OK, its a sleeve bearing, don't be too picky.
Now Wiz, what I am trying to explain is that automotive designers add extra metal to increase the factor of safety. Wider tolerances and certain types of manufacturing defects are allowed and this then saves money because tight tolerances cost money.
In the aviation design equation, the $ per pound number is higher. this means that the designer will design a part that is close to the theoretical stress limit for the conditions encountered and then use tight tolerances, materials control and so on to ensure that there are no defects.
To put it another way, suppose I have a pin joint that is designed to take an alternating shock load of whatever - say 1000 kg in shear.
The theoretical design size in an alloy steel pin might be say 5mm in diameter with a factor of safety of 4 (ie. ultimate breaking strain 4000kg).
As an automotive designer, my main driver is going to be cost. The conditions under which the pin works are going to include road salt, ice and general crap and the part is buried deep in the suspension so you want it to last the life of the car without inspection.
As a result, I might specify it as nominal 8mm diameter. Since its going to be mass produced by cold forging I'll spec it 8mm -0.00, +0.05 so that the dies can wear to blazes before they need replacing and you can use any old machine to make it. We'll simply cadmium plate it for corrosion resistance and thats the end of it. The surface finish can be as rough as bags because there is now no way the stress is high enough to cause a fatigue crack in your wildest dreams
Now look at the same problem from an aviation perspective. I need a five mm diameter to take the loads with the same factor of safety. The environment conditions lets say are the same.
I can't increase the diameter of the pin because of the weight penalty. That means that any corrosion or manufacturing defects are going to reduce the strength below the factor of 4 that I have calculated is required.
I might therefore choose a high alloy steel and hot forge it. I might then specify an ultrasonic or someother inspection process. I might heat treat it as well.
My dimensions are going to be 5mm - 0.000 + 0.005, therefore I need better quality dies and a relatively new machine to hold the tolerances. My surface finish has to be really good so as not to create stress concentrations that could start a fatigue crack.
If I cadmium plate it then I'm also going to have to add an extra step of baking the finished parts to enure there is no chance of hydrogen embrittlement.
Furthermore, just to be on the safe side you might be required to inspect the pin every 1000 hrs.
Thats where all the extra costs go.
To put it another way, a 5mm pin for aviation use may well have a load rating four or more times a 5 mm automotive pin would have - and you can't tell just by looking at them. You have to know the material, heat treatment, surface finish, tolerances - the lot.
CASA makes this point in relation to wheel bearings - automotive ones sometimes even have the same part numbers, but there are a stack of ABEC bearing grades and Aviation uses the Lower 9high quality) ones.
Same with gears - aviation requires AGMA Class One Gears. You will need a million dollar Zeiss CMM plus some very sophisticated software from Gleason to even tell the difference between class one and two.
Having said all that, I still feel for the poor bastard who specified the wrong material. It would be an easy mistake to make.
I do take your point about modern automotive quality being excellent and producing better than aviation quality. It's true in a lot of cases, for example automotive sheet metal componentry is much more accurately made than aircraft sheet metal, although the industry is catching up.
Now I'm waiting for someone to get picky about something.
Last edited by Sunfish; 15th Dec 2005 at 03:50.