Why the 4-pin linkage in heavy bolt cutters?

[Swamp Thing]

Why are these cutters made like this :

... rather than the simpler one-pin concept used in pliers and scissors?

 

[Drakkith]

Good question. My guess is that the 4-pin arrangement makes a complex lever system and gives better mechanical advantage. You can see an example and a calculator for determining the force at this site.

I input lengths of 10 inches and 3 inches for the first level, 6 inches and 2 inches for the 2nd lever, and an applied force of 100 lbs. The resultant force is over 2200 lbs, a mechanical advantage of over 22x. A single lever would need lengths of 22 to 1 to achieve comparable mechanical advantage.

 

[Swamp Thing]

You can see an example and a calculator for determining the force at this site.

If I understand correctly, the advantage of the two arrangements shown in that page is that the first and second levers overlap geometrically, allowing us to telescope the two levers into a shorter length and yet achieve a higher mechanical advantage.

In the cutters, there is no overlap. So would the mechanical advantage be greater than a simple cutter of exactly the same length?

 

[Drakkith]

Just going off of my numbers, the compound lever system is 21 inches long, whereas a single lever system would need to be 23 inches long. Not a big advantage right now, but increasing the 1st lever's 1st arm by two inches (to 12 inches) yields a total length of 23 inches and a mechanical advantage of 26.4x. A single lever system would be 27.4 inches long (26.4 to 1) to get the same advantage, an increase of 2 inches vs 4.4.

If we instead increase the 2nd lever's first arm by 2 inches we get a mechanical advantage of 29.3x. This yields a increase in length of 2 inches vs 7.3 inches for each system.

Note that my numbers were a complete guess. I don't know how long the actual parts of the cutter are.

 

[Lnewqban]

Why are these cutters made like this :
... rather than the simpler one-pin concept used in pliers and scissors?

You have one lever moving another lever.

 

[Baluncore]

The two hinge bolts, closest to the cutters, are eccentric. To adjust the separation between the cutter jaws, the eccentrics can be rotated and then locked.

The free link between those eccentrics allows for some forward and backward movement of the jaws.

 

[Drakkith]

Looking at the cutters again, I think the 1st lever is more like 12-to-1 or 10-to-1 and the second lever around 4-to-1.5. That gives a mechanical advantage of 58.8 for a length of around 19-21 inches. That's about 1/3 the length a single lever system would give you for similar advantage. Note that I assumed a load-to-fulcrum length of 1 inch for a single lever. This might need to be larger to get enough space to fit the cutters over the bolt, in which case the single lever system would be even larger.

And my estimate is probably and underestimate. This page gives a mechanical advantage of over 80 for bolt cutters (2040 kg load / 25 kg effort). This is a substantial improvement over what a comparably sized single lever system could do.

 

[Swamp Thing]

The two hinge bolts, closest to the cutters, are eccentric. To adjust the separation between the cutter jaws, the eccentrics can be rotated and then locked.

The free link between those eccentrics allows for some forward and backward movement of the jaws.

There are arrow marks on those bolt heads. I was wondering why.
The adjustment feature may explain why the arrows are there.

(The arrows are on the cutter I bought recently, not in the picture in my post. The pic is not the same product, and anyway the bolt heads are facing the other way.)

 

[jrmichler]

Getting my bolt cutter from the workshop, and photographing it in both the open and almost closed positions...

Open position:

Almost closed position:

The leverage is different between the open and closed positions. In the open position of my bolt cutter, the leverage is 9/1 X 2.25/1 = 20:1. In the almost closed position, the leverage is very large. I measured slightly over 100 mm of handle movement for the last 1 mm of jaw closing, so the leverage when almost closed is about 100:1.

Those who have tried to cut a "too large" wire with a (side cutter pliers) (diagonal cutter pliers) (pair of diagonals) found that the cut starts a lot easier than it finishes. It's easy to nick the wire, difficult to get a complete cut. So the bolt cutter is designed for enough leverage to start the cut, then increase the leverage as the cut progresses.

 

[Baluncore]

It's easy to nick the wire, difficult to get a complete cut.

That is because the opposed V jaws progressively increase their contact with the material being cut. That material is initially deformed, then as the remaining section is reduced, is stretched beyond its ductile elongation limit. The jaws do not need a sharp edge, and need not meet, so long as the stretch of the bar exceeds the elongation limit.

Lubricating the jaw faces can make cutting rebar bar easier, as it reduces the friction of the steep wedge faces on the steel. It also makes it easier to slide the jaws further onto the bar, increasing the leverage, as the section is reduced. The disadvantage is an increased tendency to spit out the bar, a disadvantage that reduces as the jaws become irregular or serrated with use.