Pulleys

Pulley basics, blocks and tackles

Pulley is a device that can increase the magnitude of the effort force. 

Without Pulley

pulley force - no pulley

With no pulley - the effort force is similar to the load - in opposite direction.

S = F         (1)

where

S = effort force (N, lb)

F = load (N, lb)

Single Pulley

Fixed Pulley

pulley force - single fixed pulley

With a single fixed pulley the effort force is similar (or more due to efficiency) to the load.

S = F         (2)

The advantage with the single fixed pulley is that the direction of force is changed - it is possible to pull down instead of lifting up.

Movable Pulley

pulley force - single movable pulley

 

With a single moveable pulley the effort force is half (or more due to efficiency) of the load.

S = 1/2 F         (3)

Combined Pulleys

pulley force - combined pulley

With a combined moveable pulley as above - the effort force is half (or more due to efficiency) of the load.

S = 1/2 F         (4)

pulley force - combined pulley

With two pulleys and the ropes as above -  the effort force is 1/3 (or more due to efficiency) of the load.

S = 1/3 F         (5)

General Equation for Blocks and Tackles

The general effort force equation for block and tackle to raise or pull a load can be expressed as

S = F / (μ n)

   = (m g) / (μ n)  (6)

where

S = effort force (N, lb)

F = load, often weight (N, lb)

m = mass (kg, slugs) (when lifting a mass)

g = constant of gravitation (9.81 m/s2, 32.17405 ft/s2) (when lifting a mass)

μ = mechanical efficiency of the system (equal to one for an ideal frictionless system, a fraction less than one for real-world systems with energy losses due to friction)

n = number of ropes between the sets of pulleys

Example - Pulley and Effort Force

pulley example 4 ropes

The effort force for a pulley with 4 ropes, no friction loss (μ = 1) and a load 100 kg can be calculated as

S = (100 kg) (9.81 m/s2) / ((1) (4))

   = 245 N

Related Topics

  • Dynamics - Dynamics Motion - velocity and acceleration
  • Mechanics - Kinematics, forces, vectors, motion, momentum, energy and the dynamics of objects

Related Documents

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