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Saturday, 31 October 2015

BASICS OF SHAFT ALIGNMENT

We have discussed in our previous post “Gear terminology” and “Gear classifications” about the various aspects in gear and maintenance of gear mechanism.

Today we are going to see the concept and guidelines for securing the suitable shaft alignment. If you are interested to see the concept and basics behind the alignment of shaft (driving and driven shaft), take one cup of tea and read this post thoroughly.

So, first we will have to understand, what is shaft alignment???

Let us think, we have a machine such as pump and electric motor arrangement for delivering the liquid at given head with required flow. This equipment will only work smoothly if driving and driven shafts are aligned properly.

Shaft alignment is basically defined as the positioning of driving and driven shaft in such a way that at point of transferring of power from one shaft (driving) to other shaft (driven), the axis of rotation of both shafts must lie in same line under normal operation or equipment.

I have seen and heard lot of times, people use word coupling alignment, but in my point of view instead of saying it as coupling alignment; we must say it as shaft alignment. During alignment of shaft, we must have to compare the positioning of coupling hub of both shafts at the point of power transfer only for securing the better precise alignment result.

Also read "Shaft alignment terminology"

These days, every organization has decided to provide specific budget for operation and maintenance on annual basis. We, as engineers, are responsible to reduce the downtime of equipment hence we need to align properly all rotating machinery whether the equipment is low energy or high energy and low speed or high speed equipment. 

For easy understanding we will see one example of pump which is taking input energy by electric motor for completing the required task as shown in figure. Main objective of shaft alignment is to increase the availability of equipment by reducing the downtime.


Image Courtesy : Google

Let us see the various types of problems we may face due to misalignment of driven and driving shaft i.e. Pump and Motor shaft as we have considered the example of pump and motor above.
  1. Equipment will operate with excessive noise.
  2. There will be excessive vibration in drive mechanism of unit which pump and motor shaft are misaligned.
  3. Bearing life will also be reduced due to misaligned pump and motor shaft.
  4. Mechanical seal or gland rope will also be damaged due to continuous operation of misaligned shaft.
  5. Due to misaligned shaft of motor and pump, shaft of motor or shaft of pump might be damaged or sheared under continuous operation of drive unit where shafts are misaligned.
I want to share one case of shaft alignment. Before few months, we had one breakdown at one drive unit of a crane. Drive unit was used for operating the hoisting and lowering mechanism. There was one AC electric motor of 350 KW, one horizontal gear box of 3 stage type, brake drum connected with input shaft of gear box, gear coupling for connecting the shafts of motor and gear box input shaft, magnet braking mechanism and output of gear box was connected with wire rope drum with one coupling.

We had faced problem with input drive, Input shaft having enough play where brake- drum was assembled. We had decided to replace the input shaft with brake-drum and gear coupling as we had one set in our workshop.

After replacement, we had carried out the alignment activities and we had tried to secure the required dial indicator reading but we were unsuccessful.We had taken cold trial and observed heavy vibration in drive unit. Finally after completing 24 hours, we were able to find out the root cause of wasting our 24 hours in alignment.

Could you answer? What might be the causes of being unsuccessful?

When i will answer, you will surely say that the writer of this article is completely mad. Let me answer, input shaft of gear box was bent by 4 mm.

Why i have mentioned this case?

I have mentioned this case to feel that before going for shaft alignment, we must have to check run out for individual shaft so that we will not repeat the above case where we had wasted 24 hours to secure the root cause of being unsuccessful in shaft alignment.

Let us go to the next post in this subject/topic of shaft alignment...

Shaft alignment terminology

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