Section A - Mechanical Devices Flashcards
Define mechanism
A mechanism is a device that changes an input motion into a different output motion
What is a lever
A lever is a mechanism that moves around a fixed point (a pivot)
What are the four basic types of motion found in mechanical systems
Linear motion
Reciprocating motion
Rotary motion
Oscillating motion
What is linear motion
Linear motion is the movement in a straight line such as on a paper trimmer (or a train moving down a track)
Example of a device that uses linear motion
Linear motion is the movement in a straight line such as on a paper trimmer (or a train moving down a track)
What is reciprocating motion
Reciprocating motion is the movement backwards and forwards in a straight line such as the movement of the needle on a sewing machine (or a piston or a pump)
Example of an device that uses reciprocating motion
Reciprocating motion is the movement backwards and forwards in a straight line such as the movement of the needle on a sewing machine (or a piston or a pump/bicycle pump)
What is rotary motion
Rotary motion is movement round in a circle, such as a wheel turning (a children’s roundabout)
Example of a device using rotary motion
Rotary motion is movement round in a circle, such as a wheel turning (a children’s roundabout)
What is oscillating motion
Oscillating motion is the movement swinging from side to side, such as a pendulum in a clock (or a swing)
Example of a device using oscillating motion
Oscillating motion is the movement swinging from side to side, such as a pendulum in a clock (or a swing)
Describe the types of movement
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- Linear motion
- Rotary motion
- Reciprocating motion
- Oscillating motion
What are the three parts of a lever called
Load (the object to be moved)
Effort (the force applied to move the load)
Fulcrum (the point where the load is pivoted)
How many basic types or orders of lever are there
There are three basic types of orders of lever
Describe the set-up for a first-order lever
A first-order lever has the fulcrum between the effort and the load.
What happens if the fulcrum is moved closer to the load, regarding a first-order lever
If the fulcrum is moved closer to the load, less effort is needed to move it (although the load does not move as far)
What happens if the fulcrum is moved closer to the effort - in a first-order lever
If the fulcrum is moved closer to the effort, more effort is needed to move the load (although the load moves much further)
Draw a first-order lever
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pivot/fulcrum
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Give an example of a first-order lever
A seesaw is an example of a first-order lever
A pair of scissors
A pair of pliers
Describe how a second-order lever is set up
A second-order lever has the load and the effort on the same side of the fulcrum.
The load is nearer to the fulcrum, therefore less effort is required to move it.
Draw a second-order lever
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Give an example of a second-order lever
Wheelbarrow
Describe how a third-order lever is set up
A third-order lever has the load and effort on the same side as the fulcrum. The load is further from the fulcrum (the effort is nearer to the fulcrum), so the effort required to move the load
is greater
When are third-order levers used. Why are they set up this way?
These levers are used for items such as barbeque tongs and tweezers where the article being picked up is small or awkward, or when handling something that could be squished or is fragile
With levers, how do you make the effort smaller.
What is the effect on the distanced moved by the lever?
Placing the fulcrum nearer to the road - less effort is required to move the load.
To make the effort smaller, the distance the lever moves is greater
Draw a third-order lever
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Give examples of third order levers
Barbeque tongs
Tweezers
Stapler
What is a linkage
A linkage is a mechanism that transfers force and can change the direction of movement
What are the 3 stages of a simple control system called
Input
Process
Output
What can the number and shapes of the linkages change
The number and shapes of the linkages can change the direction of the force
What can the position of the pivot on the linkages change
The position of the pivots can change the size or magnitude of the force
The closer the pivot to the output end of the lever __________
The closer the pivot to the output end of the lever, the larger the fore is at the output
Types of linkages
Bell cranks
Push/pull linkage (or parallel motion linkage)
Describe the bell-crank mechanism
In a bell-crank mechanism the direction of motion is turned through 90 degrees, but the magnitude of the output force is greater than that of the input force when the fixed pivot is closer to the output lever
Draw a bell-crank mechanism
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State one application of a bell-crank mechanism
A bicycle braking system uses a bell-crank mechanism
Describe the push/pull linkage (parallel motion linkage)
In a push-pull linkage (parallel motion linkage), the direction of motion (for input and output) and magnitude of the forces are the same
Why are push/pull linkage (parallel motion linkage) used
They are used when the force cannot be transferred directly in a straight line
State one application of a parallel motion linkage (push/pull linkage)
A sewing box uses parallel motion linkage
What is a Cam
