axis calculation

[McJe]

I'm calculating an axis for the wheel of a rotator to rotate large and heavy pressure vessels. (up to 500 tons)

Basically, it's nothing more then an axis, supported by 2 bearings, and a large wheel in the center.
The wheel is in fact a custom build part, made by welding 3 circular plates (20mm) on the axis and a strip (40mm thick) is welded on these plates to have the wheel surface.

If I calculate this, I tend to be conservative, for safety reasons:
- The load will be carried by 4 wheels, so I calculate 1/2 of the load weight on 1 wheel.
- I calculate the load as a single force on the center of the axis.
- I assume the axis has to carry the load on it's own.
I know this axis is strong enough to carry the load, and if anything goes wrong (misalignment, wrong usage, ...), it will still be strong enough.

My colleague tells me this is way to conservative, and tells me to include the metal wheel in the moment of inertia calculations. This way, we can reuse an old axis which is thinner.

I know he's right in some way, but I don't like his approach.
In his approach, we have to rely on the strength of the weldings. If a weld breaks, the load is carried directly by the axis, and it will break/deform.
Also, I think I cannot add the wheel this easy to the moment of inertia. The load force is still transfered to the axis by the 3 plates.


My question: can I use my colleagues calculations? Or is it better to have some extra safety?

It's not a safety factor 10 I'm talking about, too much overdesign is also not good.


Jeroen.

 

[rb1957]

axis = axel ?

it comes down to "does your design Need to be fail-safe ?"

in the normal run of things, your approach is conservative (as you recognise) 'cause the welds are intact, distributing load into the wheel. but there are many welds to the wheel, no? can all of them break without being noticed ? maybe it's safe/conservative enough to allow 1 of the 3 discs to be ineffective ??

maybe your colleague has good reasons for using an existing design ? was it designed for a similar load ?? is your colleague your boss (in a position to tell you how to analyze someting and to counter-sign the results, taking responsibility). if you fell nervous (about maybe being hung out to dry), get an email message to support the direction given.

 

[McJe]

Yes, I mean axel...
It's way to hot over here to think ... ;-)

To be complete:
The previous axel was designed for 250T vessels.
My coleague wants to reuse this one. The distance between the bearings will be smaller, because we replace the standard bearing houses by our own construction.

Since the motor, reductor and bearings (not their houses) have much overdesign, it is possible to build a 500 ton rotator. Only thing to do is changing the design, and avoiding castings (bearing houses).

The bearing houses got damaged regulary in the past.
According to the catalogues, the used bearing houses should be strong enough to carry the 250 tons but the vibrations, peak loads, misalignments, ... make them break.
Therefore, I'm more into the conservative approach.
There are real situations where the wheels get more load then designed for. This is fact that we learned the hard way.

 

[handleman]

You mean axle?

-handleman, CSWP (The new, easy test)

 

[desertfox]

Hi McJe

Can you post a sketch with dimensions it might help us to understand your problem better.

desertfox

 

[chicopee]

Oh,boy! I don't know if I should cry or laugh...axis... axel...axle...what's next.
Yeah, a sketch would be a good idea so that we call all be on the same page.

 

[McJe]

I'm sorry about the axis-axel-axle-... issue.
English is not my native language.
And in Dutch, axis and axle are both the same word: "as"
Let's call it a shaft for a change.

I have attached a preliminary sketch of the wheel.
- Black: shaft
- Blue: wheel
- Purple: bearing
- Green: housing/frame

The left side of the shaft is extended and leads to the motor/reductor.

The frame is bolted, so is shouldn't be a problem to assamble the wheel.