Say I was compressing a spring with high k value in a vice (not sure why I’d want to do that but it’s hypothetical).
As F =k(dx), there would be a force acting back against the vice when I compress the spring. In practice, how can the vice stay put and not unwind when a force is pushing back on it?
In: 3
It’s friction between the surfaces of the threads. The more the vise is compressed the greater the force on these surfaces.
Mechanical advantage. The acme screw inside the vice has I higher mechanical advantage to compress than the object being compressed has pushing back on the incline plane of the screw.
The angle of the thread(the spiral of the screw) is the reason why.
There is a lot more force pushing perpendicular to the tread than there is pushing parallel to the tread.
ElI5. Pushing a miniature car forward up a wheel chair ramp is easier than pushing forward a car up a regular stair ramp or something steeper
The force is applied at an angle. This angle causes the force to increase or decrease in strength. This basic principle allows screws to work.
A rotational force is amplified quite a bit to close the jaws. Conversely, any force on the jaws generates a pretty weak rotational force on the screw.
Both forces create friction that scales with the force. Ten times as much force means ten times as much friction. Since the force from it being backdriven *starts off weaker than the friction*, it will *always* be weaker than the friction, unless the friction decreases somehow.
Same way a wedge can stop a door. The more you push on the door, the more the wedge is compressed. The friction of the wedge (screw threads) with the floor and door (vice) is more than the force that you apply to the door (vice).
You can make a different outcome if you change the angle of the wedge (screw thread). For metal on metal, you need a very coarse screw, with the lenght of a twist similar or above the screw diameter.
Example twisting pliers: https://www.u-buy.jp/productimg/?image=aHR0cHM6Ly9tLm1lZGlhLWFtYXpvbi5jb20vaW1hZ2VzL0kvNzE4Q2RGd013NUwuX0FDX1NMMTUwMF8uanBn.jpg
Or, you can change the friction coefficient, for example, using a screw with recirculating balls, that is basically a frictionless screw. There are more applications for screws with high friction than the ones with no friction, but they exist.
Example recirculating balls:
https://www.larescorp.com/media/1353/recirculating_balls.jpg
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