Homework 4 Gears Gears are designed to transmit rotary motion. Often they are arranged in a gear train (meshed together). Gear trains provide a change in speed, torque (turning force) and direction (clockwise or anti-clockwise). The simple gear below is called a Spur Gear. When more than one of these gears are meshed together they are called a Gear Train. Spur Gear Gear Train Drawing gears is very difficult so there is a simple graphical symbol which represents a gear. The outer circle represents the outer tips of the teeth and the inner circle represents the bottom of the teeth. Graphical representation of a gear Gears are different to belts, pulleys, chain drives and sprockets in that the direction of rotation is in the opposite direction to the input. To achieve motion in the same direction you need 3 gears meshed together (a gear train). Input CW Output ACW Input CW Idler ACW Output CW CW- clockwise ACW- Anti-clockwise
There are a range of mechanisms which use gears to transmit motion, change direction of motion or change force. Spur Gear A spur gear is the most common type of gear and consists of a wheel with teeth which are cut straight (unlike bevel edge gears). They are excellent at transmitting torque (turning force) and motion (velocity). Different size gears can be used to change the output (torque and velocity), these are called Gear Ratio and Velocity Ratio. Increase Speed and Reduce Torque To increase speed and reduce torque use a large driver gear meshed to a smaller driven gear as shown below. Reduce Speed and Increase Torque To reduce speed and increase torque use a small driver gear to turn a larger driven gear.
Bevel Edge Gear Bevel gears are like spur gears with teeth that are cut at 45. They are used change the direction of the rotation through 90 degrees. Teeth cut at 45 Input Output at 90 angle, no change in speed but a change in the direction of rotation anticlockwise. They can rotate in both directions. A common application is on a hand drill. Worm & Wheel A worm drive is a gear arrangement in which a worm (which is a gear in the form of a screw with a single tooth wrapped around a shaft) meshes with a wheel which has similar in appearance to a spur gear. This is used to transmit rotary motion through 90 degrees. It can be used to give very large speed reductions in a small space so is ideal for controlled transmissions like clockwork toys or compact motors. It is also a one way system, the worm gear can drive thewheel but the wheel can not be used to drive the worm wheel. Worm Gear A single tooth Wheel Gear A spur gear
Rack & Pinion A Rack & Pinion Gear System is primarily used to convert rotary motion into linear motion. The pinion is a small gear which meshes with a flat gear called a rack. Rotary motion applied to the pinion will cause the rack to move forwards or backwards depending on the direction of rotation. Output Linear Motion Input Rotary Via Crank The pitch of the rack is the distance between 2 teeth. The number of teeth on the pinion gear will determine the distance moved by the rack. For example, if the pinion gear has 20 teeth and is rotated one full revolution (turned once) and the pitch of the rack is 10mm, then the total linear distance moved by the rack will be 200mm.
Questions 1. How many teeth does a worm gear have? A, 3 B, 5 C, 1 D, depends on the number of teeth on the wheel gear. 2. A pinion gear has 30 teeth and the rack gear has a pitch of 10mm. What is the total linear distance moved by the rack once the pinion gear has been rotated 3 times? A, 9000mm B, 900mm C, 90mm D,9mm 3. Below is a diagram of a compound gear train. If A is the driver and rotates clockwise, what direction will C rotate? A, Clockwise B, Anti-clockwise 4. What are bevel edge gears used for? A, They are used change the direction of the rotation through 90 degrees. B, They are used to increase the speed of rotation. C, They are used to reduce the speed of rotation. D, They are used to change rotary motion into linear.
5. What are gears generally used for? A. Gears transmit motion. B. Change direction of motion. C. Change force (torque). D. All of the above. 6. The diagram below shows a vehicle and its steering system. The most important part of the mechanisms is missing. This allows the steering wheel to turn the wheels left and right so that it can be steered. The gear system required must convert rotary motion into linear. A. In the space available, add a drawing of the gear system that allows direct turning of the wheels. B. Label the important parts. C. Explain how the mechanism works Explanation