mechanisms 1

Technology and DesignDepartment

Mechanisms and Machines

Outcomes• identify different mechanisms and there uses.• analyse the input, control, output of mechanical systems.• analyse the motions and forces involved.• know about - Mechanical Advantage - Velocity Ratio and Gear Ratio. - Efficiency• calculate moments of force.• understand and calculate Torque.• calculate input and output speeds.


Useful Websiteswww.technologystudent.com

www.dtonline.org

www.howstuffworks.com

www.ajkids.com

www.learningni.net


http://www.ajkids.com/

Different types of mechanisms• Levers• Linkages• Gears• Wheels•Cranks and ratchets• Cams• Chain & Sprocket


A mechanism is something that changes anA mechanism is something that changes an

input motion and forceinput motion and force

into aninto an

output motion and force.output motion and force.


A machine is something that usesmechanisms to do useful work.



All kinds of machines make work easier for us by changing the direction or size of the applied force. The amount of force we save by using the machine is called mechanical advantage.

Mechanisms require some type of Motion (movement)There are four types of motion:

Linear

Rotary

Reciprocating

Oscillating


Mechanisms are activated by forces.The different types of forces are:Static - no movement (still force) Dynamic - moving forces Compression - squashing force Tension - pulling force Bending - compression and tension Torsion - turning or twisting Shear - cutting Equilibrium - all forces are balanced


Mechanisms can give an advantage when lifting a load. This advantage is called MechanicalAdvantage (MA). It is calculated as a ratio -

MA = LOAD EFFORT

Example: a lever uses an effort of 10N to lift a load of 50N. MA = 50N/10N = 5:1 (or just) = 5

This lever gives can lift 5 times the effort.

10N 50N


http://www.dtonline.org/apps/infopage/app.exe?1&6&1&8&1&0

Efficiency

Efficiency is a comparison of the useful work energy provided by a machine or system to the work energy applied to the machine or system.

The formula for efficiency is: output/input = work out/work in

All the parts of a machine or system and how they are connected together will affect the machine's or system's efficiency.

Other forces such as friction will affect an object's movement.


Velocity Ratio(VR)

Velocity Ratio(VR) is a comparison of the distance a load moves to the distance travelled by the force needed to move it.

VR = Distance moved by the Effort = de Distance moved by the Load dl

Example: if a lever moves a load 1.0m by pushing down at the other end by 2.0m, what is the VR?

VR = de = 2.0m = 2 = 2 dl 1.0m 1



Levers


A lever is a rigid beam that can rotate about a fixed point called the fulcrum. An effort applied to one end

of the beam will cause a load to be moved at the other.

Effort

Fulcrum

Load


http://www.dtonline.org/apps/infopage/app?1&6&1&5&1&0

There are three types(or classes) of lever.

Class 1. The Fulcrum is in the middle. example: see-saw Class 2. The Load is in the middle. example: wheel barrowClass 3. The Effort is in the middle. example: tweezers

F L E 1 2 3 F - 1L - 2E - 3




Class 2. The Load is in the middle.example: wheelbarrow

F - 1L - 2E - 3

F

E

L



Class 3. The Effort is in the middle.example: tweezers

F - 1L - 2E - 3

This fire extinguisher uses a third-class lever on its handle


Torques cause changes in rotational motion. If an object is at rest, torque exerted on it will cause it to rotate.

When you use a lever, you exert a torque on it and the lifting bar rotates about the centre.


http://library.thinkquest.org/10170/torque

There are two forces that can be applied to the see-saw to cause it to rotate. The first force is the weight of person 1 and the second force is the weight of person 2. The torque acting on each end is calculated by taking the weight multiplying by the distance from centre of the see-saw.


To calculate the torque, you need to know :the magnitude of the force,the direction of the force,the distance from the application point of the force to the turning point. t = F x d

When more than one force is involved, the torque is calculated for each separately and then they are added together taking care to include the proper sign.

Example: m1 = 1 x d1 m2 = 2 x d2 = F1d1 = F2d2


Mechanisms can give an advantage when lifting a load. This advantage is called MechanicalAdvantage (MA). It is calculated as a ratio -

MA = LOAD EFFORT

Example: a lever uses an effort of 10N to lift a load of 50N. MA = 50N/10N = 5:1 (or just) = 5

This lever gives can lift 5 times the effort.

10N 50N


http://www.dtonline.org/apps/infopage/app.exe?1&6&1&8&1&0


Linkages


A linkage is a mechanism made by connecting together levers.

