Why Coupling Selection Gets Overlooked
Couplings are small, inexpensive, and easy to ignore — until one fails. A jaw coupling that is undersized for the application fails by spider wear, broken hubs, or transmitted vibration that damages connected equipment. The selection process, however, is straightforward when you follow the catalog method.
Step 1: Determine the Service Factor
Every flexible coupling catalog — including the Martin ML-series data RBC stocks — starts with a service factor table. The factor depends on the driven unit and the prime mover:
- 1.0–1.5 (Light): Uniform, steady loads — agitators, blowers, centrifugal pumps, fans, generators.
- 1.5–2.5 (Moderate): Heavy inertia or moderate shock — rotary pumps, compressors, cranes, machine tools, mixers.
- 2.0–3.0 (Heavy): Shock loads or frequent reversing — reciprocating pumps, crushers, ball mills, vibrating screens, punches, shears.
Step 2: Calculate Adjusted Horsepower
Multiply the service factor by the driven unit's (or motor's) horsepower. This gives the adjusted HP the coupling must handle.
Example: A 20 HP motor driving a reciprocating pump (service factor 2.0) needs a coupling rated for at least 40 HP — not 20 HP. Selecting on nameplate HP alone is the most common sizing mistake we see.
Step 3: Match Torque Rating and Spider
Using the ML-series data: the ML035 starts at 3.5 lb-in of torque, and the range runs up through the ML276 at 7,860 lb-in. The load-carrying member is the spider:
- Buna-N spiders: general-purpose, oil- and dirt-resistant, good damping.
- Hytrel® spiders: higher torque capacity and better chemical resistance for demanding duty.
Bore tolerances matter too — standard bores run +0.001/–0.000 for diameters up to 1.75", and +0.0015/–0.0000 from 1.8125" to 2.625". A sloppy bore fit turns a flexible coupling into a vibration generator.
The Result: A Drive You Can Ignore (in a Good Way)
Properly sized jaw couplings need no lubrication, allow angular and parallel misalignment within the spider's limits, and provide easy inspection of the load-carrying member. That translates directly into lower maintenance cost and fewer unplanned stops.
Related catalog item: RBC Industrial stocks Martin jaw couplings (ML and MS series), roller-chain couplings, Quadra-Flex and go-flex couplings, and complete coupling catalogs with bore and keyway data. Whether you need a replacement spider or a full drive-train spec, we can cross-reference across manufacturers.
Send us your motor HP, driven unit, and shaft sizes — we will size the coupling before you order.