# Block and tackle - efficiency approximation - with friction factor

## Description

A block and tackle is a system of two or more pulleys with a rope or cable threaded between them, usually used to lift or pull heavy loads.A more precise estimate of efficiency is possible by use of the sheave friction factor, (which may be obtainable from the manufacturer or published tables. The relevant equation is:

Related formulas## Variables

eff | 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) (dimensionless) |

K | sheave friction factor (which may be obtainable from the manufacturer or published tables) (dimensionless) |

N | the ideal mechanical advantage of the system (which is the same as the number of segments of line extending from the moving block) (dimensionless) |

S | number of sheaves in the purchase (dimensionless) |