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- Jan 12, 2013
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For starters, this is an opinion piece. I have not done scientific testing to come to my conclusions, and I am totally open and welcome other opinions and dissenting commentary, so 'have-at-er' if you wish. 
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I read often that one of CRK's large folder pivots is stronger than the other (IE "the sebenza 25 is like the 21 with a more robust pivot" etc, or "the umnumzaan is for hard use with it's oversize pivot"). Kiddies, I am here to tell you that all of the pivots on these three models (Lrg 21, 25, Umnumzaan) are exactly the same size (diameter). Although they differ in design, chances are they are in most-ways equal in strength. I might even argue that based on my observations, that the 21 pivot is the strongest of the three. At the moment, I am leaving blade thickness out of this discussion. I agree it is a factor, but whether it's a positive or a negative when it comes to pivot strength is not clear to me.
Let's start with some info about pivots. A large pivot is desirable because it does a few things that come to my mind:
1: The larger diameter will allow less angular deflection of the blade side to side when manufacturing tolerances are held equal to a smaller pivot. This is basic geometry. If a large pivot and a small pivot are both machined such that they allow, say, .05mm of play, the larger pivot will result in less of an angle of deflection on the blade. So big pivots make for truer knives, and give the knife maker some slack when it comes to how tight they need to keep their tolerances. obviously there is a point of diminishing returns, where the blade hole has become so large, that the blade is weakened, so it's a trade-off, but in general "bigger is better".
2: The large pivot has more surface areas for the same amount of force when opening or closing the knife, so will wear down more slowly, and any wear on the pivot will have less affect on blade play and deflection for the reason described in note #1.
3: The larger pivot can hold more lubricant, and perhaps feel smoother as a result. A larger pivot may also slow down a knife for the same reason, but we are talking strength here.
4: Importantly, the larger pivot has a greater area of interface with the slabs of the knife and thus sideways forces are less likely to cause blade deflection as the slabs have greater leverage against the pivot in the larger diameter.
First up: The 21 pivot (bushing)
Here is the myth I would like to dispel: "the 21 has the smallest pivot". The 21 pivot is not the screws! The 21 pivot is the bushing!. Glancing at the outside of a 21 in comparison to a 25 or an Umnumzaan is not the whole story! The screws only serve to sandwich the bushing between the TI slabs and are not part of the rotation of the blade. The 21 bushing is identical in diameter to both the 25 and Umnumzaan pivots, so has similar properties as far as the notes I have listed above. No normal human could shear through the screw and sleeve that pass through the bushing when using the knife, so I feel the strength of the screw and pin sleeve that pass through the bushing is not the greatest factor in this pivots strength. What is very important, in my opinion, is that the 21 bushing is firmly held between the TI slabs (sandwiched), and that those slabs are very rigid due to both the pivot screw and the stop pin screw being so close to the pivot bushing. Unlike the 25 and the Umnumzaan, the 21 bushing is butted up against the inside of the Titanium slabs (the bushing is larger than the screw holes in the slabs). I suppose my point is that unless you think you are going to press so very incredibly hard on your 21 that the blade shears through the bushing screw and sleeve, the 21 bushing pivot is a very strong and robust system, equal or greater in strength to that of the large screw in the 25 and umnumzaan. The ultimate goal of the interface of the slabs and the pivot is not to allow the blade to be torqued to one side or the other. Both the pivot screw and stop pin on a 21 are working to keep the pivot bushing true and square, and preventing deflection of the blade.
Photo of a 21 bushing in a 25 blade
Photo of a 21 bushing in an Umnumzaan blade
Second up: The Umnumzaan Pivot (old style):
Again this pivot is identical in diameter, to both the 21 pivot bushing, and the 25 pivot screw. The primary difference is the the umnumzaan (old style) pivot screw has side bolsters that sit in machined pockets in the slabs. I would argue CRK added these bolsters to make up for a percieved lack of strength due to the Umnumzaan having no stop pin to assist in the "sandwich effect' that the 21 bushing benefits from. In the case of the Umnum, the pivot passes right through the slabs, so it needs something to help prevent sideways deflection of the pivot. The thickness of the TI slabs, resting against the screw in not enough. By adding these wide bolsters to the pivots screws, CRK achieves some torsional stability if the blade is torqued left or right, that otherwise could be provided by the bushing being butted against inside of the Ti slab, like it is on the 21.
Photo of Umnumzaan pivot in a 21 blade
Photo of Umnumzaan pivot in a 25 blade
Third up: The 25 Pivot Screw
Again this pivot is identical in diameter to both the Umnumzaan screw and the 21 bushing. It does not have the large bolsters of the Umnumzaan (old style) pivot, but is protected from sideways torsion by the stop pin holding the slabs rigid. The screws themselves are counter-sunk so they have some of the bolster affect of the Umnumzaan old pivot. If my theories are correct, this would make the 'new pivot' Umnumzaan the weakest pivot of the line-up of CRK large folders (is has small bolsters, and no stop pin), but still likely strong enough for any human's purpose.
25 Pivot in a 21 blade
25 Pivot in an Umnumzaan blade:
A few more pictures:
What was really interesting to me is that the washer are all proprietary to each knife, including the non-locking side washers. In this photo you can see a (left to right) 21, 25, and Umnumzaan washer. I know the 21 has a small washer on one side, and the 25 has the one with the tab, but you would think the other ones would be similar, but they are all unique.
