1. Lever and Linkage Systems
Principles of EngineeringTM
Unit 4 – Lesson Mechanisms
Forging new generations of engineers
Project Lead The Way, Inc.
Copyright 2007

2. Lever and Linkage Systems
Methods of Power Transmission

3. The 3 Classes of Levers
Each class has the fulcrum, effort and load arranged in a different way.
The three classes are:

4. Class 1 Lever
Fulcrum between the load and the effort

5. Class 2 Lever
Load between the fulcrum and the effort

6. Class 3 Lever
Effort is between load and fulcrum

7. 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

8. 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

9. BELL CRANK Reciprocating motion Top 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

10. DOUBLE BELL CRANK PUSH, PULL TYPE PUSH requires stiff connecting rods
can use flexible wires,cables, ropes

11. 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

12. 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)
Types of four-bar linkages:
PARALLEL LINKAGE
SCISSOR LINKAGE

13. 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

14. 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

15. 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

16. 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

17. 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

18. SCISSOR LINKAGE Two pair of links of different length
Less force at input pivot point - close to pivot point
output is identical

19. SCISSOR LINKAGE Two pair of links of different length
More force at input pivot point - farther from pivot point
output is identical

20. 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

21. Latching Non-latching
TOGGLE LINKAGE
Latching
Non-latching

22. 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

23. 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 crushers
hay balers