The indicator card, and the devices that produced them, have been around about as long as there have been steam engines. The basic idea is simple: as the piston of the engine moved, a pressure indicator moved a needle and pen up and down on a paper (usually a rotating drum) and produced what’s called in thermodynamics a pV diagram, shown below.
The steam engine (or any reciprocating engine, they’re also used with internal combustion engines as well) is somewhat straightforward in that it has a fixed stroke. With an impact pile hammer, you have a free piston engine whose stroke is not determined by the rotation of the crankshaft. Vulcan’s attempt to attach an indicator mechanism to its smallest hammer (the DGH-100,) shown above, is a little tricky. The telltale rod coming out the top of the ram moves the pen radially around the drum through a lever and cord mechanism. The pressure from the hammer moves the pen up and down on the drum. Either the pressure above the piston or below can be fed to the indicator mechanism, as shown below.
Vulcan’s motivation in looking at this was probably due to questions about the downward assist of the air pressure on the top of the piston. This question challenged Vulcan during the whole life of its differential-acting hammers.
Whether Vulcan actually did this is unknown. Vulcan used a more modern approach to determine pressure-time and pressure-displacement histories in its linear vibrator, and the analysis software Vulcan developed electronically generated indicator cards for its single-compound hammer.
Mechanical indicator mechanisms are still used for slower, reciprocating engines (usually steam) but no matter how they’re made, indicator cards–and the concept behind them–are important in the performance analysis of reciprocating engines of all kinds.