Engineering FAQs - Coupling
Q: What is a coupling?
A coupling is a device that connects two generally coaxial (inline) shafts at their ends in order to transmit power between them. A coupling can be incorporated with a clutch to serve as a clutch-coupling or a torque limiter.
In applications where the ends of the two shafts to be coupled are separated by a large axial distance, a line shaft or floating shaft can be used. A line shaft/floating shaft mounts a coupling to each shaft end and connects an intermediate shaft between the two couplings.
Many varieties of coupling designs exist, each offering a different blend of torque capability, size, price, shaft misalignment, and other performance factors. Shaft misalignment can be offset (the lateral distance between the two shaft axes, also known as parallel offset), angular (the angular difference between the two axes), or a combination of both.
Disc
A disc coupling is comprised of two hubs (to connect to each shaft) bolted to a disc between them. The hubs are not directly bolted to each other. Torque is transmitted from one hub’s bolts to the other hubs bolts via the disc. The deformation of the disc accommodates for shaft misalignment. Two discs with a spacer bolted in between can also be used instead of a single disc to tolerate greater parallel misalignment.
Pin and Bush/Bushed Pin
A bushed pin coupling has two flanged hubs connected to each other with bushed pins or bolts. The pins are rigidly bolted in one flange and fitted into an elastomer bushing in the other flange. The elastomer bushing provides for shaft misalignment.
Bellows
As the name implies, a bellows coupling connects two hubs using a metal bellows. The bellows permits misalignment while still maintaining high torsional stiffness. This is critical for maintaining identical angular position between two shafts for applications that require high positioning accuracy.
Elastomer/Jaw/Spider
An elastomer/jaw/spider coupling consists of two hubs with jaws located on either side of an elastomer insert with radially extending lobes (the spider). The jaws from each hub extend parallel to the axis of rotation (like a crown gear) toward the opposite hub. The jaws from each hub fit between the lobes so that torque is transmitted from one hub’s jaws through the lobes and to the other hub’s jaws. Based on the elastomer material used, the spider can provide varying degrees of vibration absorption.
Schmidt
A Schmidt coupling permits significant and dynamic parallel misalignment during operation. Two hubs attached to either shaft connect to a disc between them via a number of links bolted to a common bolt circle. The two shafts can have any parallel misalignment up to twice the length of the links. The central disc is free to move and locates itself according to the extent of parallel offset.
Grid
A grid coupling has two hubs with slots to accept a steel grid. The slots are circumferential and parallel to the axis of rotation. The steel grid is shaped in a zig zag style pattern with each “zig” and “zag” fitting into and spanning a slot from each hub in order to transmit torque between the two. The grid is free to slide along the slots to provide for misalignment.
Gear
A gear coupling consists of two external gears located within an internal gear at a 1:1 gear ratio (equivalent to a splined shaft in bore). The three gears have nominally collinear axes of rotation. The teeth of the external gears are crowned to permit angular misalignment between the two shafts to which either gear is connected. Torque is transferred from one external gear, to the internal gear, then to the other external gear.
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Couplings offered by R.M. Hoffman Company:
