Transcript Mechanical Systems - University of KwaZulu

Mechanical Systems
Unit 2:Mechanisms
LEVERS and Linkages
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.
• Linkages are mechanisms which allow force or motion to be
directed where it is needed. Linkages can be used to change:
– The direction of motion
– The type of motion
– The size of a force
Examples of linkages
neac2.eu
Fixed and Moving Pivots
• A linkage consists of a system of rods or other rigid
materials connected by joints or pivots.
• The ability of each rod to move will be limited by moving
and fixed pivots. The input at one end of the mechanical
linkages will be different from the output, in place,
speed, direction and other ways.
• A fixed pivot is one that turns around one point, but does
not move away from that point. Symbol is :
A moving pivot is one that can move away from it’s original
position. Symbol is:
Types of linkages
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Reverse-motion linkage
Bell-crank linkage
Parallel-motion linkage
Treadle linkage
Toggle clamp linkage
Reverse-motion linkage
Reverse-motion linkage
A linkage which makes things move in
opposite directions is called a REVERSE
MOTION LINKAGE. As the top rod moves to
the left the bottom rod moves to the right. A
linkage which makes the output move in the
same direction as the input is called a PUSHPULL LINKAGE. Examples of this are found in
fold up tables and chairs.
Bell-crank linkage
Bell-crank linkage
• This linkage allows horizontal movement to be converted
to vertical movement. It also works the opposite way
round. A practical example of this is the brake
mechanism on a bicycle
• If you want to change the direction of movement or force
through 90° you can use this type of linkage
• If the fulcrum is at an equal distance from the input and
output then the movement of the output will be equal to
the movement of the input.
• A common device which uses this mechanism is the
brakes of a bicycle.
Parallel-motion linkage
Parallel-motion linkage
• The parallel motion linkage can be used to
make things move in the same direction at a
stipulated distance apart.
• Parallel motion is only achieved if the levers at
opposite sides of the parallelogram are equal
in length.
• Many tool-boxes use parallel-motion linkages
Treadle linkage
Treadle Linkage
A treadle linkage shows how linkages can be used
to change one type of motion into another. In this
case, the rotary motion of the cam moves a
verical linkage in a linear motion.
Tow Treadle linkages are used to move
windscreen wipers
DESIGNING AND MAKING MODELS: LEVERS
Practical task 1
Construct a model of a 2nd class lever system
using Fischer-Technik components. (30 mins)
Practical task 2
Construct one linkage out of the materials
available to you. (15 mins)
References:
• http://www.cableorganizer.com/articles/toolbox-roundup.html
• materials-handling.apluswhs.com
• neac2.eu
• caravantech-shop.co.uk
• best-b2b.com
• http://www.bbc.co.uk/schools/gcsebitesize/d
esign/systemscontrol/mechanismsrev3.shtml