Kevlar vs Steel Cord Timing Belts: How to Choose

The cord inside a polyurethane timing belt carries the load. Changing from steel to Kevlar (aramid) changes how the belt stretches, what pulleys it can run on, and where it can be used. This guide covers the differences across T, AT and imperial profiles, with published manufacturer numbers and how to choose.

How a Polyurethane Timing Belt Is Built

A PU timing belt has two parts: the urethane body that forms the teeth, and the tension members (cords) molded inside. The cords carry the tensile load. The urethane transfers it to the pulley teeth.

Profile and cord are separate choices. The same cord options are offered across metric T profiles (T2.5, T5, T10, T20), AT profiles (AT3, AT5, AT10, AT20) and imperial profiles (XL, L, H), though not every cord is made in every profile. The common cords are standard steel, Kevlar (aramid), high performance steel, hi-flex steel and stainless steel.

The cord does not change the tooth shape. A T10 belt runs on T10 pulleys whatever the cord. Profiles are not interchangeable, though: T5 is not AT5. Both have a 5 mm pitch, but AT5 uses a larger tooth and requires AT5 pulleys.

Steel Cord vs Kevlar Cord

Is Kevlar Stronger Than Steel? It Depends on the Profile

Published manufacturer data (Megadyne Megalinear, open-end belts) for a 25 mm wide belt, or 1 in wide for XL, L and H. Max load is the maximum allowable working tension. The last column compares Elatech's published open-end ratings at the same width. Average catalog values; "-" means not listed or not shown.

Kevlar comes out well ahead in T5 but behind steel in T10 and H. The ranking also changes by manufacturer: Elatech's catalog rates aramid below steel in T10, T20, AT10, L and H. Truly endless belts are rated separately again: in Megadyne's truly endless line at 25 mm width, T5 is 800 N steel vs 1,795 N Kevlar, while T10 is 2,290 N steel vs 1,795 N Kevlar. Confirm the rating for the exact profile, width and construction.

Minimum Pulley Teeth by Profile and Cord

Undersized pulleys are a leading cause of early cord damage. Minimum tooth counts for a simple two-pulley drive with no backbending, from the same published open-end data:

"-" means the cord is not listed for that profile in the catalog. Other manufacturers publish similar but not identical minimums.

Backbending (serpentine drives with backside idlers) raises the minimums. Minimum teeth / minimum backside idler diameter in mm:

Pitch diameter = teeth x pitch / 3.1416. A 12 tooth T5 pulley is about 19 mm (3/4 in); a 15 tooth T10 pulley is about 48 mm (1.9 in).

A drive running at or near the minimum will work, but belt life drops. If the drive needs smaller pulleys or tighter idlers than standard steel allows, hi-flex steel is the usual answer: it uses finer wires, tolerates tighter bends, and stretches only slightly more than standard steel (0.5% vs 0.4%).

Other Cord Options

• High performance (HP) steel: a stronger steel cord for high load and high repeatability drives, with the same 0.4% stretch. It needs larger pulleys than standard steel (for example 15 vs 10 teeth in T5, 25 vs 15 in AT5 and AT10).
• Hi-flex steel: for small pulleys, multi-shaft drives and severe reverse bending.
• Stainless steel: for wet, washdown and food environments. Rated lower than standard steel, roughly 5 to 40% depending on manufacturer and profile, with equal or larger minimum pulleys. Still metal, so not for metal detector lines.
• Food-contact belts: some food-contact lines use Kevlar as the standard cord; Megadyne's FC food-contact T5 and T10 belts are an example. Others, such as Megadyne's FC-S line, use encapsulated steel cord, so confirm the cord when specifying.

Choosing a Cord by Application

What About Rubber Timing Belts?

Steel versus Kevlar is mainly a polyurethane belt decision. Rubber (neoprene or EPDM) timing belts such as HTD, GT and classical trapezoidal belts use fiberglass cord as standard, chosen for length stability and flex life. Some rubber belts use aramid cord for shock loads, at the cost of more permanent stretch. In Gates' industrial catalog, steel-cord rubber belts are offered only as open-end long-length belting.

High-performance belts like Poly Chain are polyurethane belts with aramid or carbon cord rather than steel; see our Poly Chain GT Carbon belts page. For body and cord materials across all belt types, see our timing belt materials guide.

Belt Construction: Open-End, Welded, and Truly Endless

Cord material and belt construction are separate choices, though not every cord is offered in every construction.

Why welded belts are weaker. The cords are cut at the splice, so the urethane weld carries the load across the joint. Manufacturers rate welded belts at roughly half the tensile and tooth load and recommend them for conveying only. Using a welded belt in a high load drive typically ends in a break at the splice.

