Figure 7-9.—Roller bitt saves line.The roller bitt in figure 7-9 is another example ofhow you can cut down the wear and tear on lines orcable and reduce your frictional loss.When you need one surface to move over another,you can decrease the friction with lubricants such asoil, grease, or soap. You can use a lubricant on flatsurfaces and gun slides as well as on ball and rollerbearings. A lubricant reduces frictional resistanceand cuts down wear.In many situations friction is helpful. However,many sailors have found out about this the hardway—on a wet, slippery deck. You’ll find rough graincoverings are used on some of our ships. Here youhave friction working for you. It helps you to keepyour footing.EFFICIENCYTo make it easier to explain machine operations,we have neglected the effect of friction on machinesup to this point. Friction happens every time twosurfaces move against one another. The work used inovercoming the frictional resistance does not appearin the work output. Therefore, it’s obvious that youhave to put more work into a machine than you getout of it. Thus, no machine is 100 percent efficient.Take the jack in figure 7-6, for example. Thechances are good that a 2-pound force exerted on thehandle wouldn’t do the job at all. You would need apull of at least 10 pounds. This shows that only 2pounds out of the 10 pounds, or 20 percent of theeffort, is employed to do the job. The remaining 8pounds of effort was is in overcoming the friction inthe jack. Thus, the jack has an efficiency of only 20percent. Most jacks are inefficient. However, evenwith this inefficiency, it is possible to deliver a hugepush with a small amount of effort.A simple way to calculate the efficiency of amachine is to divide the output by the input andconvert it to a percentage:OutputEfficiency = InputNow go back to the block-and-tackle problemillustrated in figure 7-5. It’s likely that instead ofbeing able to lift the load with a 120-pound pull, thesailor would have to use a 160-pound pull throughthe 100 feet. You can calculate the efficiency of therig by the following method:OutputF2xS2Efficiency = Input = F1 x S1and, by substitution,600x20Efficiency = 160 x 100 = 0.75 0r 75 percent.Theoretically, with the mechanical advantage of12 developed by the cable winch in figure 6-11, youcan lift a 600-pound load with a 50-pound push on thehandle. If the machine has an efficiency of 60percent, how big a push would you actually have toapply? Actually, 50 + 0.60 = 83.3 pounds. You cancheck this yourself in the following manner:OutputEfficiency = InputF2xS2= F1 x S1One revolution of the drum would raise the600-pound load a distance S2 of 2pr, or 7.85 feet. Tomake the drum revolve once, the pinion gear mustrotate six times by the handle, and the handle mustturn through7-5
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