Vocabulary Practice

1. Describe friction.

2. What is the coefficient of friction?

3. How do static, sliding, and rolling friction differ from each other?

4. What is babbitt metal?

5. What is teflon?

6. What is antimony used for?

7. What does lubrication mean? What is a lubricant?

8. What are polymers?

Friction

Friction reduces the efficiency of machines but it is also indispensable. When you try to walk on ice, slipping and sliding and perhaps falling down, you realize the importance of friction. Ice has a low coefficient of friction and that is what makes it so difficult to walk on. Without friction it would be impossible to walk at all and our trains and cars could not move.

There are three different kinds of mechanical friction: static, sliding, and rolling. Static friction is the resistance to motion between two bodies in contact but at rest. The resistance of static friction is greater than that of sliding friction which is resistance to continued motion after one body has started to move. Rolling friction occurs when resistance is reduced to its lowest degree by rotary motion not on the same axis.

Note these three kinds of friction at work: it takes a stronger effort to put a box into motion (static friction) then it does to keep it moving across the floor once started (sliding friction); if there an rollers under the box it takes still less effort to keep it in motion (rolling friction).

One way to reduce friction in machines is through the materials for the parts that contact each other. The coefficient of friction is the constant ratio of the friction to the force pressing the surfaces together. Coefficients have been equated for different common materials using the three types of friction. Steel on steel 01 glass on glass have high coefficients but some new substances have much lower coefficients. One of these is babbitt metal, an alloy made of tin, copper, and antimony, another is teflon, a plastic containing fluorine that is sometimes used in cooking utensils.

Another way of reducing friction is by means of lubrication, applying oil or grease to the points or surfaces where the parts of a machine contact each other. Petroleum products are the principal modern lubricants; some of them include polymers, the long, heavy, complex molecules that occur in plastics.

Discussion

1. What does friction do to the efficiency of machines?

2. What does friction make possible?

3. Name the three different kinds of friction.

4. Which kind of friction offers the greatest resistance? The least? Explain and give an example.

5. How can the choice of materials in a machine help reduce friction?

6. What are some substances that have low coefficients of friction?

7. What is another way of reducing friction?

8. Name the principal lubricants and some new variations.

9. Discuss a common mechanism used to reduce friction.

BEARINGS

Special Terms

Bearing: That part of a machine which supports parts in relative motion.

Ball Bearings: Bearings containing small round balls, sometimes known as anti-friction bearings.

Cage: A device to hold and separate bearings.

Races: Rings within which cages of bearings are sealed.

Boiler Bearings: Bearings with rolling elements in the shape of cylinders or tapered cylinders or needles; they have a greater load-carrying capacity but more friction than ball bearings of similar size.

Non-contact Bearings: Bearings that touch only at rest; in motion they are separated by gas or fluid.