CamCalc. The weird lobe-calculator

[MuellerNick]

 
The graph is right now. Might find a better method for the graduations (that change with size). But finding the right intervals and start values by software is quite tricky. I'll leave it that way for now.
Added colored regions to signal the contact zones. Pale red is flank, pale green is tip (in case you don't see that at the acceleration graph.
Also, maximum acceleration is right now. There is no easy way to calculate it. Now I search for it (and find it to about 5 digits precision).


Got the idea to add "Lift area". This is the area beneath the lift curve. If you increase the tip-radius, the area increases (and acceleration). Also (obviously) if you increase the timing angle.


Also got a great idea for a modified harmonic lobe that will increase lift area substantially if you have left room for more acceleration but don't want to increase the lift. I wonder, why I didn't find that in the literature. 


I'll add "(equivalent) weight", so the minimum spring's force can be calculated. That aint complicated at all.


And for the screen-shot ... if you find a typo, or things are named different, let me hear!

Edit:
Dont ask for imperial units. That won't happen. 
Enjoy!
Nick

[MuellerNick]

I actually managed to make an app out of it. As long as you are a happy Mac-owner, it runs (or it should). 
That's not at all the final version, but you can already play with it (fixed a bug where I confused flank radius and ramp radius).
Also added parameter checking, so it should get harder to make hang the app.


Nick

[MuellerNick]

Does anybody care?


Well, I do! I implemented the idea about the modified harmonic lobe. It now is a harmonic lobe with a plateau. Not that the physical lobe has a plateau, but the plot of the lift has one. The lobe has a circle touching the tip radius and that circle is concentric to the center-line.


Say you have a given lobe with some certain timing:
By adding a plateau (without trying too long), you can get almost 5% more lift are but still keep the maximum acceleration. With more tweaking: 17%


I made 4 screenshots. You'd have to study them for a few minutes to see the difference.


"No feedback is bad feedback"


Nick

[stevehuckss396]

Sorry for not commenting Nick but to be frank I have no idea what good or bad data is.

Timing angle I assume is the same as duration
I have never heard of plateau angle
Tip radius i assume is nose radius
Clearance and clearance angle?
Max and min acceleration I wouldnt know a good number from a bad number.

Sorry buddy but i suspect the lack of feedback is due to the fact that you are working beyond mine and most peoples understanding. I respect what you are doing I just dont understand it.

[petertha]

Couple questions Nick (I haven't downloaded)

- if one wanted to evaluate a flat side cam for comparison (to harmonic), is this be a non-starter for your program because it is confined to harmonic input parameters & shape development?

-presuming a typical harmonic cam output profile, does this completely remove the issue of tappet bottom profile & cam tangency? My understanding is with harmonic it doesn't matter if the tappet bottom is flat or curved or roller etc, it will always 'open' & 'close' at the defined angular input values? Hopefully this makes sense, I'm talking about the situation where a flat bottomed tappet edge can 'dig' on a (non harmonic / flat sided) cam profile & thus lift prematurely & (suspect also) remain open longer. I thought I read somewhere with a properly developed harmonic profile, this can't happen, its always tangential contact between tappet & cam.

- is there any way to develop or evaluate typical ring gear cam lobe profile like on radial engines with your tool? Or is this involve a different algorithm?

[stevehuckss396]

I'm talking about the situation where a flat bottomed tappet edge can 'dig' on a (non harmonic / flat sided) cam profile

Have you given thought to calculating the minimum diameter lifter so this cannot happen?

[petertha]

Have you given thought to calculating the minimum diameter lifter so this cannot happen?

Hi Steve. I think I worded my sentence poorly. Personally I don't think a flat bottomed tappet + flat sided cam profile make a very happy marriage because of edge rubbing & related off timing issues. I suspect a dome shaped or radiused tappet bottom might work better & more what I was wondering about. I've seen this configuration on some model engines. If the program facilitated entry of these parameters a guy could compare valve timing & information on the plots. But I suspect I already know Nicks answer. 

[MuellerNick]

OK!


