SOME CAUSES OF POUNDING.
To an engineer with a well-regulated mind, a pound about the engine is a source of continual irritation. If a pound arises from a cause which can be remedied by an engineer, the careful man will soon perform the necessary work to end the noise. Sometimes the origin of a pound is hard to discover: very often it is beyond the power of the engineer to stop it. Some makes of locomotives always pound when working in full gear. With such an engine, a nervous engineer will fuss, pushing up wedges until they stick fast, and cause no end of grief to get them down again. He will key up the main-rod connections till they run hot, and he will prophesy that the engine is going to pieces. But the engine hangs together all the same, and is only suffering from want of lead, or want of compression. Where an engine is deficient in the cushioning to the piston, due to compression or lead, the momentum of the piston and connecting-rod is suddenly checked at the end of each stroke. The concussion to these working-parts is so great that pounding will be produced. As the engine gets hooked towards the center, this pounding will cease, because the lead opening increases as the motion is notched back. The most common causes for pounding with locomotives are worn main-rod connections, and driving-boxes too loose in the jaws, or the brasses loose in the driving-boxes. If side-rods are out of tram, or have the brasses badly worn, they sometimes pound when passing the centers. A cross-head will pound when the guides are worn very open. This last defect is liable to cause a bent piston-rod. A piston makes a tremendous pound when a badly connected rod allows it to touch a cylinder-head, and a very ominous pound is produced when the spider gets loose on the piston-rod, and a piston-rod loose in the cross-head will make itself heard all over the engine.