Open PU timing belts by the metre – complete technical guide to profile selection, reinforcement, nylon (PAZ/PAR), splice and dimensioning

1) What are open PU timing belts – and when do they make sense?

An open PU timing belt is a extruded timing belt in thermoplastic polyurethane, which is supplied “open” (not endless). The tooth profile engages a gear, giving you a positive form closure and thus a slip-free power transmission – unlike friction drives. In practice, open timing belts are used especially when you need:

• Flexible length: meterial makes prototypes and service faster.
• Linear motion: The belt acts as a “timing bar” where the carriage runs along the belt.
• Repeatability: tooth engagement provides stable positioning (depends on profile and splice).
• High cycle / automation: PU is durable and stable over time when properly set up.

2) Material and mechanical properties: PU , Shore, friction and wear

Polyurethane ( PU ) is often chosen for open timing belts because of its. high wear resistance, good dimensional stability and the possibility of several variants (cords, nylon, special surfaces). Many PU timing belts are typically around ~92 Shore A in hardness, which provides a good balance between flexibility and resilience.

In technical practice, it is important to distinguish between belt tooth engagement (position/power transmission) and surface contact (e.g. when the belt runs against a slide rail, or when items are on the back of the belt). Here, nylon (PAZ/PAR) and/or coating play a major role in friction, noise and service life.

3) Profiles and tooth pitch: T, AT, HTD/RPP and inch profiles

Profile selection is one of the biggest “SEO drivers” because many people search for specific profiles: T5, T10, AT10, HTD, RPP etc. The basic rule is simple: profile and tooth pitch must match the gearsIf you already have gears, the profile is locked. If you are designing from scratch, the profile can be selected based on load, precision and space.

T-profiles (metric trapezoidal profile)

T-profiles are classic trapezoidal profiles and are typically found in T2.5, T5, T10 and T20They are used in many “standard” automation and transportation tasks where the requirements are robust and the cost/benefit must be reasonable.

Link: Open PU timing belts in T-profile

AT profiles (improved tooth geometry – higher capacity)

AT profiles (e.g AT3, AT5, AT10, AT20) has a geometry that typically gives better power transmission per width, higher stability and often better start/stop characteristics. It is therefore a pu profile in handling, positioning and high cycle and dynamic applications.

Link: Open PU timing belts in AT profile

HTD / RPP / other curve profiles

HTD and RPP are examples of profiles with a more curved tooth shape (instead of the classic trapezoid), which are often used in applications where there is a focus on load distribution, noise and “smooth” engagement. If you have a system designed for HTD/RPP, the belt and pulley must be of the same standard.

Inch profiles (imperial)

On older or imported machines you often see XL/L/H/XH (inch profiles). Here it is critical to identify the correct profile, as the tooth pitch and tooth shape do not match metric T/AT.

4) Reinforcement (cords): steel, aramid/kevlar and stainless steel

The reinforcement in a PU timing belt (cords) largely determines tensile strength, longitudinal stability and how the belt behaves under dynamic loads (accelerations, rebound, high cycle). Typical choices are:

• Galvanized steel cord: high tensile strength and good longitudinal stability – often the standard choice in industrial environments.
• Aramid/Kevlar: often used when flexibility and lower mass are relevant, or where certain dynamic properties are desired.
• Stainless steel cord: relevant for moisture, cleaning, aggressive environments or corrosion requirements.

Technical consequences of cord choice

• Positioning: Higher length stability provides better repeatability in linear systems.
• Voltage: Correct tension is critical – especially if you want to avoid tooth skipping and uneven loading.
• Environment: Stainless steel is often chosen to minimize risk in wet/chemical zones.

5) Nylon/surfaces: PAZ & PAR – friction, noise and wear

Nylon webbing is used to control friction, wear and noise. This is especially relevant when the belt contacts belt sliding guides, or when you want to reduce friction between the tooth and the pulley – or protect the surface.

• PAZ: nylon on the teeth – can reduce friction against gears and improve driving characteristics.
• COUPLE: nylon on the back – for sliding contact against the rail/backplate.

In practice, it's about where the contact and wear is in your machine: is it in the tooth engagement, on the back, or both? The right surface can significantly extend the life in high cycles.

