Hey Alan:
I just did a short and simple experiment that I suspect you'll find quite interesting.
I rummaged through my bolt bucket and came up with a square head grade 5 bolt, with 3/8 x 16 threads, about an inch and a half long. The nut I found is softer, but I don't know what grade it is. I selected it because it is unused and has a square head, which is suitable for chucking in a vise. Then I found a stack of 3/8" washers such that, with all the washers and the nut on the bolt, there was about one thread showing. I cleaned the threads of the bolt and the nut, and cleaned the washers, all with solvent, then I cleaned off the solvent with Birchwood Casey Gun Scrubber (which is a REALLY good degreaser), and finally blew it dry.
I began by measuring the length of the bolt with calipers. The head is slightly rounded and the thread end is uneven, so I measured from the round head to the "high spot" at the end, with the shaft parallel to the caliper axis. I was careful.
Then I put the head in a vise, stacked on the washers, and tightened it to 55 ft-lb. The torque wrench clicked quickly and easily.
Then I measured the length again, using the same procedure as before.
Then I removed the nut and washers and measured it again.
Then I stacked on the washers, oiled the threads of the nut and the stud using 3-in-1 oil, and torqued it again. Actually, I "tried" to torque it again. I could not get it to 55 ft-lb. It felt "soft", as if the threads were stripping. So, I backed off the torque wrench to 20 ft-lb and tried again. I increased the torque wrench setting until I couldn't get it to click, which happened at 41 ft-lb.
There are TWO interesting results.
First, here are the length measurements:
1.523" starting
1.526" at 55 ft-lb with dry threads
1.523" after relaxing
1.529" at 41 ft-lb with lubricated threads
As you can see, the results with oil was TWICE the stretch of the bolt, meaning TWICE the internal tension, with 25% LESS torque on the nut.
Second, the nut and bolt are in fine shape. The "softness" I felt was the washers being compressed and pushed onto the square head of the bolt, meaning it was trying to pull the square head of the bolt through the round hole of the washer. There is 0.243" of thread now showing past the nut, which means that the additional bolt stretch was caused by less torque using less of the bolt length. Unfortunately, I can only approximate the amount of thread showing when it was tight with dry threads, as I didn't measure it and now the washers are buggered. My best estimate is about 0.08" was showing.
Here's my simple analysis:
The bolt length that was elongated with dry threads was 1.523" - 0.080" = 1.443". The elongation was 1.526" - 1.523" = 0.003".
The bolt length that was elongated with oily threads was 1.523" - 0.243" = 1.280". The elongation was 1.529" - 1.523" = 0.006".
The "stress improvement ratio" due to the use of oil is computed as the ratio of "stretch per unit length per unit torque" of one case vs the other case:
K = (0.006" / 1.280" / 41 ft-lb) / (0.003" / 1.443" / 55 ft-lb)
K = 3.0 (to two significant figures)
Your mileage may differ, but I think that's quite significant. Ordinary 3-in-1 oil on the threads produced three times the stress in the bolt, other things being equal, compared to clean, dry threads.
I've attached a picture of the result.
Addendum: My original post had a really dumb goof in the computation. Since nobody had posted past it yet, I edited it. Sorry, it's late and I'm tired. Mea culpa.
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