Mechanical Control Systems

Mechanical Control Systems

1. Definition

   • A mechanism is a device which transmits energy in the form of movements
     • Changes an input force and movement to an output force and movement

2. Input motion and force

   • Process (mechanism)
     • Output motion and force

3. Mechanical Control Systems make our work easier i.e. lifting a car with a car jack, this is known as a Mechanical Advantage

4. Types Of Motion

   • Rotary
     • Moves in a circle
     • I.e. Motor
   • Oscillating
     • Moves back and forth in an arc
     • I.e. Clock pendulum
   • Reciprocating
     • Moves back and forth in a straight line
     • I.e. Sewing machine needle
   • Linear
     • Moves in a straight line
     • I.e. Train

5. Mechanical Components

   • Wheel and axle
     • The axle is the rigid bar that must be attached to the centre of the wheel
     • Windmills, ferris wheels, door knobs
   • Shafts
     • Used to transmit power from one part of a machine to another
       • Mechanisms like pulleys and gears are mounted on to it, in larger machines its made from mild steel but in smaller projects it can be made with brass or even a wooden dowl
   • Gears
     • Wheels with teeth on the outer edge
     • The teeth mesh/interlock with the teeth in other gears
     • Fixed onto shafts to transfer motion from one shaft to another, shafts must be parallel
     • Produce very precise and efficient movement

6. Levers

   • A lever is a rigid rod pivoted about a fixed axis called a fulcrum, they change the direction of a force and magnify or diminish it
   • F 1 L 2 E 3
   • First class levers have the fulcrum in between the effort and load, the effort force can be magnified or diminished, i.e. scissors, see-saw
   • Second class levers always have the load in between the effort and the fulcrum, always magnifies the force, i.e. wheelbarrows, doors, gates
   • Third class levers always have the effort in between the load and the fulcrum, the effort force is always diminished but the distance moved is magnified, i.e. fishing rod, tweezers, brooms

7. Gears

   • Gear systems change and transfer rotary motion, gears teeth are an equal distance apart on the rim of the wheel but sometimes on the face and a system of gears is known as a gear train
   • Gears can change speed and direction, input gear = driver, output gear = driven
   • Gear ratio/velocity ratio = driven/driver, measures how much the speed or power is changed by a gear train
   • The idler gear allows the driver and driven gear to spin in the same direction and will not affect the gear ratio
   • Chain and sprockets; a sprocket is a toothed wheel that fits into a shaft and a chain connects two sprockets, motion and force can be transmitted via the chain from one sprocket to another and are called power submission chains
   • Power Transmission is when energy is moved from where its been generated to the place its needed to do useful work, mechanical systems need energy to work, a motor could be used as the input
   • The smallest gear will turn faster than the larger gear

8. Pulleys

   • A wheel with a belt or rope that goes around it. The wheel has a groove in it to prevent the belt/rope from slipping off, they transfer motion and can increase/decrease the relative speed of 2 shafts
   • ADVANTAGES; quiet, no lubrication required, cheap, used in domestic appliances like hoovers
   • DISADVANTAGE; the belt may slip off causing the machine to break down, sometimes useful as it can protect the user from injury
   • Speed changes can be made by using two different sized pulleys on the driver and driven shafts, velocity ratio calculated by diameters, stepped cone pulleys allow machines to operate at a range of speeds like a pillar drill
   • Vee belts are the most common as they fit tightly reducing risk of a slip
   • Round belts are used when the belt has to twist or small forces are involved
   • Toothed belts don't slip as they are flexible with teeth moulded into the inside and are used widely in mechanical devices

9. Cams and Followers

   • Cams change input rotary motion into output reciprocating motion, usually made from metal or a hard wearing plastic and fixed to a rotating shaft
   • Circular/eccentric cams produce a smooth motion and can be used in a steam engine
   • Pear cams remain motionless for around half the cycle then during the second half it rises and falls, can be used in shafts of cars
   • Heart cams allow the follower to rise and fall with uniform velocity
   • A follower is held against the cam by either its own weight or a spring, as the cam rotates the follower rises up and down in a reciprocating motion
     • Flat followers have a flat bottom and cope well under load but aren't very accurate and cause a lot of friction
     • Knife followers have a narrow point and are very accurate and have low friction but are quick to wear away
     • Roller followers have a ball bearing roller attached to the bottom of the follower and are very accurate, low friction and can withstand load but are more costly

10. Safety

    • Ensure all parts are lubricated frequently to ensure smooth running
    • Ensure there is no loose clothing involving ties before running a machine
    • Eye protection must be worn