To connect the levers together you can use any type of fastening which allows free movement, for example screws, pins, paper fasteners, pop rivets etc.

The linkage can be made to change the direction of a force or make two or more things move at the same time.


Reverse motion linkageinput

output

control

Input and output motions are in opposite directions.



Push/pull linkageinput output

control

Input and output motion is in the same direction



Parallel motion linkageinput output

control

The sides stay parallel as they move.



Equalising linkageinput output

control

Equal outputs from a single input.

output


Bell-crank leverinput output

control

A linear input changes to a rotary output.



Toggle Clamp

input

outputcontrol

The input motion pushes the output block to act as a clamp.



Gears



The Gear Train Gears work in teams. Two gears working together is called a gear train. The gear on the train to which the force is first applied is called the driver. The final gear on the train to which the force is first applied is called the driven gear. Any gears between the driver and the driven gears are called the idlers. Notice the arrows on top of the gears. They are showing that the gears move in different directions.

GearsGears are toothed or pegged wheels meshed together to transmit motion and force. In any pair of gears the larger one will rotate more slowly than the smaller one, but will rotate with greater force. Each gear in a series reverses the direction of rotation of the previous gear.


Meshed Gears The diagram below shows five meshed gears. The first gear that the force is applied is called the driver gear. Notice that the arrows show how the gears are turning. Every other gear is turning clockwise. The very last gear is the driven gear. All of the gears in between are called idlers.


Worm and WormwheelA gear which has one toothe is called a worm. The tooth is in the form of a screw thread. A wormwheel meshes with the worm. The wormwheel is a helical gear with teeth inclined so that they can engage with the thread-like worm. The wormwheel transmits torque and rotary motion through a right angle. The worm always drives the wormwheel and never the other way round. Worm mechanisms are very quiet running.


Internal gears Internal gears have better load-carrying capacity than external spur gears. They are safer in use because the teeth are guarded.

Why Do Clocks Have Brass Gears?Brass gears are often used in clocks where they work well without any lubricant. Oil causes dust to adhere to the gears and this causes gear-tooth wear. An advantage of brass gears is that constant meshing work hardens their teeth. Because of this, the brass gears in well used old clocks often show little sign of wear.

Gears

Simple Gear System Simple Gear System with Idler Gear




Compound Gear System

More complex 'compound' gear trains can be used to achieve high and low gear ratios in a compact space by coupling large and small cogs on the same axle.



Gear Ratio = Velocity Ratio (VR)

VR = the number of teeth on the driven gear = N = 20 = 1 the number of teeth on the driving gear. R 60 3

The velocity ratio of a compound gear train is calculated by multiplying the velocity ratios for all pairs of meshing gears. VR = n X n X n r r r

driven = 20 teethdriver = 60 teeth



Rack and PinionA single gear, the pinion, meshes with a sliding toothed rack. This combination converts rotary motion to back and forth motion. Windshield wipers in cars are powered by a rack and pinion mechanism. A small pinion at the base of the wiper meshes with a sliding rack below.


A rack and pinion mechanism is used to transform rotary motion into linear motion and vice versa


Bevel GearsGears that mesh at an angle change the direction of rotation.


Bevel Gears They are used in pairs to transmit rotary motion and torque where the bevel gear shafts are at right angles (90 degrees) to each other


Driven Gear: the output motion and force are transmitted by this gear

Driver Gear: the input motion and force is applied to this gear

Gear Ratio: the gear ratio is defined as the rotation speed of the output shaft divided

by the rotation speed of the input shaft.

Gear Train: a group of gears working together; They are arranged so that their teeth

closely interlock (mesh).

Gear Wheel: a basic mechanism. A gear is a wheel with accurately machined teeth

round its edge. Its purpose is to transmit rotary motion and force.

Meshed Gears: when the teeth of one gear are engaged with the teeth in the other

Spur Gears: two spur gears of different sizes mesh together; The larger gear is called

a wheel and the smaller gear is called the pinion.


Cams


CAMS, CRANKS AND RATCHETS


A CAM changes rotary motion (circular movement) to linear motion (one that moves in a straight line). They are found in many machines and toys.

A CAM has two parts, the FOLLOWER and the CAM PROFILE. Diagrams one to six show a rotating cam pushing a follower up and then allowing it to slowly fall back down.


The cam is used to convert rotary motion to reciprocal motion (backwards and forwards motion)


Cams can be shaped in any number of ways and this is determined by the way the follower is to move. The shape of the cam is called the PROFILE. Examples of various cam profiles can be seen below.

