Those Stubborn Screws
Last Thursday, during one of Dr. Crankshaft's early morning walks, he passed by Dabney Dufus' place. As he walked by Dabney's garage he heard some mighty discouraging words, and decided to stop by to see what Dabney was up to. Besides Doc knew that Dabney always has a great pot of coffee going in his garage.
Dabney was trying to get a piece of interior trim off his '58 Chevy. "Dabney, how come you sound so flustrated?" Doc said, heading straight for the coffee pot.
"I stripped some of these screws that hold this trim and I'm afraid I'm gonna' have to drill them out. I sure hate the thought of damaging the trim, though" said Dabney. "Maybe I should just order some new trim pieces and be done with it."
"Leapin' lifters, boy! Don't be so eager to throw your money away. Sometimes I think you're a brick shy of a full load! Just use your head and you won't ruin your trim." Doc found a cup that was moderately clean and poured some coffee.
"I wish I could be so sure, Doc. Trim always seems so fragile."
"Well, you just got to be careful. Besides, there's more than one way to remove those stubborn screws. I'll show you".
"Ok, Doc, but why are so many of these old screws and bolts so hard to take out, anyhow? They couldn't have been put in that tight in the first place," Dabney said, bracing for one of Doc's lengthy explanations.
Doc took a sip of his coffee. "Lots of reasons, Dabney, my boy. Try rust, for a starter. Screws and nuts and bolts that are located in seriously rusted parts have become chemically and metallurgically integral with the pieces they hold together, so they won't come unfastened no matter what you do. Fortunately, they tend to shear off easily," the Doctor explained.
"And remember that the amount of twisting force or torque required to remove a fastener is always greater than that used to tighten it in the first place."
Dabney could see that the Doc was getting wound up. So he pulled up a stool and settled in for the duration.
Doc had another sip of java and continued: "Here's why. First, the threads have been slightly deformed during the tightening process and now have greater turning friction in the opposite direction. The second reason is that the fastener's metal content is different from that of the material in which it is threaded, so over time galvanic action cements the threads together.
The Doc noticed the confused look on Dabney's face.
"Galvanic action is the electrical transfer of metal atoms through chemical action that takes place over time, and as I said, it actually cements the threads together."
"Now, where was I?" Doc said, "Oh, yeah...the third reason. Sealants may have been used on the threads. Also it's possible that the fastener was tightened too close to yield. That means that the torque is so great that the fastener is close to shearing off and it has become 'work-hardened.' You know, Dab, that's when the metal becomes more brittle and is more likely to break off," the Doctor explained.
"Yeah, like when you bend metal back and forth until it breaks."
"Right, Dabney," the Doctor chuckled a little, "There's hope for you yet."
"So how do we get these screws out," Dabney asked, hoping there wouldn't be another lecture.
"Well, your first problem was that you should have used the right screwdriver. Don't forget your basics, boy. You used a Phillips screwdriver alright, but this puny little thing swims around in the slots and tears them up. Always, ALWAYS, use the largest, longest screwdriver that fits all the way into the slots. That way, you will be able to get the greatest amount of metal contact and twisting force on the screw," the Doctor admonished. "In this case, it's too late for that, isn't it?"
Once again, the Doctor made Dabney's face turn red.
Doc ignored Dabney's embarrassment and continued... "We could drill these screws out and use a screw extractor, but that's too risky. It's be too easy to damage the trim. So, we'll use a little dose of physics."
"Physics? I just want to get the screws out so that I can get the trim loose, not take a science lesson," replied Dabney.
"Consarn it, you young-uns are so impatient," said the Doctor. "Physics is one of the best tools you have in your toolbox."
"Go over there and get that small cold chisel and sharpen it up on the grinder," Doc said, sounding impatient.
Without a thought Dabney obediently did as he was told. The Doctor went back to the coffee pot for a refill.
"Now, hold the chisel in a counterclockwise direction on the outer edge of this screw and lightly tap it with a hammer. Just enough to get the chisel to bite into the screw. Then we can firmly tap it until the screw starts turning."
And, sure enough, the screw was coming out. "That's amazing," said Dabney. "It looks so easy, just like the screw wasn't tight in the first place. How did that happen?"
"Ok, Dabney, here's your physics lesson. The screw was 'fixed' in place with rust and a little galvanic action between the body steel and the stainless steel trim. While a screwdriver - even the right size - probably wouldn't have released it, the energy from the hammer blows concentrated itself in one spot on the circumference of the screw. That force, even though momentary, was far greater than any that could be applied with a screwdriver and was enough to break the rust bond. Had the bond not broken we eventually would have created enough force to shear the screw off."
"Thanks Doc, I'm really impressed," said Dabney. "How is it you know all that stuff?"
"Well, my boy, about twenty-five years ago... it came to me in a vision."
The Doctor put down his empty cup and started back on his walk, laughing at his own joke.