Fea
Hi
After reading more through the forums, i expect that i can apply the calculated g-factor and restrain at the bearings for the static case. I expect that this is not the problem anyhow.
For the dynamic case, i will run a frequency check and check that the structural member that cracked has not got a natural frequency close to the operating frequency of the motor, hence causing resonance.
Any suggestions welcome!
Barry ■
Cracked Vibrating Grid
Wow, welcome to our forum: Quite a bit of math in your question...looks like a question for JOHN MC Kenzie.
but, on a troubleshooting basis........give us some field data to ponder in regards to why this member cracked.
I would bet ABUSE from drop height, bashing on the grid by the loader bucket or some other like reason may be the culprit.
Best regards, ■
Re: Fea Analysis Of Vibrating Grid
George
Thanks for the reply, i have seen the photos etc. This is not a field issue, the same isssue has been seen on multiple screens, with the member cracking in the same place. I would expect that the problem would occur if the machine was tested for a few 100hrs below leaving the workshop.
Regards
Barry ■
Thanks For The Reply
Hopefully one our other members can help here. ■
Re: Fea Analysis Of Vibrating Grid
Originally posted by barryodonnell
Hi
Im a new group member so please forgive any omissions etc..
I have a fair bit of experience with FEA, but in the aerospace sector. I have been asked by a mate to look at a vibrating grid design which has cracked following 8 months in-service.
By basic questions are, how can I apply FEA to improve the design. To date I have considered the following:
static analysis
-----------------
Calculate equivalent G load and apply to structure (shell & beam), restrain at bearing and at btm of spring pads (linear springs with representative longitudinal and in-plane stiffnesses). Perhaps the appropiate G load could be calculated as:
F=m x deltar x omega^2
were m is the mass of the eccentric part of the throw weight, deltar is the distance from the radial c of g of the throw weight to the centre of the shaft, and omega is the shaft speed.
Then, the inertia load is:
a = F/m
where m is the mass of the spring mounted structure.
This might allow some sort of representative stress distribution.
dynamic analysis
-----------------------
Im not sure that the problem is a static fatigue issue, perhaps the forcing frequency is causing a dynamic excitation. How is this usually modelled.
I welcome any suggestions regarding these issues.
Regards
Barry
Shake a slinky on a table top or in a box and you will get
the idea.
Where is the crack in your screener?
1. is it a primary screener or secondary screener?
a. If its a primary screener how many decks does it have?
Is the screen suspended on cable, springs or air bags?
Are you wet screening ?
b. What is the screen set up on the top deck? is it set up with
two separate screen sizes on the top deck?
c. Is the material flow even across the top of the screen deck?
d.Is flow to the screen continous or intermittent?
e.If it has mounting rubbers are the mounting rubbers clean
free of dust?
f. is screen bearing lubrication done with oil or grease?
g. Has any one checked the orbit of the screener on both sides
of the screener with paint dots to make sure it is proper?
Are the shaker arms properly balanced?
h. Has a bearing or bearings failed? what type of bearings
are you using?
If you have repeated crack at a weldment or the same weldment it is either fatigue from over heating during welding, poor welding, poor substandard steel or a poor welding set up with out welding jigs, or tack welding before major welding to hold the piece in position.
Is the steel heated to temperature before welding?
How well are the weldments grounded before welding?
2. how big is the screener?
3. is it an inclined screen or flat deck linear screen?
4. is it electric driven through belts or a power take off driven scr- eener? ■
Re: Fea Analysis Of Vibrating Grid
Barry
There are several ways to investigate the problem.
Unbalanced force you can get calculating the mechanism - you will get mr value and the force is a function of the speed. Another way - buit a model, restrain it properly (boundry conditions is everything) and than apply required acceleration ( most of the guys check for 5 g). Another part is to check the drawings - pay attention to welds ( overwelded, full penetration, stress relieving, etc..) and check it against BS7608 in your case - I have some welds impossible to make but they look very nice on the design. Once you finish in the office do an impact test on the screen, and check also the supporting structure.
regards
Ziggy
www.vibfem.com.au ■
Re: Fea Analysis Of Vibrating Grid
about bearings I can suggest you following site: http://www.who-sells-it.com/c/indust...rings-323.html that's the best overview I have found of bearings made by several manufacturers. hope I was able to help you with this - please let me know...
Mark ■
FEA analysis of vibrating grid
Hi
Im a new group member so please forgive any omissions etc..
I have a fair bit of experience with FEA, but in the aerospace sector. I have been asked by a mate to look at a vibrating grid design which has cracked following 8 months in-service.
By basic questions are, how can i apply FEA to improve the design. To date i have considered the following:
static analysis
-----------------
Calculate equivalent G load and apply to structure (shell & beam), restrain at bearing and at btm of spring pads (linear springs with representative longitudinal and in-plane stiffnesses). Perhaps the appropiate G load could be calculated as:
F=m x deltar x omega^2
were m is the mass of the eccentric part of the throw weight, deltar is the distance from the radial c of g of the throw weight to the centre of the shaft, and omega is the shaft speed.
Then, the inertia load is:
a = F/m
were m is the mass of the spring mounted structure.
This might allow some sort of representatiev stress distribution.
dynaminc analysis
-----------------------
Im not sure that the problem is a static fatigue issue, perhaps the forcing frequency is causing a dynamic excitation. How is this usually modelled.
I welcome any suggestions regarding these issues.
Regards
Barry ■