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Castoring Nose Wheel Assembly Failure (Well at least just the Pivot Pin) Departing FALA to take my Cozy to FASY, two up and loaded to gross, my approach at FASY was a little fast and flat. Runway is at 5380 Ft and 3100 Ft in length. Approach speed was 100 Mph and touch down at 90 Mph. Held the nose wheel off down to about 80 Mph and on the nose wheel touching a severe shimmy resulted after hard braking. The whole tire was worn to a slick from the shimmy. At first I thought that the tire lost some pressure, but was confirmed the next day not to be the case. It was a whole lot worse ... The Pivot shaft in the MKNG16 had pulled out about 5/16" and bent. There was a gap of 1/4" is can be seen in the picture between the large bronze friction washer between the MKNG16 and the assembly attaching to the strut.
My home made strut stood the test of this severe shimmy. Also the securing method as suggested by Ken Miller saved the day and no evidence of loosening of the assembly from the strut could be seen. There have been struts broken as a result of shimmy and even castoring nose wheel assemblies departing the strut
The badly damaged tire can be seen below. Even the side walls took a beating and gouges are evident all over.
On closer inspection I found that there was no High Tensile Rolled pin installed by the manufacturer that secures the shaft into the MKNG16. This caused the shaft to force it's way out of the MKNG16 casting resulting in the severe shimmy. The damage so far is only the shaft that is bent and a shredded tire. My suggestion is that nose wheel assemblies be inspected for the presence of the rolled safety pin in the MKNG16.
As can be seen from the above picture the shaft is bent and needs replacing. The Problem and the Fix Many emails resulted between me and the supplier in an effort to ascertain ourselves what was the cause of the failure. There are differing opinions and I will try and address these without prejudice. I claim that the supplier was neglectful by selling me a castoring nose wheel assembly that had no safety roll pin installed into the MKNG16. This roll pin is required to prevent the Pivot Shaft from slipping the casting under maximum static and dynamic loads. There is also no evidence that the roll pin was installed or thrown out during shimmy. The inside of the cross hole has no shiny marks suggesting there was a roll pin installed and no evidence on the pin shearing. See picture below and note that the hole for the roll pin is black inside and has no shiny marks suggesting there was a roll pin pressed in. The same goes for the casting, it also has no evidence of damage as a result of a shed roll pin or score markings
The Supplier claims that the roll pin was installed and it could have shed as a result of the shimmy. Roll pins are however hard to remove from their under sized holes and one has to hammer/press them out with a good bit of force. No shimmy would ever dislodge, let alone throw that roll pin. Now whether there was a roll pin or not the question now arises - Did the Pivot Shaft slip as a result of the shimmy or was the shimmy as a result of the Pivot Shaft slipping? I have never had a shimmy before but admittedly there is always a first time. I believe strongly that if there had been a roll pin installed I may still have had a shimmy, but there would have been no damage. The only way that the pivot Shaft can bend is if enough space cab be created between the MKNG16 MKNG15 allowing the bending forces to bend the 4130 Pivot Shaft. To bend 4130 tubing (and this one has a 3/16" Wall) one would have to over bend it to allow for spring back. In this case the only way this shaft could have bent is that enough space was available by the slipping from the casting. To claim that it had bent before slipping is unfounded. If one looks closely at the detail drawing in the construction manual Chap 13 Page 12, the pilot sleeve will prevent the Belleville washers to compress more than 1/16" resulting in a similar space between the MKNG16 & MKNG15 that is much too small to effect bending of the Pivot Shaft. By the supplier's own admission the castoring nose wheel assembly that he sells is only designed for a maximum front seat loading of 400 Lbs. The Cozy MKIV POH states the max front seat loading is 425 Lbs. This is of concern to me and the design needs to be revisited and this item should then not be made available for Cozy MKIV builders until the problem is resolved. I am also frustrated by the fact that the supplier will now willingly sell me a new unit and takes no responsibility in this failure through the omission of the roll pin. So I will be out of pocket with $400.00 through no fault of my own. So here is my fix and upgrade that will lessen the chances of this failure mode. I have had a local machine shop make up a new solid Pivot Shaft. I elected to use EN19 forged shaft steel that has a normalized tensile strength of 900 MPA opposed to the 600 MPA of 4130. the shaft has a flanged end that will prevent it ever slipping the casting again.
You can see the new shaft against the bent stock shaft. The bending in the stock shaft is evident of the forces capable on the the assembly. I had the bottom side of the casting machined out in countersunk fashion for the flange to seat against.
New custom EN19 Pivot Shaft installed.
A roll pin is now not necessary since the shaft is hydraulically pressed in with force after heating the casting for a little expansion. I will install only two Belleville washers in the same orientation to maintain he same pressure force as the standard arrangement and in doing so will lessen the amount of downward movement of the Pivot shaft to one quarter of the original travel. Flat washers will make up the required thickness to prevent the pilot sleeve binding or interfering. This way the required 9 Lbs side preload on the castoring nose wheel can be attained with very limited downward travel of the Pivot Shaft during high force. This should be a reasonable modification for suitable for the Cozy MKIV and live up to the max front seat loading of 425 Lbs. The Belleville washers used in the castoring nose wheel assembly has a flat load of 700 Lbs. by placing four of them in differing orientation like - / \ / \ as the original plans suggest the force required to flatten them out will the be only 700 Lbs, but the deflection will be four times the amount of a single washer.
My modification by using only 2 Belleville Washers stacked parallel the force will be 1400 Lbs ( I may even go to 3 Washers ) and the deflection of only one washer. The idea is not to have any downward movement of the pivot shaft in order to maintain the necessary friction between the MKNG16 & MKNG15 preventing shimmy. The Belleville Washers are incorporated to allow the friction without continually readjusting the tension. Total cost of this fix and modification = $10-00 Results will be posted soon. Check back regularly. So back to the airport and after a good inspection of the nose gear box laminations and MKNG6 to strut juncture I only found that the MKNG6 had a flox to strut bond failure as well. The nose box section was intact and after cleaning all the old flox/epoxy from the MKNG6 I re-bonded the strut to it. This time round I replaced the four 3/16" countersunk screws that he plans calls for with 1/4" bolts. After installation of the nose gear strut and setting the castor friction to 12 Lbs (9 Lbs did not feel right to me) I went for a couple of high speed taxi runs. The brakes soon started to fade and I called it a day. With serious braking so that he nose dips quite low there was no shimmy. I had my 50 Lbs ballast in the nose and weight came to 1880 Lbs and CG at 100". No shimmy at all and it seems that my modification is successful. Time will tell. Recipe for success:
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