Thursday, 19 March 2015

Dave Hawley, ABB’s UK General Manager for motors and generators writes: Anecdotal evidence from coupling manufacturers suggests that 50 percent of machine breakdowns can be traced back to shaft misalignment.

Despite the statistics, many engineers are sceptical and still ask if it is really necessary to align a coupling so accurately?  You can see their point too, after all a flexible coupling is designed to cope with misalignment between shafts.

This is sadly a myth.  While a flexible coupling will accommodate some degree of misalignment and allow the motor and machine to still run, the additional forces created by misalignment puts additional strain on motor bearings, seals and coupling components.

The degree to which these additional forces are affecting the motor and coupling can be difficult to measure during operation, but can be analysed with the use of detection equipment.  Using vibration analysis probes and infrared thermal imaging will allow you to identify where the additional stresses on the motor are and the severity of their effects.

This doesn’t mean expensive analysis equipment is the only way. Even without detection equipment, there are still some tell-tale signs that could point to imminent coupling failure.  These include:
  • Loose or broken foundation bolts or coupling bolts
  • Loose shim packs
  • Excessive oil leakage at bearing seals or inside coupling guards
  • Unusually high rate of coupling failures or wear
  • Shafts breaking or cracking at, or close to, the installed bearings or coupling hubs
Despite the scepticism, there are major benefits to be had aligning shaft couplings accurately. A well aligned coupling brings:
  • Improved plant operating life and reliability
  • Reduced need for spare parts like seals and bearings
  • Reduced maintenance labour costs
  • Higher production availability
  • Lower standby plant needs
  • Better plant operating safety
  • Greater energy efficiency
The 3 types of misalignment that can cause issues are:
  1. Mechanical alignment.  This occurs when the motor twists out of shape or moves during       operation.  No matter how rugged or deep in cross-section the motor frame may be, operating load can alter alignment.
  2. Parallel alignment.  This is the offset between two centrelines of the two coupling shafts and can occur when the motor base plate or mounting feet of the motor and driven machine are at different horizontal positions.
  3. Angular misalignment.  This is the amount by which the faces of the coupling used to connect the motor are out of parallel and occurs when the motor and driven components are mounted at differing vertical positions.
All of these types of misalignment are very easy to overcome.  A common approach to adjusting alignment is to:
  • Use a dial indicator or laser alignment tool to identify the correct alignment planes. Aligning shafts by eye is not reliable enough.
  • Adjust the motor base plate by repositioning the mounting bolts or fit adjustable base plates that allow for adjustments.  These make any future adjustments straight forward.
  • Use shims to adjust the height and angle of the motor so it has a firm base when operating.
  • Re-tighten the motor mounting bolts use a cross-torque pattern tightening opposite corners each time to ensure even torque on all bolts.
Plant managers must ensure that accurate alignment is achieved on all motor installations, with checks and adjustments done every 2,000 hours. Best practice training should also be considered for staff and alignment equipment made available to engineers so couplings can be aligned simply and accurately.

To find out more about how to tackle misalignment, download ABB’s motor tips e-book.


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