Mixer Platform Vibration 1

[mrMikee]

We are experiencing vibration problems with one of our mixer platforms. The mixer weighs about 15 kips and the contents are about 20 kips. This customer is apparently mixing a very wet batch and is getting a sloshing effect that rocks the machine up and down 1/8" (0.25" total) on each side in addition to the 3/8" deflection when the machine is first loaded. I think the steel support should have been designed with an AISC impact factor of 20% to 50% to account for the unknown dynamics of its operation, but we didn't, and now have problems both with the rocking of mixer and the capaity of the support steel.

To better understand the dynamics of the problem, I am now trying to calculate torsion on the platform due to the vertically mounted motor and reducer. This is a 40 hp motor that that turns the mixer blades at 30 rpm resulting in an available output torque of 7000 ft lbs. Some of this will be internal to the mixer frame but some may go into the platform. I have another platform where torsion such as this could be a problem.

If anyone has a suggestion or a good reference/standard that could be a guideline for this type of design, it would be a big help.

Thanks,
-Mike

 

[mrMikee]

By carrying this through a little further I see that the 7000 ft lbs will cause a lateral force of only 228# at each column, certainly not too much to worry about.

The mixer is at the center of the platform withs columns spaced at 10.88 feet each way, or 15.4 ft across corners. I will plug this into the model but don't expect to see much as far as stress in concerned.

-Mike

 

[SlideRuleEra]

mrMikee - When we had platforms with this type problem we would stiffen the troublesome beams by welding a "tee" to the bottom flange. Made for a funky, deeper composite wide flange beam that looked like this

The increase in the moment of inertia would often be enough to solve all of the other problems. Need a really good welder to make those overhead fillet welds, however.

For future industrial equipment installations like this, suggest going with the upper range (50%) of the AISC impact factor. Nothing at a plant seems to fall into the "light machinery" (20% impact factor) category

http://www.SlideRuleEra.net

 

[JStephen]

Those impact factors are intended to allow for increased stresses due to dynamics, not to prevent vibration. You can either approach this as a Vibrations problem, and calculate stiffness you need (to get natural frequency where you want it), or guess, or go off past experiences.

I've designed one or two mixer bridges and in those cases, the owner specified a design static load (actual + some extra for dynamic effects) but also specified a maximum deflection under those loads, and that deflection controlled the design.

 

[mrMikee]

SlideRuleEra,

My first fix was to add a channel to the bottom flange with the legs down; an idea similar to yours except it was an easier down weld. Both details result in the weld being well above the area of high-tension stress, a good idea for fatigue loading. I also wanted run two beams perpendicular to the original beams to pick up some of the load, which would help reduce the rocking. The boss didn't want to do that so he recommended only the bottom flange reinforcement.

The customer rejected our recommendation and said that we should install the perpendicular beams because that was the best way to stop the rocking. I said that was a good idea but let's put in the channel reinforcement to the bottom flange of the existing beams too. He didn't want to do that because he found out there are pipes on the existing beam. So we will have W12x26 beams intersecting with the bottom flange of one butt welded into the other, a very bad fatigue detail. The additional channel would have provided a backing for the flange to flange weld and while not the best detail it might have Ok.

I see now that I slipped into a rant, and I apologize for that, but it's getting tiring trying to fix all the stuff that the Solidworks kids come up with.

JStephen,

I agree that the impact factors address the stress issue and not vibration. My conclusion (at least for now) is the rocking is not really from vibration but simply the fact that the load in the mixer is offcenter and as it is pushed around the mixer is moving from one beam to the other causing a rocking action. I am still concerned about other possible dynamic effects and that's the reason I'm looking at the mechanical components, in part because of the next design coming up that I'm working on today. As it turned out the torsion on the frame wasn't much and the centrifugal force of the offcenter load is only about 1000#. On a well designed structural support including wind or seismic, the dynamics of the mechanism should not be very significant.

To SlideRuleEra and JStephen, I appreciate your assistance.

Regards,
-Mike

 

[zcp]

"it's getting tiring trying to fix all the stuff that the Solidworks kids come up with"

3 cheers and a star for mrMikee

ZCP

http://www.phoenix-engineer.com

 

[mrMikee]

zcp,

Thanks for the star. Of course there's nothing wrong with Solidworks, and it's really not the fault of the tech school kids that we have making the drawings. The issue in my opinion is that there are people who think that anyone who can sit down at a computer and make a nice looking drawing, and Solidworks for example makes nice drawings, is an engineer.

But that is a topic for another day.

-Mike