Nobody argues that shaft alignment matters. What gets argued about is how much it matters, and the answer is usually worse than people expect, because the damage happens in a place you cannot hear and on a timescale that outlasts the commissioning engineer.
What misalignment does to a bearing
When two coupled shafts do not line up, every revolution pushes a sideways load into both machines. That load adds to whatever the drive was already carrying, and bearing life is extremely sensitive to extra load: even a modest increase can cut the expected life in half, and a large one can reduce years of service to months. None of that is audible on day one. The unit sounds fine, the coupling takes the flex, and the life that is being consumed is invisible.
Why the coupling hides it
A flexible coupling is designed to accommodate misalignment, which is exactly the problem: it accommodates it by generating a restoring force, and that force lands on your bearings. A flexible element will happily absorb a visible offset without complaint while quietly pushing a constant side load into the shafts either side of it. The element usually fails first, gets replaced, and the real cause never gets found.
Sensible tolerances
Modern laser alignment tools make precise alignment a twenty-minute job once the machine is on its feet. There is no reason to accept “looks straight” on any drive that is expected to run for years, and fast-running drives need to be held even tighter than slow ones.
The parts people skip
- Check the feet first. A gearbox that rocks on three of its four mounting pads will pull itself out of alignment the moment you tighten the bolts, and it distorts the housing enough to disturb the gears inside as well.
- Allow for heat. A drive aligned cold and run hot moves. On larger machines, setting the cold alignment to allow for the expected movement is standard practice, not a refinement.
- Watch the pipework and cables. A conduit that has been sprung into place is a permanent side load. So is a cooling line pulled up tight.
- Re-check after the foundations settle and after the first hot run. Both move things. Neither is optional on a base-mounted unit.
Or design the problem out
The most reliable alignment is the one nobody has to do. A shaft-mounted gear unit on a torque arm has no coupling and no baseplate: the gearbox hangs directly on the driven shaft and the arm only steadies it. Our parallel-shaft and right-angle shaft-mounted frames exist largely for this reason. Where you can hang the drive instead of coupling to it, hang it.