A cam is a mechanism with a cam, slide and follower. When the car rotates, the follower moves up and down
What are the three parts of a cam mechanism
Cam
Slide
Follower
What does the shape of the cam influence
The pattern the follower moved up and down in is governed by the shape of the cam
State the three things the follower can do
The follower can:
go up (rise)
go down (fall)
stay still (dwell)
Why do cams come in different shapes
Cams come in many different shapes to create different combinations of rise, fall and dwell
What is lift
The distance the follower travels from top to bottom is called the ‘lift’
Draw and label the key parts of the eccentric cam mechanism
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Draw some different shaped cams
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If a cam follower rises for 60 degrees of the rotation, falls for 60 degrees of the cam rotation and does not move for the remaining rotation of the cam, what will the dwell angle be
360-60-60 = 240 degrees
How can the duration of the rise and fall of a cam be represented
The duration of rise and fall of a cam can be represented as an angle
What is a gear train
A gear train is a mechanism for transmitting rotary motion and torque
What is torque
Torque is the turning force that causes rotation
What do gears have. What do these things do
Gears have teeth, and interlock or mesh with one another to transmit the rotary motion
Draw a simple gear train
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Draw the symbol for a simple gear train
note 24T means 24 teeth
How can the velocity ratio for a simple gear train be worked out
Velocity ratio = Number of teeth on the driven gear/Number of teeth on the driver gear
What is the velocity ratio if the driver gear has 20 teeth and the driven gear has 60 teeth
Velocity ratio = Number of teeth on the driven gear/Number of teeth on the driver gear
Velocity ratio = 60/20 = 3/1 = 3:1
In a simple gear train what does the drive gear cause
The drive gear causes the driven gear to turn in the opposite direction
What types of gears will rotate faster than others
Smaller gears with fewer teeth rotate faster than larger gears with more teeth
What is gear ratio
The number of turns the driven gear will make for every turn of the drive gear
The driven gear has 60 teeth and the driver gear has 15 teeth.
What does this mean for each rotation of the driver gear
For each rotation of the driver gear, the driven gear would rotate four times.
Driver gear will rotate 4 times faster (or a quarter the speed) than driven gear
What is friction
Friction is the resistance of motion when one object rubs against another
What is the co-efficient of friction
The co-efficient of friction is the amount of friction a material has
What does the amount of friction objects may have depend on
The amount of friction depends on the materials from which the two objects are made
Materials that offer little resistance to a substance moving over them are said to have a __________
Materials that offer little resistance to a substance moving over them are said to have a low co-efficient of friction
Materials that offers a high resistance to a substance moving over them are said to have a __________
Materials that offers a high resistance to a substance moving over them are said to have a high co-efficient of friction
How can friction be reduced
Reduced by choosing material that have a low co-efficient of friction
Reduced by lubrication
Give examples of mechanisms that need friction in order to work
Pulley systems
Braking systems
What is the materials many gear mechanisms use for the gears.
Why is this.
When is it only suitable
Many gear mechanisms use brass for the gears
It is only suitable for applications where forces are small
Brass has a low co-efficient of friction so does not need lubrication
What material is used in gears in clocks - why is this
Many clocks have brass gears since the forces involved are very small
What material is used in gears in cars
Why?
Steel - because it is strong and hardwearing (this is important because a lot of torque is transmitted through these gears)
What material is in mechanical devices for marine application
Stainless steel is used to make pulleys, winches, and an assortment of riggings (since it will not corrode or rust)
What are the advantages of nylon gears in some systems
These gears can operate with minimum or no lubrication - as they have a relatively low co-efficient of friction
These gears are corrosion-resistant
These gears are quiet in operation
What do simple gear trains consist of
Simple gear trains consist of two gears.
The gears may be different sizes.
What can gear trains be used for
Gear trains can be used to change the speed, direction and/or power of the output motion.
Examples of gear trains
Rack and pinion mechanism
Crank and slider mechanism
Chain and sprocket mechanism
Function of a rack and pinion mechanism
Rack and pinion mechanisms can turn rotary motion to linear motion or vice versa
State an application of a rack and pinion mechanism
A pillar drill uses a rack and pinion mechanism
Most cars in the steering system uses a rack and pinion mechanism
What does a rack and pinion mechanism consist of
Rack (a flat or straight gear whose teeth mesh with the teeth on a pinion gear)
Pinion
What material do most rack and pinion systems use and why?
Most rack and pinion systems use steel because it is hardwearing
Explain how a rack and pinion mechanism works
If the pinion is rotated about its centre, the rotary motion created will be converted into the linear motion of the rack causing the rack to move in a straight line in one direction.
