Figure 7-9.—Roller bitt saves line. The roller bitt in figure 7-9 is another example of how you can cut down the wear and tear on lines or cable and reduce your frictional loss. When you need one surface to move over another, you can decrease the friction with lubricants such as oil,  grease,  or  soap.  You  can  use  a  lubricant  on  flat surfaces and gun slides as well as on ball and  roller bearings.  A  lubricant  reduces  frictional   resistance and cuts down wear. In  many  situations  friction  is  helpful.  However, many  sailors  have  found  out  about  this  the  hard way—on a wet, slippery deck. You’ll find rough grain coverings  are  used  on  some  of  our  ships.  Here  you have  friction  working  for  you.  It  helps  you  to  keep your footing. EFFICIENCY To  make  it  easier  to  explain  machine  operations, we  have  neglected  the  effect  of  friction  on  machines up  to  this  point.  Friction  happens  every  time  two surfaces move against one another. The work used in overcoming  the  frictional  resistance  does  not  appear in  the  work  output.  Therefore,  it’s  obvious  that  you have to put more work into a machine  than  you  get out of it. Thus, no machine is 100 percent efficient. Take  the  jack  in  figure  7-6,  for  example.  The chances are good that a 2-pound force exerted on the handle wouldn’t do the job  at  all.  You  would  need  a pull  of  at  least  10  pounds.  This  shows  that  only  2 pounds  out  of  the  10  pounds,  or  20  percent  of  the effort,  is  employed  to  do  the  job.  The  remaining  8 pounds of effort was is  in  overcoming  the  friction  in the jack.  Thus,  the  jack  has  an  efficiency  of  only  20 percent.  Most  jacks  are  inefficient.  However,  even with this inefficiency, it is possible to deliver a huge push with a small amount of effort. A   simple   way   to   calculate   the   efficiency   of   a machine  is  to  divide  the  output  by  the  input  and convert it to a percentage: Output Efficiency =   Input Now   go   back   to   the   block-and-tackle   problem illustrated  in  figure  7-5.  It’s  likely  that  instead  of being able to lift the load with a 120-pound pull, the sailor  would  have  to  use  a  160-pound  pull  through the  100  feet.  You  can  calculate  the  efficiency  of  the rig by the following method: Output      F₂ x S₂ Efficiency =   Input   =  F₁ x S₁ and, by substitution, 600  x  20 Efficiency = 160  x 100 = 0.75 0r 75 percent. Theoretically,  with  the  mechanical  advantage  of 12  developed  by  the  cable  winch  in  figure  6-11,  you can lift a 600-pound load with a 50-pound push on the handle.   If   the   machine   has   an   efficiency   of   60 percent,  how  big  a  push  would  you  actually  have  to apply?  Actually,  50  +  0.60  =  83.3  pounds.  You  can check this yourself in the following manner: Output Efficiency =   Input    F₂ x S₂     =  F₁ x S₁ One revolution of the drum would raise the 600-pound load a  distance  S₂  of  2 p r,  or  7.85  feet.  To make  the  drum  revolve  once,  the  pinion  gear  must rotate six times by the handle, and the handle must turn through 7-5