TEXT: 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 continue 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 are 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 or 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, 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.

A vital mechanism for reducing friction is the bearing which basically is a device that bears the friction of parts in motion. Often one of the parts will be moving and the other will be stationary. Logs used to move heavy stones in early times were the primitive or of a beaming. They were efficient because they changed sliding friction to rolling friction, thereby decreasing the effort necessary to move the stones.

Reducing friction between the parts of a machine is the principal purpose of bearings. Different types have been de­signed for use at various points of contact to fit the kinds of mo­tion at work. Probably the most familiar are ball bearings which are used in many machines. Small balls are fitted into a cage, a container that separates them. Cage and bearings are then sealed, often in a lubricant, between rings which are called races. The entire assembly is a ball bearing.

Another familiar type is the roller bearing, a modern version of the logs that were used as primitive bearings. Roller bearings contain small cylinders on which the bearing races can roll. They are usually fitted with the same kind of holding cage and races as ball bearings. In order to sustain pressure from different directions, bearing rollers are sometimes tapered and set at an angle to the races. A variation is the needle bearing with cylinders of very small diameter. Needle bearings need not be contained in a cage or between races. Their advantage is greater load-carrying capacity with more friction than ball bearings of comparable size.

A modern development is the non-contact bearing in which there is contact between the machine parts only at rest; when in motion they are separated by a thin layer of gas or fluid. This pre­vents wear between the moving parts. Non-contact bearings have been developed for such complex and sophisticated systems as missile guidance. The possibility of using cushions of compressed air in transportation systems has been discussed frequently in recent years.

While bearings are used to minimize friction other mechanical devices put friction to work. The car that rolls without acceleration is gradually brought to a stop by friction. A long gradual stop is far from suitable, however, to traffic conditions where speed must be controlled and where sudden and frequent stops are necessary. Braking devices put friction to work to provide the necessary con­trol over motion in automobiles and other kinds of machines.

The most common types of brakes ordinarily consist of a rotating component that is brought into contact with a friction compo­nent designed so that the mechanical energy is changed into heat which is dissipated into the air. The friction material may be metal, ceramic, or a substance like asbestos. Old-fashioned automobile brakes were made of a steel band that could be pressed down against the outside of the brake drum. When they got wet, however, their friction coefficient was often dangerously re­duced, a condition known as fading. This led to the development of drum brakes with a friction lining on the inside of the drum. A still more recent braking device is the disk brake. It consists of metal disks that turn with the wheel and can be brought into contact against friction pads.

Another mechanism necessary for road vehicles is a clutch device so that the motor shaft can be connected or disconnected from the wheel shaft while the motor is running. The type in com­mon use today is the disk clutch in which connection is made by applying pressure so that pairs of disks lined with friction material are brought into contact or released to increase or decrease power to the output shaft. In many cars pressure comes when the driver steps on the clutch pedal; with some automatic transmissions pressure is applied automatically through fluids as speed changes. Any kind of clutch depends on friction.