Design of steel frame mount and vibration isolation

[Guy_Incognito]

i am still abit concerned with trying to model this system with software or with equations.

to me there is too much to consider and too many unknown factors at least for me to be able to come up with useful information.

i still think i can get there faster and cleaner thru good solid design and tuned in by empirical testing

I don't think we'd ever be able to get it 100%. But it will be interesting to see how it holds up with various people's mounts and weights. If it's reasonably close, it could turn out useful for all the brave souls that venture off the beaten path..
With plenty of caveats and "your mileage may vary" style statements, of course.

And if it isn't much good, well then we can say, "Don't bother trying to calculate it ,there's too many factors to get meaningful results."

Who was it - Edison? - that said, "Today I have found a number of methods that don't work." ?

[oldnslow]

Since the engine requires the block or it's equal by virtue of it's design, you have to provide it in some way. GuyFawkes mentioned lead. What if you take Mobil Bob's torque box and fill it with lead shot, perhaps a bit of oil. Like a dead-blow hammer. You would need the sections of the frame to have enough volume. If the whole frame and it's filling equalled the mass of the concrete block you would have your resonance dampener as required by Lister but in a smaller package. Mount the whole shebang on a resiliant mount system and you're good to go, having the best of both worlds.  My two cents.

[gpkull]

oldsnow that would probably work just fine. after seeing and trying different methods i beileve that it does not matter how you do it but if you have mass to take up the thump its all good. how much? dont know but i do know that concrete works real good. without getting into all the details of physics, engineering, inspecting, and rejecting. one could take a minute to figure out how much crete it will take to make a block. form, rebar, studs, and pour thats too easy

[Guy_Incognito]

I'm curious about engines and stuff mounted in boats. A few ton of concrete or lead there is not advisable, so how do they do it?
There were plenty of little "putt-putt" boats/launches back in the day with big single cylinder engines.

Some people are making a frame for portability , others are making it as a base for decent mounts for complete isolation. It's tough to get something that can satisfy all the requirements that everyone wants and yet still be reasonably cheap. That is, cheap enough that the benefits outweigh the cost. I'm willing - in my particular case - to go with some rather expensive airmounts, because the payback is that there's no thump transmitted at all.

In the case of isolation, you basically have more motion of the engine for less motion of your subsoil and surroundings. All the "jiggle energy" is kept in the area of the engine and frame as opposed to being placed into the block and subsequently into the soil, to go and rattle the neighbours china (or hotater's bedspring 80ft away). Note that you can still have a large mass to reduce the size of the jiggle (but not it's energy) and suspend that.

In the case of portability, you need it to -obviously- be fairly portable. So you can trade some of the mass and isolation and end up with a frame that still jiggles a bit, thumps through the ground a little, but doesn't weigh 2 tons, so you can still move it about relatively easily.

It's a compromise and there are always issues with compromises. This doesn't mean by default that it's going to be a case of engineering overkill, although right now we're heading in that direction simply because we're unsure of the forces and results. I can't put my finger on it precisely (which annoys me) but gut feel says that vibration of the engine isn't such an issue as we believe. Fifty more resilient mounted engines and we'll have a pretty good idea of what works, what's necessary, and what was really overkill or not.

Someone posted here way back that all they use is a bit of channel iron and it's fine for them. The trick is finding a good solution pretty much straight away, so you're not out there tinkering with frame designs for months and buying half-a-dozen different mounts to try and find the one that works by trial and error. Sure, there'll be a bit of fine tuning involved. But once you know enough about how it's "supposed" to work, you can eliminate some designs straight away without going to all the time and effort to build it and find that they were unsuitable.

[Quinnf]

Well, one thing is they weren't very high compression, so there wasn't much reverse torque on the compression stroke and most of them were petrol engines so on the power stroke they were also quite gentle.  They were also pretty well balanced.

