The two compounds split mainly on continuous temperature: polyurethane wins on abrasion, dimensional stability and cleanability up to about +80 °C, while above that — and in hot wet service — only a rubber body such as CR, NBR or HNBR survives.

Everything else in this comparison follows from that single boundary plus two secondary ones: how much abrasion the drive sees, and whether the belt has to be joined on site. Get those three right and the remaining differences are refinements.

Both families are specified under the same profile standards — ISO 13050 for metric T and curvilinear tooth forms, ISO 5296 for trapezoidal pitch codes — so the choice of body compound is a separate decision from the choice of tooth profile. Abrasion is compared on the ISO 4649 rotating-drum basis, elastic behaviour on ISO 21181, and food-contact grades are declared against FDA 21 CFR 177.2600 or EU Regulation (EU) No 10/2011.

What the two materials actually are

Rubber bodies are compounded elastomers, cured with sulphur or peroxide, usually loaded with carbon black. The family matters more than the word "rubber":

• CR (neoprene) — the classic timing-belt body. Balanced heat, oil and weather resistance, good flex fatigue, moderate cost.
• NBR — better oil and grease resistance than CR, slightly lower heat ceiling.
• HNBR — hydrogenated NBR, the high-temperature option in this family, with oil resistance retained.
• EPDM — excellent steam, ozone and weather resistance, but poor oil resistance; used where the drive sees water rather than oil.

Polyurethane bodies are cast or thermoplastic PU. Two sub-types behave differently in service:

• Ester-based PU — better abrasion and oil resistance, but hydrolyses in hot humid conditions. This is the failure mode that gives PU its bad reputation in washdown plants.
• Ether-based PU — markedly better hydrolysis resistance at some cost in mechanical strength; the correct choice for damp or intermittently wet service.

PU is harder than a rubber body, contains no carbon black (so it can be produced white or translucent), and can be welded into an endless belt on site — a practical difference that rubber cannot match.

Side-by-side comparison

Rubber and polyurethane timing-belt bodies compared — mechanical values are industry-typical ranges per compound family

*Source: temperature, hardness, tensile and abrasion figures are industry-typical ranges published for these compound families, not test data for any specific belt; a real specification should use the manufacturer's data sheet for the grade quoted. Abrasion is quoted as volume loss on the ISO 4649 basis, where a lower figure means better resistance.*

Six differences that decide the choice

1. Temperature ceiling — PU runs to roughly +80 °C continuous, CR to about +100 °C, HNBR to about +140 °C → a drive running above 80 °C next to a sealing jaw or a dryer eliminates PU before any other criterion is considered.
2. Hydrolysis in hot wet service — ester-based PU absorbs water and loses strength at elevated humidity, while rubber is unaffected → washdown lines above roughly 60 °C, steam tunnels and bottle-washer exits are rubber territory, and a PU belt there fails by cracking rather than by wear.
3. Abrasion resistance — PU loses roughly a third to a fifth of the volume that rubber loses under the same ISO 4649 test → abrasive dust, high slip, and frequent start-stop duty all favour PU, and this is the single largest mechanical advantage it holds.
4. Stiffness and elongation — PU's higher modulus means less stretch under working tension → indexing and registration duties need less take-up travel and hold position better, while rubber's compliance absorbs shock more gracefully on drives with jam-prone loads.
5. Noise and damping — rubber damps vibration, PU transmits it → a drive near an operator station or in a quiet packaging hall is often decided on noise before wear life is considered.
6. Joining and cleanability — PU can be welded into an endless belt on the machine and can be produced white or translucent, while rubber requires a vulcanised joint and is normally black → retrofit work on an installed machine, and food lines where a visible belt is an inspection requirement, both push toward PU.

The cost question, honestly

Unit price favours rubber, usually by a clear margin, and that is where most comparisons stop. The comparison that matters is the two-year figure: unit price × replacements + downtime per change × cost per minute + scrap at each restart.

Two things move that number. First, PU's abrasion advantage lengthens the interval between replacements in abrasive duty, which can outweigh the higher unit price. Second, PU's temperature ceiling shortens it dramatically in hot service — a PU belt running at 95 °C may last a fraction of the interval a CR belt achieves, and the arithmetic inverts. Neither outcome is universal, which is why the temperature and abrasion figures matter more than the price list.

