Lower coupling restoring forces can increase bearing and seal life.
by Ryan Marks
April 18, 2017

When designing a coupling, a manufacturer takes into account many variables. In some cases, restoring forces are byproducts of the type of coupling being designed as well as other factors such as cost control, torsional stiffness requirements and torque-carrying performance.

This is no different when designing disc couplings. A disc coupling transmits torque through a stack of steel laminates. These laminates act as rigid discs in the torque transmission plane but flex like a deck of cards in the misalignment plane. The ideal disc coupling would be torsionally stiff and torque-dense in the torque plane, yet require a proportionally small force to bend it out of alignment in the misalignment plane.

Disc couplings are the preferred choice for any application that demands a long life and low-maintenance coupling. Here are some competing factors in designing a disc coupling and how manufacturers can address those challenges.

1. Total disc stack thickness

In order to make a disc coupling with a low restoring force, ideally the disc laminate stack should be very thin. However, in order to transmit torque and have a stable bolted joint, the disc laminate stack needs to be fairly thick. It is possible to get the best of both by imbedding threaded bushings into the disc pack and using ultra-high-strength stainless steel to help reduce thickness and increase strain properties. This means the manufacturer has tried to get the disc stack as thin as possible without compromising coupling torque or joint stability.

2. Disc material type

Most manufacturers are stuck using a disproportionately thicker disc pack in order to achieve proper bolted joint mechanics, and as a result they only need low-strength material to handle the torque. An alternative is a high-performance disc pack that uses ultra-high-strength stainless steel, which allows for a thinner over all laminate stack translating into lower restoring forces. This ultra-high-strength stainless steel also extends the life of the discs as they are repeatedly bent because of misalignment.

3. Disc laminate thickness

Using thinner discs allows them to flex out of alignment easier—lowering restoring forces. However, thinner discs require a manufacturer to use more of them to achieve the overall pack thickness required, and this means increased costs. Using the ultra-high-strength stainless steel can result in very thin discs in order to further reduce restoring forces.

When selecting a coupling for an application, end users should make sure they check out the restoring forces of the coupling. Their bearings and seals will appreciate it.

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