Shh--shh-shhaker feeder...

[Steelforbrains]

We recently installed a shaker feeder on a 48' dropdeck trailer for a customer of ours. This was done so that they can take the conveyor on the road and demonstrate the capabilities of their system. It was a decent idea in theory but not so great in practice. The motors on the conveyor have speed controls that adjust the power from 0 Hz to 50 Hz. At certain frequencies you would think that the trailer is going to fall to shambles. Now they want us to come up with a solution to the problem. Does anyone have any ideas on how to dampen the vibration of the trailer without compromising the effectiveness of the conveyor. We can adjust the counterweights but I think that it has more to do with the frequecy, than the amplitude, which we are kind of limited to. The frame of the conveyor is now rigidly attached to the frame of the trailer. We might try some damping mounts but we can't afford to lose any performance from the convyors. Any ideas?

 

[CESSNA1]

STEELFORBRAINS: Congratulations, welcome to the world of mechanical resonance(s). I am assuming that you are using rotating counterweights to generate the vibration. Make sure they are in phase so that you only have a force in the direction that you need. You are right, it is the frequency that excites the resonance in the trailer, and the amplitude provides the enrgy to keep it moving. You need to detremine the frequencies at which the trailer is excited and then talk to isolation mount people about isolating your system from the trailer.

Good luck
Regards
Dave

 

[GBor]

If your in the US, you may want to look at information from the Forestry Department...seems odd, but since they ship trees all over the country on the back of semis, they have a wealth of data on trailer frequencies. You may be able to alter the trailer suspension system easier than remounting your conveyor? Gotta' admit...I have no idea what a shaker feeder is.

Garland E. Borowski, PE
Borowski Engineering & Analytical Services, Inc.

http://www.borowskiengineering.com

 

[DesignerMike]

I would guess that the "speed controllers" are VFD inverters. Most of them have the ability to program out certain frequencies for that very reason (they will just skip over the programs range)

This might help you out in the short term.

 

[CESSNA1]

STEELFORBRIANS: Sorry, I cannot resist this. This sounds like an idea from the "MARKETEERS".

Regards
Dave

 

[Steelforbrains]

Cessna,
It is difficult to say exactly which direction I need my force. The shaker pan is mounted on an incline, probably about 5 degrees. I can adjust the weights to cancel out forces in certain directions but I still have the harmonic frequencies to deal with, right? There was specialty sliderbed conveyor also on the trailer and we were able to isolate the vibrations from the trailer, to the conveyor, amazingly well with spring mounts. I'm afraid if we try this on the shaker it will dance like a mexican jumping bean. Modifying the trailer seems like it would just be a stab in the dark guess and check game. I would never hear the end of it if it didn't work, but would never hear a word if it did. Avoiding the certain frequencies seems like a good idea but unfortunatly the equipment works best at the higher frequecies which also correspond to the worst resonance. I might try to adjust the counterweights all of the way out and try to achieve a larger amplitude at lower frequecies.

 

[GBor]

Steelforbrains,

I'm in Nascar country where guys LOVE playing with vehicle suspension systems...unfortunately, I'm not one of them. Find a good automotive guy and explain to him that you don't need to avoid the 1Hz frequency that makes people sick, but would rather avoid the band of frequencies that are causing excitation problems. Another option may be one of the West Coast earthquake engineers...I don't think altering the suspension system would be as difficult as you seem to think.

My 2 cents.

Garland E. Borowski, PE
Borowski Engineering & Analytical Services, Inc.

http://www.borowskiengineering.com

 

[arto]

How about airmounts?

http://www.firestoneindustrial.com/Industrial/Product_descriptions/airmount_isolation.htm

 

[CESSNA1]

STEELFORBRAINS:
I assumed that for the vibratory shaker feeder only vibrated in one direction; the horizontal. Apparantly it is supposed to vibrate in at least two directions. That complicates the problem because now you are dealing with not only interactions but different resonant frequencies.

Is this correct, can you post a photo or drawing?

Out of curiosity what are the resonant frequencies?

If you phase the weights and get only - say horizontal vibration, that should reduce the vertical vibration.

If the device is hard mounted to the trailer, you may want to consider isolation mounts, either springs or elastomeric, with dampers. Conversely if it is only setting on the trailer, you may want to consider hard mounting it.

I am envisioning a full length flat bed trailer. Is this correct or is this a smaller utility type trailer? If it is the utility type you may want to consider going to a larger, more rugged trailer.

I sympathize with you plight of hearing only about errors and not about accomplishments. It is sad to see that management continues to be their same egocentric self.

Good Luck

Regards
Dave

 

[GregLocock]

Whata terrific problem. Quick easy expensive way - build a stiff heavy subframe to carry the shaker, then mount that elastomerically, or via springs, to the trailer.

You should be able to get >> 10 dB attenuation across the mounts.

What frequency range does your shaker run at? What frequencies are causing you problems?

To be honest for a one off that is probably the most cost effective, 5 tons of steel and you are done.


Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[MintJulep]

Many assumptions that the resonance is in the trailer suspension, but SFB hasn't confirmed this. Is the trailer shaking on the the suspension, or have you found the frequency of the trailer's structure?

