how screws, which have been tightened and could be located somewhere they won’t be touched, come loose. Ex) my dining room chair had a screw that was totally tight and loosened up until it fell out over one year.
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Movement of the object will do this. As they aren’t welded or bonded/glued into the material, they have wiggle room (sometimes imperceptible but always there). And there’s a bias for the screw to wiggle free rather than wiggle tighter (just think the effort it takes to screw something in vs out).
This is most apparent in things like bikes or scooters where there’s lots of vibration. Screws will often come loose unless you use a sort of glue to hold them down.
Chairs are made of wood and wood flexes, each time it flexes the screw backed out a little bit until it was barely held in.
Following some mechanical shock, a screw can either:
A. Tighten more. The act of screwing something into wood makes the wood push back on the screw. The screw is unlikely to push further into the wood.
B. Stay the same. This is the default state
C. Untighten. As with A, the wood is pushing back on the screw and can eventually push it loose. Or gravity pulls on the screw little by little over time.
Over time and through many shocks, the screw will start loosening.
There are special machine screws and threads that self-tighten. Certain types of glue can also help keep a screw in place.
There are two ways in which this can happen. It can be caused by vibration, and by thermal expansion/contraction. All objects expand and contract with temperature changes. And temperature rises during the day and descends at night, so this happens constantly. Different materials contract and expand at different rates -for example metal expands and contracts more than wood-. Even with the same materials, just by being in different spots they will expand and contract differently so the screw and the bolt don’t always touch uniformly. And when they lose contact, they lose strength. (Because friction is what keeps screws tight)
Wood is notorious for this.
The reasoning is fairly simple: As the humidity in the air changes, the amount of water in the wood change. By changing the amount of water in the wood, the wood “inflates” or “deflates” depending if water is entering or leaving the wood (this is very simplified).
If the wood deflates, the screws loosen up. When the wood inflates, the screws tighten…..but if the screws had previously been loosened due to the wood deflating then the screws have a chance to back out, and then they won’t tighten up.
The amount the wood Inflates or Deflates is very small. You would only notice if you get out precision measuring equipment. The screws are affected because they don’t need the wood to deflate that much for the screws to loosen in the hole.
The material gives way, and what used to be tight is now loose. Depending on the nature and direction of the stresses, the material itself may also act to loosen the screws.
Wood expands and contracts with changes in humidity, and everything expands and contracts with changes in temperature, different materials at different rates. Over time this can allow screws to work loose.
Repeated stresses, or vibration, can also let screws work loose.
When you just tighten a screw or nut, it’s just friction keeping it tight. If it’s important it stays tight you can use thread locking compound.
A screw works like a circular wedge.
If you lock something with a wedge, and shake it for a while, the wedge will come lose.
The factor that help the losing more are:
1 plasticity of material: plasticity is a deformation that happens at constant load. You tight something, time passes, that material gives way, and the screw loses. Wood and plastics are pretty bad at retaining screws. That’s why we use dedicated, coarse, self threading screws. It’s the screw type that guarantees the most friction, hence, self locking effect.
2 vibration: the more you cycle the joints, the more is likely to come lose. Chairs, in your case, are subject to frequent and heavy cycles when people sit on them, worse if people oscillate back and forth on the chair. This forces help to deform and widen the joint.
3 rotation: not your case, but if you put a screw in a pivot point, and the screw axis is aligned with the rotation axis, that thing will stay tight for zero time. It’s like point 2 but with the force being focused in turning the screw.
In the UK, this also extends beyond wood.
IE you can’t fit a screw terminal junction box that’s not accessible (in the ceiling, under the floor, in a wall). Because screws are not considered maintenance free, they work loose.
I worked on a 100 year old ship restoration, and the planks were fastened with a combination of metal spikes and with wooden dowels, which were called trunnels.
The spikes, due to small shifts over time and a difference in the hardness of the materials, had worn their holes oversized, and they came right out.
The trunnels, after 100 years, needed a 6 foot breaker bar to pry out. On a couple of them I was able to hang off the end of thee bar and fully lift myself off the ground.
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