Why welded belts are still common. They can be made to almost any length on short lead times, they are the usual base for welded cleats and backings, and they can be field welded on the machine without teardown.

Rule of thumb: conveying or cleated belt, welded is usually fine. Driving a load, specify truly endless. Very short welded belts may not be possible because of minimum welded length (typically 400 to 1,200 mm depending on profile and manufacturer); short lengths are normally molded endless.

Reading a Broken Belt

How a belt broke points to the cause. A clean, straight break across the belt often means crimped cords: pulleys or idlers below the minimum, a belt run too loose so it ratchets, debris in the drive, prying the belt on with a tool, or rough handling before installation. Normal cord fatigue at the end of a belt's life leaves a jagged break at roughly 45 degrees. A ragged, uneven break suggests a shock load. Published failure guides are written mainly for rubber belts, so treat these patterns as indicators, not proof. If an undersized pulley caused the failure, change the pulley or move to hi-flex steel cord.

What to Have Ready When You Request a Quote

1. Profile and pitch (T5, T10, AT10, XL, H and so on)
2. Pulley tooth count (count the teeth; do not rely on a diameter measurement)
3. Belt width and length or tooth count
4. Construction needed: open-end, welded, or truly endless
5. Any idlers or backside bends in the drive
6. Application: positioning, conveying, or power transmission
7. Environment: washdown, chemicals, temperature, food contact, metal detection
8. How the old belt failed, if replacing one

For joining methods and splice strength, see our guide to connecting timing belts.

Frequently Asked Questions

What is the difference between Kevlar and steel cord timing belts?

Both are polyurethane timing belts; only the tension member changes. Steel cord gives the lowest stretch and the best positioning accuracy. Kevlar (aramid) cord is non-metallic, so it is non-magnetic and cannot rust, but in Megadyne's published data it stretches about twice as much as steel at maximum load (0.8% vs 0.4%). Steel is the standard choice for positioning and power transmission. Kevlar is usually specified when the belt must pass through a metal detector.

Is Kevlar cord stronger than steel cord?

It depends on the profile and the manufacturer. In published Megadyne open-end data at 25 mm width, Kevlar is rated well above standard steel in T5 and above it in T20, AT5, AT10, XL and L, but below steel in T10 and H and about equal in AT20. Elatech's catalog rates aramid below steel in T10, T20, AT10, L and H. Truly endless belts are rated differently again. Always check the rating for the exact profile, width and construction rather than assuming Kevlar is stronger.

Is Kevlar cord better for small pulleys?

Usually not. In Megadyne's published tables Kevlar never allows fewer pulley teeth than standard steel, and in some profiles it needs more: 12 teeth vs 10 in T5 and 15 vs 12 in T10. Other manufacturers' tables differ, so check the exact belt. For small pulleys, tight idlers or heavy backbending, hi-flex steel cord is the better choice. It uses finer wires, allows equal or smaller pulleys and idlers, and stretches only slightly more than standard steel (0.5% vs 0.4%).

How strong is a welded timing belt compared to an open-end or truly endless belt?

Manufacturers rate a spliced and welded belt at about 50% of the tensile and tooth load of the same belt as open-end stock. The cords are cut at the splice, so the weld carries the load across the joint. Welded belts are intended for conveying. For power transmission and high torque drives, specify a truly endless belt.

When should I use stainless steel cord?

Stainless steel cord is used in wet, washdown and food processing environments where standard steel could corrode and positioning accuracy still matters. It is rated lower than standard steel, roughly 5 to 40% lower depending on manufacturer and profile, and needs an equal or larger minimum pulley. Because it is still metal, it does not solve metal detector issues.

Can I switch cord type without changing pulleys?

Usually, as long as the profile and pitch stay the same. The cord does not change the tooth shape, so a Kevlar T10 belt runs on the same T10 pulleys as a steel T10 belt. Check the minimum tooth count for the new cord, since Kevlar, high performance steel and stainless can need larger pulleys than standard steel. Profiles are not interchangeable: AT5 has the same 5 mm pitch as T5 but a larger tooth and needs AT5 pulleys.

What cords do rubber timing belts use?

Rubber (neoprene or EPDM) timing belts such as HTD, GT and classical trapezoidal belts use fiberglass cord as standard, for length stability and flex life. Some use aramid for shock loads. Steel-cord rubber belts are uncommon; in Gates' industrial catalog they are offered only as open-end long-length belting. Steel versus Kevlar is mainly a polyurethane belt decision.

Related guides: T-Series timing belts · AT-Series timing belts · imperial timing belts · timing belt tooth profiles · open-end timing belts · food-grade timing belts