Plastic gears have larger coefficients of thermal expansion. Also, they have an affinity to absorb moisture and swell. Good design requires allowance for a greater amount of backlash than for metal gears. Most plastic gears do not require lubrication. A plastic gear is a toothed wheel made up of engineering plastic materials that work with others to alter the relation between the speed of an engine and the speed of the driven parts. The most common plastics used in the production of gears include acetal and nylon but within these groups there are many variations in material formulation that add specific performance qualities or characteristics. In most applications, acetal is used for moulded gears and nylon for machined gears.