Here's a link to a video of a Frisco Standard, one of my favorites. 

http://www.oldengine.org/members/sherman/friscovideo.AVI

http://www.smokstak.com/forum/showthread.php?t=29596

Quinn

[xyzer]

I'm curious about engines and stuff mounted in boats. A few ton of concrete or lead there is not advisable, so how do they do it?
There were plenty of little "putt-putt" boats/launches back in the day with big single cylinder engines.

Back in my sailing days I me a guy with a big old schooner. It had a BIG old single cylinder Budha. You would pull a plug in the cylinder head and light a flare to heat up a disk in the combustion chamber. When it was hot put the plug bake in and start it...it was mounted to big beams and fastened to the frames in the hull...It did thump..at the dock when it was running it would send ripples out every time it fired...it had been doing that for 75 years or so.

[Guy_Incognito]

Ok, from what quinnff and xyzer have told me, and what I can tell from ye olde boat sites, they were generally solid mounted to whatever hull mass they had, and they just quiver in the water....

Hmmm.

[Guy_Incognito]

Ok, I'll clarify again where we are:

General frame design looks pretty good ,depending on what we build it from , of course.

We're pretty much onto mounts now.  Please, everyone, consider the following few things:
The converse of each statement is true as well, of course.

- More mass suspended gives less movement and a lower natural frequency.
- Stiffer mounts raise the natural frequency and increase the transmission of pulses to the outside world.
- More of the same mounts is equivalent to stiffer mounts.

There are two exciting frequencies at a speed of 650RPM, 10.8Hz from flywheel imbalance and 5.4 from firing pulses.

You do not want all your factors to come together to give you a resonant frequency near those two points. That would be bad.
How can you tell?

The undamped natural frequency in Hz = 1.88 x SQRT( spring rate in pounds/inch / mass suspended in pounds )

Undamped is close enough. Depending on how much your mounts dampen movement, the damped natural frequency could be a little bit different. But not by much.

You need the mass of your system and your spring rate. You can guess the mass pretty well. I'll say 1500 pounds for a basic engine + generator + frame. The spring rate? Squash your proposed mount and see how far it moves. If it takes 300 pounds to move it half an inch, that's a 600lb/in spring rate. Multiply that spring rate by the number of mounts to get the system rate for the use in that formula. (edit) Manufacturers will often give the deflection of the mount at a certain load - you can use that , as long as your load is close to that.

So using those two examples - 1500 pounds  and 600lbs/in spring rate, with 4 mounts, I get a natural frequency of :

1.88 x sqrt ( 600 x 4 / 1500)

Which is about 2.37Hz. To get that back to RPM , times that by 60 to give about 142 RPM.
To get that to power pulses (which are half the speed of the engine) , times that by 120 to give 284RPM.

At these two points in the rev range, your engine will jiggle. How much? Who knows? It all depends on the damping in your mounts (which takes energy away from your jiggly mass) and how long you stay at that RPM (which puts energy into your jiggly mass)

But if your mass and spring rate combo give you a jiggle that works out around the 500-700RPM range, I'd be pretty careful about proceeding with that combo.

(Edit:) So, either :

Add more weight to bring the frequency down a bit.
Get softer mounts , or use less of them to reduce the spring rate.

To be honest, I'd probably not recommend trimming weight or going for stiffer mounts if your frequency works out around 650RPM. Strictly for reasons of safety - might be fine at 650RPM... but that one time you overspeed to 800 or 1000RPM, you'll hit the natural frequency and have to deal with a jumping as well as overspeeding engine.

[mobile_bob]

i saw the biggest resilient mounts i have ever seen today

each unit had two round plates with the rubber column between

the plates were approx 36" diameter and about 1.5" thick

the rubber was approx 30" diameter and 30 inches tall

must be 100 tons to the inch spring rate...

probably weigh as much as a yard of concrete too

  wow

bob g

[Guy_Incognito]

What the heck were they for bob? Holding up a bridge or something?

I was thinking - I've got a couple of small pneumatic "trolley" type tyres - attached , as one might expect, to a trolley  . They're good for about 150-200kg each.  Not quite enough to hold a listeroid + frame/etc off the ground, unless you double up at each corner.

