THE COUPLE

APPLICATIONS AFLOAT AND ASHORE

A trip to the engine room important the wheel and axle makes you realize how is on the modern ship.

Figure 3-8.A simple torque wrench.

Everywhere you look you see wheels of all sizes and shapes. We use most of them to open and close valves quickly. One common type of valve is shown in figure 3-7. Turning the wheel causes the threaded stem to rise and open the valve. Since the valve must close watertight, airtight, or steamtight, all the parts must fit snugly. To move the stem on most valves without the aid of the wheel would be impossible. The wheel gives you the necessary mechanical advantage.

Youve handled enough wrenches to know that the longer the handle, the tighter you can turn a nut. Actually, a wrench is a wheel-and-axle machine. You can consider the handle as one spoke of a wheel and the place where you take hold of the handle as a point on the rim. You can compare the nut that holds in the jaws of the wrench to the axle.

You know that you can turn a nut too tight and strip the threads or cause internal parts to seize. This is especially true when you are taking up on bearings. To make the proper adjustment, you use a torque wrench. There are several types. Figure 3-8 shows you one that is very simple. When you pull on the handle, its shaft bends. The rod fixed on the pointer does not bend. The pointer shows on the scale the torque, or moment of force, that you are exerting. The scale indicates pounds, although it is really measuring foot-pounds to torque. If the nut is to be tightened by a moment of 90 ft-1b, you pull until the pointer is opposite the number 90 on the scale. The servicing or repair manual on an engine or piece of machinery tells you what the torqueor moment of forceshould be on each set of nuts or bolt.

The gun pointer uses a couple to elevate and depress the gun barrel. He cranks away at a handwheel that has two handles. The right-hand handle is on the opposite side of the axle from the left-hand handle180 apart.

Look at figure 3-9. When this gun pointer pulls on one handle and pushes on the other, hes producing a couple. If he cranks only with his right hand, he no longer has a couplejust a simple first-class lever! And hed have to push twice as hard with one hand.

A system of gears-a gear train-transmits the motion to the barrel. A look at figure 3-10 will help you to figure the forces involved. The radius of the wheel is 6 inches1/2 foot-and turns each handle with a force of 20 pounds. The moment on the top that rotates the wheel in a clockwise direction is equal to 20 x 1/2 = 10 ft-lb. The bottom handle also rotates the wheel in the same direction with an equal moment. Thus, the total twist or torque on the wheel is 10 + 10 = 20 ft-lb. To get the same moment with one hand, apply a 20-pound force. The radius of the wheel would have to be twice as much12 inchesor one foot. The couple is a convenient arrangement of the wheel-and-axle machine.

A wheel-and-axle machine has the wheel fixed rigidly to the axle. The wheel and the axle turn together.

Use the wheel and axle to magnify your effort or to speed it up.

You call the effect of a force rotating an object around an axis or fulcrum a moment of force, or simply a moment.

When an object is at rest or is moving steadily, the clockwise moments are just equal and opposite to the counterclockwise moments.

Moments of force depend upon two factors: (1) the amount of the force and (2) the distance from the fulcrum or axis to the point where the force is applied.

When you apply two equal forces at equal distances on opposite sides of a fulcrum and move those forces in opposite directions so they both tend to cause rotation about the fulcrum, you have a couple.