Which one to specify

• Above +80 °C continuous → rubber, HNBR where oil is also present, CR otherwise. Do not specify PU and compensate with a wider belt.
• Hot washdown, steam, or humidity above roughly 80 % at elevated temperature → rubber, or ether-based PU only if the temperature stays below about 60 °C.
• Heavy abrasive dust, high slip, frequent starts → PU, because the abrasion advantage compounds over every replacement interval.
• Indexing, registration or positioning duty → PU, for the lower elongation and the shorter take-up travel it needs.
• Drive that must be re-belted without dismantling the machine → PU, because the belt can be welded on site.
• Food contact where a white or visibly clean belt is required → PU in a declared food-contact grade; confirm the grade against FDA 21 CFR 177.2600 or EU Regulation (EU) No 10/2011 rather than assuming "PU" means food-safe.
• Oil mist or grease carry-over → HNBR rubber, or a PU grade confirmed against the specific oil; CR is only adequate for occasional contact.
• Noise-sensitive installation with a modest duty → rubber, for the damping.
• Cost-driven, high-volume, moderate temperature and clean dry duty → rubber; it remains the lower-cost answer where none of the conditions above applies.

FAQ

Can a PU timing belt replace a rubber one on the same pulleys?

Usually yes, if the profile and pitch match and the operating temperature stays below about +80 °C, but three things must be checked first: the pulley tooth form and pitch code must be the same system, the belt's higher stiffness must not exceed what the take-up range can absorb, and the duty must be dry enough for the PU grade chosen. A PU belt fitted to a drive that runs hot or wet will fail earlier than the rubber belt it replaced, and the failure will look like cracking rather than wear.

Why do PU belts crack in washdown plants?

Because ester-based polyurethane hydrolyses — water molecules break the ester bonds in the polymer, and the belt loses strength and embrittles from the inside. The crack pattern starts at the tooth root and the back, not at the working face, which is why it is often misdiagnosed as overload. Ether-based PU resists this far better, but above roughly 60 °C in continuous wet service a rubber body remains the safer specification.

Is PU always the better choice for abrasion?

For pure abrasion resistance, yes — PU typically loses a fraction of the volume rubber loses on the same ISO 4649 test, and that advantage shows up as longer intervals in dusty or high-slip duty. But abrasion is only one of the loads acting on a belt. In a hot drive, a rubber belt that lasts a year beats a PU belt that cracks in a month, and in an oil-mist environment a standard PU grade may swell where HNBR does not.

Standards referenced

• ISO 13050 — synchronous belt drives: metric pitch curvilinear profile systems.
• ISO 5296 — synchronous belt drives: belt pitch codes and dimensional characteristics (trapezoidal profiles).
• ISO 4649 — rubber, vulcanised or thermoplastic: determination of abrasion resistance using a rotating cylindrical drum device; the basis for the abrasion figures in this article.
• ISO 21181 — light conveyor belts: determination of the relaxed elastic modulus; the basis for comparing elongation under working tension.
• ISO 254 — belt drives, pulleys: quality, finish and balance requirements.
• FDA 21 CFR 177.2600 and EU Regulation (EU) No 10/2011 — food-contact compliance routes for rubber and polyurethane articles intended for repeated use.
• ISO 1813 — antistatic characteristics, where a static-dissipative belt is required.

Confirm the current edition of each standard before quoting it, and take the actual temperature, hardness and abrasion values from the manufacturer's data sheet for the grade being specified rather than from the typical ranges above.

Zhende Machinery Co., Ltd., established in 2019 and headquartered in Dongguan, China, is a specialized manufacturer dedicated to delivering high-quality power transmission solutions to global markets.

The company focuses on the design, production, and distribution of timing belts and timing pulleys, with core expertise in customized timing belts. To enhance purchasing efficiency, Zhende Machinery provides a seamless, one-stop procurement service, helping global clients streamline their supply chain, minimize lead times, and reduce operational costs.