What Shore hardness and tolerance can you hold on PU timing belts?

Question

Accepted Answer

Short answer: Most PU timing belts are made between 80A and 95A Shore A, and 85A to 92A covers the majority of industrial drives. Practical tolerance windows are around plus or minus 0.5 mm on width and thickness for common sizes, and roughly plus or minus 0.3 to 0.5 percent on pitch length. Tighter figures are possible, but only after the actual construction is fixed, because cord type, belt thickness and length all move the achievable window.

What the hardness number changes

80A to 85A

Softer compounds conform better to pulleys and grip slightly more. Useful on small pulleys and light conveying, but they wear faster under abrasion and cut edges.

85A to 92A

The general drive range: enough stiffness for load transmission, enough flexibility for normal pulley sizes. Most replacement and OEM drives sit here.

92A to 95A

Harder grades carry more load with less elongation, but they are less forgiving of misalignment and small pulleys, and noise can rise.

What it does not tell you

Hardness says nothing about tensile strength, elongation at break or oil grade. Ask for the compound datasheet rather than judging by hardness alone.

Operator machining a pulley on a lathe in a workshop — Hardness and tolerance only mean something relative to the pulley geometry the belt runs on.

Typical tolerance windows on PU belts

Feature	Common window	Notes
Width	About plus or minus 0.5 mm up to 50 mm	Wider belts move toward plus or minus 1.0 mm
Thickness	About plus or minus 0.2 to 0.5 mm	Depends on profile depth and whether a backing is added
Pitch length	About plus or minus 0.3 to 0.5 percent	Longer belts tend toward the wider end of the range
Tooth profile	Per standard T, AT, HTD, STD or imperial	A non-standard profile needs a drawing and usually a new mould

The figures above are indicative for standard PU constructions and are not a specification. Steel cord, aramid cord, post-cure and mould condition each shift the window, and a tolerance that is easy on a 500 mm belt can be difficult on a 3000 mm one. Confirm the numbers against your drawing before releasing the order.

What to put in the request

Pitch and tooth profile, plus the pulley tooth count at both ends if the drive is tight.
Width and thickness, and whether the thickness is functional or only nominal.
Required hardness, and whether it is a grip requirement or a load requirement.
Cord preference, or the load and elongation limit if you have no preference.
Working temperature and any oil, dust or washdown exposure.
Whether the tolerance is functional, cosmetic, or only needs to match an existing belt.

PU timing belt drive with tensioning and guide elements — A tolerance that ignores the tensioner and guide layout is only half the specification.

PU timing belt fitted to a conveying application — Thickness and hardness are usually chosen together with the conveying duty, not separately.

Have a drawing with a hardness and tolerance on it?

Send the drawing and the duty cycle. We will confirm which hardness range and tolerance window we can hold for that construction, and quote accordingly.

Send Drawing for Review