Mechanical Design General Concepts AHL Topic 10

Mechanical advantage This is the factor by which a machine multiplies the force put into it. Velocity Ratio A measurement of force amplification. Mechanical efficiency is the effectiveness of a simple machine. Efficiency

How do we calculate Mechanical Advantage (MA), Velocity Ratio (VR) and Efficiency for simple mechanical systems? MA= effort load VR= distance moved by effort distance moved by load Efficiency = MA VR

LEVERS There are 3 types of lever:  1 st class lever (fulcrum is in the middle)  2 nd class lever (load is in the middle)  3 rd class lever (effort is in the middle) Each class of lever has a:  Fulcrum  Load  Effort

Examples of 1st class levers On all 1 st class lever the fulcrum is in the middle. Seesaw Crowbar Scissors

Examples of 2 nd class levers On all 2 nd class lever the load is in the middle. WheelbarrowBottle openerNut cracker

Examples of 3 rd class levers On all 3 rd class lever the effort is in the middle. TweezersBroomFishing rod

All of the levers make tasks easier for us. 1 st class levers allow us to lift heavy loads, and to shear materials accurately 2 nd class levers allow us to move and lift heavier loads. 3 rd class levers allow us to carry out precision operations.

Equilibrium When a lever is in equilibrium the net moment is zero If this 10N weight was dropped onto the end of this 1 st class lever what would happen?

GEARS A gear is a component within a transmission device that transmits rotational force to another gear or device. A gear is different from a pulley in that a gear is a round wheel that has linkages ("teeth" or "cogs") that mesh with other gear teeth, allowing force to be fully transferred without slippage. Depending on their construction and arrangement, geared devices can transmit forces at different speeds, or in a different direction, from the power source. The most common situation is for a gear to mesh with another gear, but a gear can mesh with any device having compatible teeth, such as linear moving racks. The gear's most important feature is that gears of unequal sizes (diameters) can be combined to produce a mechanical advantage, so that the rotational speed and torque of the second gear are different from those of the first.

Rack and pinion A rack and pinion is a pair of gears which convert rotational motion into linear motion. Rack-and-pinion steering is quickly becoming the most common type of steering on cars, small trucks and SUVs. It is actually a pretty simple mechanism. A rack- and-pinion gearset is enclosed in a metal tube, with each end of the rack protruding from the tube. A rod, called a tie rod, connects to each end of the rack.

Bevel gears Bevel gears are useful when the direction of a shaft's rotation needs to be changed. They are usually mounted on shafts that are 90 degrees apart, but can be designed to work at other angles as well. The teeth on bevel gears can be straight, spiral or hypoid. This feature is used in many car differentials. The ring gear of the differential and the input pinion gear are both hypoid. This allows the input pinion to be mounted lower than the axis of the ring gear. This picture shows the input pinion engaging the ring gear of the differential.

Worm gears Worm gears are used when large gear reductions are needed. It is common for worm gears to have reductions of 20:1, and even up to 300:1 or greater. Many worm gears have an interesting property that no other gear set has: the worm can easily turn the gear, but the gear cannot turn the worm. This feature is useful for machines such as conveyor systems, in which the locking feature can act as a brake for the conveyor when the motor is not turning.

Gear trains To create large gear ratios, gears are often connected together in gear trains The right-hand (purple) gear in the train is actually made in two parts, as shown. A small gear and a larger gear are connected together, one on top of the other. Gear trains often consist of multiple gears in the train, as shown in the following slide:

In this train, the smaller gears are one- fifth the size of the larger gears. That means that if you connect the purple gear to a motor spinning at 100 rpm (revolutions per minute), the green gear will turn at a rate of 500 rpm and the red gear will turn at a rate of 2,500 rpm. In the Gear train to the right, the purple gear turns at a rate twice that of the blue gear. The green gear turns at twice the rate as the purple gear. The gear train shown below has a higher gear ratio:

Belts Belt drives are an alternative to chain drives. Early motorcycles often used leather belts, which could be tensioned to give traction using a spring-loaded pulley and hand lever. Leather belts often slipped, especially in wet weather, so they were abandoned for other materials and designs. By the 1980s, advances in materials made belt final-drive systems viable again. Today's belts are made of cogged rubber and operate much the same way as metal chains. Unlike metal chains, they don't require lubrication or cleaning solvents.

Pulleys A pulley (also called a block and tackle) is a mechanism composed of a wheel (called a sheave) with a groove between two flanges around the wheel's circumference. A rope, cable, belt or chain usually runs inside the groove. Pulleys are used to change the direction of an applied force, transmit rotational motion, or realize a mechanical advantage in either a linear or rotational system of motion.

Inclined plane The inclined plane is one of the original six simple machines; as the name suggests, it is a flat surface whose endpoints are at different heights. By moving an object up an inclined plane rather than completely vertical, the amount of force required is reduced, at the expense of increasing the distance the object must travel. The mechanical advantage of an inclined plane is the ratio of the length of the sloped surface to the height it spans; this may also be expressed as the cosecant of the angle between the plane and the horizontal. Screw thread with an inclined plane