Timing pulleys have either teeth or pockets, located radially around the outside diameter of the pulley body. Teeth engage timing holes in the metal belt; pockets engage drive lugs on the belt’s inner circumference. It should be noted that even in these pulleys, the driving is accomplished by frictional forces generated between the flat belt and pulley surfaces. Teeth or pockets are used only for timing, not for power transmission.
Timing elements, particularly timing teeth, must be hard. Hardness is essential to ensure minimal wear from successive engagements of belt and pulley. As an example, Belt Technologies’ patented pulley uses hardened ball bearings as teeth.
When designing a two pulley timing system, the drive pulley should be timed while the idler, or driven pulley, should be a friction drive pulley with relief channels for lugs if necessary.
Timing pulleys for metal belts are either toothed or pocketed, each engaging respective belt perforations or drive lugs.
Care should always be taken in the design of timing pulleys to ensure that all timing elements have spherical or involute radii. This ensures smooth engagement and disengagement of the belt and pulley. To avoid problems due to accumulated tolerances, the diameter difference between driving and driven components typically should be at least ±0.005” (0.127mm) to ±0.007” (0.178mm). Zero or near zero backlash applications are a special case.
When manufacturing a toothed pulley, each timing tooth is inserted into a hole machined in the pulley body. Great care must be given to the radial location of each tooth to ensure overall pitch accuracy.
While designing a timing pulley, it is critical that the pitch diameter be at the neutral axis of the belt (one half the belt thickness for a thin flat belt), not at the base. Since metal belts are generally thin, there is a temptation to neglect their thickness in calculating the pulley tape support diameter. Failure to include the belt thickness in these calculations results in mismatching of timing elements.
The tape support diameter can be determined by the formula:
D=NP/pi – t
Where:
N = number of pitch lengths or teeth on a pulley
P = perforation pitch
t = belt thickness
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