There are five main sections in the calculator, dealing with pulleys and belts (or chains and sprockets - see below), together with speed. In each section, only one variable is calculated, depending on which radio button is selected. In the first, a selection is made whether the system is belt or chain driven. Assume Belt drive for now (see later for Chain, for which all references to 'Pulley' in the following text would read 'Sprocket', and 'Belt' would read 'Chain').
In the second section, the first listed pulley is the driver. The driven pulley is the output, which may be for a drill, saw, polisher or other device. Most pulleys have a 'V' grove to accept a V-belt. If this is the case, the diameter should be measured to the root of the 'V'. If the drive pulley is larger than the driven pulley, the driven pulley speed will be higher than drive pulley speed by the ratio of the sizes, and vice versa. By selecting the appropriate radio button, the program determines the unknown from the other two values.
In the third section, the centre-to-centre measurement is the distance from the centre of the drive shaft to the centre of the driven shaft. The length of the belt is the circumference of the belt at the point it contacts and grips the pulleys. One value is calculated from the other parameter by selecting the appropriate radio button. If the belt size chosen is too short, or the centres are too close together, no calculation is done and a warning is given. If the gap between the pulleys is calculated to be less than 5% of the average of the pulley diameters, a warning is given since, although a belt might fit OK, the pulley flanges could clash.
The thickness of the belt is used to calculate its external length - see fifth section.
In the fourth section, the input RPM is the speed of the drive pulley (usually known from the motor speed) whilst the output RPM is the speed of the driven pulley. By selecting a radio button, its value is calculated from the other parameter.
The fifth section shows the minimum belt wrap angle for the driven pulley and the gap between pulleys. These are always calculated from the other factors - they are not input fields. The internal diameter of the belt (as if it were a circle) and the external belt length (allowing for its thickness) are also given since one or other of these is often quoted by suppliers.
Enter all known variables in sections above and click Calculate - the other factors are returned. If 'Calculate Automatically' is checked, clicking on any radio button will also initiate a calculation. Click on Clear Values to reset to defaults and clear the previous answers.
For Chains, the same principles apply, except it is more convenient to use chain links as the unit of measurement. This means the values entered for Sprockets (Pulleys) in the second section would be its number of teeth, and the value entered for the 'Belt length' would be the number of links in the chain. So that the distance between sprocket centres is meaningful, a Chain Pitch must be entered in the same measurement units (eg. mm) as the centre-to-centre distance. Although the chain length can be calculated from the sprocket centres, the number of chain links must obviously be an integer! (And preferably an even number.) So the answer given must be rounded up accordingly and the centre-to-centre distance recalculated. Please note this calculator can only assume a chain without a tensioner (as on a motorcycle). A bike with Derailleur gears has a tensioner, which requires extra links, the number of which depends on the tensioner. However, this calculator can give a rough guide to minimum chain length in this situation - for Derailleurs, enter the largest sprocket on each end, ie. for the lowest gear.
The Drive Ratio and Speed sections may also be used for gears, though obviously the other sections will be meaningless since gears must be on defined centres to mesh properly.
Belt, pulley and chain covers are useful to prevent inadvertent contact with fingers and clothing! Caution - ensure belts are rated for the pulley speeds and minimum bend radii.