Here are the three pivot subjects lined up:
And here are the knives broken down (note the machined pockets in the Umnumzaan slabs to support the pivot bolsters):
If you made is this far, I commend you! As Dave would say....."All the best"!
.I read often that one of CRK's large folder pivots is stronger than the other (IE "the sebenza 25 is like the 21 with a more robust pivot" etc, or "the umnumzaan is for hard use with it's oversize pivot"). Kiddies, I am here to tell you that all of the pivots on these three models (Lrg 21, 25, Umnumzaan) are exactly the same size (diameter). Although they differ in design, chances are they are in most-ways equal in strength. I might even argue that based on my observations, that the 21 pivot is the strongest of the three. At the moment, I am leaving blade thickness out of this discussion. I agree it is a factor, but whether it's a positive or a negative when it comes to pivot strength is not clear to me.
Let's start with some info about pivots. A large pivot is desirable because it does a few things that come to my mind:
1: The larger diameter will allow less angular deflection of the blade side to side when manufacturing tolerances are held equal to a smaller pivot. This is basic geometry. If a large pivot and a small pivot are both machined such that they allow, say, .05mm of play, the larger pivot will result in less of an angle of deflection on the blade. So big pivots make for truer knives, and give the knife maker some slack when it comes to how tight they need to keep their tolerances. obviously there is a point of diminishing returns, where the blade hole has become so large, that the blade is weakened, so it's a trade-off, but in general "bigger is better".
2: The large pivot has more surface areas for the same amount of force when opening or closing the knife, so will wear down more slowly, and any wear on the pivot will have less affect on blade play and deflection for the reason described in note #1.
3: The larger pivot can hold more lubricant, and perhaps feel smoother as a result. A larger pivot may also slow down a knife for the same reason, but we are talking strength here.
4: Importantly, the larger pivot has a greater area of interface with the slabs of the knife and thus sideways forces are less likely to cause blade deflection as the slabs have greater leverage against the pivot in the larger diameter.
First up: The 21 pivot (bushing)
Here is the myth I would like to dispel: "the 21 has the smallest pivot". The 21 pivot is not the screws! The 21 pivot is the bushing!. Glancing at the outside of a 21 in comparison to a 25 or an Umnumzaan is not the whole story! The screws only serve to sandwich the bushing between the TI slabs and are not part of the rotation of the blade. The 21 bushing is identical in diameter to both the 25 and Umnumzaan pivots, so has similar properties as far as the notes I have listed above. No normal human could shear through the screw and sleeve that pass through the bushing when using the knife, so I feel the strength of the screw and pin sleeve that pass through the bushing is not the greatest factor in this pivots strength. What is very important, in my opinion, is that the 21 bushing is firmly held between the TI slabs (sandwiched), and that those slabs are very rigid due to both the pivot screw and the stop pin screw being so close to the pivot bushing. Unlike the 25 and the Umnumzaan, the 21 bushing is butted up against the inside of the Titanium slabs (the bushing is larger than the screw holes in the slabs). I suppose my point is that unless you think you are going to press so very incredibly hard on your 21 that the blade shears through the bushing screw and sleeve, the 21 bushing pivot is a very strong and robust system, equal or greater in strength to that of the large screw in the 25 and umnumzaan. The ultimate goal of the interface of the slabs and the pivot is not to allow the blade to be torqued to one side or the other. Both the pivot screw and stop pin on a 21 are working to keep the pivot bushing true and square, and preventing deflection of the blade.
Photo of a 21 bushing in a 25 blade
Photo of a 21 bushing in an Umnumzaan blade
Second up: The Umnumzaan Pivot (old style):
Again this pivot is identical in diameter, to both the 21 pivot bushing, and the 25 pivot screw. The primary difference is the the umnumzaan (old style) pivot screw has side bolsters that sit in machined pockets in the slabs. I would argue CRK added these bolsters to make up for a percieved lack of strength due to the Umnumzaan having no stop pin to assist in the "sandwich effect' that the 21 bushing benefits from. In the case of the Umnum, the pivot passes right through the slabs, so it needs something to help prevent sideways deflection of the pivot. The thickness of the TI slabs, resting against the screw in not enough. By adding these wide bolsters to the pivots screws, CRK achieves some torsional stability if the blade is torqued left or right, that otherwise could be provided by the bushing being butted against inside of the Ti slab, like it is on the 21.
Photo of Umnumzaan pivot in a 21 blade
Photo of Umnumzaan pivot in a 25 blade
Third up: The 25 Pivot Screw
Again this pivot is identical in diameter to both the Umnumzaan screw and the 21 bushing. It does not have the large bolsters of the Umnumzaan (old style) pivot, but is protected from sideways torsion by the stop pin holding the slabs rigid. The screws themselves are counter-sunk so they have some of the bolster affect of the Umnumzaan old pivot. If my theories are correct, this would make the 'new pivot' Umnumzaan the weakest pivot of the line-up of CRK large folders (is has small bolsters, and no stop pin), but still likely strong enough for any human's purpose.
25 Pivot in a 21 blade
25 Pivot in an Umnumzaan blade:
A few more pictures:
What was really interesting to me is that the washer are all proprietary to each knife, including the non-locking side washers. In this photo you can see a (left to right) 21, 25, and Umnumzaan washer. I know the 21 has a small washer on one side, and the 25 has the one with the tab, but you would think the other ones would be similar, but they are all unique.
Here are the three pivot subjects lined up:
And here are the knives broken down (note the machined pockets in the Umnumzaan slabs to support the pivot bolsters):
If you made is this far, I commend you! As Dave would say....."All the best"!
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