broken shaft 1

[Prestressed Guy]

I have a 15mmø shaft that sheared off at a roll-pin hole. The shaft is connected to the input drive with a shear-bolt that did not bread or even bend. In looking at the fractured edge there is a definite difference in the appearance of the two sides of the hole. It also looks like the metal yielded and twisted at the fracture based on the taper in the hole on both ends. Any thoughts on why this shaft would fail and not the shear bolt?

 

[metengr]

The shaft material was weaker in torsional shear strength in comparison to the shear pin material.

 

[EdStainless]

That is a very sloppy hole, lots of wobble room that shouldn't be there.
The shaft was too week from the hole.


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P.E. Metallurgy, Plymouth Tube

 

[BrianE22]

Do you know what material the shaft was made from? Pin manufacturers recommend a shaft diameter/hole diameter ratio greater than about 2.5. It looks like you have that so I'm guessing the material was not very strong.

 

[Prestressed Guy]

Background. I am a structural engineer but don't deal with metallurgy. It has been many years since my college Strength of Materials labs but the fracture surface looks odd.
This is the input shaft for a Lippert RV slide gear and the hole is for a roll pin that keeps the shaft centered. This is for my motorhome and is the second gear box that has failed in two years.

 

[mrfailure]

Tough to tell if the fracture surface is unusual given the lighting of your pictures, but it looks like a lot of contact between mating surfaces (common in fatigue propagation of this type), and ratchet marks seem to suggest OD-initiated fatigue. You should contact the manufacturer and send it back - I'm sure they they want to know about the problem and will address failure analysis and any make-good you are owed.

 

[Prestressed Guy]

Thanks.

 

[EdStainless]

I am suspicious of the quality of the shaft and accuracy of the fit.
You can tell that the pin was 'wallowing' because the sides of the hole are flared.
That indicates to me that there was some slop in the system to begin with.
If the pin was only for location then what is the load on it?
The shaft may be under sized or under strength for the job.


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P.E. Metallurgy, Plymouth Tube

 

[MikeHalloran]

Lippert makes a huge array of mechanical parts for RVs of all sizes.
Their stuff appears properly engineered, to a weight and especially to a price.
Their customers, RV manufacturers, all of them, appear to not be at all reluctant to misapply Lippert's, or anyone's, products, e.g., by using a part that's really one size too small for any particular application.

When you talk to Lippert and the RV manufacturer, I suspect you will find them pointing fingers at each other, which seems to be SOP for both parties, neither of whom will accept responsibility for anything.

I hope that I'm wrong in your case, and someone will step up and do something for you.




Mike Halloran
Pembroke Pines, FL, USA

 

[mrfailure]

It is very common in situations like these for the manufactures involved to perform a proper failure investigation. It is not about finger pointing - if there is a problem, then they want to identify it and take appropriate corrective actions before having a large number of failures and warranty claims. Getting a reputation for shoddy parts goes against their business. These types of failure investigations are common and I see them all the time.

 

[Prestressed Guy]

I contacted the manufacture and they are sending a new part and want my old one for investigation. Thanks for the help.