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.
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. 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.