Methods of exerting forces and displacements

introduction to the design of „basic machine”

Levers

Drawings taken from: „Give me point of support and I will lift the Earth”Galileo

Is this true?

Power screws (screw gears)

Torque that has to be applied to the nut (or screw head) in order to obtain force P

2'

2 221DPtgP

dMMM Tp

n

On the other side:Mn = Fhand x Lwrench

Screw with a nut is a basic machine able to achieve a large force in the

screw by multiplying the force applied to the nut

Power screwsVarious configuration of mechanisms

Compound and differential screws (Hunter’s)

Mechanizmy śruboweEvaluating torque and force

2'

2 221DPtgP

dMMM Tp

n

Design of a power screw

Material selection + limited specific load:Cast iron 8-30 MPabrass 8-15 MPabronze 10-30 MPaAluminium bronze 10-40 MPa

seizure of the surfaces excessive wear

Screw under tension - break Screw under compression - exceeding allowable stress

- flexural buckling (wyboczenie)

Selection of thread1. Thread type – trapezoidal thread (Acme) or butress threadlub niesymetr.2. Minor diameter

3. Thread pitch – standard values selected to obtain required mechanical advantage and self-locking properties.A possibility to use differential and summing mech.

- destruction of the bolt shank

- Destruction of the thread

Rope mechanisms

Pulley block (compound pulleys)

Hydraulic devices

Rack and pinion mechanism

Transformation of rotational motion into linear motion

Cam mechanisms

Crank mechanism

Rack and pinion mechanismPush rodCam follower

crankshaft

TransmissionsGear transmision

Belt transmision