Mechanical systems 2 Flashcards
what are rotary systems
mechanical systems using rotary motion
what is a pulley
a wheel with a grooved outer edge and a cable, rope or belt which make a load easier to lift
- changes the direction of the force required
what does using two or more pulleys do
changes the magnitude of the force making things feel alot lighter
- using one fixed pulley and one moving pulley means you would only need half the force to lift a load
what do belts do
transfer movement
what does a belt drive do
transfers movement from one rotating shaft to another
what are belt drives used in
pillar drills, washing machines
how are belt drives and pulleys used in pillar drills
a flexible belt joins two separate pulley wheels- this links the motor to the drill shaft
how does size change the speed of the wheel
- a smaller wheel will move faster, the wheel can also be put in different directions to go faster or slower
how to find the velocity ratio
velocity ratio= diameter of the driven pulley wheel / diameter of the driver pulley wheel
how to find the output speed
output speed = speed of driver pulley wheel (input speed) / velocity ratio
what are cams and what do they do
they are mechanics which rotary motion to reciprocating
- they always rotate
name the two main parts of a cam machanism
the cam and the follower
what does the follower do
rests on the cam motionless as it rotates and it follows its shape. May have a small wheel to reduce friction
- moves up and down (reciprocating motion) as the cam turns
what is it called when the cam pauses
dwell
describe a circular cam
also called eccentric or offset
- produces a uniform reciprocating motion
the circular cam rotates about an off- centre pivot which causes the follower to move up and down
describe a snail and drop cam
for half a turn, the follower will not move, then it will gently rise, then suddenly drop, only works in one direction (anticlockwise), clockwise will cause it to jam
describe a pear cam
for half a turn, the follower won’t move, then it will gently rise and fall
describe a four- lobed cam
- has four lobes (bits that stick out)
- for each turn of the cam, the follower will rise and suddenly fall 4 times, also only works in one direction
what can changing the size or shape of the cam do
can be used to change the magnitude of the output motion ( the reciprocating motion of the follower)
