Toothed Belts

What are toothed belts?

Toothed belts, also called timing belts or synchronous belts, are drive belts with teeth moulded on the inside that mesh with matching grooves on toothed pulleys. Because the teeth lock into the pulleys, the belt cannot slip, so the driven shaft turns in exact step with the driving shaft. This makes toothed belts ideal wherever precise timing, positioning, or a constant speed ratio is needed, such as in automation, robotics, and engine timing.

A standard toothed belt has four main parts:

• Teeth on the inside, which mesh with the pulley grooves
• Strong tension cords, usually glass fibre, aramid, or steel, which carry the load and stop the belt stretching
• A body made of rubber, such as neoprene, or polyurethane, which holds the cords and forms the teeth
• On many rubber belts, a nylon fabric facing on the teeth, which reduces wear and friction.

The teeth give a positive drive.

• Unlike V-belts and flat belts, which rely on friction, toothed belts drive through the teeth, so there is no slip or creep.
• This keeps shafts in exact synchronisation, with a constant speed ratio, even under changing loads.

Toothed belts are identified by their tooth profile and pitch, their length, and their width.

Why are toothed belts essential?

Many machines need parts to move in exact step, such as the axes of a robot, the carriage of a printer, or the camshaft and crankshaft of an engine. Toothed belts give this precise, repeatable movement in a quiet, light, and low-maintenance drive. They need no lubrication, run more quietly than chains or gears, and need less tension than friction belts, which reduces the load on shafts and bearings.

Toothed belts are also very efficient and can transmit high power at a wide range of speeds, from slow positioning drives to fast-running machinery. They are available in a wide range of pitches, widths, and lengths to suit precise drive requirements.

Typical applications include:

• Automation, robotics, and pick-and-place machines
• Linear actuators and positioning systems
• CNC machines, 3D printers, and machine tools
• Packaging, labelling, and printing machinery
• Engine camshaft drives
• Conveyors and indexing drives
• Office equipment, printers, and scanners
• Textile machinery and pumps

The main types of toothed belts

• Imperial trapezoidal belts (MXL, XL, L, H, XH) - Belts with trapezoidal teeth and inch-based pitches, from the very fine MXL to the heavy XH. Widely used on older and general machinery.
• Metric trapezoidal belts (T2.5, T5, T10, T20) - Belts with trapezoidal teeth and metric pitches, commonly made in polyurethane with steel cords. Popular in automation and linear motion.
• AT profile belts (AT5, AT10, AT20) - Metric belts with stronger, wider teeth than T profile belts, which carry more load and give better positioning accuracy.
• Curvilinear belts (HTD 3M, 5M, 8M, 14M) - Belts with rounded teeth, which spread the load more evenly and carry more power than trapezoidal belts of the same pitch. Widely used for power transmission.
• Modified curvilinear belts - Developed versions of curvilinear belts, with tooth shapes designed for very low backlash and high precision, used in robotics and positioning.
• Double-sided toothed belts - Belts with teeth on both sides, for drives where pulleys turn in opposite directions or the belt must drive from both faces.
• Open-length toothed belts - Belting supplied by the metre, usually polyurethane with steel cords, for linear motion drives where the belt ends are clamped.

How to choose the right toothed belt?

• Profile and pitch - The belt profile and pitch must match the pulleys exactly. Belts of different profiles will not mesh correctly, even if the pitch is similar.
• Length - Choose the correct pitch length, or number of teeth, for the drive. Toothed belts cannot be adjusted to make up for the wrong length.
• Width - Choose a width that suits the power and the pulleys. Wider belts carry more load.
• Material - Choose rubber belts for general power transmission and quiet running, and polyurethane belts for precision, cleanliness, and resistance to oils and wear.
• Precision - For accurate positioning, choose a low-backlash profile, such as AT or modified curvilinear, and a belt with low-stretch cords.
• Conditions - Check the belt suits the temperature, oil, and dust in the application.

Essential references and specialised ranges

How the numbering works:

Toothed belts are referenced by their profile, length, and width. Different profiles use different formats:

Imperial belts, for example 240L100:

240 = pitch length of 24.0 inches

L = L profile, with a 3/8 inch pitch

100 = width of 1.00 inch

Curvilinear metric belts, for example 1000-8M-20:

1000 = pitch length of 1000mm

8M = 8mm pitch curvilinear profile

20 = width of 20mm

Metric trapezoidal belts, for example T5 500, 16mm wide:

T5 = 5mm pitch trapezoidal profile

500 = pitch length of 500mm

16mm = belt width

The number of teeth can be worked out by dividing the pitch length by the pitch. For example, a 1000mm 8M belt has 1000 / 8 = 125 teeth.

Suffixes:

DL, DH, D = double-sided belt

PU = polyurethane belt

Some manufacturers add codes for the cord material, such as steel or aramid, or for special features such as antistatic or extra-strength belts.

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

FAQ - All you need to know about toothed belts

What is the difference between a toothed belt and a V-belt?

A toothed belt drives through teeth that mesh with the pulleys, so it cannot slip and keeps shafts in exact step. A V-belt drives by friction, so it can slip slightly under load. Toothed belts are used for timing and positioning, while V-belts are used for general power transmission.

Can I mix different tooth profiles?

No. The belt and pulley profiles must match exactly. For example, an HTD 5M belt will not mesh correctly with a T5 pulley, even though both have a 5mm pitch.

Do toothed belts need lubrication?

No. Toothed belts run dry and should never be oiled or greased, as this can damage the belt material and make it slip on the teeth.

How tight should a toothed belt be?

Toothed belts need less tension than friction belts, just enough to stop the teeth jumping under load. Too much tension wears the belt and bearings, and too little lets the belt skip teeth. Follow the belt or machine maker’s recommended tension.

How do I fit a toothed belt?

Loosen the motor or tensioner so the belt can be fitted without force. Never lever a toothed belt over the pulley flange, as this can damage the cords. Make sure the teeth mesh fully with the pulleys and, on timing drives, that the shafts are correctly aligned before tensioning.

How do I identify the belt I need?

Check the code printed on the back of the old belt. If it cannot be read, measure the pitch by counting teeth over a known length, count the total number of teeth, measure the width, and note the tooth shape.

How can I tell if a toothed belt needs replacing?

Warning signs include:

• Missing, worn, or sheared teeth
• Cracks on the back or between the teeth
• Exposed or frayed tension cords
• The belt skipping teeth or losing timing
• Excessive noise or whining from the drive
• Wear on the edges of the belt, a sign of misalignment