A chain drive is only as good as its tension and tracking. Run a chain too slack and it whips, slaps the guard, and accelerates wear on both the chain and the sprocket teeth. Run it misaligned and the load concentrates on a few teeth instead of spreading across the width of the drive. The quiet, low-cost fix in both cases is often an idler sprocket.
What an idler sprocket does
An idler sprocket is a free-spinning sprocket placed in the chain path to take up slack, control chain sag, and guide the chain around obstructions or over long spans. Because it is not part of the power path, it carries no driving torque — its job is purely mechanical: maintain tension, improve wrap on the driving sprocket, and keep the chain tracking true.
Bronze-bushed idler types
Stock idler sprockets are commonly built as bronze-bushed units: a steel sprocket body with a pressed-in bronze bearing that lets the idler spin freely on a stationary bolt or stud. That makes installation straightforward — the idler mounts on an existing fastener or a simple pivot, no shaft and bearing housing required. Stock bronze-bushed idlers are available for common chain sizes such as #35, #41/#40, #50, and #60/#60H, with outside diameters sized for the chain pitch and a stock bore sized for a standard mounting bolt.
The outside diameter matters because it sets how deeply the idler engages the chain and how much slack it takes up. Too small an idler on a large-pitch chain makes the chain wrap too tightly and can fatigue the links; too large and it may not fit in the guard envelope. The catalog part number typically encodes the chain size and outside diameter directly — for example, a 31E20 idler suits a #35 chain while a 61E14 suits a #60/#60H chain.
Idler placement tips
- Place the idler on the slack side of the drive so it takes up sag without fighting the tight strand.
- Keep the idler as close as practical to the driving sprocket for maximum wrap improvement.
- Check the idler bore and bushing for wear at every chain service interval — a worn bronze bushing lets the idler wobble and accelerates chain wear.
- Make sure the idler runs in the same plane as the chain to avoid edge loading.
Replacing a worn or missing idler is one of the quickest maintenance wins on a chain drive. Send us the chain size, the outside diameter you need, and the mounting bore, and we will match a stock bronze-bushed idler.
Catalog item plug: RBC Industrial stocks bronze-bushed idler sprockets for #35 through #60/#60H chain, with stock bores and mounting bushings ready to ship.