I might go measure their spring rate at various pressures, load them up a bit and put a exciter on them to see what they do. I've an old battery drill that runs at 300/600RPM, so I might tie it firmly on there with a weight on a rod to give it a bit of bounce.

Will be interesting to see whether they hold up to the calculations any.

Seeing as they're cheap, it might prove to be a good avenue for people to look at. Or as someone mentioned about 4 pages back, car tyres for support. And wheels would certainly help with people who want portability!

And a side note - George B. over at utterpower has this on his "Topic for the day" page :
http://www.utterpower.com/topic_of_the_day.htm

Have you made note of all the portable wagon mounted Lister CS engines and the other big flywheel engines?

Have you read where self proclaimed experts insist that these engines MUST be mounted to massive amounts of concrete or similar?

I asked David Edgington and others to do a little research into this and see if there was any information regarding the longevity of these engines fixed versus mobile. The outcome of this research indicates that there are no differences, and both mounts produce the same life expectancy.

There is no down side to anchoring engines to large masses of concrete or cast iron when there is no problem with that mass becoming a transducer. I would recommend it whenever possible. BUT...  there are cases where mountings transmit and cause nearby structures to seek resonance with the RPM of the engine, a typical (and real life) example of this was Jeff Maier's install where he tied his engine to the concrete floor of his attached garage. When Jeff asked his neighbor if there were any complaints, the woman of the house told him all was fine except Her dishes rattled around in the cupboards of the kitchen when his Lister 6/1 was running!

Jeff's efforts to de-couple his mounting from the floor fixed this. another example is the Columbia University Mounting in their lab.. imagine a rigid mounting inside a building, could there be a more insane thing to do?

Engineering isolated mountings requires a little understanding, I once started a Lister 6/1 on an old utility trailer, and the springs were just perfectly wrong, the trailer started jumping up and down like a kangaroo, unwittingly, I had created a near perfect mechanical oscillator!

the internet is a great tool, but it delivers both good and bad information.

All the best.

George B.

[danalinscott]

First of all I realiize I am relativly new to this forum and this is my first post in this discussion. I have read the 7 pages so I don't think I am covering something already covered..though I DO think several points have been at least touched on. Please feel free to critique this or tell me if you feel it is off topic.

Previously I have always followed the tried and true "pour a big block of concrete and bolt the Lister to it method". But I am in the proess designing my off the grid "retirement home" in the woods and really wish to avoid the lister powerplants from adding  their low frequency vibration to my ambient environment. And since I also want to use the heat of my Listers for heating my home I want to locate them as close to the house as possible..without having to live with the constant "thump".  I know this may be a tall order.

 I can muffle a Lister to the point it cannot be heard a few yards from the powerhose....but the low frequency vibration from the engine itself is a very different and more difficult matter.   And here is how I propose doing it:

I will fabricatre a very rigid fairly conventional welded steel frame to mount the engine upon and use air shocks to mount that to the floor (slab) of the powerhouse. The adjustability of these shocks should allow quite a bit of tuning if needed..and I am sure it will be. Air shocks can be had that are extremely progressive in their ability to smoothly snub movement in both directions (in and out). These shocks will be mounted  at a 45° angle (vertically) and a 135° angle from the four corners (horizontally) so they essentially supend the engine/frame above the floor and hold it suspended in space both vertiically and horizontally.

(Wait there is more)

 In order to dampen the vibration of the engine/frame a second frame will be suspended from it using 4 simple pneumatic supports which allow it to move slightly in all 6 directions but is progressivly snubbed as it does. The additional adjustability of these should prove very adaptable just as that of the air shocks. This purpose of this mass is to damp any harmonic resonances induced by the engine in the upper frame.

(Still more)

The suspended frame will be weighted with sand bags to allow even more adjustability by adding or subtracting weight and/or shifting weight on the suspended frame.

