Mechanical advantage
In physics and engineering mechanical advantage (MA) is the factor by which a machine multiplies the force put into it. The mechanical advantage can be calculated for the following simple machines by using the following formulas:
- Lever: MA = length of effort arm ÷ length of resistance arm.
- Wheel and axle: A wheel is essentially a lever with one arm the distance between the axle and the outer point of the wheel, and the other the radius of the axle. Typically this is a fairly large difference, leading to an equally large mechanical advantage. This is why even simple wheels with wooden axles running in wooden blocks will still turn freely, because the friction is overwhelmed by the rotational force of the wheel multiplied by the mechanical advantage.
- Pulley: Pulleys change the direction of a tension force on a flexible material, e.g. a rope or cable. However pulleys can be "added together" to create additional mechanical advantage by having the flexible material looped over several pulleys in turn. More loops and pulleys increases the mechanical advantage.
Mechanical advantage of pulleys
Consider lifting a weight with rope and pulleys. A rope looped through a pulley attached to a fixed spot, e.g. a barn roof rafter, and attached the weight is called a single fixed pulley and has an MA = 1, that is, no mechanical advantage (or disadvantage) however the change in direction may be more advantageous for other reasons.
A single moveable pulley has a MA = 2. An example would be a pulley attached to a weight being lifted and a rope running arround it where one end is attached to a fixed point above, e.g. a barn roof rafter, and a pulling force is applied upward to the other end with the two lengths parallel. Note: if an additional pulley is used to change the direction of the rope, e.g. the person doing the work wants to stand on the ground instead of on a rafter, the mechanical advantage is not increased.
By looping more ropes around more pulleys we can continue to increase the mechanical advantage. For example if we have two pulleys attached to the rafter, two pulleys attached to the weight, one end attached to the rafter and someone standing on the rafter pulling the rope then we have a mechanical advantage of four. Again if we add another pulley so that someone may stand on the ground and pull down then we still have a mechanical advantage of four.
Here are examples where the fixed point is not obvious:
A man sits on seat that hangs from a rope that is looped through a pulley attached to a roof rafter above. The man pulls down on the rope to lift himself and the seat. The pulley is considered a movable pulley and the man and the seat are considered as fixed points; MA = 2.
A velcro strap on a shoe passes through a slot and fold over on itself. The slot is a movable pulley and the MA =2.
Two ropes layed down a ramp attached to a raised platform. A barrel is rolled onto the ropes and the ropes are passed over the barrel and handed to two workers at the top of the ramp. The workers pull the ropes together to get the barrel to the top. The barrel is a movable pulley and the MA = 2. If the there is enough friction where the rope is pinched between the barrel and the ramp then the pinch point becomes the attachment point which is considered a fixed attachment point because the rope above the barrel does not move relative to the ramp. Alternatively the ends of the rope can be attached to the platform.
Most generally, the mechanical advantage is calculated thus:
- MA = (the distance over which force is applied) ÷ (the distance over which the load is moved)
also, the Force exerted IN to the machine × the distance moved IN will always be
equal to the force exerted OUT of the machine × the distance moved OUT. For example; using a block and tackle with 6 ropes, and a 600 pound load, the operator would be required to pull the rope 6 feet, and exert 100 pounds of force to lift the load 1 foot, therefore:
- (force in 100 × distance in 6) = (force out 600 × distance out 1)
or more basic, WORKin = WORKout
This requires an ideal simple machine, meaning that there are no losses due to friction or elasticity. If friction or elasticity exist in the system efficiency will be lower; Workin will be greater than Workout
Mechanical advantage also applies to torque. A simple gearset is able to multiply torque.
There are two types of mechanical advantage:
- Ideal mechanical advantage (IMA)
- Actual mechanical advantage (AMA)