I think, I'll answer in two or three posts.
1: Namings


Timing angle is duration. Should I change to duration? Others agree?
Tip radius is the radius on the tip of the lobe. If it is called "nose radius", why isn't the flank radius called "chest radius"?
Plateau radius: Well, if someone ever heard of it, than that's because I invented it a few days ago. This never existed before (well, at least I don't know of its existence). This is a modification to the classical harmonic lobe. Take a harmonic lobe, cut -at the very top- a slit down towards the center, bend both sides sideways and insert a radius into the gap. That radius is tangential to the tip radius and concentric to the camshaft's center line.
Clearance + clearance angle: A valve train needs clearance. So that's what it is for. If you design a harmonic lobe without clearance (and that's what the basic design exactly is), the valve/rocker/poppet will slam against the flank radius at the moment the lift starts. That clearance thing just adds a ramp to make that start of lifting smoother. It takes the base circle and subtracts the clearance (two times clearance for the diameter). Then, with that slightly smaller "base circle" (that no longer exists in the physical lobe) it adds a connecting radius from the base-circle-with-clearance to the flank. Both ends are connected tangential again. The clearance angle defines the length of that ramp.
Cam-RPM: There are two ways to display the speed and acceleration graph. One is with 1/rad, the other with m/s. The first one doesn't need RPM but the value is "useless" without further calculation. Maybe you don't care about speed, but acceleration is essential. If you double the cam-RPM, acceleration will be four-fold as big. Speed only twice.
What does that accel-number say? You can calculate the forces from it. Both for opening the valve and for closing. That's interesting, because you then know what force the spring has to have.
The formula is easy: F = m * a
F: Force in N (Newton)
m: mass in kg (kilogram)
a: acceleration in m/s^2
9.81 m/s^2 is the gravitational acceleration. So if the maximum acceleration is 10000 m/s^2 and you want to move 0.1 kg of mass, this will require a force of 1000 N. 1000 N is the force of 100 kg (that is not scientifically right! Mass and force are distinct things. But that way, it is easier to grasp for the common people).


I'll come back with lobe geometry (tangential lobe, harmonic lobe and jerk-fee lobe in about 45 minutes).
Nick

[MuellerNick]

So!
I have cigarettes and a fresh cup of coffee! 


Let's look at different lobe shapes ...


But first, we need to know the basics. They are simple (and maybe you already know them) but they are fundamental.


What is distance? Well, if you don't know that, I give up.


What is speed? Speed is change of distance (or way traveled) over time. If you need half the time for the same distance, you have twice the speed. When the unit for distance is m, the unit for speed is m/s. Yep! Distance over time.
Or, in other words: Change of distance over time.
You get much speed, if distance changes suddenly.


What is acceleration? Acceleration is change of speed over time. So if speed was m/s, acceleration is m/s*s or m/s^2. Easy, eh?!
You all know acceleration from your car (or at least, you dream of it having some acceleration). You all know, that the you harder you accelerate, the more forceful you are pressed into your seat. Deceleration is acceleration in the "opposed direction" or just with a minus sign.
You also know, that it is not speed that kills, it is the sudden change of speed that kills. It is the deceleration that kills (or ruins your valve train).
You get much acceleration, if speed changes suddenly.


What is jerk? If you look back in this posting, and remember how I came from distance over speed to acceleration, you can predict what jerk is. Right, it is change of acceleration over time. So acceleration was m/s*s, jerk is m/s*s*s or m/s^3
Now is it worth thinking about jerk? Yes and no. The influence the jerk has on lobe design is almost just noise. Modern engines have to be quiet, and non-jerk-fee-lobes do make more noise. It also might have some influence on fatigue failure of the valve train. But we certainly can ignore that.
You get much jerk, if the acceleration changes suddenly.

In other words, and looking back over jerk to acceleration, speed and distance, it all is the quickness of change of the previous one.

Nick

[MuellerNick]

So, did I forget about the lobe shapes? No.


There are three main shapes of lobes:
Tangential, harmonic and jerk-free.


We will ignore clearance here, it has no influence on selecting the lobe. Imagine you just file off a bit from the base circle.


Tangential: You have a base circle, a tip circle and both are connected tangentially.
Harmonic: You have a base circle, a tip circle and ... (you have to kinda remember the distance-speed-acceleration thing from above) ... both circles are connected with an other circle. All circles to touch each other tangential.
Jerk-free: Well ... if you remember the previous post and look back to harmonic and tangential, you will maybe realize what this is supposed to fix:
The tangential lobe has a sudden change of radius if you follow from base circle to flank. In fact, the change is infinite, as a straight line is a circle with infinite radius. Then going from flank to tip, you once more have a infinite change of curvature.
The harmonic lobe is smoother in that respect. It also has a sudden change in curvature (or radius), but at least it doesn't change to/from infinite.
So if you look at that point where the base radius and the flank radius are connect (in the harmonic lobe) ... how about cutting out a short piece and insert a radius that is bigger than the base circle's and smaller than the flank's? And after that, once more cut out a small part where the just inserted piece connects to it's neighbors? And repeat that until you get bored? That's what the harmonic lobe does. It smoothens the transition between sections. That's done with parts of a sine-curve or spirals or whatever helps.
I bet, that there are a lot of jerk-free designs to find. But literature about that is damned hard to find. I do have a thesis about the math for jerk-fee lobe design. But simply put: The writer must have been a jerk (pun intended). The math blows off your head (at least mine, that's way beyond from what I can understand).


