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Pullys system in physiotherapy, its type, and mechanical advantage

Updated: Feb 7, 2023

Hi, welcome to all of the physio's helping hand (physiogk) family in my new blog post is about the pulley system which is very important for 1st-year physiotherapy (BPT) students.

pulley system in pt | mechanical advantage of pulley system

In this blog, we will discuss pulleys, their type with examples, and mechanical advantage of the pulley system.


Content:-

Introduction and Definition

A pulley is a grooved wheel that is rotated about a fixed axis by a rope that passes around it. Axis is supported by a framework or block and the whole structure may be used either as a fixed pulley or a movable pulley or a pulley combination.

Pulleys are frequently used in exercise and traction equipment to increase or decreased the force magnitude to change the direction of the force.


The fixed pulley | what is a fixed pulley or define a fixed pulley

In a fixed pulley system, the pulley block is fixed to some suitable support and the rope which passes around the wheel is attached to the weight at one end and the effort is applied at the other.

The fixed pulley may be single fixed, two fixed pullies, or more than 2 fixed pulley types.


type of fixed pulley | single and double fixed pulley
Single fixed pulley and double fixed pulley

Uses of fixed pulley system

  • This is used to alter the direction of force and enable traction or resistance to be applied at any angle.

  • The fixed pulley system is used in the body by some muscles to allow them to be inserted at a more advantageous angle.

Examples-

  • Diagnostic and omohyoid muscles pull round fibrous loops.

  • Obturator internus muscles turn a right angle and glide on a rigid groove to its insertion.

 

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The Movable Pulley

A movable pulley has an axle in a movable block. A single movable pulley is supported by two parts of the same rope.

This device is used to gain a mechanical advantage when lifting heavy weights.

It acts as a second-class lever. Its mechanical advantage is (2) as it magnifies force.

single movable pulley
Movable pulley

Pulley Combination:

There are many types of pullies combinations and the simplest one is a combination of one fixed and one movable pulley as used in the Guthrie-Smith suspension frame in suspension therapy.

The upper pulley is fixed to the overhead support to which one of the ropes is attached. The rope is then wound around the movable pulley to which the weight is attached and around the fixed pulley the effort being applied at the free end.


Pulley Combination
Pulley Combination

What is Mechanical Advantage(MA)

It is a force-amplifying effectiveness of a simple machine such as a liver. ramp, wheel, or pulley. Mechanical advantage allows you to perform tasks such as lifting heavy objects much easier in terms of the force you must apply.

We can also define Mechanical Advantage as the ratio of the weight to the effort in the pulley system.









Mechanical advantage formula for a pulley:

Formula 1:- The formula used to calculate the mechanical advantage(MA) of a pulley system is

MA=2n

where n is the number of movable pulleys.

Formula 2:- For a more complicated pulley system, we can use the formula

MA= m

where m is the number of strands that support the pulley system.

Formula 3:-We can also use the formula

MA = weight(with)/effort(e)


How to calculate the mechanical advantage(MA) of the pulley system

We will calculate MA in 2 steps:-

  • Step 1: Calculate the no. of strands supporting the movable pulley or count the total number of movable pulleys or notes the weight and effort in that combination-type pulley system

  • Step 2: To calculate the MA of the pulley system use the formula of MA.

Examples 1:- Using the above image calculate the mechanical advantage of the pulley system using the above formula.


From formula 1,

  • Step 1: no. of movable pulley (n) = 1

  • Step 2: We will use the formula of mechanical advantage MA= 2n

MA= 2 (1) ⇒MA=2

From formula 2,

We will calculate the no. of the strand (m) connected to movable pulley that is

2 and put it in the formula of MA

MA= m ⇒ MA=2


From formula 3,

Step 1: Weight in the pulley system is 6 kg and

The tension required in each of the two supporting ropes will be 3kg because the weight is 6 kg and the tension is the same in all parts of the ropes. So efforts(E) required will be 3kg

  • Step 2: We will use the formula of mechanical advantage of pulley

MA = weight(w)/effort(e)

MA = 6 kg/3 kg ⇒ MA=2

Example 2: Using the image below, calculate the mechanical advantage of the pulley system.









Solution:-

For the second formula, we have

MA=m⇒MA=8

So, the mechanical advantage of this pulley system is 8.



reference:- The Principles of Exercise Therapy by Gardiner M. Deena


 

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