A little engineering/design help

[Pittknife]

Hi guys,

I am starting to get into knife designs and there are some things I was hoping to pool some knife knowledge about. I made this a separate post so not to interfere with the OP of the other posts intentions.

I modeled some blade shapes and after playing around found that I can drastically change the CG positions of knives with weight distribution. I was doing this to help another member with a weight reduction question he has.

All the blades are symmetrical so you can discount the Z axis, the X & Y is where the question arises. My assumption is the X-axis is the most important, from a physics standpoint it would be for rotational speed. Just to confirm has anyone made physical blades to test something similar to this?

thanks:

Assumptions: used mass properties of Elmax @ 7.81 g/cc, relative accuracy .0012.

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[NeilB]

You're right; you can drive the blade CG anywhere you want with the blade design. And a lot of blade designs are at least partly about CG location. Clip points and fullers pull the CG back towards the hand; tantos and drop points tend to go the other way.

However, CG is only part of the issue. The radius of gyration is important to see if one blade design is more nimble than another.

And all this ignores the affect of the handle, which is at least as important in determining CG location. A knife is a system of multiple parts (usually).

Nor does any of this address the question of the intended use. For some things, kitchen knives for example, you want the knife to balance over the middle finger. For knives intended for general utility, balance over the forefinger is generally considered desireable. For larger fixed blades, just in front of the forefinger (aids in chopping).

That intended use thing will also limit the blade shapes that are appropriate. A Wharncliffe makes a poor skinner, for example.

 

[Pittknife]

You're right; you can drive the blade CG anywhere you want with the blade design. And a lot of blade designs are at least partly about CG location. Clip points and fullers pull the CG back towards the hand; tantos and drop points tend to go the other way.

However, CG is only part of the issue. The radius of gyration is important to see if one blade design is more nimble than another.

And all this ignores the affect of the handle, which is at least as important in determining CG location. A knife is a system of multiple parts (usually).

Nor does any of this address the question of the intended use. For some things, kitchen knives for example, you want the knife to balance over the middle finger. For knives intended for general utility, balance over the forefinger is generally considered desireable. For larger fixed blades, just in front of the forefinger (aids in chopping).

That intended use thing will also limit the blade shapes that are appropriate. A Wharncliffe makes a poor skinner, for example.

Hi Neil,

Thanks for the response. I am primarily focused on increasing the speed of the blade deployment. I was compartmentalize the components to have an understanding of each before analyzing the system. I have already created the rest of the knife in a more refined model. There is a lot of variable in the "feel" aspect the flipping action, so I excluded that for right now and am just focusing on the blade geometry to get a baseline understanding of that component. Next for me is the flipper tab and if there is an effect on design versus leverage.

Since, I'm rather new, I have to focus on one aspect at a time or as you know I would introduce confounding variables. I plan on making more blades and doing a frequency distribution to record the effects of the blade shapes.

Thanks for the response.

 

[NeilB]

Oh, that makes sense.

There are flavors of flippers. Some like the big ponderous thwack of a heavy blade (large radius of gyration, large mass). They do flip with some authority, but also require a lot of energy be put into the system through a fairly stiff detent, otherwise, they don't flip so well. I have several knives like these. Zero Tolerance knives generally fit here, not all, of course, but a lot of them.

Others, like the CRKT Ripple and Eros, have low mass, low radius of gyration blades that flip out very quickly even with a less stiff detent.

Some knives, are just crappy flippers. I don't want to start a war here, so I'll keep my opinions to myself.

 

[chiral.grolim]

I am primarily focused on increasing the speed of the blade deployment.

I'm not a builder, but shifting the center of gravity away from the pivot increases the momentum requirement, so to speak. Speed of deployment depends on the energy loaded into the detent by force on the flipper (F=MA). A greater mass requires more stored energy to achieve the same speed in the same amount of time. The detent will store the same amount of energy regardless of blade-mass or CoG, but by positioning the CoG further from the pivot & lever more energy is required to get the distant mass moving, so it must of necessity accelerate more slowly. However, once it is moving it requires more resistance to stop the blade from fully deploying.

Take the common illustration of a figure-skater spinning on ice - all of the energy is loaded into the spin by a move the occurs beforehand, but once he/she is spinning, controlling the speed of the spin is a matter of extending outward the center of gravity (slows the spin) or drawing it tightly inward (speeds the spin). Make sense?

