"It's been years since my engineering materials science courses! Does a collar really help? It sounds like these are failing due to interior stress/heat as opposed to external contact. It seems like a collar may make the crack smaller, but wouldn't due much for hairline fractures and impending doom. I left the engineering world for medicine, but I'll chat with some friends on this one."
Jeepin Al wrote:
The alum, can not be stressed as steel can, especiallyif it cast allum. It does fatigue, and will eventually break. I seen this in the old motorcycle engines I played with when I was younger. Due to the oils and heat, they get brittle. Next comes the action of the gears. When the power is aplied to the gear set, the pinion wants to back off the ring gear. The ring gear is held fast by two very large bearing (compared to the pinion) and doesn't move. Most of us believe that the pinion gear backs out against the thin area holding the bearing pushing the pinion shaft and bearing outwards towards the rear of the housing cracking the housing thru the thin tail section. Once this happens, a repeted load will crack it further, as seen in the picture.
The collar should prevent the intial crack from forming, hopefully.
Al
Well, I suppose I am an idiot raining on everone's parade... but I don't see that this collar will save anyone from having to buy a new diff case. What it may do, if you check the affected area as a part of your preflight/postflight- is get you home without exploding the diff.
Aluminum, unlike steel, stress fractures. Put a piece of steel under duress, but not past its strength point, and no permanent damage occurs. Not so with aluminum. Each time you impact it- it leaves a permanent (usually microscopic) stress fracture. When enough of these fractures line up-- a crack forms. Aluminum products are designed with this in mind-- enough stress fractures should not develop to cause failure within the usefull life of the product. But the product does have a usefull life and it cannot be used past it. Take aluminum wheels-- they were originally only waranteed for a few years, even for on road use. With better technology-- that waranty is now longer. For off-roading-- aluminum wheels still have a definate life time. Use them past it and your wheel is subject to failing catastrophically. Same with aluminum mountain bikes and aircraft. Aluminum aircraft parts (including the airframe) must be taken in every so-many flight hours and x-rayed to determine the amount of stress fractures. If too many fractures are found, that part of the airframe must be replaced before the aircraft is found flight worthy. When these FAR checks are not done properly (think Aloha Airlines)-- then the planes airframe is subject to fall apart mid flight.
This differential cover is basically the Aloha Airlines flight where the airframe failed. Somebody at Dana goofed and did not take into account that the pinion backing out against this cover at the case seam would cause a repetative stress fracture resulting in premature product failure. What we basically have is someone taking a chisel and hitting this point every time the gears are under torque and the wheels lockup against something. Since the collar is on the outside, it will not stop the "chisel" from hitting the aluminum-- thus, it will not stop the stress fracture from occuring. You can think of spinning and locking the tires under load on this diff like you can of using an aluminum soda can's pop top-- it is good for so many flexes before it breaks in half. The steel collar may slow the crack's spreading (and allow you to avoid explonding the diff gears on the trail) by absorbing some of the pressure from the pinion and not allowing it be used to spread the crack. The collar will not, however, stop the crack from happening.
At least that is my take on it, good luck.
Login
Register
FAQ
Search
