Transmissions and Drives

Factors Affecting Choice of Power Transmission System

The following options are available:

Gears, Toothed belts (commonly called timing belts), Chains, V - belts, Multi - V - belts.

The important features of these systems are summarised below:

System Gears Chains Toothed belts V - Belt Multi - V
Centre distance Critical (1) (1) (1) (1)
Speed Ratio Exact Exact Exact Few % slip Few % slip
Cost Low to High Medium Medium Low Low
Installation Needs care Intermediate Intermediate Easy Easy
Power Capacity Low to very high High Medium Medium Medium
Lubrication Usually needed Essential Avoid oil etc. Avoid oil etc. Avoid oil etc.

Note - (1) Not critical with an idler.

There are 30+ British standards on gears and their manufacture, 10 on belts and belt drives and a few on chains.

Metal gears almost invariably need lubrication, lightly loaded plastic gears, frequently run with very little or no lubrication.

Not only is the centre distance critical for gears, but their axial alignment must also be good. Thermal expansion must be considered in high power density systems.

Gears must be manufactured to close tolerances.

Gears used in high load density applications are invariably hardened and tempered. The smaller gear (the pinion) is normally harder than the larger gear (the wheel). Higher hardness provides greater load carrying capacity. They should not have an integer ratio, but should have a 'hunting tooth' to give even wear.

Even with excellent accuracy of manufacture and installation, high power gear systems may well give off significant noise.

To transmit loads smoothly, helical gears must be used (more than one tooth is then always carrying the load) this results in end thrust, which must be accounted for in bearing selection.

Chains are usually quite difficult to fit if they do not have a connectiong link. Although some authorities frown on the use of connecting links, the speed and load rating is the same for chains whether they have a connecting link or not.

Oil must be kept away from V and multi V belts.

Some V belts have notches to reduce the bending stresses generated when they are bent round pulleys. Multi V - belts typically have 4 shallow 'Vs' to provide adequate friction area and the wedging effect of the 'V', while having a shallow depth to keep bending stresses lower than in a belt with a single V.

At very high speeds, oil/grease is thrown off gears and chains and special care is needed to ensure the supply of the correct quantity of lubricant in the appropriate manner.

Link to example belt calculation.

Link to Renold plc who market chain and gear products.

Link to HPC Gears Ltd who market a range of gears and other transmission products.

David J Grieve, Revised: 16th February 2010, 16th October 2006, Original 20th November 2001.