Leverage - Chain and Belt Drives

Chain and Belt Drives

Mechanisms consisting of two sprockets connected by a chain, or two pulleys connected by a belt are designed to provide a specific mechanical advantage in a power transmission systems.

The velocity v of the chain or belt is the same when in contact with the two sprockets or pulleys:

where the input sprocket or pulley A meshes with the chain or belt along the pitch radius rA and the output sprocket or pulley B meshes with this chain or belt along the pitch radius rB,

therefore

where NA is the number of teeth on the input sprocket and NB is the number of teeth on the output sprocket. For a timing belt drive, the number of teeth on the sprocket can be used. For friction belt drives the pitch radius of the input and output pulleys must be used.

The mechanical advantage of a pair of a chain drive or timing belt drive with an input sprocket with NA teeth and the output sprocket has NB teeth is given by

The mechanical advantage for friction belt drives is given by

Chains and belts dissipate power through friction, stretch and wear, which means the power output is actually less than the power input, which means the mechanical advantage of the real system will be less than that calculated for an ideal mechanism. A chain or belt drive can lose as much as 5% of the power through the system in friction heat, deformation and wear, in which case the efficiency of the drive is 95%.

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