T–3
HANDBOOK OF TIMING BELTS AND PULLEYS
SECTION 1 INTRODUCTION
Timing belts are parts of synchronous drives which represent an important category of drives.
Characteristically, these drives employ the positive engagement of two sets of meshing teeth.
Hence, they do not slip and there is no relative motion between the two elements in mesh.
Due to this feature, different parts of the drive will maintain a constant speed ratio or even a
permanent relative position. This is extremely important in applications such as automatic machinery
in which a definite motion sequence and/or indexing is involved.
The positive nature of these drives makes them capable of transmitting large torques and
withstanding large accelerations.
Belt drives are particularly useful in applications where layout flexibility is important. They
enable the designer to place components in more advantageous locations at larger distances
without paying a price penalty. Motors, which are usually the largest heat source, can be placed
away from the rest of the mechanism. Achieving this with a gear train would represent an expensive
solution.
Timing belts are basically flat belts with a series of evenly spaced teeth on the inside
circumference, thereby combining the advantages of the flat belt with the positive grip features of
chains and gears.
There is no slippage or creep as with plain flat belts. Required belt tension is low, therefore
producing very small bearing loads. Synchronous belts will not stretch and do not require lubrication.
Speed is transmitted uniformly because there is no chordal rise and fall of the pitch line as in the
case of roller chains.
The tooth profile of most commonly known synchronous belts is of trapezoidal shape with
sides being straight lines which generate an involute, similar to that of a spur gear tooth. As a
result, the profile of the pulley teeth is involute. Unlike the spur gear, however, the outside
diameter of a timing pulley is smaller than its pitch diameter, thus creating an imaginary pitch
diameter which is larger than the pulley itself. This is illustrated in Figure 1. Backlash between
pulley and belt teeth is negligible.
The trapezoidal shape timing belt was superseded by a curvilinear tooth profile which exhibited
some desirable and superior qualities. Advantages of this type of drive are as follows:
•
Proportionally deeper tooth; hence tooth jumping or loss of relative position is less probable.
•
Lighter construction, with correspondingly smaller centrifugal loss.
•
Smaller unit pressure on the tooth since area of contact is larger.
Fig. 1 Pulley and Belt Geometry
NOTE: Credit for portions of this technical section are given to: Gates Rubber Co., Sales Engineering Dept.,
Rubber Manufacturers Association (RMA), International Organization for Standardization (ISO).
Pitch (circular pitch)
Pulley Pitch Circle
Belt Pitch Line
Trapezodial
Tooth Profile
Curvilinear Tooth
Profile
Pitch (circular pitch)
Belt Pitch Line
Sprocket Pitch Circle