If the rack is moved, the linear motion created will be converted into the rotary motion of the pinon, causing the pinon to rotate about its centre
Draw a rack and pinion mechanism
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Draw the symbol for a rack and pinion mechanism
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Function of a crank and slider mechansim
A crank and slider mechanism can turn rotary motion to linear motion or vice versa
Explain how a crank and slider mechanism works
By rotating a crank, the rotary motion created will be converted into the linear motion of the slider, which causes the slider to move back and forwards along a cylinder or a track
If the slider produces the input, the linear motion created will be converted into the rotary motion of the crank, causing the crank to rotate
State an application of a crank and slider mechanism
A piston in a car engine uses the crank and slider mechanism
What are the sliders (pistons - as they are called in engines) and the crankshafts in a car made of
Why?
They are made from an an aluminium alloy because of its high strength-to-weight ratio and because it conducts heat very well.
The crankshafts in a car are made from steel because of its strength, toughness and hardness
State the function of a chain and sprocket mechanism
A chain and sprocket mechanism transfers rotary motion to rotary motion elsewhere in a machine - such as bicycles and motorcycles
What is a sprocket and what are chains made up of
A sprocket is a toothed wheel
A chain is made up of links that can pivot and go over the teeth on a sprocket.
How does a sprocket and chain mechanism work
Rotary motion is transmitted between the shafts that the sprockets are attached to - by the grip that the chain links have on the teeth of the sprocket as it rotates.
What will happen if the sprocket on the motor is larger than the sprocket is is driving
If the sprocket on the motor is larger than the sprocket it is driving, then the powered sprocket will spin faster and the torque will increase
How can you work out the velocity ration for a sprocket and chain system
Velocity ratio = number of teen on the driven sprocket / number of teeth on the driver sprocket
If a bicycle has 60 teeth on the sprocket attached to the pedals and 12 teeth on the sprocket attached to the back wheels, what is the velocity ratio
Velocity ratio = number of teen on the driven sprocket / number of teeth on the driver sprocket
Velocity ratio = 12/60
Velocity ratio = 1/5 = 1:5
If a bicycle has 60 teeth on the sprocket attached to the pedals and 12 teeth on the sprocket attached to the back wheels,
What does this mean for each rotation of the driver sprocket
Velocity ratio = 1:5
For each rotation of the driven gear, the driver gear would rotate four times.
The driven sprocket will rotate five times faster than the driver sprocket
Draw the symbol for a chain and sprocket mechanism
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Function of pulley systems
Pulley systems transmit rotary motion to rotary motion in machines
What is a pulley
A wheel with a groove in its rim
How are pulleys connected.
Two pulleys are connected together by a flexible belt that will transmit rotary motion and torqueQ
What do different sized pulleys connected together cause
Different sized pulleys connected together either increase or decrease the speed of rotation and increase or decrease the torque transmitted.
Why is it important for the belt not to slip on the pulley
To ensure all movement is to be transferred
What are pulleys and bels made from and why?
Pulleys made from a variety of materials including aluminium.
The majority of belts used in pulley systems are made from rubber
When is aluminium used for pulleys
Aluminium is used for pulleys in machines using large turning forces because of its high tensile strength and relatively low weight`
Why is rubber used for belts in pulley system
Rubber reinforced with high tensile materials like Kevlar (to prevent stretching ) is used in belts so that it gets a good grip on the pulleys
What is the preferred material to make pulleys for marine applications
Stainless steel - it will not corrode or rust
What can drive a simple pulley system
An electric motor can drive a simple pulley system
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Draw the symbol for a simple pulley system
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What will happen if the pully on the motor is larger than the pulley it is driving
If the pulley on the motor is larger than the pulley it is driving, the driven pulley will spin faster but the torque will be less
What will happen if the pulley on the motor is smaller than the pulley it is driving
If the pulley on the motor is smaller than the pulley it is driving, then the driven pulley will spin slower but the torque will be increased
What is velocity ratio
the speed the pulleys turn in relation to each other
How can speed changes be achieved on pulley systems
Speed changes can be achieved by varying the diameters of the pulleys.
e.g. a small diameter pulley of 50 mm driving a larger diameter pulley of 200mm reduces the speed of rotation
How can velocity ratio for pulleys be calculated
Velocity ratio = diameter of the driven pulley/ diameter of the driver pulley
a small diameter pulley of 50 mm driving a larger diameter pulley of 200mm reduces the speed of rotation
calculate velocity ratio
Velocity ratio = diameter of the driven pulley/ diameter of the driver pulley
velocity ratio = 200/50 = 4/1 = 4:1
What does the ratio 4:1 mean in terms for each rotation of the driven pulley
Driven pulley will rotate four times slower than the driver pulley (or at a quarter the speed)
Driver pulley will rotate four times faster than the driven pulley (four times the speed)
