Pulleys

it has details of pulley, it types and uses in physiotherapy

1. PULLEY SYSTEMS AND IN
PHYSIOTHERAPY
Prof. Dr. M. Rajesh, PT, M.P.T(cardio), BCRC
TRINITY MISSION AND MEDICAL FOUNDATION
MADURAI

2. INTRODUCTION
Apulleyis awheelon anaxleorshaftthat is designed to
support movement and change of direction of a taut cable or
belt, or transfer of power between the shaft and cable or belt.
In the case of a pulley supported by a frame or shell that does
not transfer power to a shaft, but is used to guide the cable or
exert a force, the supporting shell is called a block, and the
pulley may be called a sheave.
A pulley may have agrooveor grooves betweenflangesaround
itscircumferenceto locate the cable or belt. The drive element
of a pulley system can be arope,cable,belt, orchain.

3. BLOCK AND TACKLE
A set of pulleys assembled so that they rotate independently on
the same axle form a block. Two blocks with a rope attached to
one of the blocks and threaded through the two sets of pulleys
form ablock and tackle.
Ablock and tackleis assembled so one block is attached to
fixed mounting point and the other is attached to the moving
load. Theideal mechanical advantageof the block and tackle is
equal to the number of parts of the rope that support the
moving block.

4. In the diagram on the right the ideal mechanical advantage of
each of the block and tackle assembliesshown is as follows:
Gun tackle: 2
Luff tackle: 3
Double tackle: 4
Gyn tackle: 5
Threefold purchase: 6

5. ROPE AND PULLEY SYSTEM
A hoist using the compound pulley system yielding an
advantage of 4. The single fixed pulley is installed on thehoist
(device). The two movable pulleys (joined together) are
attached to thehook. One end of the rope is attached to the
crane frame, another to the winch.
A rope and pulley system—that is, ablock and tackle—is
characterized by the use of a single continuous rope to transmit
a tension force around one or more pulleys to lift or move a
load—the rope may be a light line or a strong cable. This
system is included in the list ofsimple machinesidentified by
Renaissance scientists.

6. If the rope and pulley system does not dissipate or store
energy, then itsmechanical advantageis the number of parts of
the rope that act on the load. This can be shown as follows.
Consider the set of pulleys that form the moving block and the
parts of the rope that support this block. If there arepof these
parts of the rope supporting the loadW,then a force balance
on the moving block shows that the tension in each of the parts
of the rope must beW/p.This means the input force on the
rope isT=W/p.Thus, the block and tackle reduces the input
force by the factorp.

7. A gun tackle has a single pulley in both the
fixed and moving blocks with two rope parts
supporting the loadW.
Separation of the pulleys in the gun tackle
show the force balance that results in a
rope tension ofW/2.

8. A double tackle has two pulleys in both the
fixed and moving blocks with four rope parts
supporting the loadW.
Separation of the pulleys in the double tackle
show the force balance that results in a rope
tension ofW/4.

9. METHOD OF OPERATION
The simplest theory of operation for a pulley system assumes
that the pulleys and lines are weightless, and that there is no
energy loss due to friction. It is also assumed that the lines do
not stretch.
In equilibrium, the forces on the moving block must sum to
zero. In addition the tension in the rope must be the same for
each of its parts. This means that the two parts of the rope
supporting the moving block must each support half the load.

10. Fixed pulley
The loadFon the moving pulley is
balanced by the tension in two parts of the
rope supporting the pulley.

11. Movable pulley
A movable pulley lifting the loadWis supported by
two rope parts with tensionW/2.

12. TYPES OF PULLEY SYSTEM
Fixed:Afixedpulley has an axle mounted in bearings attached
to a supporting structure. A fixed pulley changes the direction of
the force on a rope or belt that moves along its circumference.
Mechanical advantage is gained by combining a fixed pulley
with a movable pulley or another fixed pulley of a different
diameter.
Movable:Amovablepulley has an axle in a movable block. A
single movable pulley is supported by two parts of the same
rope and has a mechanical advantage of two.
Compound:A combination of fixed and movable pulleys forms
ablock and tackle. Ablock and tacklecan have several pulleys
mounted on the fixed and moving axles, further increasing the
mechanical advantage.

13. Dig 3.The gun tackle "rove to advantage" has the
rope attached to the moving pulley. The tension in
the rope isW/3yielding an advantage of three.
Dig 3a. The Luff tackle adds a fixed pulley "rove to
disadvantage." The tension in the rope
remainsW/3yielding an advantage of three.

14. The mechanical advantage of the gun tackle can be increased
by interchanging the fixed and moving blocks so the rope is
attached to the moving block and the rope is pulled in the
direction of the lifted load. In this case theblock and tackleis
said to be "rove to advantage."Diagram 3 shows that now three
rope parts support the loadWwhich means the tension in the
rope isW/3. Thus, the mechanical advantage is three.
By adding a pulley to the fixed block of a gun tackle the
direction of the pulling force is reversed though the mechanical
advantage remains the same, Diagram 3a. This is an example of
the Luff tackle.

15. PULLEY SYSTEM IN
PHYSIOTHERAPY
Thispulleysystemisusedtogaina
mechanicaladvantagewhenlifting
heavyweights.Onesimplecombination
isincommonuseforliftingthetrunkfor
suspensionexercises.Theupperpulley
isfixedtoanoverheadsupport,towhich
oneendoftheropeisattached.The
ropeisthenwoundroundthemovable
pulley,towhichtheweightisattached,
androundthefiXedpulley,theeffort
beingappliedatthefreeend.

16. Provided the effect of friction is omitted, the tension is the same
in all parts of the rope, therefore, if the weight (W) is 6kg. The
tension required in each of the two supporting ropes will be
3kg, and the effort (E) required will be 3kg.
MECHANICAL ADVANTAGE = W/E = 6kg/3kg = 2
If double pulleys are used the effort required can again be
reduced by half.

17. THANK YOU
Prof. Dr. M. RAJESH, PT,M.P.T(cardio),B.C.R.C
TRINITY MISSIOIN AND MEDICAL FOUNDATION
MADURAI.
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