Pear Pear shaped cams are used on the shafts of cars. The follower remains virtually motionless for about half of the cycle of the cam and during the second half it rises and falls.

Circular/eccentric Circular cams or eccentric cams produce a smooth motion. These cams are used in steam engines.


Heart Heart shaped cams allow the follower to rise and fall with ‘uniform’ velocity.

Drop or SnailWhat type of movement do you think this cam profile will give ?


Different types of follower

FLAT

POINT/KNIFE

ROLLER

OFFSET


One cycle = One rotation/revolution of the cam.

Dwell = When the cam rotates but the follower does not rise or fall.

Rise = That part of the cam that causes the follower to rise.

Fall = That part of the cam that causes the follower to fall.


The Flat Plate Cam / Linear Cam: As the flat plate cam profile moves to the left the follower drops down the slope and then eventually rises up at the other end. The flat plate cam then reverses in the opposite direction and the follower drops and rises again.

The edge of the flat plate cam can be shaped to give different vertical movements of the cam follower. Flat plate cams or linear cams as they are often called are used frequently in machines which carry out the same repetitive movements.


A quick return mechanism such as the one seen opposite is used where there is a need to convert rotary motion into reciprocating motion. As the disc rotates the black slide moves forwards and backwards. Many machines have this type of mechanism and in the school workshop the best example is the shaping machine.

QUICK RETURN CRANK MECHANISM


The reciprocating motion of the mechanism inside the shaping machine can be seen in the diagram. As the disc rotates the top of the machine moves forwards and backwards, pushing a cutting tool. The cutting tool removes the metal from work which is carefully bolted down.

The shaping machine is used to machine flat metal surfaces especially where a large amount of metal has to be removed. Other machines such as milling machines are much more expensive and are more suited to removing smaller amounts of metal, very accurately.


One of the best examples of a crank and slider mechanism is a steam train. Steam pressure powers the slider mechanism as the connecting rod pushes and pulls the wheel round. The cylinder of an internal combustion engine is another example of a crank and slider mechanism

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.

The crank which is the rotating disc, the slider which slides inside the tube and the connecting rod which joins the parts together.

This mechanism is composed of three important parts: CRANK AND SLIDER MECHANISM


CRANK AND SLIDER MECHANISM

The four "strokes" of these engines are as follows.

Intake: The intake valve opens allowing fresh oxygen rich air mixed with fuel to enter the cylinder.

Compression: The piston is pushed upward by theflywheel's momentum compressing the air/fuel mix.

Combustion: As the piston reaches the top of its strokethe spark plug fires igniting the mixture. Due to the high compression of this mixture (typically around 190PSI in a typical engine) it is very volatile and it explodeswhen the spark is introduced. This pushes the piston downward and produces power.

Exhaust: After the Air/Fuel mix has been burnt the remaining chemicals in the cylinder (water and CO2 for the most part) must be removed so that fresh air can be brought in. As the piston goes back up after combustion the exhaust valve opens allowing the exhaust gasses tobe expelled.

intake valveexhaustvalve

cylinder

Piston

Spark plug

chamber


RATCHET MECHANISMS

A ratchet mechanism is based on a wheel that has teeth cut out of it and a pawl that follows as the wheel turns. Studying the diagram you will see that as the ratchet wheel turns and the pawl falls into the 'dip' between the teeth. The ratchet wheel can only turn in one direction - in this case anticlockwise.

The water well ratchet mechanism allows the person to rotate the handle in an anticlockwise direction. The bucket of water is heavy and so the person can rest by taking his/her hands away from the handle. This is because the pawl has fallen into the 'dip' between the teeth and so the bucket cannot fall back into the well. Ratchet mechanisms are very useful devices for example, they are used in mechanical clocks. They are also very useful when using a system, such as the one shown, to lift heavy weights.


MechanismsMechanical Advantage

Velocity RatioGear RatioVelocityMoments

Force

TorqueLevers

LinkagesPulleysCranks

RatchetsCams

Chain and SprocketMotionLinear Rotary

ReciprocatingOscillating

Static

DynamicCompression

TensionBendingTorsionShear

Equilibrium

LoadEffort

Fulcrum or PivotParallel

EqualisingToggle

LinkageBell-crank

Simple gear trainWorm and worm wheel

Internal gearsLubricant

Compound gear trainAxle

Rack and pinionBevel gearsSpur gearsEccentric

Rise

Dwell Fall

Crank and sliderCompression Combustion

Exhaust

mechanisms 1

Documents

applied force

output motion

different mechanisms

moments of force

efficiency efficiency

load moves

types of motion

load effortexample