OpenStax-CNX module: m23856
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Pulleys and gears
∗
Siyavula Uploaders
This work is produced by OpenStax-CNX and licensed under the
†
Creative Commons Attribution License 3.0
1 TECHNOLOGY
2 Grade 9
3 MECHANISMS AND MOTION
4 Module 3
5 Pulleys and Gears
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Pulleys and gears are used to transfer rotary motion from one place to another.
Pulleys are wheels with grooves in the rims, in which a belt or rope can run.
Both are wheels.
Pulleys can transfer
rotary motion from one shaft to another or can be used to lift heavy loads.
•
Gears are wheels with teeth evenly spaced around the rims. Several gears can be placed, so that their
teeth interlock, or mesh. When two or more gears are used together, they form what is called a gear
train. Gears can be used to change the speed and direction in which something rotates. When chains
are used to connect the gears, a chain and sprocket system is formed, for example on bicycles and
motorcycles.Simple pulley
Figure 1
Twisted pulley
∗ Version
1.1: May 24, 2009 8:41 am -0500
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Figure 2
Compound pulley
Figure 3
Two gears
Figure 4
A Gear train
Figure 5
Compound gears
Figure 6
Rack & Pinion
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Figure 7
Worm and wheel
Figure 8
Bevel gears
Figure 9
Gear Systems
Figure 10
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The illustration shows gears xed to parallel shafts, forming a simple gear train. The gear xed on
the drive shaft is called the driver gear and is connected to the driven gear. The shafts will turn in
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opposite directions and, because the gears have dierent sizes, they will turn at dierent speeds. The
dierence in their speeds (velocity ratio) can be calculated from the number of teeth on each gear.
Figure 11
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This means that in this case the driver shaft must turn twice for the driven shaft to complete one turn.
Pulley Systems
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Pulley systems use a belt to transmit motion and force from the driver shaft to the driven shaft. Cars
use a pulley system with V-belts running in grooves on the pulley wheels. Speed changes are made by
using dierent sizes of pulleys on the driver and driven shafts respectively. By comparing the sizes of
the two pulleys you can calculate the velocity ratio (VR) of the system.
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If the diameter of the driver pulley in a pulley system is 100 mm and that of the driven pulley 25 mm,
Figure 12
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This means that one turn of the driver shaft will give four turns of the driven shaft. The speed of the
driven shaft can be calculated as follows:
Figure 13
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If the input speed is 2000rpm and the VR 1:4 the output speed is 8000 rpm.
Activity 1
1. Add arrows to the sketches to indicate the output direction.
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Figure 14
Figure 15
1. Sketch the cross-section of a V-belt and pulley.
2. Give two examples of the use of pulley systems.
1. The gure shows two gears in mesh:
Figure 16
4.1 If the input speed is 1 250 rpm what is the output speed?
4.2 How could the driver and driven gears be made to rotate in the same direction?
1. What are the advantages and disadvantages of chain and sprocket systems compared to pulley systems?
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Advantages
Disadvantages
Chain and sprocket systems
Pulley systems
Table 1
LO 2.3
Table 2
6 Assessment
LO 2
TECHNOLOGICAL KNOWLEDGE AND UNDERSTANDING
The learner will be able to un-
derstand and apply relevant technological knowledge ethically and responsibly.
We know this when the learner:
systems and control:2.3 demonstrates knowledge and understanding of interacting mechanical systems
and sub-systems by practical analysis and represents them using system diagrams:
•
•
•
•
•
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gear systems;
belt drive pulley systems with more than one stage;
mechanical control mechanism (e.g. ratchet and pawl, cleats);
pneumatic or hydraulic systems that use restrictors;
one-way valves;
systems where mechanical, electrical or pneumatic or hydraulic systems are combined;
Table 3
7 Memorandum
ACTIVITY 1
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1.
Table 4
2.
Figure 17
1. Drilling machine, Block and tackle
•
•
80
1250
= 250 ohm
4 × 1
Insert idling gear
4. Advantages of tooth gears and chains
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•
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Greater forces can be transferred
Chains do not slip/slide
Chains can be disassembled for removal
5.Advantages of pulleys and belts
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•
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Belts are not noisy
Belts can be stretched for tting and for removal
Crossed belts can change the direction of rotation
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