doubled.
  • By increasing the coefficient of friction the torque capacity is increased.

Conversion Charts

KW HP HP KW
100 134 100 75
150 201 150 112
200 268 200 112
250 335 250 187
300 402 300 224
350 470 350 261
400 536 400 299
450 603 450 336
500 670 500 373

Multiply Kw x 1.34 to get HP
Multiply HP x 0.7457 to get Kw

NM FT LBS FT LBS NM
100 74 100 136
200 147 200 271
300 221 300 407
400 295 400 542
500 368 500 678
600 422 600 814
700 516 700 949
800 590 800 1085
900 663 900 1220
1000 737 1000 1356

Multiply Nm x 0.74 to get Ft lbs
Multiply Ft lbs x 1.356 to get Nm