Simple Machines: Gears, Velocity Ratios and Mechanical Advantage What is a simple machine? A device which uses basic mechanisms to make work easier for the user. Example: Lifting a heavy box with a rope and pulley system Simple machines are found in many transportation devices such as the following: Cars Bicycles Trains What are gears and why are they so useful? Gears are toothed wheels which interlock to form simple machines. The tighter the joint, the less chance of slipping Gears range in size but the important number is how many teeth a gear has. What is a gear ratio? A gear ratio is the ratio between the driver gear (the one being powered) and the driven gear (the one connected to the driver) Gear Ratios are expressed as fractions and can be written a number of ways. 1 to 3 1/3 1:3 Always remember that the driven gear will turn in the opposite direction of the driver gear What is a pulley? A wheel with a grove carved around its circumference in which a rope or belt is placed to drive the wheel. Pulleys are used in a number of applications including: Elevators Cranes Automobiles A Pulley System Drawbacks of Pulleys Pulleys work great as long as the belt which connects them is tight and both pulleys are aligned. Pulleys tend to slip when too much strain is placed on the system. How do I calculate my gear ratio? A simple equation is used to find the ratio of your gearing system Number of Teeth on the Driven Gear / Number of Teeth on the Driver Gear. Size of the driver pulley/ Size of the driven pulley What is Velocity Ratio (VR)? When calculating for velocity or speed, it is best to have a ratio in which the driver gear is larger than the driven gear. For maximum speed you should always have a larger driver gear/pulley paired with a smaller driven gear/pulley. What is Mechanical Advantage (MA)? Reciprocal of velocity ratio. Mechanical advantage describes how much work can be done based on the input. If my system required me to apply 50 pounds of force into the system to lift 100 pounds of weight, I have used MA. So which do I want? Higher velocity ratios are better when working on straight and level surfaces which are smooth. Always remember that with a high VR your system will have very low power. Higher mechanical advantage systems are better with inclines surfaces that are bumpy. With a high MA your system will produce more power, but will move slower.