Mechanical devices used to produce movement

• The difference between studying Design Engineering and electronics is that you will need to be able to use your circuits to make things happen in the real world. One of the coursework projects you might be asked to complete is a humane mousetrap, for instance. The first thing to understand is the four types of motion…

Levers are simple mechanisms that create mechanical advantage. They have three parts - effort(input), fulcrum(pivot) and the load(output). There are 3 types of levers.

• In Year 7 you looked at Cams when making you CAM box project. Click on this link to go back and look over it again.
• Cams allow us to change the type of motion, direction and speed from a rotary shaft.
• They convert rotary motion into reciprocating and oscillating motion.
• The follower is the device that follows the movement of the cam profile to provide the desired output.
• They are three stages of movement - rise (follower moves up), fall (follower moves down) and dwell (follower remains stationary).
• Examples are:

Drop cam(snail) -slow rise and then a sudden fall. Used in hammers or punches to give that sudden drop. Pear cam - dwells half the time and the other half it slows rises and falls. Used in car engines to open and close valves. Eccentric cam - gives a continuous smooth motion as it rises or falls. Used in fuel pump or steam engines.

• Followers can also be different shapes.
• Roller - reduces friction good for higher speeds.
• Knife edge - used for accuracy such as in embroidery machines.
• Flat - used when higher load bearing is required.

• In Year 8 and Year 9 you looked at pulleys and belts and completed all the equations.
• Click on this link to see the Year 8 work.
• Click on this link to see the Year 9 work.
• Pulleys are used to change the speed, direction of rotation, or turning force or torque.
• A pulley system consists of two pulley wheels each on a shaft, connected by a belt. This transmits rotary motion and force from the input, or driver shaft, to the output, or driven shaft.
• A pulley system with one 40mm diameter pulley and a 120mm pulley, connected by a belt. The smaller pulley is rotating at 100rpm
• If the pulley wheels are different sizes, the smaller one will spin faster than the larger one. The difference in speed is called the velocity ratio. This is calculated using the formula:
• Velocity ratio = diameter of the driven pulley ÷ diameter of the driver pulley
• If you know the velocity ratio and the input speed of a pulley system, you can calculate the output speed using the formula:
• Output speed = input speed ÷ velocity ratio
• Velocity ratio = 120mm ÷ 40mm = 1:3
• Output speed = 100rpm ÷ 3 = 33.3 rpm

Torque

• The velocity ratio of a pulley system also determines the amount of turning force or torque transmitted from the driver pulley to the driven pulley. The formula is:

output torque = input torque × velocity ratio.

Pulley drive belts

• Drive belts are usually made of synthetic fibres such as neoprene and polyurethane, with a V-shaped cross section. It is possible to reverse the direction of the driven pulley by twisting the belt as it crosses from input to output.
• Pulley belts have the advantage over chains that they do not need lubrication (though unlike a chain, a belt can slip).
• For more information, click on this link

• This mechanism is composed of three important parts:
• The crank which is the rotating disc, the slider which slides inside the tube and the connecting rod which joins the parts together.
• As the slider moves to the right the connecting rod pushes the wheel round for the first 180 degrees of wheel rotation.
• When the slider begins to move back into the tube, the connecting rod pulls the wheel round to complete the rotation.