6) Dimensioning: width, gears, tension and geometry

Sizing open PU timing belts typically requires mastering (1) profile/gear, (2) width, (3) center distance/geometry, (4) load, and (5) speed/acceleration. Here is a practical “engineering” sequence:

6.1 Profile and gears (compatibility first)

The profile must match the sprockets. If you are designing new, larger tooth pitch and/or AT profile often gives higher capacity per width, but also requires appropriate pulleys and geometry.

6.2 Belt width (capacity and service life)

Width is selected based on load and desired service life. Too narrow a belt can give high load per tooth and increase the risk of wear/tooth skipping. Too wide a belt can be unnecessarily expensive and require more space. In linear axes, the width is also selected based on stiffness and desired positioning stability.

6.3 Voltage (critical for operation)

Too low a tension can cause tooth skipping, noise and uneven power transmission. Too high a tension increases bearing/shaft load and can cause premature wear. The optimum depends on profile, length, cord and operation (continuous vs. start/stop).

6.4 Geometry and enclosure

Ensure sufficient wrap angle and number of teeth in mesh, especially under high load and acceleration. Pulleys that are too small or too few teeth in mesh increase the risk of tooth loading and wear.

7) Connection: welding (finger) vs. pin connection – strength and tolerances

Open timing belts can be used “open” in linear systems or assembled into an endless belt. The choice depends on the installation and load. In general: a joint is a weaker pu than a factory endless belt without a joint.

Welding (finger joint)

Welding melts and presses the ends together. This provides a neat solution, but strength and life depend on tooling, temperature, pressure, cleanliness and proper alignment of the ends. In high load/high acceleration applications, you should consider whether welding is sufficient.

Mechanical pin joint

Pin joint may be relevant when the belt needs to be installed without major dissplice, or when replacement needs to be quick. It is often a “service-friendly” solution, but always consider noise, dynamics and tolerances in the system.

8) Splice & adjustment: alignment, tooth engagement and tension

Many problems are not caused by the belt – but splice. The three classic mistakes are: (1) poor alignment, (2) incorrect tension, (3) too small pulleys / too few teeth in mesh. A few practical technical tips:

• Alignment: Gears must be parallel and in the same plane. Small angular errors cause edge loading and jagged edges.
• Voltage: Measure and document tension in service – especially in linear systems with high repeatability.
• Purity: Sharp edges, chips and foreign objects quickly cause damage – especially at high cycles.

9) Troubleshooting: the typical errors – and what you do about them

9.1 The belt “jumps”, loses teeth or makes noise

• Check voltage (low voltage is a classic).
• Check that profile matches sprocket (T vs. AT etc.).
• Increase the engagement/number of teeth in engagement (idler/shift) at high load.

9.2 Frayed edges / uneven wear

• Alignment: misaligned pulleys or side loading cause edge friction.
• Consider belt width vs. load and steering.
• Check belt sliding guides/edges and surfaces (PAZ/PAR can help with sliding contact).

9.3 Splice breaks down or “breaks” during operation

• Assess whether the load/dynamics is too high for the selected joint.
• Consider pin joint or endless solution.
• When welding: verify process (temperature/pressure/time) and end preparation.

FAQ about open PU timing belts

What does “open-end” / by the yard mean?

This means that the timing belt is delivered as an open belt on a roll (by the meter). You can order a length in meters, or have it cut to the desired length.

What is the difference between T-profile and AT-profile?

T-profile is a classic trapezoidal profile and is used in many standard applications. AT-profile typically has a geometry that provides better capacity/stability, and is often chosen for more demanding start/stop and transport handling. If you have existing gears, the profile must match.

Can I get nylon on my teeth or on the back?

Yes – nylon fabric is typically used as PAZ (on teeth) and PAR (on back/back) to control friction, wear and noise.

Can I assemble an open PU timing belt for endless use myself?

Yes. This is typically done by welding (finger joint) with appropriate tools, or via mechanical pin jointing depending on the application.

Where can I buy open PU timing belts by the meter?

You can see the range and choose a profile here: Open PU timing belts (per meter).