Steel cord or aramid cord for PU timing belts: which do I need?

Question

Accepted Answer

Short answer: choose steel cord when you need maximum rigidity, minimal elongation and precise positioning (linear drives, robotics, long spans). Choose aramid (Kevlar-type) cord when the drive sees shock loads, frequent starts, small pulleys, or humid and corrosive conditions. Both run inside the same PU body — the cord decides stretch, flexibility and durability.

Side-by-side comparison

Property	Steel cord	Aramid (Kevlar-type) cord
Elongation under load	Very low — best for positioning accuracy	Low, slightly higher than steel
Flexibility over small pulleys	Lower; larger minimum pulley diameters	Better; tolerates smaller pulleys
Shock-load resistance	Moderate — sensitive to impact damage	Excellent — high impact toughness
Corrosion / humidity	Needs protection in wet or corrosive areas	Naturally corrosion-free and non-magnetic
Magnetic environments	Affected (steel)	Neutral (non-magnetic)
Typical duty	Precision positioning, long center distances, heavy static loads	Shock loads, frequent starts, small-diameter drives

PU timing belt in wide industrial pitch — The PU body is the same — the tension cord defines drive behavior.

Choose steel cord when…

The drive is a positioning or linear application where even small belt stretch causes lost accuracy.
Center distances are long and sag must stay minimal.
Loads are steady and predictable, without frequent hard starts.
Pulleys are large enough for the cord's minimum bending radius.

Choose aramid cord when…

The machine starts under load frequently or sees torque spikes (packaging machinery, wood processing, mixers).
Drive layout forces small pulley diameters or back-bending idlers.
Humidity, washdown or corrosive media are present around the belt.
Equipment is non-magnetic by design (some medical, electronics and food machinery).

Open-end PU timing belt showing tension cords — Open-end PU belts can be welded or joined into endless loops after cord selection.

Cord failure is usually a symptom, not a cause: under-sizing the belt, misaligned pulleys or wrong tension will destroy either cord type prematurely. If a steel-cord belt failed from shock loads, simply repeating the same construction will fail the same way — send us the failure photo and drive data for a root-cause review.

Belt inspection on a workbench — Cord selection is confirmed together with profile, width and duty data.

Which cord fits your machine?

Describe your drive — load profile, pulley diameters, environment and accuracy needs. We will recommend steel or aramid cord and quote the matching belt.

Get a Cord Recommendation