Slide 1
Rope redirects a pull
A fixed pulley changes the direction of a rope force. On its own, an ideal fixed pulley does not reduce the force needed to support a load.
More to Read
Pulling downward may still be ergonomically useful because the operator can use body weight or a convenient stance. Mechanical advantage and practical convenience are different benefits.
Slide 2
Support with several rope segments
A movable pulley can let multiple rope segments support one load. In an ideal continuous rope, each segment carries the same tension.
More to Read
Two supporting vertical segments each carrying 50 newtons can support a 100-newton load, neglecting pulley weight. Counting the segments attached to the moving assembly gives the ideal force advantage.
Slide 3
Pull more rope
Reducing required force increases the length of rope that must be pulled. Lifting the load a short distance uses a longer effort movement.
More to Read
With two supporting segments, raising the ideal load one metre shortens each segment by one metre. The free end must therefore take up two metres of rope.
Slide 4
Real tension is not perfectly uniform
Bearing friction, rope bending and other losses make real pulley systems depart from the ideal model. More pulleys can add more loss.
More to Read
The rope’s stiffness and pulley diameter affect bending resistance. A system with a large theoretical advantage may therefore feel less helpful if many small pulleys repeatedly bend a stiff rope.
Slide 5
The support carries a reaction
A pulley’s axle and attachment must carry the combined rope forces. The supporting structure can experience more load than one rope tension.
More to Read
For a fixed pulley with two nearly parallel downward rope segments, the attachment carries approximately twice the tension. Designing only for the operator’s pull would miss that support load.
Sources
- Simple Machines
OpenStax
- Basics of Mechanical Design
MIT Center for Bits and Atoms