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(a) Simple 3:1 using MPD™

(b) Simple 5:1 using MPD™
Figure 176- Simple pulley systems utilizing the MPD™ (Multi-purpose Device) as a ratchet

COMPOUND PULLEY SYSTEM
Compound pulley systems are identified as one simple pulley system acting on another
simple pulley system (Figure 177). Travelling pulleys in the system move towards the
anchor at different speeds. Compound pulleys systems are useful because they can
provide greater mechanical advantage than simple systems for the same number of

pulleys.

The TMA of a compound pulley system is calculated by multiplying the individual TMA
of each simple pulley system together. A simple 2:1 pulling on a simple 3:1, results in a
compound pulley system with 6:1TMA.

Using a compound system that is comprised of two dissimilar MA simple pulley
systems, the system with the smaller MA will collapse first. To maximize efficiency in
raising a load with the fewest resets, locate the pulley systems so the system with the
higher MA is pulling on the lower MA system (e.g. have a 3:1 pull on a 2:1 in a
compound 6:1).

Positioning sufficient distance between the anchor points of the individual simple pulley

systems in a compound system can improve the throw distance per reset by allowing
both systems to collapse at the same time. If a compound pulley system is comprised
of two simple 3:1 systems, then the rearward (furthest from load) system must have a
reset distance of three times the of the forward system. This is because the three times
more rope must be pulled through the rearward 3:1 than the forward 3:1.

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