Mechanical advantage device advantage for a screw can be calculated using the following equation: where Note that the actual mechanical advantage of a screw "system" is greater, as a screwdriver or other screw driving system has a mechanical advantage as well. Mechanical advantage device A simple machine that exhibits mechanical advantage is called a mechanical advantage device - e.g.: 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 to the weight is called a "single pulley". It has a mechanical advantage (MA) = 1 (assuming frictionless
Block and tackle output force), formula_5 is the ideal mechanical advantage of the system (which is the same as the number of segments of line extending from the moving block), and formula_6 is the mechanical efficiency of the system (equal to one for an ideal frictionless system; a fraction less than one for real-world systems with energy losses due to friction and other causes). If formula_7 is the number of sheaves in the purchase, and there is a roughly formula_8% loss of efficiency at each sheave due to friction, then: This approximation is more accurate for smaller values of formula_7 and formula_8. A