CamCalc. The weird lobe-calculator

[Admiral_dk]

Very interesting, though I'm not thinking about Four Strokes now (have been preoccupied with workshop, lathe and other projects) and the fakt that I only program in Assembler, Basic or Pascal - but I'll try a comment or two.

Looking at your screen print - those sharp angled "edges" looked wrong, so I googled "speed profile harmonic cam" and found some examples and though they more or less looked like yours, it dawned on me what I thought was wrong (in my mind). I we look at your first cam profile on top of this page (2) - I would like to see a speed profile looking like a full 360 degree sine, with the start of the sine starting at the start of lift, max speed half way up the lift and down to zero at max lift, followed by max "negative speed" (opposite direction) at the half closed state and back up to zero where the valve closes. This would give us a accellearion curve being Cosine => starting at max accelleration going to zero at mid lift and down to max negative at max lift, back to zero at half closed and max accelleration at closing. Obviously we can't start or stop with max accelleration, so we'll have to have a more gentle start and end acelleration curves and this will also modify the start and end of the sine curve.

I'm also aware of the fact that we don't do negative speed, so that will modify both curves as well. Now yours look more the part, although to abrupt in the transitions.

You're right about the fact that all of us who end up making a spring closed valve will have use for this program. But for that to work, you'll have to include the weight of the valve + spring + cap etc.
That then brings the last possible and very interesting calculation max flow through valve. That requires diameter of the stem, head and seat in addition to the rest of the data you already requested. The result could point to too much or too litle amount of gas to the desired rpm for our engine.

I'm not sure that any of this helps you - but it cleared my mind a bit on the subject 

[MuellerNick]

I would like to see a speed profile looking like a full 360 degree sine,

Simple answer: You don't get that with harmonic lobes. Maybe you read (again) what I wrote on the previous page.
I'm sorry

But for that to work, you'll have to include the weight of the valve + spring + cap etc.

Why "but"? If you want to *know* the force required, you have to give these values. If you don't know them (too lazy to use your scale? CAD doesn't calculate volume?) you wont get that answer. CamTastic has no built in crystal ball. It calculates but doesn't guess.
I'm sorry.


"How do I get to this place?"
"To *what* place?"
"I dont know"
"F*ck off"
I'm sorry.

That then brings the last possible and very interesting calculation max flow through valve.

CamTastic is a lobe designer, a designer for harmonic lobes, not a digital flow-bench. And it never will be. Buy a flow simulator package for your CAD and you will know why.
I'm sorry


Honestly, I have very little understanding for your posting.
Nick

[MuellerNick]

And here is a screenshot with both windows: The main window, and the "Forces window".
If the resulting force from acceleration exceeds the spring's force, that area is hatched red to signal a fault.
I had to use quite funny values. That spring is more than odd (but I'll re-check the values).


Nick

[dieselpilot]

Interesting work Nick. Area pressure is probably contact stress which results in fatigue failure? Real life valve train is very complex. Models can use simple cams and run just fine you just don't want them to wear out to fast. I don't understand talk of valve time area for an engine that will never do much other than an shaft run.

An interesting read if you like valvetrains. The Right Lift.

Greg

[MuellerNick]

Hi Greg!
I looked it up, it is called "contact pressure" in English. You are right, for a slow running engine that will maybe just run 100 hours during its life, it is of no interest. For a high rev engine that should output a lot of power, it is interesting. The more you go to the extreme with the timing, the more stress you get.


That link is great! I partially read it yesterday. What they did there, is part of my idea I had. But as I said, it is way too much work. Especially when unpaid.


Nick

[MuellerNick]

Next round ...
That "resulting forces" window from above was quite wrong.
The thought that I can calculate the unloaded spring's length was a brain fart.
And the resulting forces from acceleration were the wrong way round. Oh well ...


Anyhow! I implemented the Hertzian contact stress graph. And it is much more interesting than I thought. Initially, I considered it as a gadget, but now it is extremely helpful.
Why? If you chose the design parameters in such way, that you get a big flank radius, the acceleration on the flank jumps upwards to frightening values. And you might think that that won't work. BUT! Hertzian stress goes down when the radius goes up. And that helps tremendously to reduce the stress at the flank. In fact, it decreases!


See attached screenshot.


Next, sort out all the glitches with window handling. At least, if you change values in the main window and recalculate (or press <Ret>), the forces window gets updated.


