Balance Bob

A large counterweighted lever, attached to a shaft's pump rods, that offset the rods' own weight so that a Cornish pumping engine only had to do the work of lifting the water itself.

Historical usage

Balance bobs were standard equipment on deep Cornish-engine-pumped shafts, since the pump rods (or 'pitwork') running the full depth of a mine could themselves weigh a very great deal, quite apart from any water being lifted.

How it worked

A surface balance bob sat on a pair of plinths or a masonry support beside the shaft, with a large box of scrap iron or rock as its counterweight working up and down in an adjacent stone-lined pit; on deeper shafts, further balance bobs were installed in chambers cut into the shaft wall itself at intervals, to counterbalance the rods' weight at that depth as well.

Why it was used

Without a balance bob, a pumping engine would have to expend most of its power simply raising and lowering the pump rods themselves before it could do any useful work lifting water; the counterweight let the engine's power go almost entirely into pumping, which mattered enormously as shafts reached ever greater, and heavier, depths.

Risks & limitations

A balance bob had to be sized correctly for the pump rods it served — too light, and the engine still laboured under the rods' weight; too heavy, and the engine instead fought the counterweight on the return stroke.

Regional variation

The device and its name are strongly associated with Cornish mining engineering specifically, developed alongside the distinctive Cornish beam pumping engine.

Sources

1. (primary) Definition of pump bob — Mindat
2. (secondary) A Cornish mining glossary — Cornish Mining World Heritage Site (UNESCO)

Record created: 21 August 2026 · Last researched: 21 August 2026