Product design: Mechanical systems Recall Mechanisms can: change direction of movement, e.g. from clockwise to anticlockwise or from horizontal to vertical; change type of movement, e.g. from rotating to linear or from reciprocating to oscillating; alter axis of movement, e.g. from horizontal to vertical; increase speed and distance while output force is reduced; increase output force while speed or distance is reduced; apply and maintain a force; transmit force and movement. Here are some examples. Tin opener Can you see how the mechanism alters the axis of movement, uses a pair of levers to increase force and a friction drive to transmit movement? Car window Can you see how the rack and pinion plus linked levers turn rotary movement into linear movement? Pillar drill Can you see how the belt drive increases the speed of the drill?
Starting and stopping Driver Chooser Chart You can use this information to help you to choose the best driver for your mechanical system. Type of driver d.c. motor clockwork windmill/ waterwheel/ pneumatic solenoid ramrod motor wind turbine water turbine cylinder Type of movement or or or Power source battery human muscle wind moving compressed battery battery energy stored water air in spring Output force low low-medium low medium high low medium or torque Output speed high low low-medium low-medium low-high medium-high low Cost * * Other features Can be Runs for Requires wind. Can be Fixed stroke. Fixed stroke. Limited reversed by limited time Very low reversed Only stops at Only stops at stroke. Can reversing in-between power for size through gear one end or one end or be stopped polarity rewinds box or pulley other. other. mid-stroke. system Reversed by Reversed by Reversed by spring (sac) switching off reversing or air (dac) polarity *if home made Here is an example. Sarah was designing a system to gently rock a tray of photographic chemicals. It could be powered by battery or spring. She wanted to be able to switch it on and forget it. The tray had to rise 25 mm at one end, 15 times per minute. Low cost was important.
Sarah looked at the pros and cons of several drivers: She checked the Mechanisms Chooser Chart, a separate download, to make sure. Sarah s initial choice was a d.c. motor, driving a worm and wheel, driving a cam and follower. This was low cost, and would raise and drop the tray without the need for splash-proof reversing switches. Testing the initial choice If you know exactly what force/torque, speed, etc. you need, you may be able to choose the right driver from a catalogue. Often it will be cheaper to use a supplier who doesn t specify all the technical details. In this case, it is useful to set up a test rig. To keep the cost low, she chose the d.c. motor, even though it produced rotary motion, not reciprocating motion. Sarah set up this rig to test the motors that her teacher had. By trial and error, she found a motor and worm wheel which gave enough force and the right speed. She knew that this choice of driver would work. She remembered two useful mechanisms: worm and wheel, to reduce speed and increase force; cam and follower, to change rotary motion to reciprocating motion.
Transmission Shafts A shaft transmits rotation along its own axis, as in this sewing machine. shaft Belts and chains Belts and chains transmit rotation from one axle or shaft to a parallel axle or shaft, as shown in these examples. Universal joints are used to transmit rotation between shafts that meet at an angle as in this socket set. A shaft must be stiff enough to resist twisting and sagging. The angle between shafts must be less than 20. Chains and toothed belts: If a single universal joint is used the output shaft will not rotate at a constant speed even though the input shaft is rotating at a constant speed. For a constant speed input and one universal joint, the output speed is not constant. To achieve a constant output speed you need to use two universal joints as shown. For the transmission of low forces you can use rubber tubing or springs for universal joints. don t slip; give positive drive; transmit larger forces than belts; require more accurate spacing and alignment than belts; are more expensive and noisier than belts; often need lubrication. Plain belts: slip under excessive loads; rarely need lubrication.
Rods and links Rods and links transmit forces from cranks and levers, as shown in the aquarium pump and the squeezy mop. Rods and links must be stiff enough to resist buckling when in compression. crank pin piston rod cylinder valve block outer cable Cables Cables transmit pulling forces along straight or curved paths, as shown in this bicycle brake. cable Cables cannot provide pushing forces and any reversal of the input movement requires an additional force, usually provided by a spring or gravity. Other functions Many transmission systems will include belt or gear systems, which will enable them to change direction of movement, alter axis of movement and change speed.
Clutches Clutches connect or disconnect two shafts that are in line with one another. Positive clutches are used for shafts at rest; slip clutches are used when one or both of the shafts are rotating. For shafts that are parallel you can use a belt tensioner as a clutch. Operating lever Engage Usually one shaft must be able to slide as well as rotate so that it can engage and disengage with the other shaft. Sometimes neither shaft slides but part of the clutch can slide on one of the shafts. You will need to design a means of moving the sliding parts and you may need to use a spring to hold the clutch parts together. Disengage Mass extends downward force on jockey wheel and belt Engaged A dog clutch Driver turning Belt tight, moving Driven pulley turning Belt tensioner Moves this way to engage Moves this way to disengage DISENGAGED Driver shaft in motion ENGAGED Driven shaft stationary Both shafts in motion Friction plate clutch
Brakes and governors Brakes Brakes bring moving parts to rest, keep moving parts stationary (or parked) and reduce the speed of moving parts. They all work by pushing a stationary surface against the surface of a moving part. Friction provides the slowing force and heat is generated. Caliper brakes Wheelrim (wear resistant) Spokes Pull this way to apply brake Governors Governors smooth out variations in speed and prevent parts from moving too fast. A centrifugal governor is used to control the speed of a rotating shaft. The masses move outwards as the shaft rotates more quickly. As they move outwards, it takes more energy and force to move them, so the rotating shaft slows down. As the shaft slows down, the masses move inwards and require less energy and force to move them, so the rotating shaft speeds up. For any one power input a given pair of masses will maintain a particular speed. The governor can control the power used to move the shaft by connecting the governor collar to a control device a fuel valve, steam valve or electrical resistor. As the collar rises, the amount of fuel, steam or electrical current is reduced and vice versa. You will need to experiment with both the geometry and the masses of a centrifugal governor in order to get the speed control that you need. Slow rotation Faster rotation Wheel (wear resistant) Band brakes Choose materials which don t soften, melt or distort when they get hot. You will need to choose how force will be applied to put the brake on and to take it off. Fuel or energy supply to control device Centrifugal governor Fuel or energy supply to control device You will need to decide if your brake needs to stay on when the operator moves away, like a car handbrake. If so you will need a mechanism that applies and maintains force (see the Holding, Supporting, Attaching and Adjusting Chooser Chart, a separate download).