

Feb 04, 2013, 11:33 AM  
Vienna, Austria
Joined Apr 2007
2,132 Posts

from acceleration to balance grade (maybe)
Quote:
if we can take the vibrating movements to be pretty similar to those of a harmonic oscillator, we can use simple relations between the max acceleration and the max velocity (they are of course phase shifted, when the speed is max the acceleration is momentariy zero, and in the return points, we have max acceleration), both vary like the sine (or cosine :) function. Here's where I got it from, long live Wikipedia: http://en.wikipedia.org/wiki/Simple_harmonic_motion In the formulae you often have "omega", that is the angular velocity, and that's just 2*pi*f (f...frequency), I will only use frequency. A is the amplitude, or max. displacement from zero position. I was worried we would need to know A, but it turns out we do not. But we need the rpm value, and revs per minute are divided by 60 to get the revs per second which is identical to the frequency f = rpm / 60 So the two formulae for acceleration a and velocity v are: a (max) = A * (2*pi*f)^2 (actual acceleration varying like cosine) v (max) = A * (2*pi*f) (varying like cosine, but phase shifted relative to a) Now let's make a calculation using the mentioned 3 m/s^2 for acceleration: If we got a = 3 m / s^2 and that was mearsured at, e.g., 6000 rpm, then f = 6000/60 = 100, and we can write: 3 = A * (2 * pi * 100)^2 and so the amplitude A will be approximately A = 3 / 400,000 m = 0.0075 mm Now we have everything to calculate v (max): v (max) = 0.0075 * 628 = 47 mm/s (rounded)  does that make sense? cheers Clemens 

Feb 04, 2013, 02:25 PM  
Canada, ON, Hamilton
Joined Nov 2012
314 Posts

Quote:
v (max) = 0.0075 * 628 = 47 mm/s (rounded) I belive you forgot a decimal point . Should it not read 6.28 (2*3.14) ???? v (max) = A * (2*pi*f) 

Feb 04, 2013, 04:53 PM  
Australia, VIC, Melbourne
Joined Nov 2006
11,754 Posts

I thought it was quite interesting that a few of the Apps let you 'remove the force of gravity'.
I meant to test it, but haven't so far.... eg what does it really mean? And for any axis? What happens if the phone is at some angle? LOL It seems they apply it, or something(!), just to one axis. Not a vector if the phone was, say, at 45deg. It just seems wishy washy to me.... but I need to investigate that more. But the readouts (graphs) are still a waveform on top of any offset anyway, so you still get to see an absolute value of the variation anyway. I don't worry too much about one session to another, I just aim at reducing the value seen. eg memorise or record what it was and just keep striving for less. But I did find an issue with their 'detection' of harmonics.... you can have the fan reverberate some amount, at some RPM, and that is not shown in the graph much at all, if at all! Or not as plain as the large value it would/should be  seeing you can plainly see/tell it is a notable issue. You need to go through the RPM range and find those (they will repeat at harmonics from what I can work out), and also find the largest graphed areas, and use them all as factors you are aiming to reduce! I am still working on it to have a more fixed/repeatable way to do it as simply and quickly as possible. eg So far finding the first reverberation point (could be 7000rpm for eg) and fixing that range does a pretty good job of overall balance. But I have had to also do a visual shaft check also, as I could get it to low value "X" on the graph, yet still have the same area of values (it wavers a bit) for rotor rotation adjustments over a fair angle range, but you can see shaft motion (motor shaft flex) change as you go across that angle range. And picking the angle of least flex of course makes those 'graph doesn't seem to show well' reverberations reduce. That seems to be tied in with the phone sensor, or software polling, etc somehow missing out on certain cases of frequency or something. Thus not very well indicated in the graph at all for those cases. 
Latest blog entry: F86 Sabre  FlyFly 90mm  1420mm...


Feb 04, 2013, 11:05 PM  
Australia, VIC, Melbourne
Joined Nov 2006
11,754 Posts

I have the fan mounted in a 'very lightly sprung cradle', so that it can still vibrate very well. If you bolted it to some rigid mount it would 'erase' some vibrations.
I tried laying the phone across the top, on a small piece of double sided tape.... plus then angled, from housing to the cradle plate, again with the bit of double sided tape to keep it there. Both gave the same results.... just in different X,Y, Z waveforms. So both ways were useful to get readings anyway. 
Latest blog entry: F86 Sabre  FlyFly 90mm  1420mm...


Feb 05, 2013, 03:54 AM  
Australia, VIC, Sale
Joined May 2007
3,593 Posts

Quote:
Here's a quick rundown It'd be nice to get to G3 or so, but even G10 is pretty good for these little things. We have the tools, we have the knowledge, there is a Standard way to do things, why must the EDF fraternity ignore the lessons of those who have been doing this since 1940? 

Feb 05, 2013, 08:23 AM  

Quote:
I got it from this video.


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