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A Quick Refresher on Converting Torsional Stiffness Values

Customers often ask us for a bit of assistance in comparing torsional stiffness values between flexible couplings made by manufacturers using Imperial versus SI units.  This may be considered rudimentary to some but a quick refresher never hurts.

Any drive component which transmits torque will twist a bit when torque is applied. For example, if a shaft was marked on each end very precisely notched on the same plane along its axis, the notches would move apart as torque is applied.

The amount they move apart is proportional to the applied torque. Therefore, torsional stiffness is called out in units of torque applied per angular unit of twist. North American manufacturers tend to use a variety of different units while Europeans typically use Newton meters per radian.

The Conversion

I've provided an example conversion below.  In this example calculation, we are converting a given stiffness value of 76000 N*m/Rad  to in*lb/Deg , this will require a bit of unit conversion. As many of us may remember from college chemistry, we are allowed to multiply ratios of different units that mathematically equal 1 to cancel unwanted units and convert to the desired ones. Below, we can see that the units in red cancel. In just a couple of steps the desired units emerge.

Or conversely, converting a value given in in*lb/Deg to N*m/Rad:

Coupling Stiffness

As we see in the example, if we we’re comparing flexible couplings, the top one would be more torsionally stiff. Having a high level of torsional stiffness is not always desirable, especially if someone wanted to use the flexible coupling to damp vibration.  However in many cases machine builders need to maximize torsional stiffness in order to reduce settling times in highly dynamic applications or to maximize rotational positioning accuracy.  In the case of flexible shaft couplings, elastomeric (elastic) couplings are typically used for vibration damping, and metallic couplings are used for maximizing torsional stiffness.

This example can be used for comparing any multitude of different units as long as the correct conversion ratios are employed.