- #1
- 1,047
- 790
Why are these cutters made like this :
... rather than the simpler one-pin concept used in pliers and scissors?
... rather than the simpler one-pin concept used in pliers and scissors?
Why the 4-pin linkage in heavy bolt cutters?
Click For Summary
Discussion Overview
The discussion centers on the design of heavy bolt cutters, specifically the use of a 4-pin linkage system compared to simpler one-pin mechanisms found in pliers and scissors. Participants explore the mechanical advantages, lever systems, and adjustments in the design, as well as the implications for cutting effectiveness.
Discussion Character
- Technical explanation
- Debate/contested
- Mathematical reasoning
Main Points Raised
- Some participants propose that the 4-pin arrangement creates a complex lever system that provides better mechanical advantage than a single lever system.
- Others argue that the overlapping geometry of levers in some designs allows for a shorter overall length while achieving higher mechanical advantage, though this may not apply to bolt cutters.
- A participant estimates various mechanical advantages based on hypothetical lever lengths and configurations, suggesting that the compound lever system can yield significant force amplification.
- Some contributions note the eccentric hinge bolts that allow for adjustments in the cutter jaws, which may enhance functionality.
- One participant discusses the leverage differences between the open and almost closed positions of the bolt cutter, indicating a substantial increase in leverage as the jaws close.
- Another participant explains the cutting mechanism, emphasizing how the design allows for progressive deformation of the material being cut, which aids in achieving a complete cut.
- There are mentions of practical considerations, such as the effect of lubrication on cutting efficiency and the potential for material to be expelled during the cutting process.
Areas of Agreement / Disagreement
Participants express a range of views on the mechanical advantages and design features of bolt cutters, with no clear consensus reached on the superiority of the 4-pin system over simpler designs or the exact mechanical advantages achievable.
Contextual Notes
Some estimates provided by participants are based on hypothetical values and assumptions, and actual measurements may vary. The discussion includes various interpretations of mechanical advantage that depend on specific configurations and lever lengths.
- #2
Drakkith
Mentor
- 23,200
- 7,681
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.
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.
- #3
- 1,047
- 790
Drakkith said:
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?
- #4
Drakkith
Mentor
- 23,200
- 7,681
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.
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.
- #5
Lnewqban
Homework Helper
Gold Member
- 4,146
- 2,360
You have one lever moving another lever.Swamp Thing said:
- #6
Baluncore
Science Advisor
2025 Award
- 16,791
- 10,434
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.
The free link between those eccentrics allows for some forward and backward movement of the jaws.
- #7
Drakkith
Mentor
- 23,200
- 7,681
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.
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.
- #8
- 1,047
- 790
There are arrow marks on those bolt heads. I was wondering why.Baluncore said:
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.)
- #9
jrmichler
Mentor
- 2,447
- 3,535
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.
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.
- #10
Baluncore
Science Advisor
2025 Award
- 16,791
- 10,434
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.jrmichler said:
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.