Mechanical power transmission: How it works

How gears, ropes and pressurised pipes carry power

Before electricity, energy was moved as motion or pressure: spinning shafts and belts in factories, moving ropes on cable cars, and pressurised water or air in city-wide pipe networks. These methods still run inside every machine and many industrial sites, but over distances of more than a few kilometres electricity has almost entirely replaced them.

Step by step

1. A prime mover makes motion or pressureAn engine, motor or water wheel turns a shaft, or drives a pump or compressor that pressurises water, oil or air. Pumps can approach 90% efficiency under ideal conditions but often fall below 75% with varying loads.
2. Gears and belts carry rotationMeshing gear teeth or a belt wrapped round pulleys pass torque from one shaft to another and change speed. The best high-power gears reach 98 to 99% efficiency per mesh; a synchronous (toothed) belt holds about 98%.
3. Ropes and chains carry pulling forceA steel haul rope driven by one motor in a station pulls many cabins along a line; urban ropeways run at about 10 to 20 km per hour and carry 1,000 to 2,000 passengers per hour each way.
4. Pipes carry pressureWater or oil under pressure (hydraulics) or compressed air (pneumatics) flows through pipes to rams, motors or tools. London's hydraulic network ran at a nominal 800 psi (about 55 bar, calculated) through up to about 180 miles (290 km) of mains; Paris's compressed-air network grew to about 1,000 km of pipes serving some 10,000 customers.
5. Energy is used at the far endRams lift loads or open bridges, air motors turn tools, and the spent fluid is exhausted (air) or returned or discharged (water and oil). London's water powered workshop machinery, lifts, cranes, theatre stages and the backup mechanism of Tower Bridge.

The science

Mechanical power is force times speed, or for rotation torque times rotational speed. Gears and belts lose energy to friction where surfaces slide or flex, plus oil churning and bearing drag. Fluids carry power as pressure times flow rate. Water and oil barely compress, so a hydraulic system stores little energy and responds instantly. Air compresses a great deal and heats up as it is squeezed; that heat is lost from the pipes, and the air cools as it expands at the tool, which is why compressed air is one of the least efficient ways to deliver energy.

The engineering

Gearboxes use hardened helical or planetary gears running in oil, with bearings and seals; their efficiency depends on gear ratio, tooth angle and load. Belt drives need correct tension: V-belts slip and wear, while toothed synchronous belts do not slip. Ropeways use a single central drive, sheaves, tensioning systems and emergency brakes. Hydraulic networks need pumping stations, accumulators (weighted pistons that store pressure), cast-iron mains and pressure-relief valves. Compressed-air systems need compressors, dryers, receivers, filters and leak control.

Key numbers

Sources

1. Replace V-belts with notched or synchronous belt drives (Motor Systems Tip Sheet #5) US Department of Energy, Advanced Manufacturing Office, 2012
2. Estimating the Impact (Energy, Emissions and Economics) of the U.S. Fluid Power Industry (ORNL/TM-2011/14), Love, Lanke and Alles Oak Ridge National Laboratory, 2012
3. London Hydraulic Power Company Science Museum Group Collection, 2025
4. Victor Popp et l'horlogerie pneumatique à Paris Ville de Paris, 2024
5. Using the Box-Behnken response surface method to study parametric influence to improve the efficiency of helical gears (Machines 9(11), 264) MDPI, 2021
6. Innovation in the air: using cable cars for urban transport World Bank (Transport blog), 2017