What is crankshaft web deflection?

Answer 1

An engine/compressor crankshaft has several places where rods to the pistons are connected. The rods are on a pin which is offset from the center of the crankshaft.

Thus, as the crankshaft rotates, the pin orbits in a circle around it.

The connecting rod is thus forced back and forth, due to the pin's motion.

The pin is connected to the crankshaft by two pieces called WEBs.

On a perfectly-aligned crankshaft, the distance between the two webs is constant, even as the shaft rotates.

However, after months of constant use, the mounting of the engine/compressor can change: ground settling, worn support structures, bolt tension changes, etc.

When this happens, the distance between the webs can start to vary with rotation. The amount of variation in this distance is called WEB DEFLECTION.

That means the webs are subject to cyclic stress every rotation, and if severe enough, the crankshaft can be destroyed.

WEB DEFLECTION is thus an indicator of the state of crankshaft alignment. Even a few mils variation is indicative of trouble. Manufacturers recommend checking this periodically, and re-aligning the shaft if the deflection is over a given limit.

Here is a drawing of a crankshaft, with the web labeled:

http://www.motorera.com/dictionary/pics/c/crankshaft.gif