Design of cast iron pulleys and their types and terminologies.

1. G.H PATEL COLLEGE OF
ENGINEERING & TECHNOLOGY
2 1 8 2 0 0 4
D E S I G N O F M E C H A N I S M S I I
DESIGN OF CI PULLEYS
G U I D E D B Y : D R . V I N O D P A T E L

2. HELLO!
This is a presentation by:
150110120060: Tirth Parmar
150110120062: Vaibhav Solanki
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3. “
“A designer knows he has achieved
perfection not when there is nothing left to
add, but when there is nothing left to take
away.”
- Antoine de Saint-Exupéry
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4. 1.
CAST IRON PULLEYS
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5. INTRODUCTION
◈ The pulleys are generally made of cast iron, because of their low
cost.
◈ The rim is held in place by web from the central boss or by arms
or spokes.
◈ The arms may be straight or curved as shown in Fig. 19.1 (a) and
(b) and the cross-section is usually elliptical.
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6. DETAILS
◈ These are made of grey cast iron of grade FG200.
◈ The rim and the arms are made of low carbon steel while the hub which
is subjected to crushing stress is made of medium carbon steel.
◈ The rim is roll-formed from a steel plate and joined either by bolts or
welded.
◈ The arms are screwed to the hub if it is made of cast iron.
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7. BIG CONCEPT
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9. CROWNING
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◈ The cast iron pulleys are generally made with
rounded rims.
◈ This slight convexity is known as Crowning.
◈ The crowning tends to keep the belt in centre
on a pulley rim while in motion.
◈ The crowning may be 9 mm for 300 mm width
of pulley face.

10. OBJECTIVES OF PROVIDING CROWN
◈ Helps to hold the belt on the pulley in running condition.
◈ Prevents the belt from running off the pulley.
◈ Brings the belt to running equilibrium position near the
mid-plane of the pulley.
◈ The crown on the pulley is essential, particularly when the
pulleys are mounted inaccurately or there is a possibility
of slip due to non-parallelism between connected shafts.
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11. SPLIT CI PULLEYS
◈ When it is necessary to mount a pulley on a shaft which already carrying
pulleys etc. or have its ends swelled, it is easier to use a split-pulley.
◈ There is a clearance between the faces and the two halves are readily
tightened upon the shafts by the bolts as shown in Fig. 19.2. A sunk key
is used for heavy drives.
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13. ARMS OF CI PULLEY
◈ There are three important things about the arms of the pulley. They are as follows:
(i) The arms of pulley have elliptical cross-section.
(ii) The major axis of elliptical cross-section is in the plane of rotation.
(iii) The major axis of elliptical cross-section is usually twice the minor axis.
◈ The design of these arms illustrates the application of simple formula for bending
stresses.
◈ It is assumed that the belt wraps around the rim of the pulley through
approximately 180° and one-half of the arms carry the load at any moment.
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14. TORQUE TRANSMITTED
◈ The torque transmitted by the pulley is given by:
◈ Mt = torque transmitted by the pulley (N-mm)
◈ P = tangential force at the end of each arm (N)
◈ R = radius of the rim (mm)
◈ N = number of arms
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15. ◈ The major axis of the elliptical cross-section is in the
plane of rotation. For this elliptical cross-section with
a and b as minor and major axes respectively,
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16. ◈ where sb is the permissible bending stress.
◈ If we consider the minor axis in the plane of rotation as illustrated in Fig. 13.21(b),
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17. ◈ It is observed from Eqs (f) and (g) that keeping the minor axis in
the plane of rotation increases the cross-sectional area.
◈ It is therefore ‘economical’ to keep major axis of elliptical cross-
section in the plane of rotation.
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18. EXAMPLE
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A pulley, made of grey cast iron FG 150,
transmits 10 kW of power at 720 rpm. The
diameter of the pulley is 500 mm. The
pulley has four arms of elliptical cross-
section, in which the major axis is twice of
the minor axis. Determine the dimensions
of the cross-section of the arm, if the factor
of safety is 5.

19. SOLUTION
◈ Given
◈ kW = 10 n = 720 rpm D = 500 mm
◈ For pulley, Sut = 150 N/mm2 (fs) = 5
◈ N = 4
◈ b = 2a
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