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The trapezoidal shape timing belt was superseded by a curvilinear tooth profile which exhibited some desirable and superior qualities.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.
Pitch (circular pitch)
Belt Pitch Line
Sprocket Pitch Circle
Curvilinear Tooth
Profile
Pitch (circular pitch)
Trapezoidal
Tooth Profile
Pulley Pitch Circle
Belt Pitch Line
Tension Member
Trapezoidal Curvilinear
Fig. 2 Stress Pattern in Belts
• Greater shear strength due to larger tooth cross section.
• Lower cost since a narrower belt will handle larger load.
• Energy efficient, particularly if replacing a "V" belt drive which incurs energy losses due to slippage.
• Installation tension is small, therefore, light bearing loads.
In Figure 2, the photoelastic pattern shows the stress distribution within teeth of different geometry. There is a definite stress concentration near the root of the trapezoidal belt tooth, with very low strains elsewhere. For the curvilinear tooth, there is a uniform, nearly constant, strain distribution across the belt. The load is largest in the direction of the tension member to which it is
transferred.
Because of their superior load carrying capabilities, the curvilinear belts are marketed under the name of Gates' HTD drives. This is an abbreviation of High Torque Drives.
As a result of continuous research, a newer version of the curvilinear technology was developed by Gates, which was designated as Gates' PowerGrip GT belt drives.
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