Easy question, Hard solution

[CBNDude]

Could anyone offer me some advise on accomplishing this task.
Lets say I have chased threads into a part (impeller) 7.750 29 degree double lead. My major and minor bore is +.020 oversized, the root is .05 to wide and the crest is .05 to small. The part (impeller) weights approx. 6000lbs. What I want to know is will the shaft (which is in tolerance) center itself in the impeller, given the weight of the impeller? My opinion
would be yes, due to the degree of the threads. But with what program can I use to prove this? Perferably in 3-d so I could visaully see the threads self centering.. Hope this is not to bad of a task to ask.. Any help
would be great. A program or such that you could plug in different dimensions and weight to see how it effects the mating of the locking side of threads wolud be Great(ok I'm reaching a lil)

Please any advice or suggestions needed.

CBNDude

[HuFlungDung]

CBNDude,
Are you referring to a centralizing Acme thread? At least, that is what you would have preferred to cut?
I do not think it will centralize, particularly if the impeller seats against a shoulder, unless you manage to somehow lift it to center at the moment it stops against the shoulder. Due to the weight of it, I am pretty sure it would "work the thread", fretting it away in service.

[Dave's_Not_Here]

CBNDUDE...

Do you know anyone with a copy of AutoDesk Inventor that might be able to run the animation of the assemblies interactions during assembly?

[mxtras]

I can't say I have ever mounted an impeller that weighs 6K pounds, but it does seem odd that the design calls for threads to position the impeller to the shaft with such a high degree of precison.

Does the impeller stop against a shoulder or location feature?

I would be very interested to see this assembly! I think I would look for options if it is really that critical. I would not recommend placing the burden of precision on threads alone, but that's just me, and I don't really know all of the details of the application.

Scott

[DieGuy]

I suspect that this was a failed attempt at creating an intolerance thread? As others have stated I doubt that a 29 deg. thread will pull the impeller into the center of the shaft. Have you tried an over wire measurement of the shaft to determine the real thread size? It may not be as far off as you think. Have you tried an assembly and does the thread "rattle around" during assembly? I assume this needs to be assembled with the shaft in the horizontal position. An alteration of the assemble may be possible with the use of a cone shaped washer to align the shaft and thread if a shoulder exists on the shaft. What locks the impeller to the shaft? Is there a nut?

[HuFlungDung]

FWIW, a standard Acme thread does have a clearance specification on the major and minor diameters. It appears that this spec was met, but, a better design would have been to control the OD of the shaft (easy) and control the major diameter of the internal thread (difficult), to try to make a centralizing thread. This way, the hub major thread ID sits on the OD of the shaft, and the actual clearance of the flanks of the threads is not critical.

[mxtras]

It just seems like an odd thing to expect a threaded assembly to run true within .005" on something so large, doesn't it? It seems like there needs to be a location feature.

I don't think we have the full story here.

Scott

[CBNDude]

Ok I see a little more info is needed so here it is:
Look at the attachment.
Our impellers go upto a 16" plug and 110" O.D. with a weight of 18,000lbs.
9194s plugs being the smallest
material of impellers range from cast iron(80d,65d)
gasite also known as white iron (4g,6g,20g,28g,29g,30g,38g) hardness is about 795bH

[BillPSu]

The designer of the part didn't review the tolerance of the thread when he designed the part and put the tolerance he needed on the assembly and put the problem on manufacturing's back. A solution would be to have a turned diameter on the shaft and a counterbore on the impellor to locate the part and using the thread to retain the part.

I'm sure the answer will be we've been doing it this way for years. Have they ever inspected the assembly before this to make sure of the runout? I bet they haven't.

[CBNDude]

These impellers do have a counterbore .250" deep for alignment which is normally size of major dia. But counterbore is also oversized. The impeller does pull up tight to a shoulder on the shaft.

[HuFlungDung]

If they want to reclaim this one, maybe they should bore out the counterbore, and shrink fit a sleeve in it, then rebore it on size to center the hub properly.

[mxtras]

Duct tape and bailing wire should do it, I think. Just use the good stuff - you should be fine.

Scott