inertial block- large shaker 1

[jbthiel]

I am trying to determine the inertial mass needed for a large bedplate to be built that may be used for off-road vehicle shaker tests. The test subjects weight 2-9t and the actual vibration spectrum is not known.

I have found a rule of thumb (from thread 384-119694) about inertia block type foundations being at least 3X equipment weight. I have also seen other numbers between 5 and 10 thrown about.

Do you have any better source of information or experience? What number should I use?

Thanks in advance

 

[GregLocock]

Well, it all depends what you want. We often use a shaker hanging from a stinger - the shaker weighs say 20 kg, to excite a car body weighing say 200 kg.

Unless you know what vibration spectrum you'll need, and the acceptable vibration of the inertial block, you'll just have to guess. If high levels of vibration on the inertial mass are acceptable then there is no real limit. If, on the other hand, the inertial mass is the floor, you probably want to make sure it doesn't vibrate 1/10 as much as the test specimen.





Cheers

Greg Locock

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

 

[jbthiel]

{{Well, it all depends what you want. We often use a shaker hanging from a stinger - the shaker weighs say 20 kg, to excite a car body weighing say 200 kg.))
I was invisioning hydraulic actuators under the vehicle to reproduce "road" vibration. Main concerns are structural/bracket fatique and major squeaks/rattles. Low level NVH is overshadowed by the diesel engines and many of the machines have open operator compartments.

((Unless you know what vibration spectrum you'll need, and the acceptable vibration of the inertial block, you'll just have to guess. If high levels of vibration on the inertial mass are acceptable then there is no real limit. If, on the other hand, the inertial mass is the floor, you probably want to make sure it doesn't vibrate 1/10 as much as the test specimen.))
Yes, the bedplate will be even with the floor, but it will be isolated from the rest of the building/floor/foundation. My two concerns are: 1-vibrating the building 2-failing the bedplate foundation. I don't care if the bedplate moves a little, I'm not testing a Rolls Royce or satellite.

 

[GregLocock]

OK, you'll probably end up with a concrete block standing on rubber (?) mats as isolation. With hydraulics you'll probably be able to put seriously damaging forces into the block, so I'd go big and heavy. Let's see, 3 metres wide, 8 metres long, half a metre deep (is that enough?) , that's 36 tonnes. So I don't think you are going to run into any mass ratio problems, directly.

However, bear in mind you'll be applying dynamic shock forces of up to 10 tonnes at each wheel, you'll find that a single isolation system is going to be more like limiting the annoyance of everyone else in the building rather than eliminating it. You could work back from an acceptable vibration spectrum elsewhere in the building, once you've decided what sort of input loads you are aiming at.

Cheers

Greg Locock

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

 

[Rob45]

And if you're simulating road vibration (for off-road vehicles?), you're not really looking at periodic inputs, are you?
Are you planning to do simulations using a drive file from actual data?
If so, the only periodic vibrational inputs will be things like tire imbalance, at quite low frequencies, say, less than 5 Hz perhaps.

 

[jbthiel]

I've seen this type of testing done to a small tractor. A drive file was created from ~10 different scenarios including driving over a bump, over cobbles and over a rough field. And yes the major forces will be at a very low frequency. That is why I'm afraid that using an inertial block that is too small will result in too much vibration being transfered to the building. I think that below 5Hz the soil doesn't do much dampening anymore (depending on the composition of course).

all thanks to all that have posted so far.