Rotating rotor

[dkien.mills]

Hello members,

I am trying to declare a time function of a machine placed on one floor of my building.

I don't know which form of equation to enter to describe 3 phases of this vibrating load : start phase, constant phase and ending phase.

Someone could lead me ?

Many thanks.

[Stephane.kapetanovic]

hi @dkien.mills 

https://knowledge.autodesk.com/support/robot-structural-analysis-professional/learn-explore/caas/sfd...

Best regards

[dkien.mills]

Hi @Stephane.kapetanovic

actually I just watched these videos several times; 

But The author did not show how to define this function with 3 phases, and that's my question.

[dkien.mills]

Hi @Anonymous, actually I just watched these videos several times;
But The author did not show how to define this function with 3 phases, and
that's my question.

[Pawel.Pulak]

Hi,

the constant conditions of rotating machinery are described in this forum thread.

As concerns starting conditions, is linear increase of rotation frequency sufficiently precise for you? 

In such case I can try to find some example I made years ago.

Regards,

[Stephane.kapetanovic]

hi @dkien.mills 

it is important to consider transient regimes. you can create functions even add them and also load or create a *.thf file

the points contained in the *.thf files can be obtained using a spreadsheet especially if your start and stop functions are not linear

Best regards

[dkien.mills]

hi @Pawel.Pulak 

thanks for your sms

My machine dryers runs from 0Hz to 5.75Hz and then constanly at 5.75Hz.

Like you said, I wanna model the transient and constant phases.

[dkien.mills]

Re @Stephane.kapetanovic 

thanks for your advice, yeah that's I wanne do. But I don't find anywhere the type of function in the transient regime.

if you ever have an exemple about it ?

[Stephane.kapetanovic · Accepted solution]

hi @dkien.mills 

https://forums.autodesk.com/t5/robot-structural-analysis-forum/conversion-of-txt-or-excel-file-in-th...

https://forums.autodesk.com/t5/robot-structural-analysis-forum/how-can-i-import-accelerograms/m-p/80...

Linear start function example

this type of curve depends on the type of start: gradual or direct, no-load or under load, etc.

Best regards

[Stephane.kapetanovic]

In the case of a complete use you must have at least three functions: start, normal regime, stop.

Duration of normal regime is also function of your structure response to obtain all results.
You must "add expression" three times with different parameters

Function Start, beginning time = 0 , end time = duration(Start)
Function Normal, beginning time = End of Start, duration(Start + Normal)
Function Stop, beginning time = End of Normal, end time = duration(Start + Normal + End)

After that you can mixt between values from a file and formulas by adding expression

[dkien.mills]

Hi @Stephane.kapetanovic 

Thanks for your return

You answered my question. I have now an idea of the function at transient stage.

[Pawel.Pulak · Accepted solution]

Hi @dkien.mills ,

apologies for delay of my response.

Finally I have found the old example illustrating it which I made years ago.

It was necessary to translate to English the short doc with mathematical/mechanical explanations and comments in the test spreadsheet. All together (example model, doc and spreadsheet) are contained in the first attached ZIP.

This example discusses only the start of machinery and steady state (without stop) and neglects the increase of the amplitude of centrifugal force with the square of rotation frequency.

I have also found the model made several years later, used in the video linked above by @Stephane.kapetanovic 

I have attached it to this post too.

Here both start, steady state and stop of machinery are considered, linear change of rotation frequency is assumed during start and stop, and the influence of rotation frequency on the centrifugal force is taken into account.

Unfortunately I have not noted the expressions of forcing functions - but it was easy to derive them again basing on doc I mentioned.

This model contains time history with linear increase of rotation frequency to 10 Hz between 0 and 4 seconds, then constant 10 Hz frequency between 4 and 8 seconds and then  linear decrease of rotation frequency from 10 Hz to 0 Hz between 8 and 12 seconds.

Here are the expressions to generate forcing functions for above conditions (for angles expressed in Radians):

"Force FY" excitation in "2_start and stop of rotating machine THA.rtd")
start 0=<t=<4
1/16*t^2*sin(10/4*3.1416*t^2)
steady state 4<t=<8
sin(2*3.1416*10*t)
stop 8<t-<12
-1/16*(t-12)^2*sin(10/4*3.1416*(12-t)^2)

"Force FZ" excitation in "2_start and stop of rotating machine THA.rtd")
start 0=<t=<4
1/16*t^2*cos(10/4*3.1416*t^2)
steady state 4<t=<8
cos(2*3.1416*10*t)
stop 8<t=<12
1/16*(t-12)^2*cos(10/4*3.1416*(12-t)^2)

Regards,

[dkien.mills]

hi @Pawel.Pulak 

thanks you for your return

it's very useful not only for me but also for other users.

Regards,