While  I can appreciate that the problem can probably solved mathematicallly for a specific engine in order to provide a solution adaptable to a variety of engines I believe that infinate adjustability is the key.

What do you think?
Does this have any potential?

[hotater]

A properly built concrete mount transfers no vibrations of any frequency to the soil that I can find.

Your mounting system seems just complicated enough to assure at least ONE bad part at the worst possible time....

My previous engine was bolted to a 'slab' that moved and turned the whole canyon into a sympathetic bass drum, but the new one is perfectly still and the running sound is more cam and lifter clatter than anything else.

[danalinscott]

Hotater,

Even with well balanced engines I have not found a 2 yard concrete mount to be sufficient to prevent some transfer of low frequency vibration to the ground. Perhaps I am simply more sensitive to the pulse than average.

But since I intend to live in this home for the rest of my life (hopefully another 20-30 years) and want to place the powerplant as close to it as possible (for co-gen) I do not find the "large mass"  mount I have used up to this point desireable for this project.  I know how to do that but want a mount that isolates the engine from the ground as much as possible.

Airshocks are exremely dependable and unlikely to fail. If they do it should be a progressive rather than a catastrophic failure.

Simiilarly the pheumatic supports (really just very heavy duty aibags) are extremely dependable and unlikely to fail without some warning ... if at all.

I know my welds will not fail.

I incorporate a low oil/high temperature auto shutdown in all of my gensets..and can easily incorporate a $3 shutdown sensor for excessive engine frame movement as well...which is probably smart just in case of an un-noticed mount progressive failure on this untested design.

I do appreciate your feedback...but don't understand which element of this proposed design do you feel is most likely to fail?

[SCOTT]

Dana
I too have issues with the low frequency thump from my Lister type engine.  I think a viable alternative to the block of concrete is to use the block AND some form isolator.

Imagine if you will a rectangular hole dug 8” deeper than the standard block. And a few inches wider in each direction

Fill the bottom 8” with fiber reinforced concrete and plenty of rebar.

On top of this 8” thick pad add standard dome shaped rubber mounts with the correct  durometer      specs and spacing to support the weight of the engine/frame/gen  and the  concrete block.

Build a box with a ¼” sheet metal bottom and attach the dome mounts to the steel.  The rest of the box is made of plywood.

So now you have a box within a box

If you want you can add isolators to the sides of the block or just leave a small air gap.

If it turns out that there is too much movement of the block you could fill in the air gap on the sides with sand while the engine is running, this would allow the sand to fill the gaps in the bottom and sides 

The sand would make it more rigid but may still offer a level of dampening.  You now have a one way one time variable rigidity mounting system

Concrete, sand, and domed isolators are cheap.  If I were to start with a blank slate I would go this route, it offers the best of both world, a rigid base with tons (literally) of mass and a level of decoupling of the engine from its surroundings, it is also very simple not much to go wrong.

Best regards
Scott

[danalinscott]

Thanks Tater,Scot, and those who emailed me wishing to remain nameless.

Summarizing:
----------------------------
The air shocks are probably over kill and can be added later if needed.
Why not just use very squshy tires to isolate the unit from the ground?

I like the extreme variability of your proposed mount..but I think it could be simpler.
How about just usning regular shocks?
How about adding weight to both the engine frame and the subframe?
----------------------------

Ok..how about using "squishy tires" (salvaged ATV wheels/axles/tires)  as the interface between the ground and the mount and mounting cheap shocks with coil spring slipped over them to connect the subframe that the ATV axles are mounted to and the main frame the engine is mounted to? The main frame would be welded onto a  deep "box" so it can be variably weighted with sand(bags) for adjustability. 

It really is just an automotive type frame now...but in this case  the ground is  isolated from the load instead of the load isolated from the ground.

I really want this mount to be "moveable" enough so I can test for sound transmission prior to building the power house/engine room.

To help with the current proposal....here is a crude drawing. If a picture is worth 10,000 words this would be worth at least 50.