Synchronous / Timing belts are basically endless flat belts which pass over pulleys- the belts having grooves which mate with teeth on the pulleys.  These belt drives, unlike flat and vee belt drives are positive.   Any slip of the belt relative to the pulleys is minor in degree and is due to belt stretch, or erosion of the grooves.  These belts are used for power transfer and for synchronised drives to ensure that the driven pulley is always rotating at a fixed speed ratio to the driving pulley.

The first synchronous belts had a trapezoidal tooth profile, and is identified as timing belts.   The belt tooth profile is a trapezoidal shape with sides being straight lines    The profile of the pulley teeth which mates with the belt is involute.  These belts are based on imperial (inch) pitch sizes and can provide power transmission up to 150 kW.

The development of the classical timing belt with has a rounded tooth (curvilinear tooth profile) and is identified as as the high torque drive, or HTD.  Advantages of this belt design include..

HTD sprockets have metric pitches (3 5 8 14 & 20) and can transmit up to 1000 kW.

The most advanced synchronous belts, has a modified rounded tooth profile with a higher tooth angle and shallower tooth.   These belts e.g Gates Powergrip GT have available pitch sizes of 2mm, 3mm & 5mm and can powers up to transmit up to 600 kW .   The belts have the advantages that they provide a smoother drive at higher accuracy,

A correctly designed and installed synchronous belt drive should operate successfully for between 8000 and 12000 hrs and have an operating efficiency of about 98%.

Synchronous belts have a number of advantages such that they are often used for applications not requiring shaft synchronization.   Their section and flexibility enable timing belts to operate very well on miniature drives and in applications involving high speeds or small pulleys.   They are extremely efficient when correctly installed.   They can also be specified to continuous high loads.    For these reasons, synchronous belts have proved to be cost effective in non-synchronous applications as drives for power saws, motorcycles, and domestic appliances.

The disadvantages of synchronous belt drives are that they are generally more costly compared to other belt drive options and the require accurate alignment of the pulleys for efficient reliable operation

Synchronous belts are made with elastomer e.g natural rubber,neoprene, polyurethane, polychioroprene, core with reinforcement to provide increased tensile strength.  These belts were originally reinforced with steel to provide the necessary strength.   In modern drives the most common reinforcement is glass fiber, but aramid is used if maximum capacity is required. Synchronous belts are often provided with nylon facings to provide the necessary wear resistance and can include conductive coatings.

Synchronous drive pulleys are often made of ductile or cast iron.   Aluminum is a often selected for drives that require low weight.   These applications can include high speed drives with low inertia.   Steel(and Stainless Steel )is preferred to iron when the drive will exceed the safe operating limits for cast iron (2000 mpm) or ductile iron (2500 to 3,000 mpm).
Plastic pulleys e.g. nylon are low-cost options when power requirements are low as in office machines or home appliances such as vacuum cleaners.   Plastic gears may also be acceptable when it is acceptable that the belt service life is short, as in some power tools, or lawn and garden equipment.

Pulleys are mounted to shafts using pins, keyways or by using proprietory shaft locking bushes such taperlock bushes. Pulleys can have one or two flanges to ensure the belts are retained in place.   For drives with horizontal pulley axes it is normal to have two flanges to retain the belt (two flanges on one pulley or one flange on each pulley on opposite sides).    On pulleys with vertical shaft axes the lower face of each pulley should include a flange and one pulley should include two flanges.

The British Standard for timing belt drives was
BS 4548:1987 :Specification for synchronous belt drives for industrial applications . This standard is still in use but is declared as obsolescent the current standard in europe for timing belt drives is
ISO 5294:1989: Synchronous belt drives -- Pulleys
ISO 5296-1:1989:1989: Synchronous belt drives -- Belts -- Part 1: Pitch codes MXL, XL, L, H, XH and XXH -- Metric and inch dimensions
This is not equivalent and belts and pulleys to the British Standard are not interchangeable with the ISO standard.

Note : The various notes below relate to the classical timing belt drives. For the more advanced drive belt design refer to manufactures literature... I will include notes on these belt drives at a later date...

Designing a Synchronous Belt System
Belt design procedures can be based on torque calculations or they can be based on power calculations.

1) The driven speed and the maximum driven torque required (including inertia load, shock loads, friction, etc) are used to calculate the required driven power

2) From information on the driver, driven equipment and operating period a service factor is obtained - see below

3) A design power is obtained based on the product of the Driven Power required and the service factor .

