Failed Lift Tag Axle, 1997 WB (youtube)

[davidbrady]

Anyone have any help for a stranded bird? (video published on March 23, 2019)

[Kev]

https://www.youtube.com/watch?v=fTSrv_IZJaw

We're working on getting a new axle made, however, we will continue to explore other options until we have the new axle in-hand, or at least until it's in the process of being manufactured.

Over the past 6 weeks we've learned a lot about how Blue Bird built their liftable tag axles as well as what options are available when one breaks. For anyone interested, we'll share our findings here once the axle is replaced and the work is complete.

[ernie ekberg]

wonder how it broke?

[davidbrady]

(04-03-2019 10:02)ernie ekberg Wrote:  wonder how it broke?

Hi Ernie, To put some numbers to it, here's my envelope drawing for loads, shear, and moments:
  1997WB_TagShearMoment.pdf (Size: 296.61 KB / Downloads: 608)

The max bending stress is: s = M/Z where M is the max moment, Z is the section modulus and for a 5" diameter tube with 0.5" wall thickness Z = 7.25, which gives us a max stress (s) of s = 80,000/7.25 = 11034 psi. Suspensions are built to handle 3G of vertical load. This simple analysis shows this axle has the required safety factor and stays clear of the metal fatigue limit during normal driving which is less than half its UTS (Ultimate Tensile Strength), or around 32000 psi for mild carbon steel. IOWs, something else is going on.

[davidbrady]

(04-03-2019 10:02)ernie ekberg Wrote:  wonder how it broke?

You know how Wanderlodges fall off the jacks sometimes? Often we park for the night, dump the air, and deploy the HWH jacks to level the coach. When it's time to leave some of us may neglect to air up the suspension before we retract the jacks. I'm sure most of us have experienced the result when we hit the HWH jack retract button w/o first airing up - the bus drops onto the bump stops. What if the tag bottoms first, what if the tag carries all of that falling weight and the drive axle none? This could hit double digit g forces. Always good to air up first.

[Kev]

SUCCESS! Our broken tag axle has been replaced with a new 8" drop center axle made by Dexter Axle in Ontario. Along with the new axle we also replaced the hubs, drums, shoes, brake chambers, airbags and shocks.

Since the tag axle is welded to the Ridewell tag suspension we thought that replacing the tag axle would require removing the entire tag axle suspension assembly from the coach. However, we found that the broken axle could be cut away from where it attaches to the suspension, thereby leaving the suspension in-place and saving a lot of time. The new axle was simply set in-place, lined up, and welded. Removal and installation of the axle was about a days work from start to finish. We expected it to take much longer.

The most difficult part for the guys in the shop was the removal and installation of the drive axle airbags and the tag lift airbags. Those particular bags are just hard to access!

We haven't driven the coach since having the work done other than the drive from the shop back to the RV park. Our drive tires still need replacing so we're waiting for those to get installed before we start logging more miles. I will say, however, that the ride from the shop was noticably smoother than before.

All in all it was a great learning experience, albeit a very frustrating one at times! We looked at every repair option that was available to us - from welding the old axle to finding a used axle to building a new axle - and we are confident that we made the right decision for our situation. We now know that our new tag axle is stonger than the original axle that Blue Bird created and should be good for at least another 22 years! That is a great peace-of-mind for us and hopefully reasurring for whomever the next caretaker of our coach happens to be.

Our thanks to everyone who was willing and able to help us gather information as we learned what it takes to fix our bird's broken leg.