Agitator run out check

[svkd]

Guys , How to check run out for an Agitator shaft with steady bearing,the shaft diameter is 75mm and length 11meters,The vendor recommends to check run out on extension shaft at the end.It is supported at one end only(at the top) and other end free so when try to manually turn the shaft for run out measurement it starts shaking so Not able to measure the actual run out at end.
Shaft Drawing is in attachment.

 

[drawoh]

svkd,

Are you interested in static run-out or dynamic run-out?

--
JHG

 

[geesamand]

That is a long, slender agitator shaft design that requires the support of the lower bearing.

The shaking you describe could be a natural frequency or mechanical issue, but in any case it would make no sense to check "runout" of that hanging shaft other than to best align the lower bearing to the agitator shaft's natural centerline. With the shaking behavior you might need to hold the shaft end by hand while rotating it and stop to let it hang naturally at 45 degree or 90 degree increments.

The hanging shaft end will trace a circle, maybe a large one, and it's generally ok. It takes only a light force to hold that shaft end on center so it won't harm the bearing. Once you have the lower bearing in the center of that hanging shaft end, you can install the bearing and do your normal runout and spin checks.

If this shaft is still insufficiently straight, look at the shipping containers and installation/handling. Shaft straightness in agitators is easily lost in rough handling.

It is a common misconception that agitator shafting must be perfectly straight and held to runout limits of higher-speed machinery. There are numerous specs belonging to big-name companies that grossly (ignorantly) overspecify the required straightness of slow-turning agitator assemblies.

 

[svkd]

geesamand , appreciate your inputs
We already checked the run out by holding the shaft end by hand while rotating it and stop to let it hang naturally at 90 degree increments but we got erratic readings.

Now with the steady bearing installed , There is no space available to put the dial from the bottom nozzle,to measure the shaft run out.Just have a look the drawing attached in my Ist post.

 

[svkd]

drawoh , It was to check dynamic run out by rotating the shaft manually at the bottom shaft end as mentioned in drawing attached in my Ist post

 

[geesamand]

Does the shaft turn freely in the steady bearing? Did the steady bearing slide on without undue effort? Is the lateral clearance of the shaft assembled in the bearing close to the actual difference between the steady bearing ID - shaft OD? If the above questions are answered and results are satisfactory, I would run the machine. I don't see what relevance a runout measurement offers if you measure the shaft right next to a bearing. If the agitator mfr is competent and supportive I suppose this thread topic wouldn't exist(?)

I'm still curious about the erratic movements of the shaft. Is the top mounting surface of the agitator deflecting?

To Drawoh's question, "static" runout means turning by hand, "dynamic" is at operating speed with imbalance forces and hydraulic forces included. Dynamic runouts of in-tank shaft sections are very rarely measured in production agitators.

 

[svkd]

geesamand , Shaft run out should be under allowable limits to ensure long life of steady bearing. It is OEM recommendation, But He didn't mention the method to check this run out.
Is the top mounting surface of the agitator deflecting? No

The shaft is like cantilever ,fixed at top and free at bottom so in static condition also it is shaking around 1-2mm ,that why difficult to check run out.

The OEM recommendation is to check extension shaft run out at bottom end before installation & alignment of steady bearing.
So We still needs to perform below activities:
Does the shaft turn freely in the steady bearing? Did the steady bearing slide on without undue effort? Is the lateral clearance of the shaft assembled in the bearing close to the actual difference between the steady bearing ID - shaft OD?

 

[Tmoose]

I think the intent of the runout check is to be a final check to confirm the accuracy/quality of the manufacturing of the shafts and flanges, as well as the quality of the installation.
For instance, burrs or dirt on a flange face could tip the shaft significantly. Or, the runout on the 4 flange faces may stack up to create quite a bit of shaft "bow."
Thus needing the steady bearing "out of the picture."

To rotate the shaft for runout checking, I'd gently turn the uppermost coupling flange by hand, or even better, turn the motor or gearbox input shaft well above the upper support bearing to reduce the influence on the dangling shaft.

=====================

A secondary check would be to place and indicator at the top of the motor/gearbox, with the mag base on the top of the vessel well away from the welded agitator mount.
Then, with the lower steady bearing installed, rotate the motor fan etc watching for the drive assembly to be wobbling on the vessel.

Any mechanical motion suggests the motion/vibration while running will be at least as much, and probably more.

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Is this agitator, in this exact size, on this exact vessel, a proven design, with several users to contact as references ?

 

[geesamand]

I would like to know the OEM limits and method of measurement.
We can make up our own limits but clearly they have no relevance in your contract and OEM warranty.

On the other hand if you are entirely free of the OEM warranty then give up on trying to follow their poorly specified and unknown limits.

Also a rule of thumb for the allowable orbit before installing the lower bearing is to literally use your thumb. If you can push the shaft back to center with a the man then the side load is so low it's ok.

I wouldn't be offended if this mixer arrangement is unique. Most are.

David

 

[Tmoose]

A big name agitator supplier supplied us 20 agitators. Prior to the failures they failed to mention They just "scaled up" an existing design to create ours, which it turns out was by far the largest in that product family, and very definitely FOAK. First of a Kind.

Initially there were beaucoup cracking issues with the welded fabricated impellers made of a tricky duplex stainless steel ( everybody else casts them).
A year or so after the delayed installation, after MANY failures of varying types, and lots of money and embarrassing repairs and updates later our customer simply ripped out all those agitators and had a competitor install theirs.

Shortly thereafter The BNAS website revealed they had revised their product line to eliminate the scaled up version and replace it with an entirely new product.