This is an hidden answer (needs more reflecting, but I'll come back to it if I don't forget): The tangential lobe is is a harmonic lobe with an infinite flank radius.


Nick

[MuellerNick]

Now, we will come into a dark area of my CamTastic and all the other cam-design programs that are free. It is the valve train that they all ignore.
Valve train is the link between the lobe and the valve's stem. You might drive the valve directly (with a poppet in between). You might have a rocker arm, you might have a poppet then a push-rod  and then a rocker.
The rocker arm itself might have different shapes: You can have the pick up side flat or curved. That curve might be off-center or not. It might have a big or a small radius. It might also well be a more complex shape. And the driving side also might have different shapes.
As there are so many possible configurations, I just ignore them. I once thought about a software that can handle that, but it would be absolutely $$$ and not at all for the hobbyist (that includes me).


But there is a simple or partly ignorant answer: The valve train can solve some problems that you get from a tangential lobe. But also, it most often will distort the plot of the lift at the valve. It even might distort the lift asymmetrical. Again, that's a complex story and you better ignore it (with one exception).


Back to the hidden answer "The tangential lobe is a harmonic lobe with infinite flank radius".
First conclusion from it is, that CamTastic can compare a tangential lobe and a harmonic lobe. But just almost! As CamTastic assumes a flat follower (flat poppet), the resulting acceleration would be infinite. You can not (currently you can, but that hangs CamTastic) get a infinite flank radius. You get that, when tip-radius + lift >= base circle radius. So if you enter a tip radius that is close enough to that limit, you *almost* get a tangential lobe.
Second conclusion is, that a tangential lobe doesn't work on a flat poppet. You will get infinite acceleration ("It is not speed that kills, ...") and the tangent will simply hammer against the flat. A tangential lobe only works with a rocker (with a curved pickup surface) or a poppet with a curved surface.


And yes, the minimum poppet width can be calculated. I'll add that (for a flat poppet).


I *hope*, that I have answered all questions. If *your* answer is missing, please feel free to repeat it. I won't get embarrassed at all!


Nick

[MuellerNick]

You get that, when tip-radius + lift >= base circle radius.

Edit: Screenshot and numbers didn't mach, so I fixed them...

Well, that aint' true I think. But, by increasing the tip radius, you will find the hot spot. That's when the flank radius snaps over to negative (that is not allowed in CamTastic).
I made a screen shot. The resulting flank radius in that example is 70907 mm (70 m) so that is almost infinite compared to the other dimensions.
But you also see, that the maximum acceleration is about 12.5 million m/s^2. That is about 1.2 million times gravity. "1 kg suddenly weights 1200 tons"
Also, the graphics fail to display that extremely narrow and acute peak of acceleration. The software find the peak, but the plotting routine simply misses that point (it only calculates values that are one screen pixel apart.

Nick

[MuellerNick]

<Steve Jobs> "One more thing! </Steve Jobs>


All these diagrams are a bit useless, as long as you can't draw consequences out of them. And one parameter that is very interesting is the required spring!
I'll plot a graph of resulting forces from acceleration and a graph of the spring's force (assuming a linear one), with the values of force on the base circle and force on the top of lift. With know travel, that gives the required spring constant and the required compression from the spring's length in relaxed state.


And from these forces (spring and acceleration), there is an other interesting value to be derived: Area pressure (in German: Hertzsche Flächenpressung, can't find my technical dictionary). It depends on force and radius of the surfaces. If you have a small radius, the area pressure increases and might lead to failure of that surface (pitting). Cam followers are often hardened and ground. With area pressure, you know what you have to expect.


TBC ...
Nick

[steamer]

Nick

I'm going to be off for a few days after today.. so I will be reading up....

[MuellerNick]

<Steve Jobs> "One more thing! </Steve Jobs>
An other interesting parameter is average lift. It is the lift area divided by duration. The higher the average lift, the more you get through the valve. Average lift is always less than lift. That's obvious.
You need that value, in case you want to flow-bench your port. Just measure and compare with the valve opened by the average lift.
That number is similar to the lift area, but is easier to grasp. If you add a plateau, the average lift increases (as does lift area) without increasing lift.


An other pro for the plateau:
With it, you can go down with the lift. One advantage is, that the valve maybe does no more hit the piston, the other is better flow at the valve's tulip (is it called tulip?) when the valve is full open.


Hope the ideas stop flooding in. 


Nick