Welded Shaft

Good Day from Scotland

I am replying to your shaft problem -

You say the shaft is heavily loaded. What is the application? what is the RPM? - I take it the shaft is shearing or failing at the Flange location - if it is failing at the coupling then it will be failing by a twisting motion. Are there Bearing problems further down the rotating element?

Can the shaft Dia be inceased??? and a Coupling be shrink fitted? - the design that you have come up with does not have any undercut between the Flange face and the shaft - this is a recipe for disaster.

If you weld the shaft - which I do not recommend - the Grain structure will be upset and your strongest point will be the weakest point of the weld....

Rgds

Doug Michael Engr ( Mech) T.M.I.E.T. C&G M Navy Engr ( mech)

The corner there is a undercut with radius, not shown very clearly (rough sketch )

The failure of shaft is not that common to be worried of (minimum time to fail is approx 5 yrs of 24x7 service and normal is 10 yrs+). But the problem is it is almost the only point of failure in the gear box. All components are designed (and had been running including the gears, bearings etc, until the shaft fails).

The point is under existing condition itself, the failure is from the corner.

And what you have pointed out, and I had in the mind, is the corner can not be weakened by the nearby weld.

The shaft dia can not be increased (just above the step is the journal portion).

It is a low RPM shaft (30-50 RPM) driving upto 1MW power.

This is the output shaft of a gearbox (planetary chain).

Hi sb,

The HAZ width in any weld is dependent upon the heat input, but also the specific material, the geometry, number of ways to dissipate heat, etc. It's just a heat transfer problem combined with a metallurgical one (HAZ definition is that portion of base material not melted but reached a temperature in which some metallurgical changes occurs, such us grain growth, phase transformation,...)

Anyway, all possible detrimental effects in the base material HAZ can be eliminated by an adequate subsequent heat treatment, in your case for example a full recrystallization annealing.

Some questions: The actual design is really "all forged"? How is made the flange to shaft transition? Is that area machined cutting the "fiber direction"? Can the radius between shaft and radius be increased to diminish the stress intensity?

You've said: The shafts of this type are known to fail in operation in case of compromises/ overlooks (eg un-identified surface/subsurface defects) and it's right, but probabilities of having such defects is greater in the proposed welded design than in a really well forged design.

Kind regards

Annealing will compromise strength .

Normal SR of PWHT will not be enough to change the grains in HAZ.

The original forging (flange is forged along with the shaft portion) is in HT condition so annealing is going to have a mojor effect.

In fact the equation I know is complex for HAZ i just was curious is there any thmb rule say you carry out a full penetration X weld on a plate say t thick, the weld location should be n.t away from any notch where n= ?

I am trying to brain-wash our engineering people against it (let DTC go to ). But this is an open discussion which I can some-times use for other usages?

PS: DTC= Design To Cost (but sometimes this is more used as DTRV - Design to reduced Value) and We production line people/Customer some-times suffer due to this.

Form here onwards OT

A classic example is 200MW TG sets used to generate earlier easily 250MWs (it was infact designd with that margin). Now re-designed to cost the 200MWs some times touch 195MW making customers extremely noisy and this also reduces our margin of error.

We have a few German/ Cxech old machines - CNC as well as Non CNC may be 25 Years old. They still run and machine to the al most original accuracy. Of course we had to compromise about 20-25% on the capacity - old fellows . But these are non-DTC with rugged single piece cast bodies and columns.

But a few later machines with fabricated structures have already reached- reaching death-beds.

Some times I feel a bit over-design at a higher cost is much better for all than the exact-proper design with low margin.

I see...

Of course I agree PWHT (stress relieving) doesn't change grain structure.

Anyway, any welding process between shaft and flange will produce a HAZ, even the best and quicker methods (Laser, electron beam..) which use to give it narrow, produce some HAZ. And even if you calculate exactly the HAZ width, you'll have in any case a weaker zone. If the steel has HT enhanced properties, the only way to restore completely the mechanical properties is to repeat the complete HT (surely quenching and tempering) and this may cause distortion...

