Solid axle or no?

[sands]

I am a relative newbie in a foundry setting. Whole nother world here. We have a "mobile mixer" running on rails carrying ~100 tons of various elements on it. It has dual 7.5 HP Demag gearmotor systems for travel (drive motors, gearboxes, DRS trucks etc.) It keeps destroying said travel gearboxes, motors, trucks, bearings etc. It is a difficult environment. My boss wants to replace these systems with a solid axle and single motor, for speed & ease of maintenance, and other factors. My contention is that any misalignment will cause major stresses in a single drive system. Any thoughts?

Thanks for a great teaching forum. I have enjoyed perusing for awhile now.

 

[MintJulep]

Clearly what you have now isn't working so well.

It sounds like the magnitude and nature of the loads are not very well known. Once they are identified and understood it should be possible to design a solid axle that will work.

 

[Cockroach]

You need to model shaft deflection accurately, either by hand or using a "canned" computer program. Clearly the use of gears implies a maximum allowable shaft deflection in order to minimize misalignment.

Once this is done, you can easily play with the geometry of the shaft cross section and arrive at the obvious answer, solid verses bored. A good place to start is the elementry textbooks in Strength of Materials, Statics or the various case loads given in Roark's Formulae for Stress and Strain.

This is a very well documented problem.

Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada

 

[ornerynorsk]

Are your dual motors out of synch with each other, "fighting" each other?

 

[sands]

Our maintenance people say no. It's just that sand gets on the tracks (and other things) because production does not keep it clear and this causes tremendous stresses with that kind of weight. One of the major difficulties is dealing with production. We cannot have the machine for any real length of time for testing. That was really why I was asking for any experience. I can do the calculations after I find the prints, manuals, etc. and I guess I will have to. C'est le vie. Thanks for the responses.

 

[arunmrao]

Sands you have identified the problem right. Silica sand is very abrasive in nature and . If you can avoid spillage,keep the tracks clean, and have rubber hoods or cover over the moving parts and the gear boxes you can reduce your breakdown. Instead of moving the mixer around,can you consider transfering the sand mix,by belt conveyors,makes life easier and simpler.

I am not a design engineer but a foundryman,in a small unit.

 

[MikeHalloran]

Could you fit air nozzles at the wheel/track interface to blow the sand away?



Mike Halloran
Pembroke Pines, FL, USA

 

[Warpspeed]

Standard electric railway locomotive practice is a solid axle shaft with single motor, so it can work.

Keeping sand and dust out is really a seal design problem, and perhaps more than one layer of sealing needs to be provided. Actively pressurising the seals from the inside might also help.

 

[sands]

All excellent ideas.

I did think of the air blow off and this may be a partial future solution. The difficulty is getting air to this unit.

I was not aware that loco's had solid axles, though now thinking about it, it should have been obvious. Jeez, the boss is right again.

arunmrao
Moving the mixer is a necessity. We do medium to very large castings (largest to date was 145 ton). We will be doing one this year that hits 210 ton, although this bay does not produce those larger ones. In any case, this mixer moves ~210 feet. Agreed that sand is one of the worst system contaminants to see in any arena. The problem here appears to be undue stresses on the drive units though. I see internal fractures and a broken shaft. Yes I feel some slight grittiness in the lube, but I don't see a need to do an expensive oil analysis. For one thing, I suspect the grit may have come from the R&R process. My boss wants to bring in a fab company to redesign the drive, whereas I agree with some of the previous posts, IE. we need more info! Boss sees that as one more unnecessary failure, and he's already getting alot of heat over this. Strain gauges & recorders are not that expensive. Whereas, $165K quote to fab a new drive sure as heck is in my book. One thing I've learned from reading this board & experience is when you fix one problem, ie beef something up, the next weakest link shows itself, and I may be right back where we were. Better to do a root cause analysis. That is what I am pushing for, more info. Get it right the first time.

Thanks to all. Appreciate the input.

 

[MikeHalloran]

Lacking air, you could use rotating brushes like a street sweeper, or static polyurethane wipers.

Since your boss is under pressure, this would be a very good time to be perceived as doing something. He's already made it clear that gathering data is not doing something, by his definition.

What may be helpful, if you do it fast, is back- engineering the failed drives, well enough to _estimate_ the load that caused them to fail. That will give you a number that you have to exceed; only the necessary margin is unknown. Take a wild, conservative, guess.

Then maybe you can scrounge some usable locomotive axles and sturdy drives. This is easier if your rails are standard gauge, of course.



Mike Halloran
Pembroke Pines, FL, USA

 

[Warpspeed]

I have never seen them used for this application, but a small ring compressor as commonly used for spa blower applications might be worth a thought. Small domestic units typically provide 50 to 100 CFM of air against perhaps 3 to 4 psi back pressure. A simple labyrinth seal with air pouring out of it is not going to allow much dust or grit to enter.

Keeping the blower induction air clean is probably the next hurdle. Trucks and off road vehicles have by far the best system. A centrifugal cyclonic dust extractor in a big drum, followed by a standard automotive paper filter element is the most efficient in VERY dusty conditions. All standard easily obtained hardware.

 

[sands]

Thanks Mike. You are right of course. This one resolution could put me in good sted, or bad. Excellent solutions & thots. Thank you. I will post final solutions later.

 

[sands]

Gosh, where do I start.
Three different track sizes for this machine from day one it appears. The managers at the time went and got whatever track they could from where ever they could to save a few bucks. That means different heights, different size ties underneath, different shimming. Some tie points are welded and ground off, some have tie plates (correct method). Sand in track grooves and on track itself exacerbates problems further. Common American induction motors (without brakes)are now installed with adapter flanges instead of the Demag gearmotor systems (with brakes)the manufacturer engineered in. Clutches are welded solid. Seals & covers are missing.
Do all businesses operate this way? It's no wonder they have gone through 3 maintenance managers in 5 years. Sorry to waste your time.