Design of roller hub/shaft to improve fatigue life 2

[user71]

I need to design a hub/shaft on a roller that is subjected only to high bending stresses that can lead to fatigue failures of the shaft. Which design is best from a fatigue point of view? - machine a single piece hub and stub shaft with a large radius (actually a compound radius) in the critical area OR a 2 piece assembly of press fit shaft into a sufficiently thick hub, welded on the backside well away from any bending stress. I'm guessing in the two piece design it would still be required to have a radius on the hub where it meets the shaft...I dont have much experience with press fits, but have seen them used to fix fatigue problems like this before.
Any tips, or leads on where I could find such info?
Thanks
Larry

 

[CoryPad]

I don't see how press fit is going to help improve fatigue life due to bending stress. I think a properly designed single shaft with hub should be better. You can improve the fatigue life of the shaft by using high strength steel, shot peening, roller burnishing, etc.

 

[unclesyd]

Like stated in the previous post I've never seen a press fitted hub improve the fatigue life of a shaft. The ideal would be to use a one piece forging where you have an integral shaft and hub. There are several ways to transition from hub to shaft and the one method that I've had very good luck with is to use an elliptical radius at the transition. If possible a min. of 3::1.

Comeback with your sizes and loads which will generate a little better feedback.

 

[user71]

Sorry, I should explain better - the key here is that the original design had weld at the high stress area, and the press fit design removes the weld from the high stress zone increasing life. I suspect as you both point out it still falls short of a properly radiused one piece design but I am not sure how to quantify the difference.

I like the elipse idea, because that is almost exactly where I ended up with the compound radius using fea to optimize, but I bet ellipse is the magic shape.
Thanks!

 

[unclesyd]

I agree that a press fit will be better than a weld which creates both a mechanical and metallurgical notch. I would go with a shrink fit even though press and shrink are used interchangeably.
Another thing that I've done is on the hub end of the shaft is to use a change in section, slightly larger, with a corresponding radius.

 

[Cockroach]

Very intertesting UncleSyd, lower stresses in bending and/or torque?

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

 

[unclesyd]

I've resolved problems with shaft failures from both bending and torsional stresses using either or both approaches. In the OP"s case there is a tremendous bending moment imposed on shaft by the roll movement and one hell of a torsional load if a bearing seized. The formal analysis has always been a point of contention among engineers involved as it is never as straight forward as it seems. I was involved with this type failure long before the advent of computers and FEA so my approach could be called Edisonion though in reality was culmination of a lot of rudimentary analysis, understanding of the failure modes, application of some principles of mechanics and materials.
Sans any formal analysis for bending failures I tend to go with the elliptical radius for the reduction of stress concentration at any change in section. I like the change in diameter if as in the above case where there is potential for higher torsional loads on the hub flange. The change in diameter (step) can also be used to get weld area out of the highly highly stressed area. I believe that this approach has worked so well is because the original designers were never very far off. Some that worked I would really like to go back and do a formal analysis.



Anecdotal:
The Hub From Hell:
In the manufacture of a nonwoven fabric there are two sets of what are called Crown Rolls, a roll that is forced to operate with an induced bow. These rolls are approximately 14" in diameter and 12' long and are mounted 3 ft off the floor on stands. All these rolls have bolted on hub flanges, 2 piece welded 4140, and you can imagine there unknown forces from unknown directions acting on the hub and shaft during every revolution.
Under our operating conditions the original hub flanges sheared the mounting fasteners, hex head grade 5, We reworked the HF's to take SHCS, We went with our plant standard , H-11 at 220,000 ultimate TS. The manufacture of this products requires the use of HCL and very shortly we started have failures of the H-11, You would hear a bang and the bolt heads would come off like bullets and make a trip around the area. We rebolted with standard SHCS at around 160,000 ultimate. It took around 4 weeks and bang when enough went the roll went bouncing across the floor. We again reworked the flanges to used more socket heads capscrews where we drew the standard back to 120,000 UTS.
This worked then we started having preferential corrosion to the weld metal, so we clad the highly stressed area with Hastelloy C. imperfect but better. Then we started having fatigue failures on the shaft All the king's calculations were saying this shaft was more than adequate as the stresses were low. We upped the diameter as much as possible as we didn't want to raise the strength level due to H2 attack from government. This was causing so much concern that a select committee from corporate engineering and two consultants were brought to our site. The conclusion after a week that the existing hub was more than satisfactory. Before they left I presented them with a newly broken hub shaft. Broken shafts and bearing failures, another story, were about to put us out of the nonwoven business. I convinced management to go with a test of a 3 step Hastelloy C shaft welded to a many bolt 4140 flange. This worked extremely well and despite the astronomical cost they went across the house.
When everything was under o semblance of control a young engineering, who I will put in the class of one of the sharpest I've worked with started work with our group. During a period while we were a little slack I gave him the hub shaft problem. His first iteration was that everything looked fine and there should be no problem. He didn't present his work at the time and said he wanted to work some more on it. All of a sudden he was ready to leave the company, another long story, and on his last day he showed me a stack of computation papers with all type of force diagrams and stated he knew why the shafts were mechanically failing, discounting the H2 exposure. I never got to see the answer.

