1.5 - Mechanical Devices Flashcards
What are the different types of movement?
- Linear
- Reciprocating
- Rotary
- Oscillating
Exam Style Question:
Which type of motion best describes the movement of a sewing machine needle?
[1]
Reciprocating motion.
What is a lever?
- A lever is a very simple way to gain mechanical advantage (MA), making lifting or moving something much easier.
- It consists of a rigid bar that pivots or rotates about a fulcrum with a load applied
What does a typical lever consist of?
- Fulcrum - a pivot point around which a lever turns.
- Effort - an input force.
- Load - an output force.
What is the formula to calculate a lever’s mechanical advantage?
MA = Load/Effort
What is a first order lever?
- First class levers have the fulcrum between the force and the load.
- Example: Pliers.
What is a second order lever?
- Second order levers are most easily remembered as having a wheelbarrow action.
- The fulcrum is at one end with the effort at the opposite end.
- The load is anywhere in the middle.
What is a third order lever?
- A Class 3 lever has the fulcrum at one end, the load at the opposite end and the effort applied in the middle.
- Example: Tweezers.
When a lever is balanced it has ___________ - the load is balanced on either side.
equilibrium
Define the term ‘Mechanical advantage’.
Mechanical advantage is the amount of help you get using a machine in comparison to doing something with just human effort, and it is created by levers.
What is a linkage?
- A linkage is a mechanism made by connecting rigid parts.
- Linkages can change the magnitude of a force, change the direction of a force or transform it into a totally different motion.
What are some different types of linkages?
- Reverse motion
- Parallel motion / push pull
- Bell crank
- Crank and slider
- Treadle
What ia a ‘reverse motion’ linkage?
- Reverse motion linkages change the direction of input so that the output goes the opposite way.
- A fixed pivot forces the change in direction. These are often used on foldable clothes horses.
What is a ‘parallel motion’ / ‘push pull’ linkage?
- Parallel motion linkages use two fixed pivots to make the input and output travel in the same direction, through a link arm.
- Changing the placing of the fixed pivots changes the amount of force exerted, while keeping the direction the same.
- For example, a toolbox with drawers opens up effectively using parallel motion linkages.
What is a ‘bell crank’ linkage?
- Bell crank linkages change the direction of force through 90°.
- The amount of output force can be changed by moving the fixed pivot.
- When used in bicycle brakes, the rider can pull the brakes, which changes direction through the bell crank to make the brake pads touch the wheels.
What is a ‘crank and slider’ linkage?
- Crank and slider linkages change rotary motion into reciprocating motion.
- A fixed pivot is attached to a crank, which turns around and pushes and pulls a slider.
- When used in a car engine, the ignition of petrol by the spark plugs pushes the slider up, moving the connecting rod and turning the crank.
What is a ‘treadle’ linkage?
- Treadle linkages use a rotary input to turn a crank on a fixed pivot.
- A connecting rod joins two moving pivots to another fixed pivot.
- This is how a pair of windscreen wipers works on a car, moving backwards and forwards together.
What is a rotary system?
- Rotary systems are used to drive mechanisms in equipment and machinery.
- They transfer the direction of force along different paths and through changes of angle and direction.
- They include a cam and follower.
What is a cam and follower? (Rotary systems)
- A cam is mainly used to change rotary motion into reciprocating motion through the use of a follower.
- A crank is used to rotate the shaft, which rotates the cam and moves the follower up and down.
What are some different types of cams?
- Circular cam
- Eccentric cam
- Pear cam
- Snail cam
- Heart shaped cam
Describe the reciprocating motion produced by circular / eccentric cams?
- Steady rise and fall.
Describe the reciprocating motion produced by pear cams?
- Rapid rise and fall followed by long dwell (rest).
Describe the reciprocating motion produced by snail cams?
- Long dwell followed by steady rise and sudden drop.
- This can only turn in one direction, otherwise the mechanism would jam.
Describe the reciprocating motion produced by heart shaped cams?
- Slight rise and fall with no dwell period.
Exam Style Question:
A child’s toy crocodile has a mouth that slowly opens and snaps shut again when a handle is turned.
Suggest the most suitable cam to be used to create this mechanism.
[1]
A snail cam.
What is a gear?
- Gears are toothed wheels, that mesh to transfer motion from one part of a machine to another.
- Gears are found in many things from cars and bicycles, to clocks and can openers.
What are the three main types of followers?
- Flat
- Point or knife
- Roller
What are the qualities of a flat follower?
- Flat followers have a flat bottom that sits on the cam.
- These cope well under load but aren’t very accurate and have a lot of friction.
What are the qualities of a point/knife follower?
- Point or knife followers have a narrow point that sits on the cam.
- These are very accurate and low friction but are quick to wear away the pointed edge.
What are the qualities of a roller follower?
- Roller followers have a roller such as a ball bearing attached to the bottom of the following.
- These are accurate, low on friction and can withstand load, but are more costly to produce.
What is a gear?
- Gears are toothed wheels, that mesh to transfer motion from one part of a machine to another.
- Gears are found in many things from cars and bicycles, to clocks and can openers.
What is a gear train?
- Gear trains are when two or more gears are joined together.
- A gear train transmits rotary motion and torque.
- Different sized gears connect to increase or decrease the speed of rotation.
What is torque?
Torque is a force that causes rotation.
What is a ‘drive gear’?
- The drive gear (input) turns the driven gear (output).
- The gears turn in opposite directions.
What is an ‘idler gear’?
- An idler gear is used to change the direction of rotation so that the driven gear turns in the same direction as the drive gear.
- It transfers movement from the drive gear to the driven, so size does not affect speed.
How do you calculate gear ratio? (Gear trains)
Number of teeth on driven gear : number of teeth on driver gear
Or
Number of teeth on driven gear / number of teeth on driver gear
How do you calculate velocity ratio? (Gear trains)
Velocity ratio = drive gear / driven gear
What are pulley systems?
- Pulleys can help to lift a load providing a mechanical advantage.
- In machines, pulleys transmit rotary motion and force from the drive shaft (input) to the driven shaft (output).
- Pulleys are not ideal for transmitting high torque due to their tendency for the belt to slip.
- Belts are often made of reinforced rubber or high strength materials such as Kevlar.
What are the two types of pulley systems?
- Fixed pulley
(no mechanical advantage) - Block and tackle pulley
(has mechanical advantage)
What is a ‘fixed pulley’?
- A fixed pulley uses a wheel with a groove in it and a rope that sits in the groove.
- It allows you to change the direction of the force needed, which makes lifting easier, but the weight will feel the same (no mechanical advantage).
What is a ‘block and tackle pulley’?
- A block and tackle pulley uses two or more pulleys, one fixed and one moveable, to help reduce the amount of effort needed to lift a load.
- It provides a mechanical advantage making a load easier to lift.
How do you calculate the effort needed to raise a load using a pulley system?
Effort = load / number of pulleys
How are belts used in pulley systems?
- A drive belt (loop of flexible rubber) connects two or more pulleys together.
- The belt transfers power from the pulley to the receiving system.
- In the instance of a car, a belt transfers engine power to the alternator to recharge the battery.
