Timing Belt Pulleys

What are timing belt pulleys?

Timing belt pulleys, also called synchronous pulleys or toothed pulleys, are pulleys with teeth around their outside that mesh with the teeth of a timing belt. Instead of relying on friction, like a V-belt drive, the belt and pulley teeth lock together, so the drive cannot slip. This keeps the driving and driven shafts perfectly in step, making timing belt pulleys essential for automation, positioning, and any drive where exact speed and timing matter.

A standard timing belt pulley is made up of:

• A toothed rim, with teeth shaped to match a specific belt profile and pitch
• Side flanges on one or both sides, which stop the belt from running off the pulley
• A hub or body that connects the toothed rim to the bore
• A bore, either a pilot bore for machining or a tapered bore for a taper bush.

The tooth profile and pitch must match the belt exactly.

• The pitch is the distance between the centres of neighbouring teeth, such as 5mm, 8mm, or 14mm.
• The profile is the shape of the teeth, such as curved or trapezoidal, and a pulley will only run correctly with a belt of the same profile and pitch.

Timing belt pulleys are described by their number of teeth, pitch, profile, belt width, and bore type.

Why are timing belt pulleys essential?

Timing belt pulleys give a positive, slip-free drive that keeps shafts in exact synchronisation. This accuracy is vital in machines that index, position, or must keep several parts moving in time with each other.

Compared with chain drives, timing belt drives run quietly, need no lubrication, and wear very little. Compared with V-belt drives, they need less belt tension, which reduces the load on bearings, and they keep their speed ratio exactly under load.

Typical applications include:

• Automation and robotics
• Packaging and labelling machinery
• CNC machines and linear actuators
• Printing and textile machinery
• Conveyors and indexing tables
• Food processing machinery
• Pumps, compressors, and motor drives
• Office equipment and 3D printers

The main types of timing belt pulleys

• Pilot bore pulleys - Pulleys supplied with a small bore, which is machined out by the customer to suit the shaft, usually with a keyway and set screw added.
• Taper bore pulleys - Pulleys with a tapered bore that takes a separate taper bush, so one pulley can fit a range of shaft sizes and is quick to fit and remove.
• Flanged and unflanged pulleys - Flanges guide the belt and stop it running off. Smaller pulleys in a drive usually have flanges, while larger pulleys are often supplied without.
• Curvilinear profile pulleys - Pulleys for belts with rounded teeth, such as 3M, 5M, 8M, and 14M, which carry high loads and run quietly.
• Trapezoidal profile pulleys - Pulleys for belts with straight-sided teeth, in metric pitches such as T5 and T10, and imperial pitches such as XL, L, and H.
• Bar stock timing pulleys - Long toothed bars that are cut and machined to make custom pulleys, ideal for one-off sizes, prototypes, and repairs.
• Aluminium and steel pulleys - Aluminium pulleys are light and used for smaller, faster drives, while steel and cast iron pulleys are used for higher-power drives.

How to choose the right timing belt pulleys?

• Profile and pitch - Choose a pulley with the same tooth profile and pitch as the belt. Mixing profiles causes noise, wear, and teeth skipping.
• Number of teeth - Choose the number of teeth to give the speed ratio needed. Avoid very small pulleys, as fewer teeth in mesh reduce the power the drive can carry.
• Belt width - Choose a pulley wide enough for the belt, as wider belts carry more power.
• Bore type - Choose a taper bore pulley for quick fitting and easy removal, or a pilot bore pulley where the bore will be machined to suit the shaft.
• Flanges - Make sure at least one pulley in the drive is flanged, or both on drives with vertical shafts or long belts.
• Material - Choose aluminium for light, fast-running drives, or steel or cast iron for heavier loads.

Essential references and specialised ranges

How the numbering works:

Metric timing pulleys are usually referenced by the number of teeth, the pitch and profile, and the belt width in millimetres. For example:

32-5M-15 = 32 teeth, 5mm pitch curvilinear profile, for a 15mm wide belt

44-8M-30 = 44 teeth, 8mm pitch curvilinear profile, for a 30mm wide belt

Imperial pulleys give the belt width in hundredths of an inch. For example:

20XL037 = 20 teeth, XL pitch, for a 3/8 inch (0.37 inch) wide belt

24L100 = 24 teeth, L pitch, for a 1 inch wide belt

Taper bore pulleys may also show the taper bush size, such as 1610 or 2012.

Suffixes:

TB = taper bore, for a taper bush

PB = pilot bore, ready for machining

F = flanged

Suffix codes vary between manufacturers, so check the supplier’s catalogue for the exact meaning.

FAQ - All you need to know about timing belt pulleys

What is the difference between a timing pulley and a V-belt pulley?

A timing pulley has teeth that mesh with a toothed belt, so it cannot slip and keeps exact timing. A V-belt pulley has smooth grooves and drives by friction, so a small amount of slip is normal.

Can I use a T5 belt on a 5M pulley?

No. Although both have a 5mm pitch, the tooth shapes are different. The belt will not mesh correctly and will wear quickly or skip teeth.

Why do timing pulleys have flanges?

Timing belts tend to move sideways as they run. The flanges keep the belt on the pulley. Usually at least one pulley in a drive needs flanges.

What is bar stock used for?

Bar stock is a long length of toothed material that can be cut and machined into pulleys of any width and bore. It is ideal for custom sizes, prototypes, and urgent repairs.

How do I fit a taper bore timing pulley?

Clean the bore and bush, fit the bush into the pulley, slide it onto the shaft, and tighten the screws evenly to the recommended torque. Line up the pulleys with a straight edge before fitting the belt.

How do I identify the pulley I need?

Check the code on the pulley or belt. If it cannot be read, count the teeth, measure the pitch between teeth and the belt width, check the tooth shape, and note the bore type and size.

How can I tell if a timing pulley needs replacing?

Warning signs include:

• Worn, hooked, or polished teeth
• The belt skipping teeth or jumping under load
• Bent or damaged flanges
• Rapid belt wear or wear on one edge of the belt
• Increased noise or whining from the drive
• A loose pulley, worn bore, or damaged keyway