7. Group the synonyms.

1)straight,2) general, 3)usual, 4)complete,8) apply, 5)single, 6)point, 7)magnitude,9)refer, mean, 10)value, 11)total, equal, way,13) dot, 14)one, 15) vertical

The theory of machines and mechanisms

UNIT 5

1. Learn the notions and their definitions.

Simple machine A device that changes the direction or magnitude, or both, of an applied force and thus do useful work.

Linkage A mechanism consisting of rolls connected to each other by joints that permit motion.

Wheel and Axle A basic machine consisting of a wheel that rotates on a shaft called the axle.

Spring An elastic material that returns to its original shape after being forced out of that shape.

Inclined plane Flat surface that slants or slopes

Wedge Two inclined planes joined together

Screw An inclined plane wrapped around a cylinder

Crank A bent shaft or arm for transmitting motion or changing from rotary to reciprocating motion and vice versa.

Pulley A basic machine con­sisting of a wheel with a grooved rim through which a rope, wire, or chain is passed.

Block A pulley contained in a housing; the combina­tion of a fixed and a movable block together with a rope is known as a block and tackle.

Helix The corkscrew-shaped figure that results from wrapping the line of an inclined plane around a cylinder; the plural is helices.

Jack A device used to raise heavy weights for short distances.

Lever A basic machine con­sisting of a rigid piece or bar that turns on a point.

Fulcrum The point on which a lever turns.

Text 1

2. Look through the text for the problems described. Examplify them using your experience. Translate and retell the text.

Simple Machines

The term simple machine is often used to identify those devices that change the direction or magnitude, or both, of an applied force and thus do useful work. Among the simple machines are the lever; wheel-and-axle; inclined plane, wedge, screw, and pulley.

Linkage. Machines are often classified by the arrangement of their linkage systems. For exam­ple, an internal-combustion engine may be de­scribed by a four-link system called a slider crank. Another four-link system is the four-bar linkage. The punch press is an example of a slightly more complicated device that com­bines the slider crank and the four-bar linkage into a six-bar arrangement.

Most simple machines operate in a single plane or in a series of parallel planes and have an output motion that is rotational, translational, or a combination of the two. The four-bar linkage illustrates the use of a crank for both input and output motions. Depending on the configuration of the links, the four-bar linkage with a constant rotational input motion can have an oscillating,

identical, or nonuniform output motion

Inclined Plane. The inclined plane is an ex­ample of a simple machine that exchanges in­creased distance for less effort. An inclined plane is a flat surface that slants or slopes. If two inclined planes are joined together, they form the simple machine called a wedge. Wedges are very helpful in cutting and splitting things. Knives, axes, and our front teeth are good examples of wedges.

An inclined plane A wedge.

Fig 8.

The inclined plane has various applications, (a) When used as a loading ramp for a truck, the inclined plane has a mechanical advantage equal to L/H, where L is the length of the ramp and H is the height of the truck's tailgate above the ground, (b) A wedge is equivalent to two inclined planes joined together. A downward force is transformed into sideward forces, useful for splitting things, (c) If an inclined plane is wrapped around a cylinder, we have a simple machine known as a screw. A screw-type automobile jack is an ex­ample of this machine.

The simplest pulley is called a fixed pulley. A single pulley at the top of a flagpole is a good example. Ideally, omitting friction, this pulley has an MA of 1—that is, our force is not multi­plied; only its direction is changed. A downward pull on the rope sends the flag up the pole. It is easier to pull down than to pull up; gravity works with our body weight to make this easier.

A second kind of pulley is called a movable pulley. This kind moves along with the weight. In a movable pulley, a support above gives the added effort. So pulling upward on a movable pulley results in lifting an object with

Fig. 9 Fig. 10

only half the effort (neglecting friction), but the object rises only half the distance pulled. The reason is that to lift the object 1 foot, we must pull up 1 foot on the effort side and take up 1 foot of slack on the supported side.