Gear Driven Pump - Some makes of engine were equipped with a displacement pump that were driven by a gear mounted on the crankshaft, so that the pump operated whenever the engine rolled over. The geared pump was thought by some to be better than a crosshead pump in that it could be geared down to a more advantageous speed, could be made double-acting (thus smoothing out the pressure surges), and was nearly impossible to get out of alignment.
    The pump is actually two displacement pumps that share common inlet and discharge chambers. As the ram works back and forth in the cylinders, the check valves alternately open and close, pulling the water from the inlet chamber and forcing it into the discharge chamber, and thence into the boiler. The discharge line has a tee in it, with a bypass valve regulating how much water may pass back into the tank. Closing the valve forces all the water into the boiler, opening it allows all of the water to return to the tank. It is considered best to run with an intermediate setting, ideally one that introduces exactly as much water as the engine is using. Given the pressures in use, this can be as little as 1/8 of a turn open.
    Like many things on a steamer, the geared pump is too simple to not work. Barring leaky plumbing and dry tanks, about the only way they fail is by getting pieces of foreign matter into the check valves, allowing the water in the cylinder to pass back into the inlet chamber. You do want to be sure that the line to the boiler is equipped with a functioning pressure relief valve, though. If the boiler inlet valve is left closed, the geared pump is capable of generating hundreds of pounds of pressure, and something is guaranteed to fail before the load stops the engine. Most often, the pressure dome on top of the pump ruptures, tossing bits of metal around, and making a small replica of Old Faithful.
    The output of a geared pump is often routed through a feedwater heater, which transfers some of the heat energy of the exhaust steam to the incoming water, improving economy and making the feedwater less likely to cool the boiler.