To find the critical speed of a vertical shaft

[xxChrisxx]

I'd say cantilever over simply supported, as you essentially arent allowing movement.

I'd like to point out that I'm not 100% sure though.

 

[vikkispike117]

I'd say cantilever over simply supported, as you essentially arent allowing movement.

I'd like to point out that I'm not 100% sure though.

the shaft slides while declamping the tool...but sliding takes place when the shaft is not roatating...are u able to get my point...so can i take it as a cantilever only with no mass attached at the end?

 

[minger]

You want to simulate the condition as well as possible. So, your failures occur when your part is rotating at a fixed speed. So, what is happening there? I still am not sure how it's not fixed in the x-direction. It must be fixed in that direction somehow during operation?

Either way, you're going to have a cantilevered beam case and just essentially neglect the fixed part and say that the beam starts where it's not clamped. Don't neglect the mass, that mass at the end of the shaft will be the driving force in the critical speed.

Ya know, if you're just looking a quick estimate, throw me some quick beam numbers (i.e. lengths and diameters) and I'll see if I can spit you out some numbers quickly.

 

[vikkispike117]

So my constraints must be Ux,Uy,Uz and ROTy,ROTZ...is it correct



the main aim is to determine the cause of failure of the shaft...so i think that might the shaft is running near the critical speed...there is one more possibility but i wil finish this case first and proceed to the next...

about the declamping i wil xplain 2 u in detail...

the shaft is rotating at a high speed...CNC machine is doing a particular operation(for eg:drilling)...now after the drilling operation,,,,,milling should be done so the tool change must take place...so the spinle stops and the decalmp shaft which is inside the spindle moves down (the declamp shaft moves down due to the application of pressure from above)
now the declamping takes place and the milling tool gets attached to the spindle...




i am trying to explain that the shaft rotates along with the spindle during the operation...
but the shaft is stationary when the declamping(i.e when tool change takes place) is taking place...

Now i think u might be clear about the operation.....

 

[vikkispike117]

and about the constraints it's at the left extreme end...
i.e the starting 86.5 mm there is a line contact...
i wil give u the dimenions where it is supposed to be constrained

Ro:13.5 mm

length:86.5 mm

 

[minger]

OK, so based on what you told me I threw the shaft quickly into the rotordynamics program that we use. Here is the geometry

The coupler on the right, Station 7 is on another "level" in the program, this basically will assume that the coupler is completely rigid. In the real-world it's not, but unless we have a number to put in, that's the best I'm going to be able to do.

The left spring is a bearing, bearing by definition only, I realize this is actually a fit. I have made the bearing very rigid in the lateral directions, essentially cantilevering the rest of the shaft off of it.

Hopefully, the rest of it looks good. The first few mode shapes that the program computes are:

The table of computed critical speeds are:

           *******   Spin(1)/Whirl Ratio =  1.000    *******

              no     rpm           R/S          Hz

               1    .173939E-01  .182148E-02  .289898E-03
               2    6617.14      692.946      110.286    
               3    23774.2      2489.63      396.237    
               4    54967.9      5756.23      916.132    
               5    117665.      12321.9      1961.09    
               6    186694.      19550.6      3111.57    
               7    546068.      57184.2      9101.14    
               8    631345.      66114.3      10522.4    
               9    .159996E+07  167547.      26666.0    
              10    .244668E+07  256216.      40778.0    
              11    .281101E+07  294368.      46850.1    
              12    .492635E+07  515886.      82105.8    
              13    .685237E+07  717578.      114206.    
              14    .719070E+07  753008.      119845.    
              15    .334042E+08  .349808E+07  556737.    
              16    .504299E+08  .528100E+07  840498.

If you want the data output file, then it's located at:
http://www.eng.utoledo.edu/~mheminge/shaft_test.OU2 The file is open-able in wordpad, it lists the station properties, displacements, energy distributions, etc. Well, hopefully this helps, moreso, hopefully it matches what you tried to compute analytically. If there is any small changes in the inputs, then let me know and I'll try and find time to change it and rerun it.

 

[vikkispike117]

thank u so much ...
ya i wil do it analytically and verify it...
can't u change the constraint from a "all DOf" and making it free to translate along "x-direction" and rotate along "x direction".Is it possible if yes wil it make a huge difference in the solution

 

[minger]

Change what constraint?

In my model, I have not modeled any thrust bearings, so the shaft is able to translate along the x (z in my program) axis. I have only given it stiffness in the x/y directions, which are lateral in this situation.

 

[vikkispike117]

oh k den itz right...

 

[vikkispike117]

thank u once again

i'll let u knw bout the analytical result...

now the second part comes into picture...
a impulse load acts on the sleeve during declamping i think that the impact load might be causing the shaft 2 fail...can there be other reasons for the shaft failure?

 

[vikkispike117]

can you just help me in getting me the trial version of this software...where can i get this one?

can this stuff be done in ansys also?

 

[minger]

To try and get a trial version, contact the creator, I'm sure you can find an email/phone at dyrobes.com Yes, this can all be done in Ansys.

 

[vikkispike117]

i had sent a mail to the creator but i haven yet got any reply...but in ansys i have created the model using pipe16element...but i did not get the results as u got it using the dyrobes software...so i m not able to make out in which part i have messed up...i think i might have done mistake with the constraints (OR) in the method used actually there r three methods
1) block lancoz
2)substep
3) reduced
may be i have done mistake in these 2 cases...just can u help me out...

 

[vikkispike117]

actually the frequencies in the results start from

27.8hz only...
65hz
100hz...
and keeps increasing
i think the answer which u have got is right ...i don knw were am i messing up...

 

[kampher]

no entiendo nada :P