Friction

Reduction of friction

It is beneficial to reduce the friction between surfaces to make movement easier or reduce the wear and tear on a surface. There are a number of ways to reduce friction:

1. Make the surfaces smoother:Rough surfaces produce more friction and smooth surfaces reduce friction. Some swimmers wear suits to reduce underwater resistance. These suits mimic the smooth skin of sharks.
2. Lubricationis another way to make a surface smoother. A lubricant is a slippery substance designed to reduce the friction between surfaces. You might use oil to stop a door from squeaking – the oil reduces the friction in the hinge. Water can be used as a lubricant – think of how a floor becomes slippery after it has been mopped.
3. Make the object more streamlined:A streamline shape is one that allows air or water to flow around it easily, offering the least resistance. Compare a boxy old car with a new car that has a rounded shape, allowing it to move with less effort.
4. Reduce the forces acting on the surfaces:The stronger the forces acting on the surfaces, the higher the friction, so reducing the forces would reduce the friction. If you apply the handbrake when you try to drive a car, the car will have a lot of difficulty moving because of the force immobilising (stopping the movement of) the wheels. If you release the handbrake, the wheels will move more freely because there is no extra force acting on them.
5. Reduce the contact between the surfaces:Have you ever tried to roll a cube? Spheres are the best shape for reducing friction because very little of a spherical object is in contact with the other surface. Several types of wheels, such as skateboard wheels, contain small spheres called ball bearings to reduce the friction between the moving parts. You can witness the effect of ball bearings by comparing the friction between sliding a book on a table and then doing the same, but using marbles between the book and the surface of the table. Notice how the marbles act as ball bearings, reducing the friction.

LUBRICATION

Lubrication is simply the use of a material to improve the smoothness if movement of one surface over another; the material which is used in this way is called a lubricant. Lubricants are usually liquids or semi-liquids, but may be solids or gases or any combination of solids, liquids, and gases.

The smoothness of movement is improved by reducing friction. This is not, however, always the case, and there may be situations in which it is more important to maintain steady friction than to obtain the lowest possible friction.

In addition to reducing or controlling friction, lubricants are usually expected to reduce wear and often to prevent overheating and corrosion.

TYPES OF LUBRICANTS

Lubricants are usually divided into four basic classes.

(a)   Oils: A general term used to cover all liquid lubricants, whether they are mineral oils, natural oils, synthetics, emulsions, or even process fluids.

(b)  Greases: Technically these are oils, which contain a thickening agent to make them semi-solid. It is convenient, however, to include the anti-seize pastes and the semi-fluid greases under the same heading.

(c)   Dry lubricants: These include any lubricants, which are used in solid form, and may be bulky solids, paint-like coatings, or loose powders.

(d)  Gases: The gas usually used in gas bearings is air, but any gas can be used which will not attack the bearings, or itself decompose.

The advantages and disadvantages of oils stem from their ability to flow easily. Thus, on the credit side, it is very easy to pour them from a container, to feed them into a bearing by dripping, splashing or pumping, and to drain them out of a machine when no longer fir for use. Other advantages are the cooling of a bearing by carrying away heat, and cleaning it by removing debris.

The behavior of greases is very similar to that of oils, but the former are used where the advantages of easy flow are outweighed by the disadvantages. Thus grease do not easily leak out of a machine, or container, do not migrate away, and will form an effective seal against contaminants.

The advantages and disadvantages of solid lubricants are rather like the extremes for greases, where the lubricant will not flow at all. Similarly, the advantages and disadvantages of gas lubricants are like the extremes of oils, where the flow properties are almost too good.

ASSESSMENT

1. Which of the following actions will reduce friction?
   (a) Make the surfaces rougher
   (b) Make the surfaces smoother
   (c) Increasing the contact between the surfaces
   (d) Exerting more force on the surfaces
2. Lubrication is a way to make a surface
   (a) clean
   (b) smooth
   (c) rough
   (d) dirty
3. The stronger the forces acting on the surfaces…
   (a) the higher the friction
   (b) the lower the friction
   (c) the higher the smoothness
   (d) the lower the smoothness
4. One of these is not a class of lubricants
   (a) solid
   (b) liquid
   (c) gaseous
   (d) flat
5. Greases contain a thickening agent that makes them
   (a) solid
   (b) semi-solid
   (c) liquid
   (d) gas