Why Don't All Knives Have a Pivot Bushing?

Lenny

Lenny

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Oct 15, 1998
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This just makes so much sense.
With my sebenza, you just tighten the screws down till they don't tighten any more. Voilla! Perfect opening and closing tension.
With most other knives, you gotta fiddle with the pivot bolt, then LocTite it at your preferred tension. Then, when you want to disassemble the knife to clean or relube, ya gotta go thru the whole trial and error thing again.
What gives?
Is it that much more expensive to design a knife with a bushing?
Inquiring minds want to know.
Lenny
 
I'm guessing there are 5 reasons.

- most knives are not made with tight enough tolerances for a bushing to work.

- it's more expensive

- NIH or similar syndrome. "My grandpa didn't have one so I don't need one"

- users enjoy messing with their pivot tension

- users don't know better.
 
Not to hijack the thread, but believe it or not the Buck 110 uses a somewhat similar bushing. I believe it was introduced with the 4-dot in 1984(?).

110parts.jpg



Anyway, I agree with Dave's reasons and would add that it's something you don't fully appreciate until you take apart a Sebenza and put it back together.
 
Is the bushing the copper-colored insert in the pivot hole of the blade? How does it work, how does it make tensioning the pivot pin easier?

I'm trying to keep up with you master knife nuts....
 
I have sebbie and if I overtighten the pivot screw, blade won't move! I always thought that the pivot bushing was to minimize wear on the blade itself. The bronze bushing also has less friction and makes the blade move easier.My thoughts.
 
What the bushing does, at least with the sebenza, is make the opening/closing action of the blade independent of pivot tension. You essentialy tighten the pivot until it is tight, the bushing is just enough longer so that the blade glides between the washers. Though usually the lock bar adds significant friction. The best thing is you don't have to spend forever trying to adjust pivot tension against blade play, and never have to use loctite.

I have sebbie and if I overtighten the pivot screw, blade won't move!
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If that's the case it has been assembled incorrectly.
 
I just picked up a W.R. Clark tactical tanto with titanium scales and liner. This has the same bushing set up as my Sebbie has but it is just bigger. The bushing is bigger and the pivot is bigger. It opens so easy now it is unbelivable. It had a bunch of blade wiggle and that may be why the gentleman I purchased it from got rid of it. Had 0 lubrication on anything inside. Knife had never been apart in 15 or 16 years! Relubed it and tightened everything down to where it was hard to open and then backed off a scoche and that thing is smooth now for sure. Nice way to to put together a knife. keepem sharp
 
Lenny said:
Is it that much more expensive to design a knife with a bushing?
Click to expand...
Not at all. But it is more expensive to mass produce knives with a bushing.

In mass production, seemingly identical parts really aren't; each part will have slight variation in sizes, angles and thicknesses. So long as these variations aren't too big ("within tolerance"), it's not a problem, and they can be treated as identical.

Slight variations in dimensions can be accomodated in various ways; in that Buck 110, as with most slipjoint pocketknives, the bolsters, pins and blades are hammered together under a specific pressure, to prevent blade play while still allowing the blade to move smoothly. Modern "tactical" folders adjust for any variation in the blade tang via screw tension.

The problem with the Sebenza-style bushing is that it only works correctly when parts involved are made to a much tighter tolerance (higher precision) than other knives. In particular, the blade tang thickness and flatness, and the thickness of the bushing, must be machined to several times the normal level of precision.

That level of precision doesn't come easy, and "not easy" means "not cheap". The parts have to be either hand-fitted (added labor, more scrapped parts that don't meet spec) or require extra precision machining operations (lapping, special grinding jigs), all of which takes more time, labor and money.

Could the likes of Spyderco or Benchmade execute a Sebebza-style bushing? I'm quite sure that they could. But it would add to the cost of each knife, reduce the number of knives they could produce in their current facilities, require upfront investment in special precision tooling... all while they're trying to compete with Chinese knifemakers who steal their R&D and sell their crappy-to-decent (but getting better every month) knives at a fraction of what Spyderco or Benchmade can.

Not enough potential buyers understand or appreciate the bushing, much less are willing to pay extra for it, to make it economically feasable. Sorry, that's just the way it is.
 
Great explanation Gryffin.
I had a feeling that was the case.
Thanks.
Lenny
 
Does anyone know who would install a bushing in an 806d2? Mine has very slight side-to-side blade play, open or closed, if I adjust the pivot for smooth opening. Suggestions?
 
nevertoomany said:
Does anyone know who would install a bushing in an 806d2? Mine has very slight side-to-side blade play, open or closed, if I adjust the pivot for smooth opening. Suggestions?
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This would require precise machining of a bushing, the equally precise machining of the blade to fit the bushing, and machining a an entirely new pivot screw assembly, if you really wanted it to work like a Sebenza. I don't know how much something like that would cost, or who if anyone would do it, but it very easily might approach or even exceed the cost of a Sebenza.

Your post points out one of the reasons the pivot bushing design is superior --it completely eliminates the need to adust the pivot.
 
Well here's the deal with bushing set ups like this. You should be able to tighten the pivot screw all the way down. That is the way the set up is made. You take the guess work out of adjustment. Very simple. You have to make sure when you take the knife apart for cleaning etc. that everything goes back the same way with the bronze bushings not over riding anything on the pivot etc. Now this is key. You tighten everything down as you should you have made sure the pivot and everything else is put together correctly and you still get binding.

Frustrating isn't it. Now I'll let you in a little secret I discovered by accident when this happened to me. Others undoubtedly know about this too but here it is . Loosen slightly the screw going to the blade stop on top of the knife. Voila! The blade should swing freely with no binding. If it swings to freely after the pivot is tightened down tighten that scew up a bit. If it is 2 screws securing the blade stop do each side a little at a time. Hope this helps and keepem sharp
 
Your post points out what I should do: stop messing around and buy a Sebenza!!
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I'm not so sure that it's worth $400.00 just to have a folder that you don't need to adjust the pivot screw....of course I never need to adjust the pivot on my Buck 110 and it was only $23.00 dollars.
 
The Buck set-up isn't ideal, either. You may not have needed to adjust it yet, but if you use it hard for long enough, it's possible you might, only to find that you can't!

That pivot pin is just peened into place; sufficient abuse can pull the frame apart and lead to blade play. And if it does, you can't just tighten a screw, you have to re-peen the pivot. And if you don't do that just right, you might find you can't open the knife at all!

(And no, this isn't theoretical; I bought a Schrade LB7 my senior year of high school, and beat the crap out of it for years. After about 15 years, it had developed a lot of blade play. I fixed it with an anvil and a hammer, but luckily for me, didn't ruin it. I tried it a few years later on a cheap Puma I found at a garage sale, and screwed it up permanently.)
 
KeithAM said:
This would require precise machining of a bushing, the equally precise machining of the blade to fit the bushing, and machining a an entirely new pivot screw assembly, if you really wanted it to work like a Sebenza.
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If you made the bushing with the right inside diameter, you could probably re-use the original pivot screw. Problem is, that would mean drilling a much larger hole in the blade tang.

And there's the hard part: drilling a large, precision hole in a hardened blade. Unless you were real good or real lucky, that'll destroy a few very expensive carbide or cobalt drills. And if you're off-center even a little, you'll ruin the lock-up for good.
 
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