Applications aflot and ashore

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Curved Lever Arms

Basic Machines

Chapter 2 - Block and tackle

APPLICATIONS AFLOAT AND ASHORE

Doors, called hatches aboard a ship, are locked shut by lugs called dogs. Figure 1-10 shows you how these dogs are used to secure the door. If the handle is four times as long as the lug, that 50-pound heave of yours is multiplied to 200 pounds against the slanting face of the wedge. Incidentally, take a look at the wedgeits an inclined plane, and it multiplies the 200-pound force by about 4. Result: Your 50-pound heave actually ends up as a 800-pound force on each wedge to keep the hatch closed! The hatch dog is one use of a first-class lever in combination with an inclined plane.

The breech of a big gun is closed with a breech plug. Figure 1-11 shows you that this plug has some interrupted screw threads on it, which fit into similar

Figure 1-12.-Using a wrecking bar.

interrupted threads in the breech. Turning the plug part way around locks it into the breech. The plug is locked and unlocked by the operating lever. Notice that the connecting rod is secured to the operating lever a few inches from the fulcrum. Youll see that this is an application of a second-class lever.

You know that the plug is in there good and tight. But, with a mechanical advantage of 10, your 100-pound pull on the handle will twist the plug loose with a force of a half ton.

If youve spent any time opening crates at a base, youve already used a wrecking bar. The sailor in figure 1-12 is busily engaged in tearing that crate open.

Figure 1-11.-The breech of an 8-inch gun.

Figure 1-13.-An electric crane.

Figure 1-14.-A. A pelican hook; B. A chain stopper.

The wrecking bar is a first-class lever. Notice that it has curved lever arms. Can you figure the mechanical advantage of this one? Your answer should be M.A. = 5.

The crane in figure 1-13 is used for handling relatively light loads around a warehouse or a dock. You can see that the crane is rigged as a third-class lever; the effort is applied between the fulcrum and the load. This gives a mechanical advantage of less than 1. If its going to support that 1/2-ton load, you know that the pull on the lifting cable will have to be considerably greater than 1,000 pounds. How much greater? Use the formula to figure it out:

Got the answer? Right. . . E = 1,333 pounds

Now, because the cable is pulling at an angle of E, you can use some trigonometry to find that the pull on the cable will be about 3,560 pounds to lift the 1/2-ton weight! However, since the loads are

Figure 1-15.-An improvised drill press.

generally light, and speed is important, the crane is a practical and useful machine.

Anchors are usually housed in the hawsepipe and secured by a chain stopper. The chain stopper consists of a short length of chain containing a turnbuckle and a pelican hook. When you secure one end of the stopper to a pad eye in the deck and lock the pelican hook over the anchor chain, the winch is relieved of the strain.

Figure 1-14, part A, gives you the details of the pelican hook. Figure 1-14, part B, shows the chain stopper as a whole. Notice that the load is applied close to the fulcrum. The resistance arm is very short. The bale shackle, which holds the hook secure, exerts its force at a considerable distance from the fulcrum. If the chain rests against the hook 1 inch from the fulcrum and the bale shackle is holding the hook closed 12 + 1 = 13 inches from the fulcrum, whats the mechanical advantage? Its 13. A strain of only 1,000 pounds on the base shackle can hold the hook closed when a 6 1/2-ton anchor is dangling over the ships side. Youll recognize the pelican hook as a second-class lever with curved arms.

Figure 1-15 shows you a couple of guys who are using their heads to spare their muscles. Rather than exert themselves by bearing down on that drill, they pick up a board from a nearby crate and use it as a second-class lever.

If the drill is placed halfway along the board, they will get a mechanical advantage of 2. How would you increase the mechanical advantage if you were using this rig? Right. You would move the drill in closer to the fulcrum. In the Navy, a knowledge of levers and how to apply them pays off.

SUMMARY

Now for a brief summary of levers:

Levers are machines because they help you to do your work. They help by changing the size, direction, or speed of the force you apply.

There are three classes of levers. They differ primarily in the relative points where effort is applied, where the resistance is overcome, and where the fulcrum is located.

First-class levers have the effort and the resistance on opposite sides of the fulcrum, and effort and resistance move in opposite directions.

Second-class levers have the effort and the resistance on the same side of the fulcrum but the effort is farther from the fulcrum than is the resistance. Both effort and resistance move in the same direction.

Third-class levers have the effort applied on the same side of the fulcrum as the resistance but the effort is applied between the resistance and the fulcrum, and both effort and resistance move in the same direction.

First- and second-class levers magnify the amount of effort exerted and decrease the speed of effort. First-class and third-class levers magnify the distance and the speed of the effort exerted and decrease its magnitude.

The same general formula applies to all three types of levers:

Mechanical advantage (M.A.) is an expression of the ratio of the applied force and the resistance. It may be written: