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

Have a think...

Lever Mechanisms Fulcrum 4 m1 m Effort 10N Load xN Fulcrum 4 m1 m Effort 10N Load yN

Mechanical Advantage = Load Effort 4 m1 m Effort 10N Load 40N Mechanical Advantage

Velocity Ratio 4 m1 m Effort 10N Load 40N 50mm 200mm Velocity Ratio = Distance moved by Effort Distance moved by Load

Efficiency 4 m1 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 Ratio Pulley 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 = 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 = = 1 : 3

Example

Pulleys and Lifting Devices The pulley is a form of Class 1 lever

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

Four Pulley System Velocity Ratio = Distance moved by Effort Distance moved by Load Velocity Ratio = 4x x Velocity Ratio = 4:1

Cams

Uses Pear shaped cams are used in valve control mechanisms

Cams used in a four cylinder engine com/watch?v=OXd1 PlGur8M&feature=re lated

Cam motions

Types of cam follower

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

Cam Profiles

Displacement graph for a pear shaped cam

Displacement Graphs

Bearings

Thrust Bearings

Bearings

Bronze Nylon PTFE Air White metal Cast Iron Sintered

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

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

Gears

com/watch?v=9NoQ m0wnK_c&feature=r elated com/watch?v=K4Jhr uinbWc&NR=1

Basic Gear Geometry si.com/D190/PDF/D190T25.PDF

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:

The screw thread

Screw thread terms

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