Rotating machinery is not usually perfectly balanced at all speeds. (unless it can be perfectly symetrical such as an electric motor or centrifugal pump)
When machinery is designed, the speeds at which vibration are calculated to occurr will be compared with the normal running speed. If the machinery is to be run at only one speed it is easy to alter the design slighly to make sure that the vibration is negligible at the normal running speed and while accelerating through lower speeds.
Similar to balancing car wheels by adding weights to the rim, counterweights are bolted onto the crankwebs of engines to balance them.
For slow speed main engines which can operate at various different speeds for prolonged periods, a rotating balancer is often fitted. A Lanchester balancer is a rotating shaft driven by the engine which has out of balance weights fitted to it to produce vibration that will cancel out the vibration produced by the engine, this will usually be incorporated in the camshaft drive chain.
Critical speed or barred speed range is a speed at which a slow speed engine will suffer damaging vibration and has to be manoevred through as quickly as possible.
Hope this helps.
Last edited by Big Pete
on Wed Apr 03, 2013 12:14 am, edited 1 time in total.
It is always better to ask a stupid question than to do a stupid thing.