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[VintageLambo] Urraco and Jalpa differentials

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Hi Mike, Thanks for the very descriptive explanation, almost as if I had the gears in front of me. True the meshing is not adjustable, but I have already plans on how to make it so, namely by machining eccentric aluminum sleeves for the bearing hubs and turning the hubs slightly down accordingly. A risky venture, but in my judgment necessary now for more than noise reasons. All else being equal, straight cut gears are stronger that helical gears. While helical gears have contact with more teeth, each of them only have a theoretical point contact and have an additional axial force component, while straight cut gears have line contacts and no axial forces. The only justification for helical gears is noise, since they engage gradually. If straight cut gears do not mesh well, the teeth can almost slam together (shock force) making them they particularly noisy and weakening them. Since your fatigue break started at the center the shafts must be parallel (a good thing). On my crown wheel teeth show wear (shiny part) from 2mm from the root to the very top. I actually cut my hands on the teeth. How was/is the noise level of your differential compared to other Urracos? And where was/is the wear (shiny part) from bottom to top of your teeth? It is actually amazing that a tooth can be welded back on. I thought it was only dentists, who could do such things. Also, good catch of the problem before it damaged more of your differential. Laust --- In VintageLambo@yahoogroups.com, "Mike Dodge" wrote: > Hi Laust, > > Well the story is like this! > I was driving on a navigation rally and during an urban section of > the run I started to hear a ticking sound from the back of the car. > (almost as if it was a stone caught in a disc brake or similar > sound) I stopped to check it out jacked up and spun the left rear > wheel while i was doing this the ticking was eminating from the diff > housing and not at the wheel! I then trailered the car home and > proceded to remove the engine/gearbox for closer inspection. > > Once the cover was of the differential it was apparent what the > problem was! I couldnt find the missing tooth but removed the > crownwheel and it was sitting in a small hollow under the crownwheel > hidden from view. (Incidently the backlash was about .010" and not > adjustable! but wear on all faces minor and no problems with the > mesh evident)At this point i was glad i trailered the car home to > avaoid further dammage to the pinion which is part of the output > shaft! > > The tooth had a large Fatigue crack that had started in the centre > of the thrust face of the tooth right at the root radius. > After further magnetic dye penetrant inspection (I am an aircraft > engineer by trade) we discovered another tooth that was also > cracking in the same area but aprox 120deg from the other one. I > took the crownwheel to a well known gear repairer and manufacturer > here and he repaired the broken one with totally new special welding > material. He suggested leaving the other one as the crack did not > apear to be very large. I decided to repair this at the same time. > It was indeed cracked through aprox 30% of its thickness in an arc > eminating in the same area as the first. > They do crack !! My car had covered 63,000 mls from new at that time. > The teeth are large and straight cut so one tooth is always under > load at any one time where as helical/hypoid types often have two or > more teeth carying some of the load at any one point. > > The rest of the gearbox had no cracking evident and is very strong! > I heard that it is very similar to a Muria box? Being of the same > era I would think that to be true. Someone once told me that early > Muira cars had the engine oil intermixed with the gearbox oil? > > Cheers > Mike > To visi