Hacker News Reader

Comment by msandford | Hacker News Reader

I don't follow how stacking two terminals cuts the clamping force in half.

If a screw provides 10lbs of clamp force, the equal and opposite reaction is that the thing it is screwed into must resist with 10lbs.

If you put one terminal in between it must transmit all 10lbs through itself or else the forces don't balance out and something must be accelerating.

If you then stack another one in there all the force must be transmitted through it also. So the screw clamps with 10lbs of force and both terminals feel 10lbs of clamp force.

I just can't figure out where you got the idea of "it cuts the clamp force in half" but I'm interested to hear.

View on HN

The clamping force is distributed over an area: \sigma = F/A. Adding ring terminals increases the total clamped area, reducing the pressure seen at any point on the surface. Since increasing from 1 to 2 doubles A (assuming each has the same contact area) these surfaces see half the distributed force at every point.

It's easy to visualize if you replace the two rings with one enormous ring (and fastener, etc.) while F remains the same: obviously the distributed force at any point will be low.

The distributed force is crucial. Friction in real mechanical systems is non-linear. Conductors made of real materials vary in yield strength. A correctly engineered terminal must account for force, yield strength, area, vibration, dissimilar metals and other factors to prevent back off, gas ingress (thus corrosion,) high resistance etc. Real engineers don't do all the materials science involved here and no one would trust it if they tried: they rely on published standards, authored in blood.

Stacking ring terminals torpedoes all that: what was (relatively) simple with one ring becomes unanalyzed and prone to failure when stacked.

Ah I see. That analysis works if you neglect the free body diagram of the system. I agree with your analysis if one larger terminal were analogous to two smaller ones. But it is not.

When a bolt applies clamp force it does so to every thing in between the bolt head and the anchoring threads. All of the force is transmitted along the bolt shaft from the mating threads to the head of the bolt and then back down whatever is in between the bolt head and the mating threads.

If this were not the case no fasteners would work. The only exception is when you have multiple mating threaded regions, rust, etc.

If you put one washer in between the bolt head and the threads obviously it must feel the full force of the clamping, or else some force went missing.

If you put two washers in between they're both still trapped in this identical clamp force situation.

This can be extended by induction about as far as you'd like. Certainly 2,3,5,10 washers. Even to 100. Eventually gravity and other things start to creep in at much, much larger scales. If you wanted to clamp 1 million washers this simple analysis would fall apart of course.

Reply on news.ycombinator.com