Flexible couplings connect the motor to the driven machine while absorbing misalignment and shock. The jaw coupling is one of the most popular designs in industry because it is simple, needs no lubrication, and lets you inspect the wearing element — the spider — in seconds.
How a jaw coupling works
Two cast metal hubs with curved jaws interlock around a flexible elastomer spider. The spider transmits torque while accommodating angular and parallel misalignment, and it cushions shock loads that would otherwise hammer the motor and driven-equipment bearings. Because the spider is the only wearing part, maintenance is a five-minute swap when the coupling is sized and aligned properly.
Sizing: service factor first
The catalog selection procedure is straightforward:
- Determine the service factor for the application. Uniform loads such as centrifugal pumps, fans, and light conveyors with an electric-motor prime mover use a 1.0 factor. Moderate shock or frequent starting — compressors, mixers, hoists — moves to 1.5. Heavy shock and reversing loads such as crushers, mills, and reciprocating equipment use 2.0.
- Multiply the driven load or motor horsepower by the service factor to get the adjusted horsepower.
- Select a coupling whose torque and horsepower rating at your operating speed equals or exceeds the adjusted value.
In the Martin ML Universal series, catalog numbers ML035 through ML276 cover torque ratings from 3.5 lb-in up to more than 4,700 lb-in, with maximum bores from 0.375-inch up to about 2.875-inch. The same coupling family offers stainless-steel variants for corrosive service.
Spider material changes the rating
The standard spider is Buna-N, which handles oil, dirt, sand, and moisture well. An upgraded Hytrel spider roughly triples the torque rating of the same coupling size — useful when you need more margin without changing the hubs. When replacing a spider, match the durometer and profile to the coupling catalog number; the wrong spider changes the load rating even if it fits.
When jaw couplings are not enough
Jaw couplings tolerate modest misalignment and light shock. For heavy shock, reversing loads, or applications needing higher torque density, gear couplings or roller-chain couplings are better choices. Gear couplings transmit higher torque in a smaller envelope but require lubrication; roller-chain couplings are no-lube and forgiving of parallel offset, with a chain-and-sprocket wearing element that is easy to inspect. The right choice depends on torque, speed, misalignment, and how much maintenance you want to do.
Keep a spider on the shelf
RBC Industrial stocks Martin ML jaw couplings and replacement spiders in Buna-N and Hytrel, plus gear and roller-chain coupling lines. Know your service factor and shaft sizes, and we can size the coupling on the spot.