Roller Chain Wear Elongation Replacement Check

Roller chain does not stretch, it WEARS: the pin and bushing clearances grow and the chain gets longer, so the replacement decision is a tape measure over a known number of pitches and one percentage. Measure across the run with the chain pulled taut, pin centre to pin centre, and compare against pitch times the count. MEASURING OVER TWELVE OR MORE PITCHES RATHER THAN ONE IS THE WHOLE ACCURACY TRICK -- the wear per joint is tiny and only accumulates into something a tape can read over a span. A #50 chain at 0.625 in pitch reading 7.66 in over twelve pitches is 2.13% elongated; the same chain measured over ONE pitch would show 13.3 thousandths of difference, which nothing in a plant will resolve reliably. The 1.5% figure is not arbitrary. A chain riding a sprocket is a polygon, and as the pitch grows the chain contacts fewer teeth and rides higher up the flanks; past roughly 1.5% on a normal tooth count it begins to jump, and the sprocket teeth wear into a hooked profile. Once that happens the sprocket is scrap too, and a new chain on a hooked sprocket wears out in a fraction of its life -- which is why chain and sprockets are replaced as a SET and why a replace verdict here is also an instruction to inspect the teeth. The threshold rises for sprockets with many teeth, where the chain has more engagement to lose before it climbs, and 3.0% is accepted on large, slow, low-tooth-count drives; the limit is entered rather than fixed for that reason. This is one measurement against one limit. It does not inspect the sprockets, which is the other half of the decision and which no length reading reveals; it does not evaluate lubrication, which is what actually determines the wear rate and whose failure is the usual root cause; and it does not address elongation from a single overload event, chain fatigue, or plate cracking, none of which shows as elongation and any of which can fail a chain that measures fine. It does not size a chain or select a replacement. The chain and sprocket manufacturers' data and the drive designer govern.

nominal length = chain pitch x the number of pitches measured; elongation = (measured - nominal) / nominal; allowable length = nominal x (1 + the limit).

The roller chain wear-elongation check as standard drive maintenance practice, by name. The 1.5% replacement figure for a normal hardened-tooth sprocket and the 3.0% accepted on large, slow, low-tooth-count drives are ENTERED, not fixed. Chain and sprockets are replaced as a set. The chain and sprocket manufacturers' data and the drive designer govern.

A tape measurement against a nominal length; no manufacturer table is reproduced.

Estimate. AHJ and licensed professional govern.

Field names used by the API: chain_pitch_in, pitches_measured, measured_length_in, elongation_limit_pct, sprocket_teeth, nominal_length_in, elongation_in, elongation_pct, allowable_length_in, per_pitch_wear_in

• The limit is entered, not fixed 3.0% is accepted on large, slow, low-tooth-count drivesdrive maintenance practice
• The sprockets are not inspected here and that is the other half of the decisiondrive maintenance practice
• Overload damage does not show as elongation nor do fatigue and plate crackingthe chain manufacturer's data