and in this case, T.M.A.   = 21tR 2R nR  –  m = (R   -    r)” If A is a large wheel and B is a little smaller, the value of 2R becomes large and then  (R – r) becomes small. Then you have a large number for 2R (R  -  r) which  is  the  theoretical  mechanical  advantage. You can lift heavy loads with chain hoists. To give you an idea of the mechanical advantage of a chain hoist, suppose the large wheel has a radius  (R) of 6 inches and the smaller wheel a radius  (r) of 5 3/4 inches. What theoretical  mechanical  advantage  would  you  get?  Use the  formula Then substitute the numbers in their proper places, and solve Since  the  friction  in  this  type  of  machine  is considerable, the actual mechanical advantage is not as high  as  the  theoretical  mechanical  advantage.  For example,  that  theoretical  mechanical  advantage  of  48 tells you that with a 1-pound pull you can lift a 48-pound load.  However,  actually  your  1-pound  pull  might  only lift a 20-pound load. You will use the rest of your effort in overcoming the friction. SUMMARY The  most  important  point  to  remember  about  block and tackle is that they are simple machines. And simple machines multiply effort or change its direction. You should also remember the following points: A pulley is a grooved wheel that turns by the action of a rope in the groove. There are different types of pulleys. Pulleys are either fixed or movable. You attach a fixed pulley to one place. The fixed pulley helps make work easier by changing the direction of the effort. You hook a movable pulley to the object you are lifting. As you pull, the object and the pulley move together. This pulley does not change the direction of the effort, but it does multiply the effort. You can use fixed and movable pulleys together to get a large mechanical advantage (M.A.). 2-6