I find difficult that the welded construction give a better or equal real working life than forged one. I'd try better to improve the forging/machining/HT process.

Some times I feel a bit over-design at a higher cost is much better for all than the exact-proper design with low margin: I fully agree. This remind me some years ago when scheduling the heavy equipment transportation for some power plants here in Spain. The few roman empire made bridges were O.K. we just had to reinforce new designed bridges!!!

Kind regards

I think you may need to do a post weld heat treat to normalized all stresses introduced from welding and get "clean slate" as far as stress goes on the item. This is very important for the HAZ because this is the most stressed area other than the weld itself but weld metal is usually stronger to begin with. I have seen and done similar shaft flanges for things like tank agitator shafts both hollow and solid shafts. We generally use a low carbon steel alloy for the shafts and heat treat according to the alloy and recommendations for normalizing after the welding. This may not be the best way to do this on your application but it has worked for me in the past.

In fact before posting I had been scanning through the net and other literature for this type.

When I googled Shaft fatigue HAZ (I think that was the logic) I got n number of papers where shaft has failed in fatigue (including turbine shaft) where the fatigue crack has originated from HAZ (not the weld proper). Now I can control my weld property by electrode but HAZ is out of control

PS: If I try to normalize the shaft - may be the cost benifit I am gaining (rather the designer is trying to gain) is lost

Then again the H&T forging is likely to give me far better properties than a H&T forging and then normalised.

Speaking of turbine shafts I have a completely refurbished shaft, rotor as well as a new bearing set for a 50 HP Westinghouse brand steam turbine. I also have several other parts for this turbine such as oil pump etc. Just mentioning it in case you ever run across someone that may need one.

welded ?

I am not sure about welded. It was a spare shaft and rotor assembly we had in the store room. We used to run two of those turbines for ID fans and a line shaft. I got it when I purchased a bunch of obsolete store room items where I work. I do that every now and then mainly just to see if I can stuff one more item of no practical use into my already overflowing shop. I know it will probably end up in a scrap yard one day but I just didn't have the heart to scrap it before at least trying to find it a home.

Hi,

Your question has a qualitative aspect. I would prefer to think on quantitative a basis.

Could you please give some dimensions and indicate the loads in order to estimate the stress level? This will allow a better and safer approach for a solution which could be interesting.

If you want a certain degree of confidentiality we can use a private message way for the informations.

Nick

Dimensions are indicated on sketch (people don't read )

Shaft dia 300mm (actually taper on top portion for hub fit) ,flange OD approx 900mm

shaft length 400mm (300mm taper +100mm straight journal - flange side)

Current forging flange thickness 85mm (proposed 63mm plate)

transmitted power approx 500-700KW @40 RPM

Heavy torsional + bending fatigue (the peak may alternate to -ve also)

Earlier shaft (slightly different design but same diameters) had 40CrNiMo6 material (HT condition) Now it is plain carbon steel forging (We have our own internal std but I think it is equivalent to 30C8)

We have seen. FOS is very high (we have done ANSYS and everything looks OK but these sometinme fail even with the 40CrNi )

I am scared to reduce the FOS deliberately.

I apologize, in fact I looked at the first mail only as information about welding zones and did not look at all details (too small!).

I have an idea which shall be more analysed but it is based only on EBW. Do you have any possibility to do it at a convenient price?

The idea is to avoid a full welding but instead have a step and have at 2 different dias. two weldings only on half the flange thickness.

This way in every critical section welding is only half flange thickness.

Use of EBW is an advantage since the HAZ is a lot smaller due to less energy, narrower and does not ditord the welded parts. Due to it the diameters are not far from each other.

If my explanation is not clear enough I shal send a sketch.

Nick

Hey apologies to are not in order. No sarcasm meant only

Sorry no such facilities in vicinity

But still the sketch will be useful (post it here since as I have earlier noticed, my firewall blocks all email links . (Direct email is no problem)

It will amplify my knowledge, I can use it for some destructive purpose somewhere