Mechanical Efficiency

1. Mechanical Efficiency

2. Introduction • Machines are designed to help make things easier for us to do • A machine can : transform energy, transfer forces from one place to another, change direction of a force, change the magnitude of a force and / or increase and / or decrease speed Machines as systems • Most machines can be thought as a system made up of subsystems • A subsystem or each subsystem performs a different function and together they make up a bigger system that performs a task or larger function • Many subsystems are composed of mechanisms • A mechanism is a system of moving parts that changes an input motion and force into a desired output motion and force • For a system to operate, each subsystem needs to perform it’s function • For a machine to be successful, it must have a control

3. Levers Levers : How they work • A lever is a rigid bar that pivots at a point called fulcrum • Levers can multiply a small force into a large force Types of levers • Levers are found in all sorts of tools and in complex machines such as cranes and robots • There are only 3 types of levers : Class 1, Class 2 and Class 3 • The classification is based on relative positions of the effort, fulcrum and load • A class 1 lever can move a heavy load with a small force. The fulcrum in this type of lever is between the load force and the effort force( load force = is the force exerted by the load. Effort force = is the force required to move the load) E.g. Screwdriver , see-saw, hammer’s claws and scissors • A class 2 lever always moves a large load using a small effort force as the fulcrum is at one end . E.g. Wheel Barrow, bottle opener nut cracker and stapler • A class 3 lever makes it more difficult to move/lift instead of easier . In fact, the effort arm is always shorter then the load arm . E.g. Fishing rod, tongs , baseball bat and hammer

4. Mechanical Advantage • Mechanical advantage is the number of times by which a machine can increase or decrease the effort force • To determine the mechanical advantage of a mechanism you need to know the load and effort force • After you can calculate the mechanical advantage by calculating this ratio: Mechanical Load Force (N) Advantage = ----------------------- (MA) Effort Force (N) • Mechanical advantage has no units • However, if the mechanical advantage is less than 1 , this means a larger effort force is required to move a smaller load (class 3) • Machines with mechanical a mechanical advantage higher than 1 require less effort as they are able to carry larger loads with less effort (class 1 and 2 levers) • When we are dealing with levers , the mechanical advantage is affected by the distance of the point of application of load and effort forces form the fulcrum Mecanical Length of effort arm Advantage = ---------------------------------------- (MA) Length of load arm • In real life friction would act on the mechanical advantage. So the equation used to calculate mechanical advantage by the length of the arms is only good/ useful for predictions without friction . In fact, the effort force that is needed will always be greater than the effort force(that-you predict using the length of arms) as it takes extra effort to overcome friction • To calculate real mechanical advantage we need to measure the forces

5. Velocity Ratio • When we compare the distance that the effort force moves , we are looking at the velocity ratio Velocity = Distance effort force moves Ratio ------------------------------------------- Distance load force moves Efficient Lever Mechanisms • To calculate the efficiency of a mechanism we use the following ratio: Percentage = MA Efficiency -------------------- x 100 Velocity Ratio Connecting levers together • Machines and devices that use a combination of levers is called linkage • A linkage is 2 or more levers connected Pulleys, wheel and axle and gears • Rotation is a common type of motion found in all machines • Rotation in mechanisms wouldn’t be possible without pulleys, gears and wheels wheel and axle • The wheel and axle is the most common mechanism • It is composed of a large diameter disk(wheel) , which is attached to a small diameter shaft (axle) • The effort force on the wheel magnifies the load force on the axle • E.g. a screw driver Pulleys • A single pulley is a wheel and axle mechanism that is used to change the direction of a force or motion • There are 2 types of pulley systems: fixable and movable • The simplest system is a single fixed pulley. The function of this pulley is to change the direction of the force. While the other systems are designed to multiply the force Gears • Gears are toothed wheels, usually made from metal or plastic and are used to speed up or slow down motion • A gear train is made up of 2 wheels with meshed teeth • Gears work by reducing the needed revolving force or by increasing it • In fact, the ratio of the circumference of the gears is called the gear ratio

6. Friction Everywhere • The inefficiency in machines is caused by friction . This inefficiency in machines is created when two parts in a mechanism rub together, they lose mechanical energy which is then transferred into thermal energy • Friction occurs between any solid surfaces that are in contact. However, this force is alos present when an object is moving through a fluid Reducing Friction • Since friction reduces the efficiency of mechanisms and releases heat that can damage parts • So to reduce friction, we reduce the surface areas that are in contact • Ball bearings (small steel spheres) and lubricant (oil, grease and graphite) are often used to reduce friction Testing friction • Minimizing friction is important way of having a machine work efficiently • Since it is impossible to predict how much friction will there be, the only way to find out is actually measuring the effort force needed to overcome the force of friction Force and Pressure • A force applied to one part of a solid is transmitted directed through it to any solid object it is in contact with • For e.g. in a thumb tack to the large surface area is transmitted through a tiny pointed end. This means that the distribution of the force being spread out over a large area has become concentrated on the tiny surface area of the sharp point Reducing Pressure • If you have a larger surface area you can reduce the pressure you exert Calculating Pressure • Pressure = Force or P = F ------------ ---- Area A • Force is measured in Newtons (N) is measured in meter square (m2) • One N/ m2 is also equal to one Pascal (Pa)