Mechanisms

Dean Hackett March 2012

Types of motion

• Linear

• Rotary

• Reciprocating

• Oscillating

Simple (Basic) Machines

• Two ‘families’– Inclined plane– lever

Simple Machines

• Inclined plane

• Wedge

• Screw

• Lever

• Wheel and axle

• Pulley

Work = Force x distance

Classes of Lever

• Class 1

• Class 2

• Class 3

Types of Linkage

• Parallel

• Reverse motion

• Bell crank

• Treadle

• Crank slider

bellCrank.swf

CrankSlider.swf

parallelLinkage.swf

reverseLinkage.swf

treadle.swf

Have a think...

Lever Mechanisms

Fulcrum

4 m 1 m

Effort 10N

Load xN

Fulcrum

4 m1 m

Effort 10N

Load yN

Mechanical Advantage = Load

Effort

4 m 1 m

Effort 10N

Load 40N

Mechanical Advantage

Velocity Ratio

4 m 1 m

Effort 10N

Load 40N

50mm

200mm

Velocity Ratio =

Distance moved by Effort

Distance moved by Load

Efficiency

4 m 1 m

Effort 10N

Load 40N

50mm

200mm

Efficiency = M.A.

V.R.x

100%

Lever Mechanism

What load can this person lift?

Rotary Motion

A Pulley Mechanism uses rotary motion to transmit rotary motion between two parallel shafts.

Discuss...

• How do you attach a pulley to a shaft?

Mechanisms using Rotary Motion

Pulley mechanisms can be used to increase or decrease rotary velocity

Velocity Ratio Velocity Ratio =

Distance moved by Effort

Distance moved by Load

Velocity Ratio =

Distance moved by the driver pulley

Distance moved by the driven pulley

Velocity Ratio = Diameter of Driven Pulley

Diameter of Driver Pulley

Velocity RatioPulley Shaft Rotary Velocities can be calculated using the

following formula

rotary velocity of driven pulley x diameter of driven pulley =

rotary velocity of driver pulley x diameter of driver pulley

rotary velocity of driven = diameter of driver pulley

diameter of driven pulley

rotary velocity of driver x

What is the rotary velocity of the driven pulley shaft?

rotary velocity of driven = diameter of driver pulley

diameter of driven pulley

= 30

90

revs/min

= 150 revs/min

rotary velocity of driver x

450 x

Pulleys and Belts

A section through a grooved pulley and

round belt

Vee pulley and section through a vee pulley and belt

Stepped cone pulleys provide a range of shaft speeds

Flat belts and

pulleys

A section through a flat pulley and belt

Flat belt in use on a threshing

machine

Jockey pulley in use

Chains and sprockets

Bicycle chain and sprockets

Graphical symbols

Velocity Ratio = number of teeth on the driven

sprocket number of teeth on the driver sprocket

= 12

36

= 1 : 3

Example

Pulleys and Lifting Devices

The pulley

is a form of Class 1 lever

11pulley.swf

Movable single pulley

Pulleys

Velocity Ratio =

Distance moved by Effort

Distance moved by Load

Velocity Ratio = the number of rope sections that support the load

Two Pulley System

Velocity Ratio

=

Distance moved by

Effort

Distance moved by

Load

Velocity Ratio

=

2x

x

Velocity Ratio = 2:1

21pulley.swf

Four Pulley System

Velocity Ratio

=

Distance moved by

Effort

Distance moved by

Load

Velocity Ratio

=

4x

x

Velocity Ratio = 4:1

41pulley.swf

Cams

Cams

Uses

Pear shaped cams are used in valve control mechanisms

Cams used in a four cylinder engine

http://www.youtube.com/watch?v=OXd1PlGur8M&feature=related

Cam motions

Types of cam follower

Types of cam follower

Springs are used to keep the follower in contact with the cam

Cam Profiles camSystems.swf

Displacement graph for a pear shaped cam

Displacement Graphs

Bearings

Thrust Bearings

Bearings

Bearings

• Bronze

• Nylon

• PTFE

• Air

• White metal

• Cast Iron

• Sintered

Gears

Gears

Gears are not only used to transmit motion.

They are also used to transmit force.

Gears

Mechanical Advantage =

Number of teeth on the driven gear

Number of teeth on the driver gear

Velocity Ratio = Gear Ratio =

Number of teeth on the driven gear

Number of teeth on the driver gear

Gears

Gears

Gear Ratio =Product of teeth on the driven gearsProduct of teeth on the driver gears

Gears

Gears

Gears

Gears

http://www.youtube.com/watch?v=9NoQm0wnK_c&feature=related

http://www.youtube.com/watch?v=K4JhruinbWc&NR=1

Basic Gear Geometry

http://www.sdp-si.com/D190/PDF/D190T25.PDF

The inclined plane

The inclined plane

The inclined plane

Effort required to pull trolley up slope

F = effort E

F = 1000 x sin

F = 1000 x 0.01

F = 10N

E = 10N

sin = 1/100 = 0.01

M.A. = 1000/10

= 100

Follow link to see effects of steeper incline:http://lectureonline.cl.msu.edu/~mmp/applist/si/plane.htm

The screw thread

Screw thread terms

Screw thread forms

Screw thread forms

Screw thread forms

B.S. PD7308

Newton’s Laws

• First Law– A body continues in its state of rest or uniform

motion in a straight line unless compelled by some external forces to change that state.

(sometimes know as the law of inertia)

Newton’s Laws

• Second Law– Rate of change of momentum is proportional

to the applied force and takes place in the direction in which the force acts.

(Continued force means continued acceleration)

Newton’s Laws

• Third Law– To every action there is an equal and

opposite reaction