Shaft Failures

[Robin2150]

Due to a design error a shaft supporting a horizontal rotor has failed. The manufacturer has agreed that the failure is due to a design fault and is replacing the shaft. The manufacturer has provided design calculations, which appear to be ok, but I need to check the various factors and formulae that have been used.

Can anyone recommend a good text book that will provide the detailed background and the tables of factor needed to establish the design stress.

The shaft torque is 3880 Nm and the bending moment is 878 Nm.

The shaft size 80 mm, increasing to 100 mm.

The proposed marterial is 42CrMo4.

 

[chicopee]

You need to provide more info about the shaft failure.Was it due to torsional shear, bending stress, stress concentration at keyways, bearing, vibration? Is the shaft supported on ends w/ the rotor between supports? what is the rpm? any cantilevered loads?
Any text on machine design will have analysis on the type of system mentioned.

 

[ccw]

Hi Robin2150,
It may be very dated, but we always backed checked our shafting designs against the old ASME Code for the Design of Transmission Shafting B17c-1927. You can probably find remnants of this shafting design code as well as more modern approaches to shafting design (such as maximum shear theory of failure for combined loads when loads are fluctuating) in texts such as Shigley or Spotts Design of Machine Elements.

Chicopee's advice is good that one of the major factors you must consider is the root cause of failure. In other words, what in the design was the weak link that led to the failure and does that correlate with, or support, the forensic evidence?

 

[Robin2150]

Thanks for the responses.

The failure is due to fatigue, resulting from the combined stress.

The component that failed is a stub shaft at the drive end of a horizontal rotor. One end of the shaft is bolted to the main section of the rotor, the other end supports a shaft mounted gearbox and between the two is a bearing.

I have received design calculations from the supplier, which conclude that the shaft was too small to support the combined torsion and bending load. A new shaft has been designed. but I have no reference books detailing the method used to determine the endurance limit for the material and subsequently the permitted design stress.

I have got as far as using von Mises theory to determine the maximum combined stress.

 

[jomandar]

Dear Robin,
I am agree with the use of ASME code for shaft design. Design for fatigue involves either design for finite life or infinite life. Could u plz specify the life in millions of revolutions? (expected)
I think this above data along with reliability factor will give u the factor of safety against fatigue. It should be greater than 2.5 (I suppose). Depending on the material conditions,heat treatment (if any),cost ,application failure opportunity cost and past experience you may interate the design to get required factor of safety.
If i am going wrong plz correct me.
regards,
jo'

 

[ccw]

Two points:

1. If it was fatigue, assuming the shaft eventually separated, the cleavage planes will typically show two distinct areas: first a dull looking area (this is the area where the fatigue crack slowly propagated and the rapid stress reversal burnishes the contact points or the area actually rusts, and then a bright area that shows definite plastic flow with pull outs or twist when the shaft finally fails suddenly in tension or shear.

2. You have every right to ask the supplier to show the calculations checked independently or at least document and reference the source of code or criteria (a publicized and widely accepted critera, hopefully) he used to make the calculations so that a competent person can follow up with a check. I am not sure that using FEA to get von Misces data is going to compare apples to apples with classic shafting design codes, if that is what is in play.