Disc coupling failure modes

Figure 1[/caption]

There are three main failure modes for the flexible membranes of a disc coupling. 1, Excessive misalignment causes fatigue failure around the washer with the outside lamination failing first and then prorogating through the pack of laminations in direct contact as seen in Figure 1. Depending on the level of misalignment seen, this propagation through the membrane pack is generally gradual with torque being carried through the remaining membranes. Couplings that operate at very high misalignment can cause rapid failure. Vibration monitoring in the adjoining machines can detect the failure before becoming catastrophic.

This article contains excerpts from the paper, “Drivetrain protection through coupling design” by Oliver Doidge of Altra Industrial Motion.

 An overload in torque generally collapses the compressive leg of the membrane pack between the driven and driving bolt as seen in Figure 2.

Figure 2[/caption]

A large, single torque overload will cause a sudden failure of membranes this failure can cause the spacer to flail and be ejected from the machine. To ensure this section is controlled, an anti-flail device (Figure 15) should be incorporated into the design (a mandatory requirement under API 671). Designs in which the spacer is controlled by the coupling bolts only should be avoided. Figure 15 Note. A coupling guard is not designed to withstand the kinetic energy of a flailing or ejected spacer and should not be relied upon as a primary safety device. Its purpose is to deflect foreign articles from ingression and personnel protection only.

Disc couplings can fail due to fretting of the membrane laminations. Under misalignment the membranes bend with the outside membranes bending furthest. The action of this bending causes the surfaces to slide against one another. Over time, this micromovement wears the surface of the materials and can lead to fretting corrosion failure. Manufacturers try to overcome this with different methods. A large preload achieved by using a larger bolt can give sufficient clamp load to avoid movement at the expense of increased size and mass. The other method is to apply a low friction coating, similar to Teflon, that allows micro movement without causing fretting failures.