 

[Pittknife]

I'm not a builder, but shifting the center of gravity away from the pivot increases the momentum requirement, so to speak. Speed of deployment depends on the energy loaded into the detent by force on the flipper (F=MA). A greater mass requires more stored energy to achieve the same speed in the same amount of time. The detent will store the same amount of energy regardless of blade-mass or CoG, but by positioning the CoG further from the pivot & lever more energy is required to get the distant mass moving, so it must of necessity accelerate more slowly. However, once it is moving it requires more resistance to stop the blade from fully deploying.

Take the common illustration of a figure-skater spinning on ice - all of the energy is loaded into the spin by a move the occurs beforehand, but once he/she is spinning, controlling the speed of the spin is a matter of extending outward the center of gravity (slows the spin) or drawing it tightly inward (speeds the spin). Make sense?

Hi,

Yes that is centripetal force vs centrifugal. As I stated, I'm not looking at a system, just the blade. In that context, the CG position, primarily the X-axis should be rather relevant. By moving the X further from the pivot, the blade should have more momentum. I am not talking about pre-loading potential energy.

 

[chiral.grolim]

Yes that is centripetal force vs centrifugal. As I stated, I'm not looking at a system, just the blade. In that context, the CG position, primarily the X-axis should be rather relevant. By moving the X further from the pivot, the blade should have more momentum. I am not talking about pre-loading potential energy.

The pivot an liners restrict the blade's motion along a plane, the radius of the curvature on that plane is dominated by the x-axis since the blade is longer than it is wide, so yes it is proportionally "rather relevant"
The pre-loaded potential energy is pivotal (pun intended) since THAT is what determines how fast the rotation can be at the outset.

Moment of inertia I is defined as the ratio of the angular momentum L of a system to its angular velocity ω around a principal axis: L = Iω. If the momentum of a system is constant, then as the moment of inertia gets smaller, the angular velocity must increase. This occurs when spinning figure skaters pulls in their outstretched arms or divers move from a straight position to a tuck position during a dive...

For a simple pendulum, ... the moment of inertia I in terms of the mass m of the pendulum and its distance r from the pivot point : I=mr^2

So L=ω*mr^2, ω = L/(mr^2). Since L & m are constant, only r is changing, ω = K/(r^2), i.e. angular velocity is inversely proportional to the square of the radius-distance to CoG. If the radius is doubled to CoG, the speed of the flip is reduced four-fold. Closer CoG (smaller 'r' in the denominator) will always flip faster if mass is constant.

 

[poolhustler]

The pivot an liners restrict the blade's motion along a plane, the radius of the curvature on that plane is dominated by the x-axis since the blade is longer than it is wide, so yes it is proportionally "rather relevant"
The pre-loaded potential energy is pivotal (pun intended) since THAT is what determines how fast the rotation can be at the outset.
F=m*r*v^2. The speed of rotation (v) is inversely proportional to the square-root of the radius (r): v = √(F/(m*r)). Closer CoG (smaller 'r' in the denominator) will always flip faster if mass is constant, but may have insufficient momentum to overcome frictional resistance for the duration of the spin. The distant CoG (larger 'r' in the denominator) will always flip slower but have greater momentum to overcome frictional resistance. If you can reduce friction to insignificant levels or increase the pre-loaded energy with a stronger detent, the close CoG blade will always flip out faster than the distant CoG blade and just as completely.

I was just gonna say that.........

 

[sechip]

The pivot an liners restrict the blade's motion along a plane, the radius of the curvature on that plane is dominated by the x-axis since the blade is longer than it is wide, so yes it is proportionally "rather relevant"
The pre-loaded potential energy is pivotal (pun intended) since THAT is what determines how fast the rotation can be at the outset.
F=m*r*v^2. The speed of rotation (v) is inversely proportional to the square-root of the radius (r): v = √(F/(m*r)). Closer CoG (smaller 'r' in the denominator) will always flip faster if mass is constant, but may have insufficient momentum to overcome frictional resistance for the duration of the spin. The distant CoG (larger 'r' in the denominator) will always flip slower but have greater momentum to overcome frictional resistance. If you can reduce friction to insignificant levels or increase the pre-loaded energy with a stronger detent, the close CoG blade will always flip out faster than the distant CoG blade and just as completely.

In terms of angular momentum, L = r*m*v and L is conserved: v = L/(mr), velocity inversely proportional to radius.

What he said!

 

[Pittknife]

The pivot an liners restrict the blade's motion along a plane, the radius of the curvature on that plane is dominated by the x-axis since the blade is longer than it is wide, so yes it is proportionally "rather relevant"
The pre-loaded potential energy is pivotal (pun intended) since THAT is what determines how fast the rotation can be at the outset.



