50B21 PITCH: #50 TEETH: 21 Bore: Minimum Plain Bore

Martin numbers read left to right. The prefix is the strand count: none single, D double, E triple, F quadruple, DS double single. Next is the chain number, the pitch in eighths of an inch for ANSI chain (80 is 1"); metric sprockets use the ISO number (08B, 16B), double pitch a 20 prefix (2040, 2060). Then the type: A plate only, B hub one side, C hubs both sides, D detachable hub, BS bored to size, BTB taper bushed, a QD or MST code, or BB, NB, BR idlers. Then tooth count, H hardened teeth or SS stainless, and the bore. So D160B16H 2 1/4 is a double No. 160, Type B, 16 hardened teeth, 2-1/4" bore.

Martin makes bushed sprockets for three bushing systems. BTB sprockets take TB (taper bushed) bushings sized by number, such as 1610 or 2517. QD sprockets take QD bushings coded JA, SH, SDS, SD, SK, SF, E, F, J, M, N, P, W and S. MST (Martin Split Taper) sprockets take bushings coded G, H, P1 to P3, Q1 to Q3, R1, R2, S1, S2, U0 to U2, W1 and W2; for example 100R45 uses an R1 bushing. Martin states the styles are not interchangeable with each other, but each one is interchangeable with the same style from other manufacturers. Match the bushing style and size on the sprocket before ordering.

Martin marks hardened tooth sprockets with an H suffix after the tooth count, for example D35B19H; the bored to size tables use HT, as in 40BS12HT. Martin recommends hardened teeth on the smaller sprocket of a roller chain drive when the ratio is 4:1 or greater, or when the small sprocket has 24 teeth or fewer and runs above 600 RPM, because hardened teeth give longer chain and sprocket life. Some stock lines are hardened as standard: triple No. 35 stock sprockets with 25 teeth or fewer, and all Martin needle bearing idlers. Hardened versions are listed across plain bore, bored to size, taper bushed and MST sprockets in our Martin range.

Stock bore sprockets, shown on our listings as MPB (minimum plain bore), have a small plain bore for reboring to your shaft. Martin lists a recommended maximum bore for each, sized to leave room for a standard keyseat with a setscrew over it; slightly larger bores are possible with no keyseat or a shallow one. BS sprockets are finished at the factory to a listed shaft size with a keyway on the center line of a tooth and two setscrews; the smallest bores (3/8" and 1/2" on No. 35 and No. 40) get setscrews only, no keyway. For example, 40BS60 1 1/2 is a No. 40, 60 tooth sprocket with a 1-1/2" finished bore.

Martin codes idler sprockets by bearing type. BB is a ball bearing idler, normally with hardened steel teeth (35BB19H 1/2); an NM suffix means non-metallic teeth (40BB17NM 1/2). NB is a needle bearing idler with hardened teeth (80NB13H 1). BR is a bronze bearing idler, with a grease groove on the 7/8" and 1-1/8" bores (60BR15 7/8). Martin publishes radial load capacities for ball and needle bearing idlers based on 2, 500 hours average bearing life: a 3/4" bore ball bearing idler is rated 600 lb at 1, 000 RPM, a 1" bore needle bearing idler 1, 379 lb. Sintered bronze bushed idlers are rated to 2, 500 RPM and 50 lb. Select by chain size, bore and load.