If the problem is actually with the suspension, why not just take it out the picture when the equipment is running. Maybe outriggers like used to stabilize a wheeled crane.

You cannot mount a shaker table on isolation mounts unless the mass of the parts that are supposed to stay still is VERY MUCH greater than the mass of the parts that are supposed to shake. Something about equal and opposite forces....

And yes, shaker tables need to move in two axis, otherwise they don't move the product.

 

[GregLocock]

Ahem
"You cannot mount a shaker table on isolation mounts unless the mass of the parts that are supposed to stay still is VERY MUCH greater"

That's a fascinating observation. The vibration on the surface of an engine is up to 20g. The virbation of the body near the engine mount will be much less than 1 g. The effective mass of the body is less than that of the engine.

I think you are ignoring the attenuation seen after the first resonance.


Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[BillPSU]

I'm going to combine GregLocock and MintJulep suggestions. Normally both aerial devices and digger derrick are mounted to truck chassis with a stiff sub-base which has outriggers on the ends of the sub-base. While in use the outriggers stabilize the unit and the truck weight becomes the counter-weight for the unit. Trailer frames are usually not stiff enough to mount outrigger directly to the frame so I would consider making a sub-base and adding outriggers and mounting the shaker to the sub-base and utilize the trailer weight as counter weight. The trailer could be isolated from the sub-base yet do the job. The outrigger could be made with pilot operated check valve and be pressured one time and hold the trailer in the raised position for a reasonable amount of time (2-3 days).

How much does your combined shaker and trailer currently weigh?

 

[rob768]

I hear some pretyy god solutions in previous posts. First: make sure forces are in one plane only (the plane of travel of your the product), probably meaning that you have to align the unbalances you use for the movement. Any warping of the frame only makes things worse.
Second: careful with damping, as damping means destruction of usefull energy. Too much damping and you will not be able to meet the specifications.
The best thing to do is establish the mode shape at resonance (if correct, deflections are in the plane of excitation only), and increase stiffness in areas where you see or expect deflections. You will have to increase stiffness, preferably shifting resonances upwards and away from exciting forces. These forces are related to unbalance, so exciting frequencies are relatively low. Don't be shy on adding stiffness. Too much isn't a bad thing.

if you need more info:
rmooij@technofyscica.nl
web:

http://www.technofysica.nl

best
rob

 

[MintJulep]

Greg,

Isolation mounts will work very will to prevent the vibration energy from getting to the trailer. However the shaker table won't work correctly.

Typically shaker tables are driven by eccentric masses rotating on shafts to provide the driving force. The goal is to get most of that force into the table. However if the shaker's base is free to move, than the force will be equally happy to shake the base pretty much in the opposite phase of the table. The whole thing will jiggle around quite impressively, but the product won't move.

To continue with your engine analogy, the table would be the pistons, the base would be the block and the carbody would be the trailer.

 

[Steelforbrains]

Okay, I don't have the equipment here but I did receive an email from our customer saying that he wanted to discuss making changes, so I wanted to have some ammo for the meeting. I don't have have all of the information but this is what I remember about the project. The trailer was a 48' single dropdeck. The shaker conveyor was a scaled down model of their full version. The pan was about 16 ft long and about 3 to 4 ft wide with 4" fins sticking up about every 6" (to separate the product, which is shredded scrap metal, mostly aluminum) I believe the pan was 10 gage material. The pan was mounted to the frame via damping mounts. These damping mounts can be found on

http://www.mcmaster.com

pn#97295k53 and 97295k62. These dampers had a 6" rod conecting them. There were about 16 of these units mounted to the pan. The frame was 4x4x1/4 sq tubing and was rigidly mounted to the trailer frame. I guessing now but the motors are probably 2 hp motors, 1800rpm 60Hz motors but they were run on a 0 to 50 Hz speed controller. maximum resonance at about 46 Hz. I have to go now will be back later...

 

[aggman]

I kind of scanned through the post above so this might have been mentioned already but I will give my 2 cents. Place hydraulic jacks (Milwaukie or the like) under the support points where your frame welds to the chassis and have them support the screen. This will help isolate the vibration by forcing it into the ground and not allowing the beam frame to "span" the load. While in transport the jacks can be raised out of the way. This is common practice with portable equipment design in the aggregate industry.

 

[Steelforbrains]

That is an interesting idea that I haven't considered... Could you give me a ball park figure of what that might cost?

 

[aggman]

Depends on the hydraulic package and the such but the jacks are usually around $1000 - $1500 each, plus mounting and the hydraulic unit, I would speculate around 5 - 10 grand. Personally I doubt you will make it work without doing something like this though.

 

[Steelforbrains]

I thought that I had a good idea but then I looked back and realized that GregLocock had already said it. I can use damping mounts if my frame has much greater mass than my shaker pan. Does anyone know of any rules of thumb or calculations for determining how much more massive it should be? This trailer is not going to be permanent and some day the equipment will be removed and the trailer will be put back into service. With that said I would like to concentrate on modifying the equipment and not the trailer.