replacement of EN8 shaft with something stronger...

Are the shafts heat treated?

EN8 is pretty low carbon, but I believe it can take a bit of hardening/tempering.

i will have to enquire whether they were heat treated or not.

but i was looking for a complete replacement of the shaft rather than any treatment of the shafts.

though i will keep that option open.

You do not specify what type of shaft you are dealing with, if it is a pump shaft, propellor shaft, what type of fluid are you dealing with, what is the shaft rpm, temperature, etc.

You may be able to use stainless steel or one of the super alloys e.g. Iconel 825. Although the super alloys maybe expensive, the expense maybe woth it in the long run.

the shaft is a centrifugal fan shaft being used in ambient conditions.

constant rpm of around 900.

could i use EN24 or EN19???

the shaft is about 1200mm long and has 7 blowers hanging from it. though bearings have been provided at each nodal point, yet the problem of fatigue persists.

am sorry...the shaft length is around 2400mm (not 1200mm as stated previously).

Get IIT Metallurgy Dept. advice--the root cause.

What is the shear dtrength,Ultimate tendile strength,bending stress with 7.86 rpm

U CAN USE EN19

Perhaps the fans need dynamic balancing before assemble. Just a suggestion.

regards JD.

possible misalignment = bending shaft back and forth = fatigue.

A balanced bunch of rotors should not be a problem.

is there any way to adjust the alignment?

It sounds as though you might have a misalignment problem. If all the bearing supports are not co-axial, or the drive (pulley? or motor coupling?) are not aligned, you would be flexing the shaft every rotation... which just might result in a fatigue failure.

30mm dia seems to be under sized for 2400mm length. Redesign the dia of shaft provided no fault in alignment,balancing etc.

There is another possible root cause. There may be a hard start to spin them up to 900 RPM. On a length like that you will get the inertia of the rotors acting to twist the shaft. Repeat that and you het shear fatigue failure.

How quickly do they run up to speed when started?

In this case the shaft will fail with a ~45 degree break, a typical shear failure.

Solution add a heavy flywheel at the motor to slow down the startup

To check bearing alignment, pass a piano wire through the bearings and tension, them measure bearing alignment.

Regards JD.

hes, has a problem. He would have to unship the shaft and remove the blowers etc, a long task.

An easier way is to loosen the ones in the middle so they can walk back and forth/up and down a little. Then hand turn the shaft and see if they walk at all. Bearings like this usually have a concentric gap around the bolt with a washer on top and some shims underneath. the break will occur at the place of maximum cyclic shear and will be where the alignment is most out of line. The motor should also drive the shaft with a coupler that tolerates some misalignment. Theses are commonly use, rubber spider types.

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The motor shaft should not be directly rigidly coupled or you will get fatigue failures

or it might use a belt.

My firend!

Don't run for an alternative without doing a little homework.

En8 can be thru hardened to 25-30 HRC with UTS of about 50 kgf/sq mm. Since your shaft is subjected to fatugue loading due to cyclic loading, you need to calculate endurance limit using Soderberg's or Goodman's theorem. Check whether the fatigue stresses are coming limit. If not go for En19/24, etc.

If you don't know the proper reason, then any matl will fail. As an engineer you should try to find a calculated solution.

As others have said .

Don't solve the symptoms.... solve the problem!

My old quote was... Solving problems is easy....finding out what the problem is...that's the hard bit.

Del

touché Del

You can always repair the breakdowns caused by a problem, but, if you do not fix the problem you will continue to have breakdowns Eventually if you don't solve the problem you won't have anymore breakdowns to worry about, you will be looking at a total failure of the engine, machine, etc.

If you are convinced that a fatigue resistant steel will solve your problem, I suggest you try some heat treated S-2 shock steel. That is what they make jackhammer bits from. Also, the steels that valve springs are made from have impressive fatigue properties.

EN8 material may not require replacement. But before any suggestion I would like to know the futigue failure location. Drawing of failure location shall give more information. I think some change in machining geometry shall solve ur problem

What type of bearings are used in this setup, and have they ever been checked for wear.

It would be a good idea, if all bearings are accessible, have bearing temperatures been taken and logged over a period of time. This could give you an idea as to where you may have misalignment problems, if they exist. This way you do not have to strip everything down and it may locate the source of your problem.

Like the majority of the correspondents writing in to you, I am also in favour of checking alignment, you can as suggested, remove all bearings except the end brgs. and check the shaft alignment,& concentricty with the aid of a deflection gauge, mounted on a mag. base, having ascertained the shaft is not wind milling, then go for the intermediate bearing housings and or bearings.check also for wear in the bearings. Good luck.

Metal spraying using a flame-application process might be an acceptable repair technique for the fretted aras, provided the metal to be sprayed is of a harder-wearing composition, n'est-ce pas?

As pointed out by Del the Cat earlier:

SOLVE the problem, not repair the effects of the problem. Alas it is to no avail if one does not do this first.

If the problem is misalignment, chasing higher grade shaft materials will likely shift the problem elsewhere - something else will fatigue as the load goes up or bearings will suffer. Come to think of it so will energy consumption. If the problem is lack of torsional strength or torsional resonance then a better material will no doubt help, but to avoid the load/resonance could provide a more robust or cheaper solution.

Everybody is trying to help you. But you are not helping them help you decisively.

The best would have been for you to post a Drawing showing:

1. Where the Drive pulley is and keyway dimensions there.
2. What is Drive HP/RPM .ShaftRpm you have given-can show again.
3. Shaftall the way 30mm? With so many blowers-show locations along 2400mm!
4. How are all the loads along 2400 ?All Dia 30 H7/j7? Keyed? Very POOR!
5. Where are bearings along 2400? Show Bearing Tuype-Sleeved-over 30_ BAR?
6. What is motor Current -Rated-Actual
7. Is it possible to put sleeves on shaftjamming on Bearings' inner races? Can increase torsional resistance readily many times-will never twist and fail--even a EN 8 shaft.
8. Where is your fatigue failure? Every time same place? When You Start?
9. Anything more to Declare?

A picture tells a thousand stories .HelpCR4 TO HELP YOU BETTER. Send Drawing!

I would go with EN19 and try to machine big radii on any diameter transitions and then shot peen the area where it has been breaking.

good luck

ps EN19/24 both rust like crazy so protect them well and use rust-inhibiting marine grease.

thanks for such a good response friends...am sorry that i will not be able to provide all the info. right now...just wait for a few days...and i will have all the data...