and in this case,T.M.A.=21tR2RnR– m= (R- r)”If A is a large wheel and B is a little smaller, thevalue of 2R becomes large and then (R – r) becomessmall. Then you have a large number for2R(R - r)which is the theoretical mechanical advantage.You can lift heavy loads with chain hoists. To giveyou an idea of the mechanical advantage of a chain hoist,suppose the large wheel has a radius (R) of 6 inches andthe smaller wheel a radius (r) of 5 3/4 inches. Whattheoretical mechanical advantage would you get? Usethe formulaThen substitute the numbers in their proper places, andsolveSince the friction in this type of machine isconsiderable, the actual mechanical advantage is not ashigh as the theoretical mechanical advantage. Forexample, that theoretical mechanical advantage of 48tells you that with a 1-pound pull you can lift a 48-poundload. However, actually your 1-pound pull might onlylift a 20-pound load. You will use the rest of your effortin overcoming the friction.SUMMARYThe most important point to remember about blockand tackle is that they are simple machines. And simplemachines multiply effort or change its direction. Youshould also remember the following points:A pulley is a grooved wheel that turns by the actionof a rope in the groove.There are different types of pulleys. Pulleys areeither fixed or movable.You attach a fixed pulley to one place. The fixedpulley helps make work easier by changing thedirection of the effort.You hook a movable pulley to the object you arelifting. As you pull, the object and the pulleymove together. This pulley does not change thedirection of the effort, but it does multiply theeffort.You can use fixed and movable pulleys together toget a large mechanical advantage (M.A.).2-6