So L=ω*mr^2, ω = L/(mr^2). Since L & m are constant, only r is changing, ω = K/(r^2), i.e. angular velocity is inversely proportional to the square of the radius-distance to CoG. If the radius is doubled to CoG, the speed of the flip is reduced four-fold. Closer CoG (smaller 'r' in the denominator) will always flip faster if mass is constant.

since it has been a long time since I graduated college, let me simplify, the closer the X is towards the CoG the faster the flip, which was my hypothesis to begin with.

 

[bdmicarta]

Some like the big ponderous thwack of a heavy blade (large radius of gyration, large mass). They do flip with some authority, but also require a lot of energy be put into the system through a fairly stiff detent

I have a ZT 0561 and it works like this- the strong detent forces you to pull really hard on the flipper before it begins to move, then you are pulling so hard that you get the blade moving fast and its mass keeps it moving.

The discussions above are using "radius of gyration" incorrectly. If this is an engineering analysis then in structural engineering this term describes the resistance of a long object to buckling when put into compression. I think what you are interested in would be "mass moment of inertia" or "rotational moment of inertia" which has to do with an object's resistance to change in rotational speed.

 

[Pittknife]

I have a ZT 0561 and it works like this- the strong detent forces you to pull really hard on the flipper before it begins to move, then you are pulling so hard that you get the blade moving fast and its mass keeps it moving.

The discussions above are using "radius of gyration" incorrectly. If this is an engineering analysis then in structural engineering this term describes the resistance of a long object to buckling when put into compression. I think what you are interested in would be "mass moment of inertia" or "rotational moment of inertia" which has to do with an object's resistance to change in rotational speed.

bdmicarta is correct, that I why I extended the results to include some of the moments of inertia.

Thanks for all the input guys, I really appreciate it. I'm going to do some more blade designs and record the changes. I have Ansys so I can do for FEA (finite element analysis) for blade structure after.

*ZT is a pretty big company, I wonder if they do these type of analysis during there design?

 

[chiral.grolim]

since it has been a long time since I graduated college, let me simplify, the closer the X is towards the CoG the faster the flip, which was my hypothesis to begin with.

It isn't the "X" vs "Y" that matters, it's mass and radius. In your original diagrams, you cut out a portion of the spine, reducing mass beyond the CoG radius. If you instead removed as much material from close to the pivot along ANY axis, you would push the CoG further from the pivot.

 

[Pittknife]

The mass was removed on either side of the COG. I don't think you understand what I am trying to say but rather rehashing 1st year physics class. The x & y give you your new location of cog, which I use as a baseline. I am trying to find if there is a fundamental change to blade balance with just shifting the cg. You are talking about system level design which I am not.

 

[chiral.grolim]

The mass was removed on either side of the COG. I don't think you understand what I am trying to say but rather rehashing 1st year physics class. The x & y give you your new location of cog, which I use as a baseline. I am trying to find if there is a fundamental change to blade balance with just shifting the cg. You are talking about system level design which I am not.

Perhaps i do not understand you. I am talking about just the blade. The axes (x/y/z) are specific to how you laid out your blade, that is all. If you laid the blade vertically (along the y-axis), what then? X/Y doesn't matter, distance from pivot to CoG (i.e. radius) matters. What i am saying is that shifting the CoG in relation to the pivot impacts speed of the flip. The further away the CoG from the pivot, regardless of x / y / z location, the slower the speed of the flip.

The CoG IS where the blade balances, i.e. shifting the CoG means shifting the blade balance - they are one and the same.

 

[NeilB]

The discussions above are using "radius of gyration" incorrectly. If this is an engineering analysis then in structural engineering this term describes the resistance of a long object to buckling when put into compression. I think what you are interested in would be "mass moment of inertia" or "rotational moment of inertia" which has to do with an object's resistance to change in rotational speed.

Not incorrectly, just in a context that you as a structural engineer (civil?) probably aren't used to. In rotating machinery, we use RofG as a short hand method of discussing things like the amount of energy that can be stored in a flywheel or how fast a turbine can be accelerated. The CG will tell you only so much; the distribution of mass (the RofG) will tell you other things you need to know. Granted, the effects we are discussing here are fairly small, but you can feel it in your hands whether one knife is more nimble than another, and the CG can be in the exact same location. I find this more interesting with respect to kitchen knives more than my EDC knife.