Why does my PU timing belt keep losing tension or stretching?

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Direct answer: Polyurethane itself stretches very little. The length change you see comes from the reinforcing cords creeping, from the belt bedding into the pulley grooves, or from heat and overload. The usual cure is to re-tension after run-in, then verify that the cord type and the belt width match the real load.A seamless PU belt has no joint to creep, so tension loss on a seamless belt points to the cord, heat or the pulleys.Five causes, in the order to check themCauseWhat you seeActionBedding in after fittingA small loss in the first hours, then stableRe-tension once after run-in and record the valueCord creep under steady loadSlow, continuous lengthening over weeksCheck cord type and belt width against the torqueOverload or tooth jumpingTension drops suddenly, teeth show damageRecalculate the drive, widen the belt or add teeth in meshHeat above the belt ratingBelt feels soft, back looks glossy, tension fallsCheck ambient and pulley temperature, ventilate or shieldWorn pulleysBelt sits deeper in the groove, effective diameter dropsInspect the grooves; a new belt will not fix a worn pulleyAn open-ended belt such as 25-H-8750 is joined on the machine; a joint that creeps shows up as steady tension loss.How to tension and re-checkSet tension to the value for the pitch and width, using a deflection gauge or a sonic tension meter rather than feel.Run the drive under normal load for a few hours, then re-check. Most bedding-in loss happens early.Mark the take-up position so the next check is comparable with this one.Re-check at each service interval. A steady slow loss over months is cord creep; a sudden loss is damage.Log tension against running hours. A trend tells you more than a single reading.A 322L-13 seamless belt that keeps losing tension after re-tensioning usually has a load or heat problem, not a belt problem. Do not answer tension loss with over-tensioning. Extra tension loads the bearings and the tooth roots, and it will not stop cord creep.Send the drive dataEmail sales@goodlybelts.com with the belt marking, pitch, width, pulley diameters, motor power and running temperature, or use the enquiry form. We will check whether the cord and width suit the load and quote a belt that holds tension.Check my belt specification

Why does my PU timing belt keep losing tension or stretching?

Five causes, in the order to check them

How to tension and re-check

Send the drive data

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Cause	What you see	Action
Bedding in after fitting	A small loss in the first hours, then stable	Re-tension once after run-in and record the value
Cord creep under steady load	Slow, continuous lengthening over weeks	Check cord type and belt width against the torque
Overload or tooth jumping	Tension drops suddenly, teeth show damage	Recalculate the drive, widen the belt or add teeth in mesh
Heat above the belt rating	Belt feels soft, back looks glossy, tension falls	Check ambient and pulley temperature, ventilate or shield
Worn pulleys	Belt sits deeper in the groove, effective diameter drops	Inspect the grooves; a new belt will not fix a worn pulley