Pulleys and gears are used to transfer rotary motion from one place to another. Both are wheels. Pulleys are wheels with grooves in the rims, in which a belt or rope can run. 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
Twisted pulley
Compound pulley
Two gears
A Gear train
Compound gears
Rack&Pinion
Worm and wheel
Bevel gears
Gear Systems
The illustration shows gears fixed to parallel shafts, forming a simple gear train. The gear fixed on the drive shaft is called the driver gear and is connected to the driven gear. The shafts will turn in opposite directions and, because the gears have different sizes, they will turn at different speeds. The difference in their speeds (velocity ratio) can be calculated from the number of teeth on each gear.
This means that in this case the driver shaft must turn twice for the driven shaft to complete one turn.
Pulley Systems
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 different 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.
If the diameter of the driver pulley in a pulley system is 100 mm and that of the driven pulley 25 mm,
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:
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.
Sketch the cross-section of a V-belt and pulley.
Give two examples of the use of pulley systems.
The figure shows two gears in mesh:
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?
What are the advantages and disadvantages of chain and sprocket systems compared to pulley systems?
Advantages
Disadvantages
Chain and sprocket systems
Pulley systems
LO 2.3
Assessment
LO 2
TECHNOLOGICAL KNOWLEDGE AND UNDERSTANDING The learner will be able to understand 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:
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;
Memorandum
ACTIVITY 1
1.
2.
Drilling machine, Block and tackle
= 250 ohm
Insert idling gear
4. Advantages of tooth gears and chains
Greater forces can be transferred
Chains do not slip/slide
Chains can be disassembled for removal
5.Advantages of pulleys and belts
Belts are not noisy
Belts can be stretched for fitting and for removal
Crossed belts can change the direction of rotation
