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Lever and Linkage Systems

Methods of Power Transmission

The 3 Classes of Levers

• Each class has the fulcrum, effort and load arranged in a different way.

• The three classes are:

Class 1 Lever

• Fulcrum between the load and the effort

Class 2 Lever

• Load between the fulcrum and the effort

Class 3 Lever

• Effort is between load and fulcrum

ROCKER ARM

• Has an input and output arm which rock around a fulcrum point

• Acts as a link which transmit force or motion between moving parts

• Operate usually as a first class lever

• Change linear input motion to an opposed linear motion

BELL CRANK• A bent first class lever

that pivots at the bent point or elbow

• Used to change the direction of force or motion 90 degrees or less

• Used to convert rotary motion to straight line or reciprocating motion

• Reciprocating motionTop

– pin A moves slider to right

– pin B moves bell crank which moves slider to left

Bottom– pins move crank;

spring returns to start position

BELL CRANK

DOUBLE BELL CRANK

• PUSH, PULL TYPEPUSH

– requires stiff connecting rods

PULL– can use flexible

wires,cables, ropes

DIFFERENTIAL LINKAGE

• Used to combine several motion inputs into a resulting output

• Three pivot points, none of which are fixed but are free to float within limits

• Any pivot can function as an input

• A motion control mechanism rather than a means of transmitting force or power

FOUR-BAR LINKAGES

Types of four-bar linkages:•PARALLEL LINKAGE•SCISSOR LINKAGE

(Whatever the motion, each point on a moving link must move in the same PLANE, and all the links must be parallel)

FOUR-BAR LINKAGES

• To make a four-bar linkage a usable device, one of the four links, or one of the pivots, must be fixed

• Depending on the fixed pivot or link different motion outputs will occur

FOUR-BAR LINKAGES

• Fixed link– links pivoting from

the ends of the fixed link are either input links or input and output links

– the moving link opposite the fixed link is the connecting link

FOUR-BAR LINKAGES

• FIXED PIVOT– the point opposite

the the fixed point is normally the output point

– the remaining two pivot points are usually the input points

PARALLEL LINKAGE

• To provide parallel motion of a line or a surface as the linkage moves about two controlled pivot points

• The arms remain parallel through the operation of the connecting link

Eg: man lift, robot gripper

SCISSOR LINKAGE

• To change the direction of a linear motion input to a linear motion output that is perpendicular to the input

• Large motion output (distance advantage) with a small motion input

• Especially true of a series of connecting linkages

• One fixed pivot point

SCISSOR LINKAGE

• Two pair of links of different length– Less force at input

pivot point - close to pivot point

– output is identical

SCISSOR LINKAGE• Two pair of links of

different length– More force at input

pivot point - farther from pivot point

– output is identical

WATT STRAIGHT LINE MOTION

• Does not form a parallelogram but consists of two equal length pivoting links and a connecting link

• Produces a straight line motion in the connecting link

TOGGLE LINKAGE

•Latching•Non-latching

TOGGLE LINKAGE

• LATCHING– capable of snap

action from one state to an alternate state

– arm length may be different

– may have a toggle stop to produce latched condition

TOGGLE LINKAGE• NON-LATCHING

– operated so that the two toggle arms never quite straighten

– may have a toggle stop to produce latched condition

e.g.rock crushershay balers