T–42
SECTION 9 DESIGN AND INSTALLATION SUGGESTIONS
There are some general guidelines which are applicable to all timing belts, including miniature
and double-sided belts:
1. Drives should always be designed with ample reserve horsepower capacity. Use of
overload service factors is important. Belts should be rated at only 1/15th of their
respective ultimate strength.
2. For MXL pitch belts, the smallest recommended pulley will have 10 teeth. For other
pitches, Table 18, on the previous page, should be used.
3. The pulley diameter should never be smaller than the width of the belt.
4. Belts with Fibrex-glass fiber tension members should not be subjected to sharp bends or
rough handling, since this could cause breakage of the fibers.
5. In order to deliver the rated horsepower, a belt must have six or more teeth in mesh with
the grooves of the smaller pulley. The number of teeth in mesh may be obtained by
formula given in SECTION 24 TIMING BELT DRIVE SELECTION PROCEDURE. The
shear strength of a single tooth is only a fraction of the belt break strength.
6. Because of a slight side thrust of synchronous belts in motion, at least one pulley in the
drive must be flanged. When the center distance between the shafts is 8 or more times
the diameter of the smaller pulley, or when the drive is operating on vertical shafts, both
pulleys should be flanged.
7. Belt surface speed should not exceed 5500 feet per minute (28 m/s) for larger pitch belts
and 10000 feet per minute (50 m/s) for minipitch belts. For the HTD belts, a speed of
6500 feet per minute (33 m/s) is permitted, whereas for GT2 belts, the maximum permitted
speed is 7500 feet per minute (38 m/s). The maximum allowable operating speed for T
series is 4000 feet per minute (20 m/s).
8. Belts are, in general, rated to yield a minimum of 3000 hours of useful life if all instructions
are properly followed.
9. Belt drives are inherently efficient. It can be assumed that the efficiency of a synchronous
belt drive is greater than 95%.
10. Belt drives are usually a source of noise. The frequency of the noise level increases
proportionally with the belt speed. The higher the initial belt tension, the greater the
noise level. The belt teeth entering the pulleys at high speed act as a compressor and
this creates noise. Some noise is the result of a belt rubbing against the flange, which in
turn may be the result of the shafts not being parallel. As shown in Figure 9 (page T-9),
the noise level is substantially reduced if the PowerGrip GT2 belt is being used.
11. If the drive is part of a sensitive acoustical or electronics sensing or recording device, it is
recommended that the back surfaces of the belt be ground to assure absolutely uniform
belt thickness.
12. For some applications, no backlash between the driving and the driven shaft is permitted.
For these cases, special profile pulleys can be produced without any clearance between
the belt tooth and pulley. This may shorten the belt life, but it eliminates backlash.
Figure 10 (page T-9) shows the superiority of PowerGrip GT2 profile as far as reduction
of backlash is concerned.
13. Synchronous belts are often driven by stepping motors. These drives are subjected to
continuous and large accelerations and decelerations. If the belt reinforcing fiber, i.e.,
tension member, as well as the belt material, have high tensile strength and no elongation,
the belt will not be instrumental in absorbing the shock loads. This will result in sheared
belt teeth. Therefore, take this into account when the size of the smallest pulley and the
materials for the belt and tension member are selected.
14. The choice of the pulley material (metal vs. plastic) is a matter of price, desired precision,
inertia, color, magnetic properties and, above all, personal preference based on
experiences. Plastic pulleys with metal inserts or metal hubs represent a good compromise.
