Two Workhorse Coupling Families, One Selection Method
Flexible couplings are where most power transmission problems surface first. When a drive line starts vibrating, squealing, or throwing parts, the coupling is usually the component that gets the blame — and often it is the component that was never properly specified in the first place. This guide compares two of the most common stock flexible coupling families — jaw couplings and roller chain couplings — and walks through the same selection procedure used in the Martin Sprocket & Gear catalogs RBC Industrial carries.
The Selection Procedure Both Families Share
Catalog selection is a three-step process:
- Determine the service factor. Match the driven unit against the prime mover in the service factor guide. A centrifugal pump on an electric motor starts at a 1.0 factor (light, uniform load); a crusher or vibrating screen under the same motor is a 2.0 (heavy shock, frequent reversing); a reciprocating compressor on a 4-cylinder engine jumps to 2.5–3.0.
- Adjust the horsepower. Multiply the motor HP by the service factor to get adjusted HP.
- Select the coupling. Choose the coupling whose torque or HP rating at your operating RPM is equal to or greater than the adjusted figure.
Jaw Couplings: No Lubrication, Easy Inspection
Jaw couplings pair two machined hubs with a single elastomeric spider. The catalog's own feature list is the pitch: no lubrication required, easy installation, no metal-to-metal contact, and resistance to oil, dirt, sand, moisture, and grease. The spider is the load-carrying element and can be inspected in seconds — remove the cover, look at the spider, button it back up. Buna-N spiders handle standard service; Hytrel spiders roughly triple torque capacity where chemical or temperature resistance matters.
Martin offers two jaw styles. The Martin Universal (ML) series is completely interchangeable with industry-standard jaw coupling hubs, which makes it the safe cross-reference choice for replacement work. The Martin Super (MS) series carries approximately 20% higher horsepower for the same envelope — a good upgrade path when the original coupling was borderline. Stock torque ratings run from 3.5 lb-in on the ML035 up through the larger sizes, with max bores from 0.375 in. upward.
Roller Chain Couplings: Positive Engagement, Bored-to-Size Options
Roller chain couplings use two sprocket halves joined by a length of double-strand roller chain. They give positive engagement with modest misalignment capability, and they are available two ways in stock: bored-to-size (finished bore, keyway, and setscrew included) and QD bushed versions that let one coupling half fit many shaft diameters. Stock sizes run from the small 4012 through the 12022, with finished bores from 0.500 in. up to nearly 5 in. High-speed standard covers (HH, FF, and FH styles) are available for every stock size.
Which One Goes on Your Machine?
Use jaw couplings where you want no-lube simplicity, quick inspection, and spider-based flexibility. Use roller chain couplings where you want positive engagement, higher torque density in a compact package, and the inventory flexibility of QD bushing systems. When in doubt, run the three-step service factor procedure — it works for both families.
RBC Industrial stocks both coupling families plus covers and spiders. Call (915) 845-8188 or visit www.rbc-industrial.com to get a cross-reference and a same-day ship quote.