Friction and Lubrication
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Some times there is screeching sound while using a pulley or hand pump. Friction is a force. Whatever work we do, friction works opposite to our force. When we apply a force to move something, the friction opposes our force and there is no motion. When our force is greater than the friction force, the object starts moving. The parts where there is friction, they wear out fast. A smooth surface shows lots of uneven and rough surface under a microscope. When these rough parts rub on each other, it results in friction. Oiling and greasing reduces friction and wear & tear of parts. When there is dust or foreign particles in the area of contact, friction and wear & tear are high and rapid. To prevent this, we must first clean the friction area and put oil or grease there.
Advantages of friction
Friction is not always bad. Sometimes we require friction. For example: 1) While walking friction helps us. If we put oil on the floor, we will fall down due to slippery floor. We need friction to be able to walk. 2) It is commonly seen that in muddy area the wheel of a truck slips round and round because there is no friction between the tyres and ground. 3) Flour mill works because of friction between belt and wheel. 4) Bicycle can be stopped by putting on a brake, which works because there is friction. 5) When we tie a knot in a rope, it is the friction which holds the knot. Thus friction is useful to some extent but it will reduce the efficiency of work.
Factors affecting friction
The intensity of friction depends on following factors: i) The area involved in friction. ii) The pressure applied on the surfaces. Force = Pressure ´ Area Frictional force will increase, if the area of contact will increase or if pressure applied on the surface increased.
Methods to reduce friction
i) Polish the contact surface. ii) Put oil or grease so that it fills in the small gaps of the flat parts. iii) Use ball bearings to reduce area of contact between rotating parts.
Following methods can be used to reduce friction: Oil is either thin or viscous. It depends upon SAE No. of oil. (SAE means Society of Automotive Engineers). If we use very viscous oil, it does not reach all the parts. Very thin oil will flows away easily and gets wasted. Grease is used in such cases. It is generally used around ball-bearing. Normal grease or oil is never used where there is high pressure, high temperature and high speed. Special lubricants are used in such cases. In cold season the oil becomes thick and in hot season it becomes thin. Therefore selection of lubrication also depends on the season. It is always advisable to refer operating manual of the equipment before selecting the lubricant.
What you have learnt
In this chapter, you have learnt different types of simple machines. You also studied its application. Any big machine is combination of six simple machines. You also know about efficiency and mechanical advantage. In the end, you learnt about friction and lubrication. You also learned about advantages of friction.
i) Draw a diagram of brake system of bicycle. Mark different fulcrum points, load arm and effort arm. Discuss how force is transferred to apply brakes.
ii) On the slope, driver changes gear. Can you give reason behind it?
iii) Draw different types of forks used in kitchen? Why they are of different shape? Which type of simple machine is used in it?
iv) Observe sugar cane juice machine. Write down important parts and simple machine principles used in it.
v) Why boric power is used on carom board?
vi) Give three examples when friction is desirable and undesirable.
vii) Why bush is used around a rotating shaft?
3.8 Answer to intext Questions
3.1 i) Energy ii) distance iii) N-m iv) Six
3.2 i) slope ii) efforts, load iii) fulcrum, efforts
3.3 i) Match the following
|Lifting pump from bore well||pulley|
|Wood cutting axe||Wedge|
|Hilly road||Inclined plane|
ii) bevel, Power output / power input , distance traveled by load increases
Observe following scissors carefully i) Barber scissor ii) Gardner scissor iii) Tailors scissor iv) Doctors scissor. What is difference between them? Find out reason by actually working with it.
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