Nick

[stevehuckss396]

After the program is complete are you going to have tutorials on how to read the graphs and what all this data means. I would like to learn how to read and manipulate all this data for use in the engines I build.

[MuellerNick]

After the program is complete ...

Slow!
It ain't complete. I have fixed most of the windows handling issues. I'm in the middle of File: Save/Save as/Open. I'd like to have several worksheets open (compare lobes side by side; but that is on the end of the list). I'd like to add a graphical representation of the profile.
And I need to add the g-code part!


For instructions, I'll just copy my postings here and make a PDF out of it and be done!
Yes, there will be instructions. No, I will not give any tips what duration and lift and overlap and whatever for what engine.


Forgot:
I added "Kld". Thats the ratio between opening area of a square shaped lift and the actual lift. That was inspired by the article dieselpilot linked to.


Nick

[MuellerNick]

Finally got that clearance ramp working. 
The literature I have, has a formula for the radius of the clearance ramp. I double checked the triple check, but still, the result of the radius was wrong. Then, I searched the error on my side and tried to figure out wether I misunderstood something. I tried to match both ends (pun: the end of the flank and the end of the base circle w. clearance) but never got that working. I also made the error, that there must be a way to get a radius for the ramp that connects to the other radii tangential.
Until I decided and realized that there is no radius on earth that connects tangential on both ends. And that the formula in the book is simply wrong.


So I finally got that running! 


I also changed the name (for the last time?), as "CamTastic" was already used by two companies. I changed it to "CamCulator", like "Calculator", with the "m" just one letter off to the "l". 


Here is a screenshot of the lobe view (with extreme values to better show the ramp). I have drawn a red circle where flank and ramp touch each other. The red dashed circle is the base circle (w/o clearance).


Nick

[stevehuckss396]

So what do we (by we i mean you) have left to do before we release this thing to the beta testers (by beta testers I mean me)

[MuellerNick]

Attached is a comparison of two lobes.
Both have the same duration. But one is with a plateau angle of 10°.
The one with plateau has a Kld of 0.645, without plateau the Kld is 0.573. What does that mean? 12% more throughput. The interesting thing is, that the maximal Hertzian contact stress is NOT increased.
So without changing duration and without changing lift and without higher stress, more power.
Even more interesting is, that the Hertzian contact stress at the flank is in fact being reduced, albeit the acceleration jumps up from 1026 m/s^2 to 2456 m/s^2
So, once more, the Hertzian contact stress is an essential design parameter. Just looking at acceleration will lead you to wrong conclusions.


Next step is generating G-code. I'll have to make some GUI-decissions and machining setup decisions.
Currently, I think I'll provide single lobe milling with the C axis parallel to the cam's axis. And the other one with an A axis. The A axis requires a "stitcher", so you can arrange different lobes (intake and exhaust) along the camshaft.


Nick

[MuellerNick]

Here is a screenshot of the dialog for generating G-code. This one is for the Z-axis parallel to the camshaft, no rotary axis involved.
I think, I'l have to add wether IJ for G2/G3 is absolute or relative. I will hear.
I'll add a picture for clarification. The code generated will make some air cutting while roughing. But I think defining the stock is more embarrassing than waisting a minute of machining time.


I think, I can cover everything with that dialog. Machining will work with G41/G42 (cutter compensation), so no need for the diameter. Any deviations in dimensions can be adjusted at the control. No, there won't be a PP for Haidenhain etc.


Tried the program on Linux in a tcl-shell and it worked. I just need to add a bit of code to adjust the accelerator-keys (Ctrl-S or Option-S etc.)


Currently have about 1600 lines of code. I'm starting to like Tcl/Tk.


Nick

[stevehuckss396]

 

[MuellerNick]

Honed the G-code generating dialog. Some fine-tuning is left, but I'm quite happy with it.


If you look closely at the "Post-roughing cmd.", you see in the third line a "G00 $EndZ".
Now look at the left side of that screen shot at the generated G-code (well, *there* is work left!) what happened to the $EndZ! Yep! It was substituted with the value of End-Z And that is 30.0
Not sure wether that is useful, but I think it will be. Future will show.

Maybe I'll get that to make the first useful farts and try machining at the weekend.

I'll have two beer now!


Nick

[tangler]

I'll have two beer now!


Nick

That'll explain the farts then