Originally Posted by therusty
... I KNOW this is a real problem. I have to tighten the screws on my teaching board every day. If I tighten them too much, the screws bulge the base out...
Hi Rusty –
Sorry – poor choice of words on my part. I wasn’t at all doubting the existence of the problem you described. Rather, I was trying to say that I was in complete agreement with your earlier statement doubting that the problem was insurmountable. My sentence should have read, ” Like you, I simply can't believe this is an insurmountable problem …
To address your hesitation about un-doing screws set with Locktite, it is not a problem if it is applied correctly. I’ve personally used Locktite since the 1980’s and/or directed its use by engineers, technicians or machinists working for me on various experiments. As you start to apply torque, the screw or nut won’t move at all. Then, as you increase the amount of torque you are applying with the wrench or screwdriver, the thin plastic film that the Locktite forms on the threads will suddenly break loose and the screw / nut will turn easily.
However, using Locktite properly is not totally trivial either. The biggest problems I’ve encountered in using Locktite are:
(a) Using the wrong variety: Notice that in the link I provided, there are separate varieties for different size screws, different viscosities, permanent vs. temporary locking, etc. Technician-level folks never seem to pay attention to this. If they see a little red squeeze bottle on the shelf, they put a few drops on the screw, no matter what it says on the label.
(b) Dirt / oil on the screw: This can cause poor adhesion. Obviously, the screw should not be intentionally lubricated. You wouldn’t believe how many folks with machinist backgrounds do this as a knee-jerk reflex to prevent seizing. If you intend to re-use the fastener with Locktite, it should be cleaned with (clean) solvent.
(c) A primer may be needed for the particular metals involved: Even many real engineering techs (ie, not just poorly trained seasonal ski techs) don’t even know about this issue. For example, someone may switch from a standard steel machine screw to a stainless one (thinking it has to be better), not realize that the latter needs a quick coating of primer (whereas the former didn't), and then blame the Locktite when it doesn't work.
(d) Locktite works better if old coatings of it are removed: Either a new fastener should be used, or the old one cleaned. Running a clean, new, tight-fitting nut up and down a machine screw that previously had Locktite on it is a good first step.
(e) Problems with blind holes: The air trapped at the bottom of a blind hole can push the locktite back out of the threads before it polymerizes. The tech has to apply the Locktite to both the male (exterior) and female (interior) threads in such a case to prevent this.Lockwashers
With respect to your negative experience with the lockwashers on your board, your problem could easily be in the basic design. To inexperienced or non-professional designers / mechanical engineers, lockwashers look like incredibly simple devices, so they don’t give them much thought.
However, for lockwashers to work, certain fundamental conditions must be met. One important condition is that many lockwashers must bite into the materials on either side of it, or conversely, the surrounding materials must “bite into” the lockwasher. If the materials in contact have the wrong hardness (either too hard OR too soft), the serrations not sharp enough, etc., the user will be tempted to overtighten the screw. This can cause many problems, eg, deformation of the surrounding materials (as you observed), stripped threads, etc.. Obviously, this issue is closely coupled with the detailed design of the area, eg, number and type of threads on the fastener, length of the fastener, stiffness of the supporting structures, etc.
The problem you are encountering could also be caused by attempting to re-use lockwashers that are not designed for re-use. For example, the prongs on a toothed lockwasher are designed to partially flatten & irreversibly deform on each use. After a few normal tightenings or one good overtightening, the teeth will be pressed totally flat, and the “lockwasher” will no longer lock anything. It will act more like a regular non-locking washer. At this point, the user is tempted to tighten it even further. At the other extreme, Belleville lockwashers are basically springs which flatten reversibly on use, and can tolerate many more re-uses.
Now that I think more about the question I asked earlier about whether or not board techs use Locktite / lockwashers, the real question probably should be, "Do they use them correctly?"
Anyway, just some thoughts for a hot, wet day.
Tom / PM