 

[macthenife]

Two piece will pre-stress the joint(thus resisting fatigue).

Use tapered shaft and bore.(Enhanses pre-load and drive key
may not be required.

Welding will destroy any cold worked strength and weld decay could destroy the joint.

Tap the shaft end and fit a min. 8.8 bolt with a spiraloc
washer.

For Class1 locking, thread the shaft end and fit a castle nut instead of the above.

 

[dvd]

Here is an example of a pulley using shaft locking devices:

http://www.ringfeder.com/Applications.htm

I have used shaft locking devices on numerous pulley designs. Use a through shaft held with shaft locking devices and welded hubs. Design per Ringfeder design literature. Ringfeder is one of several manufacturers.

 

[CoryPad]

macthenife,

Spiralock is a ramped internal thread form, not a washer. Perhaps you are thinking of Nordlock washer?

A castellated nut with a cotter pin or safety wire? Neither are locking, neither retain preload, their only function is tot prevent immediate complete loss of a fastener so that corrective action can be taken.

Good options for a low-mass, low-cost, shaft-hub connection that is fatigue and vibration resistant include bolts (property class 8.8 and above is a good suggestion), splines, adhesives, press fit and shrink fit.

 

[macthenife]

CoryPad

Sorry!! I tripped over my memory.Yes Nordlock or Disc-lock.
Although a Whitworth form thread is self locking above the
"jam line"but would require a threaded bush of the correct length.
You are correct in your comments.
However Class 1 locking I understand is still required on all aircraft engines.And at a practical level it does maintain the status quo.

A bolt is self locking (eg.cyl.hd.bolts)if recommended tightening torque(plus half turn) is applied and their is at least(as a rough guide)60mm of free shank length to impart elasticity.

Use cold drawn close tolerance bar stock to finished shaft diameter.You don't cut into the grain flow.The shipwrights of old never broke the skin of a mast for the same reason.

 

[Cockroach]

Very, very interesting UncleSyd.

I ran a "textbook" stepped shaft, computed the maximum allowable torsion based on a 4.0 mm radius and generated the required FEA to support the answer.

I then reworked the shaft for replacement of the radius with a 3:1 ellipse. The resulting FEA fed back a 38% lower stress concentration factor in comparison to the former result.

Of equal note, the distribution pattern of the stress razor from fillet concentrations improved dramatically, more evenly spread out axially on the ellipse.

I've never thought of using an elliptical corner, never in twenty plus years. Thanks for the tip, a gold star coming your way!

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

 

[unclesyd]

Cockroach

Kenneth,

Sorry I haven't responded sooner but have been fighting Malware and trying to locate some old, old computations. Like many things in industry once you get a problem fixed there is no money or time to study the method that got you there. A long time ago myself and the company mathematician tried to resolve the improvement afforded by this approach and as I recall our numbers, resolved graphically, were in the range of 10-12% improvement in stress concentration. As I've stated several times in post's, a design is normally never far enough off that 10% improvement of any kind wouldn't of some help.
Again many thanks for the quantification.

Anecdotal:
I first used this approach on high pressure, (10,000 psig) piping in a hydrogenation process where the smaller branch connections were failing from fatigue.
I can't tell you how long ago this was.