4) A belt section is initially selected using a graph as typically shown below

5) A drive geometry is derived selecting suitable pulleys, and belt Centre Distance - Some Pulley sizes are provided below

6) A Basic Power for the belt is calculated and a mesh factor is calculated - see below

7) A suitable belt width is selected -Using a table as provided below- Some iteration may be required

The classical MXL belt and the Curvilinear more advanced belt options are designed based on torque levels.  The outline method for the MXL drive is provided below.    The method used for the HTD and other modern belt options will be provided at some future date...


The MXL belts operate generally at relatively low belt speeds so the torque levels are similar for the normal range of pulley rotational speed.  Torque ratings can be calculated of each of the MXL belt widths as follows: I have converted an imperial formula to a metric formula and minor differences with the original formulae results..

Torque ratings of belts T_r (Nm) at P₂ PCDs (mm)

Belt width =3.048 mm... T_r = P₂(5,03 - 9,5147.10^-6.P₂²).10^-3
Belt width =4.826mm... T_r = P₂(8,36 - 1,586.10^-5.P₂²).10^-3
Belt width =6.35 mm...T_r = P₂(11,7 - 2,213.10^-5.P₂²).10^-3

1) The driven speed and the maximum driven torque required (including inertia load, shock loads, friction, etc) are calculated

2) From information on the driver, driven equipment and operating period a service factor is obtained - see below

3) A design torque is obtained based on the product of the torque required and the service factor .

4) A belt section is initially selected (assuming MXL) using a graph as typically shown below

5) A drive geometry is derived selecting suitable pulleys, and belt Centre Distance - Some Pulley sizes are provided below

6) The design torque is divided by the teeth mesh factor (see below) to arrive at an adjusted torque

7) The table below is used to select the belt width which has a torque value equal to or larger than the corrected torque

When designing belt drives it is normal to apply a service factor to the drive operating load to compensate for allow for different driver type, driven load types and operating periods.  Typical service factor values are included on the linked page Service Factors

Synchronous Belt sizes are identified by a standard number.   The first digits specify the belt length to one-tenth inch followed by the belt section (pitch) designation.   The digits following the belt section designation represent the nominal belt width times 100.   For example, an L section belt 30.000 inches pitch length and 0.75 inches in width would be specified as a 300L075 Synchronous Belt.  A similar method is used for designating metric belt designations

When the design power has been determined (Power x Service Factor) a synchronous belt can be selected generally using a graph similar to the one below..This is provided for guidance only and is copied from published graphs generally available.

This method is based on the method shown in Machinery's handbook.  It is preferable to use the calculation tool provided by the belt manufacturers to size the belts for detail design.   Or even better let the suppliers do the design for you...

The Power ratings of belts for the basic belt widths (in brackets) are as identified below..

For Belt (width-mm) = XL (9.652)...... P_r = 0.746.Z.(0,0916 - 7,07.10^-5.Z² )
For Belt (width-,mm) = L (25,4)...... P_r = 0.746.Z.(0,436 - 3,01.10^-4.Z² )
For Belt (width-mm) = H (76,2)...... P_r =0.746.Z.(3,73- 1,41.10^-3.Z² )
For Belt (width-mm) = XH (101,6)...... P_r = 0.746.Z.(7,21 - 4,68.10^-3.Z² )
For Belt (width-mm) = XXH (127)...... P_r =0.746.Z.(11,4 - 7,81.10^-3.Z² )

1) The Pitch dia of a pulley P = No Teeth on Pulley . Pitch /p
2) The Drop distance d = [ P₁ - P₂ ] /2
3) The belt contact angle α = arcsin(d /C) ..C= Centre distance
4) The belt fall length = fl = d / tan α
5) The contact length Small Pulley= CL₂= P₂. p. [90 - α]/180 degrees
6) The contact length Large Pulley = CL₁=P₁.p. [90 + α]/180 degrees
7) The Belt Length L = 2.fl + CL₁ + CL₂
8) Total number of teeth on belt = L / Pitch
9) Number of teeth in mesh (small pulley) = CL₂ /Pitch. Rounded down to nearest whole number.

The horsepower ratings obtained above are based on the smallest pulleys having six or more teeth in mesh. For drives with small angles of lap on the smallest pulleys the mesh factor is required.

1) First establish the design power to be transferred(kW) = Service Factor x Power.
2) Select a suitable belt and calculate the basic power using the belt size, smaller pulley speed, and smaller pulley size.
3) If the basic belt power is less than the design power- change one or more of belt size , pulley size or speed.
3) Divide the Basic power/ Design power to obtain a belt width factor.
4) Use the table below and select a width with a width factor higher than the calculated width factor required

Below are listed a collection of pulley Dimensions (PCD and OD) for pulleys in the classical timing belt range.   In practice there are a vast number of pulleys available from suppliers on the belt sections shown and on other higher specification sections.   Additional data is available using the links below and preferable by contacting the suppliers.