Drilled holes in hollow rotating shafts in mixing application 1

[alegna]

I recently had to replace an agitator shaft that was hollow. This shaft is essentially a SCH 80S 6" pipe, and rotates at a very low RPM (around 5 RPM). The shaft is about 8 ft tall.

The original design has two holes about 3/8" drilled into the shaft, one closer to the bottom, and one up top. I am wondering if anyone knows the purpose of these holes?

I am told that this is to provide a means of pressure relief in the void space of the shaft. However, in this application there is no large temperature differentials, and there is not enough pressure within the tank to crush the shaft. Both ends of the hollow shaft is welded shut.

Thoughts?

 

[Dougt115]

What is the mix? Liquid, gravel...?

Could be used to mount a sensor that is no longer used?

Doesn't the mix get into the shaft through the holes?

Drain hole?

 

[1gibson]

It would keep the shaft from trying to float up due to the volume of air that would otherwise be submerged, but that doesn't seem like a big enough force to worry about (except maybe during installation?) If it wants to float, that would make it unstable and it would whip around, whereas downforce has a straightening effect on the shaft. Obviously can't run numbers without knowing how much it is submerged, and in what, but I guess it could be a substantial force.

 

[Dougt115]

If it was to prevent floating then it would have to let the mix in. This could cause the material the enters to not be properly mixed and ruin any chance at a uniform mix. Guessing it is not for floating.

A pipe of this size, length and slow speed would be for large volumes and high torque with a thick material but not liquid. Possibly a machine used with gravel or other dry medium.

 

[MintJulep]

Possibly to allow for measurement of wall thickness to monitor wear?

 

[alegna]

This shaft sits at the bottom of a thickener tank, and moves the thick sledge along so that it doesn't stick to the bottom.

I don't think that the shaft would float in the tank since there is another pice at the bottom that keeps the shaft from deflecting.

This is a grandfathered design, so I'm just trying to see if there was any reason for the holes..

 

[tbuelna]

You mentioned both ends of the hollow shaft were welded shut, so they might just be vent holes used to back purge the internal pipe volume during welding.

 

[Dougt115]

Vent holes? Is the room temperature to mixture temperature large? Would the pipe deform due to internal pressure?

 

[Compositepro]

Is the shaft galvanized or otherwise coated? Such holes are required to allow the inside to be coated. In such a design the shaft will get filled with liquid, but that must be considered to not be a problem.

 

[dhengr]

Alegna:
You’d probably best ask the original designer of the shaft or the guy who drilled the holes in the first place. Otherwise, your question is a bit like asking why didn’t they select the other piece of 6" pipe which was on the pipe rack when they fabricated this shaft? Not an easily answered question. If you are having to replace this shaft, the real question should be do these holes have anything to do with why the shaft is having to be replaced? Where they a failure starting point. This shaft only really transmits torque which drives the raking structure in the bottom of the tank, right? And, there is some sort of a bearing structure, both vert. load/thrust bearing and rotational bearing/centering, right? Does your design need those holes for anything, do you need a drain hole in the bottom of the shaft?

 

[3DDave]

Best guesses:

1) they were used to lift the pipe at some time, probably before the welding was done
2) agree on their use in welding purge gas
3) I've had some welders ask for vent holes to keep the expanding gas in a closed unit from blowing a hole in the liquified weld metal. Little known fact - the heat from a glass blob on a blowpipe causes the air in the pipe to inflate the glass, not the glass blower filling it like a balloon
4) they were left over from some other use for the pipe

 

[tbuelna]

alegna-

What welding process was used for this drive shaft? Given the wall thickness of 6" schedule 80 pipe, if GTAW was used then I imagine the internal volume would have been back purged, at least during the root pass. Efficient gas back purging usually involves a feed and discharge port at opposite ends of the volume (such as described) so that the flow of inert purging gas displaces as much ambient atmosphere as practical.

If the drive shaft was given some type of heat treatment/stress relief after welding, the heat-treater may have required the vent holes. They prefer to not put a sealed pressure vessel in their high temp furnace for obvious reasons.

 

[kingnero]

I see weld purging mentioned, but I see nowhere the kind of steel.
Agree with all other suggested possible reasons.

 

[dvd]

 

[alegna]

Hi Everyone,

We opted to re-fabricate this shaft as per the original design, with the holes in the shaft. Although it's still not exactly known why these holes were drilled in the shaft in the first place, we just thought we'd stick to the original design.

Thanks for your input.

 

[geesamand]

I can't see why you'd need spanner holes in an agitator shaft. However, they might be drain holes - one to vent air and one to drain liquid. The ones I'm familiar with are sealed.

Somewhat related - I hope you got your weld details and finishing perfect. Drain holes or not, I can't count how many "pipe" agitator shafts I've seen fail in very short order. There are many ways to set up that weld and only a couple don't form a destructive stress riser. Even at 5rpm you have a VHCF stress situation.

 

[DubMac]

How about pin holes for a sleeve.

 

[tbuelna]

alegna-

Does your drawing show the holes as part of a detail tube part prior to welding, or as features of the finished weldment? If the holes are defined as features of the finished weldment, that would mean they could be drilled after the welding is performed. And thus may not be vent or gas purge ports needed for the welding process.

 

[chicopee]

I believe that Tbuelna has it 3/4 correct as they are vent holes to relieve pressure from the welding process.