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Wednesday, 4 November 2015

SHAFT ALIGNMENT TERMINOLOGY


We have discussed in our previous post “Basics of shaft alignment” about the shaft alignment and importance of alignment also.

Let us go ahead to understand about the various terms used in alignment of shaft, we need to understand about these terms in order to understand and to secure the successful shaft alignment. 

We will have to consider these alignment terms as without understanding the alignment terms, we will not be able to secure the successful alignment. We are considering here the case of shaft alignment of motor and pump, hence we will see each term with consideration of shaft alignment for motor and pump.

Also read "Shaft alignment procedure"

So, let us come to point to understand the following alignment terms

Parallel misalignment

When we go for alignment of the shaft of pump and motor, we will see one case of shaft misalignment where shaft center lines of pump and motor might be parallel but will be offset with each other and this type of shaft misalignment is named as parallel misalignment. Parallel misalignment might be up and down i.e. in vertical plane and such type of misalignment will also be in horizontal plane also i.e. side to side.

Angular misalignment

Another type of shaft misalignment is angular misalignment, where shaft center lines of pump and motor will not be in same center line but also it will be at an angle. Angular misalignment is also possible in vertical as well as in horizontal plane also.

Combination of parallel and angular misalignment

During alignment of shaft , we may face the misalignment of combination of both above type of misalignment i.e. there will be parallel misalignment as well as angular misalignment also. 

Shims

Before understanding what are the shims and why we need them? We must understand that alignment process will only be commenced once pump installation will be completed with all piping and bolting. In shaft alignment, we will have to assume one equipment as reference and that equipment will be considered as master equipment and we will have to align the second equipment shaft with master equipment shaft.

In case of pump and motor alignment, we will have to consider the pump as master equipment because motor could be moved easily in each direction for securing angular and parallel alignment.
 
During alignment process, we will face one condition where we need to raise the motor to secure the successful alignment and motor could be raised by inserting shims between motor foundation and base plate.
We must prefer to use precut stainless steel shims instead of homemade carbon steel shims, pre cut stainless steel shims of various size could be easily purchased from market. Shim thickness, in mm, will be written over the surface of shim.

Bolt Bound

During alignment, we also face one condition where we need to move motor further (forward or reverse direction) but we could not move the motor due to restriction of foundation hole’s location and this condition is called as Bolt bound.

In order to solve this issue, we will have to remove the motor from foundation. We will have to increase the size of base holes or we can also make slot to provide path for movement of motor.

Dial indicator

Dial indicator is used in shaft alignment to determine the value of misalignment between shafts of equipments (motor and pump). Least count of dial indicators will be 0.001 mm. Dial indicators must be checked before going for alignment although dial indicators are already calibrated by manufacturer but we need to check it’s calibration after per-decided duration.


Before going for alignment we need to check dial indicators whether it is working properly or not and its needle is able to travel throughout indicator or not. We will have to install the dial indicator with one shaft and must be set on zero reading and when we rotate the shaft we will have to focus over the direction of travel of needle of dial indicator as on the basis of direction of needle travel we will be aware about the value of misalignment. 

We have taken one sign convention, clockwise travel will be considered as positive and anti clock wise will be considered as negative.

Bar Sag

Bar sag is basically defined as the deflection which will be present in the overhung bar of dial indicator because of self weight of bar and weight of dial indicator. 

If we want to determine the value of bar sag, we need to install the dial indicator on shaft and set zero reading on top position, now rotate the shaft by 180 degree i.e. at bottom position and read the dial indicator reading and this dial indicator reading will be considered as bar sag.

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

"Shaft alignment procedure"

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