The Technological World The Amazing World of Gears

Gear Theory What is the purpose of gears? To increase torque and decrease speed To increase speed and decrease torque To change the direction of rotation by 90° To reverse the direction of rotation

Gear Theory Regardless of what you do you will only ever have two main types of gears: Driving Gears and Driven Gears Driving Gears are connected directly to the power source and transfer the power to the Driven Gears. Driven Gears are in contact with the Driving Gears and either increase the torque or increase the speed.

Principles of Gearing When a small gear drives a large gear you are said to have a gear reduction. Where the driving gear would turn several times for only one turn of the driven gear. When a large gear drives a small gear you are said to have an overdrive. Where the driving gear would turn only once and the driven gear would turn several times..

Principles of Gearing When two gears of the exact same size are in contact, it is said to be direct drive. Where there is neither a torque increase nor a speed increase. An example of this would be the top gear of a transmission before they had overdrive!

Principles of Gearing If you have an even number of gears in contact, regardless of their size, the last gear will always turn in the opposite direction of the first gear in the set. Example: If the top gear drives the bottom gear, the bottom gear will rotate in the opposite direction!

Principles of Gearing If you have an odd number of gears in contact, regardless of their size, the last gear in the set will always rotate in the same direction as the first gear in the set. Example: If the first gear rotates clockwise every odd numbered gear must also rotate clockwise.

How to determine Gear Ratio To calculate gear ratio you must always use the following formula: Number of Teeth on the Driver Gear divided by the Number of Teeth on the Driven Gear. Example 1: If the driven gear has 8 teeth and the driving gear has 41 teeth, the ratio is said to be 5.13:1 41 ÷ 8 = 5.13

How Gear Ratio Affects Output If a gear ratio has a number numerically larger than 1 at the front of the ratio, you have a gear reduction. Example 4 : 1 A gear reduction means that the speed of the output gear is Χ times slower than the input gear. At the same time a gear reduction means that the output torque will be increase by X times over the input torque.

If a gear ratio has a number numerically smaller than 1 at the front of the ratio, you have an overdrive. Example 0.75 : 1 An overdrive means that the output gear will be turning X times faster than the input gear. An overdrive means that the output torque will be X times smaller than the input torque.

If both input and output gears, driven and driving, are the exact same size then there is no advantage. There will be neither a speed increase nor a torque increase. There would simply be a direct transfer of power.

Review To recap: Torque is inversely proportional to Speed When there is a Torque Increase there is a Speed Decrease of the Same Amount. When there is a Speed Increase there is a Torque Decrease of the Same Amount.