Another fine article by a guy who doesn't understand the engineering of engines.
The K (friction) factor of plain steel is generally .18 to .19, Cadmium plated fasteners are .19, Oil lubricated is in the same .19 to .20 range. So if you re-use a plated fastener, there is no real change in clamp load. The technique of torqueing is not a mystery. I would be more shy of a mechanic that is scared to pull a jug. If the A&P was taught well, an adheres to good practice, there is no danger.
Aaron,
Before giving Busch a rough time again, I think you need to review your numbers. Comparing Lubricated plain steel with 3 different lubes to cadmium results in anything from a 70% over torque to a 20% under torque if you use the nominal values of K (mu actually in the machinery's handbook).
Also having spent some time in vehicle maintenance I have learned over the years that skipping steps (aka not pulling the whole engine out to replace the jug in this case) can lead to increased room for error as cramped conditions deteriorate the ability to see and perform quality work.
I have to report that I too was surprised by some of the material. I have a engine where the Hartzell prop has to come off every 25 hrs. 6 of the wonderful Hartzell stud/nut/roll pin "bolt" assemblies. Hartzell Propeller Owners Manual 115N says dry threads when torquing them. And I see no plating on those threads. And the Hartzell instructions state that the studs can be reused when the prop is removed and replaced. I think that I have removed and replaced that prop at least 20 times. And that prop sees me do snap rolls in IAC competition. Which is a lot of stress. No issues so far.
Now prop bolts are not the same as cylinder hold down bolts or case through bolts, but they are highly stressed and the same requirement for consistent torque on all of the bolts applies. So how do I square Hartzell's instructions to torque the fasteners dry with the article's description that you can't get consistent torque unless the threads are wet?
Best of luck,
Wes
N78PS
Ok so in the chart you have I see .15 for oiled steel, and .12 for cadmium plated, not a big difference. The current fastener design handbooks using DIN 946 for the test method have slightly higher K values as I listed above. I don't see a reference for a test procedure on your data so I am not sure how it compares. Typically the K value will drop slightly with repeated tightening due to polishing of the surfaces. At least that's what we see in our testing. Honestly the biggest concern with that fastener losing clamp load is probably the base flange to case joint moving from contamination (paint, grease, sealant etc), But I did not see that mentioned. Im sorry but I do not like the way Mike pushes the "don't pull a cylinder" idea. I feel its an unsafe mindset to get into. I know he likes to be cheap, but many times damage cannot be found without disassembly. Here is a published K-factor sheet, looks like their testing has cad at .19 and plain at .20, and oiled at .21 using standard testing means.
Last edited by Aaron Novak; 06-19-2014 at 01:57 PM.
Hey Wes,Originally Posted by WLIU
Dry is fine, lubed is fine. Just use the process as the manufacturer does. In modern engine production, some fasteners designed to assemble dry, some are oiled, some have treated washers, some have moly, all with consistent clamp load. As long as the right procedure and materials are used for the specific joint, you will be fine.
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