Puppeteer

Author Topic: Design of steel frame mount and vibration isolation  (Read 80389 times)

Tom

  • Hero Member
  • *****
  • Posts: 1746
  • Green power is good.
    • View Profile
Re: Design of steel frame mount and vibration isolation
« Reply #15 on: September 28, 2006, 06:29:42 PM »
G_I,

The body mounts are Urethane and very stiff. The engine will not bounce around. The stock rubber ones were fairly soft. Also they are designed to support about 1.5 tons so I think they will be more than up to the job.
Tom
2004 Ashwamegh 6/1 #217 - ST5 just over 3k hours.

Guy_Incognito

  • Sr. Member
  • ****
  • Posts: 255
  • Just a guy, incognito.
    • View Profile
Re: Design of steel frame mount and vibration isolation
« Reply #16 on: September 29, 2006, 12:13:41 AM »
xyzer - I'm just trying to get a handle on how much it might move - there's a heap of unknowns, but the bigger unknowns (like big flywheel imbalance, or the mass balance for the reciprocating bits) can be guessed at. The smaller unknowns (imbalance between the two flywheels, for example) will have to be left out of it. I just wanted an idea of whether it would jiggle or really leap. Suprisingly, it would up as a jiggle.... which makes me instantly suspicious  :D

Tom - Sorry, didn't notice that they were urethane mounts, they just looked rubber-ish in the pic! Careful with the stiffness of them - if they're too stiff, you might as well just mount it directly. Time will tell, eh? How far along are you with the engine/frame/mount?

Tom

  • Hero Member
  • *****
  • Posts: 1746
  • Green power is good.
    • View Profile
Re: Design of steel frame mount and vibration isolation
« Reply #17 on: September 29, 2006, 05:10:45 AM »
The engine is up and running on the main frame which is now bolted to a couple of redwood logs as skids. When I finish the house I'll start playing with the urethane mounts. Unless someone wants to try them first and give us a report.  ;)
Tom
2004 Ashwamegh 6/1 #217 - ST5 just over 3k hours.

mobile_bob

  • Administrator
  • Hero Member
  • *****
  • Posts: 2940
    • View Profile
Re: Design of steel frame mount and vibration isolation
« Reply #18 on: September 29, 2006, 05:42:33 AM »
i don't know anyone that can do the math on mounts for any engine, subframe system before engineering and building the subframe.

i don't believe it is possible

if one is going to go to all the trouble to follow this path, one might as well do it right.

1. start with as well a balanced engine as you can, either purchased or made that way by you if you can or by a qualified outside shop.

2. determine how large a base you need, to accomodate all the auxillery components, such as gen head, pumps etc.

3. make provisions in your measurements for stability as Jack mentions, these things can be quite top heavy

4. then draw it all up, make it as rigid as is possible, remembering that this is a subframe and not a resilient mount. Also note that anything that is bolted to the crankcase becomes part of the crankcase, not part of the motor mount so to speak. so build this subframe as rigid, resistant to flex and twist as possible. you dont want to introduce stresses back into the crankcase, most especially twisting forces that will come back from a large frame if allowed to, this will damage your engine.

5. fill it with sand, shot, lead, concrete or whatever to dampen any harmonics that "will" be in a steel structure.

**cautionary note**
remember these are very heavy engine's,  even at 6hp there is horrendous instantaneous power available, likely several hundred hp for a second or two, which is an eternity if something fails.

also the ligher the frame the more engineering will be required and the lower the lifespan of the subframe, so don't cut corners, this is probably the strongest arguement for the use of a concrete base. further...

if one is unsure of his ability to design a  movable or resilient subframe system, he should seek professional help. it is one thing to assume risk for damage to an engine, but quite another to assume risk of personal injury.

finally

the determination of which resilient mounts that will work most effectively will as stated by xyzer (if i recall) be by empirical testing and not math.
i don't see how you are going to use math to arrive at what is needed without some rather sophisticated instrumentation to measure what actually is taking place at your point of contact with the floor.
some rough math might get you in the ballpark, but i would think there will be some margin that will have to be done by testing.

bob g
otherpower.com, microcogen.info, practicalmachinist.com
(useful forums), utterpower.com for all sorts of diy info

Guy_Incognito

  • Sr. Member
  • ****
  • Posts: 255
  • Just a guy, incognito.
    • View Profile
Re: Design of steel frame mount and vibration isolation
« Reply #19 on: September 29, 2006, 07:04:27 AM »
All your points about frame design are good points bob and the best course of action to follow.

But the math isn't really that difficult for someone who does it everyday. Unfortunately, I don't do it every day, but I've been down this path a couple of times before.... pity it was years ago now.  ::)

- You've got known exciting frequencies and a fairly well guessable excitation force.
- You've a known mass of the engine and frame.
- You've a desired maximum amount of motion in the frame
- You've presumably want the minimal amount of force transmitted to the floor.

All those can be put together to get some mounts that have the right characteristics straight up, with minimal tuning and tweaking afterwards. It at least allows you to separate the wheat from the chaff when you're trying to select a mount.
The thing to remember is that for less vibration on the outside of the mounts, there's going to have to be more vibration of the engine. The stiffer the mounts, the less engine vibration, be the more vibration is passed through them to the floor.

Have a look at this chart :

(A bigger version is here : http://listerengine.com/coppermine/albums/userpics/10075/NaturalFreqIsolation.jpg )

Pick the wrong rubber mount, and worst-case, you'll be at resonance and the engine frame will tear itself away from the ground. Too stiff, you might as well just solid mount it. For proper isolation - of the type that reduces the forces on the ground, you need a mount that has a low natural frequency.

Now, have a look at this airmount :

It's also at : http://listerengine.com/coppermine/albums/userpics/10075/1X84D_Airmount.jpg , so you can read the numbers.

It's natural frequency at 40PSI is about 1.35Hz. It's carrying capacity at that pressure is about 300 pounds, so four of them will carry a 6/1 + genset + a bit extra mass fine. Looking at that chart further up , with a forced frequency of 5Hz and a natural frequency of 1.35Hz, the transmitted force through the mount is about 10%. If we have a reciprocating force (after a 50% balance factor) equivalent to a hundred pounds of force at 5Hz, that particular mount reduces that thumping on your thin concrete slab to 10 pounds. For 10Hz flywheel imbalances, it's up around 97 percent isolation.

I've thrown together some cad drawings for the frame - basically as you describe bob, but a little longer than a normal compact frame, to allow for some extra mass to help damp it a bit and also to allow me some room to possibly mount an A/C compressor or DC alternator near the generator later on. Consists of twin 3" box rails, separated by 6 3" tubes, one on each end, two at the spacing for the engine and generator. This frame will be plated with something like 1/4" steel. It's reasonably wide (wider than the flywheels) and the base is reasonably long (to allow for risers and gussets/strengthening on each end to mount the 8" high airmount supports at crankshaft height)

A question with the top plating - does it need to be welded all round? Or will solidly clamped via bolts do? Just wondering about access for tuning with sand, etc.

Those times I was down this path before? It was when I was working in a coal analysing lab on site at an open-cut mine. The lab was three floors up, in a small room.... attached to a 7 floor washplant full of screens and centrifuges and pumps , all merrily vibrating at anything from 300Hz up to a few kilohertz. Every now and then, they'd all get into sync and the whole place would shake enough to spill your coffee! I had a balance that read to 1/10000th of a gram - (0.0001g) and I needed that accuracy. Weighing things with the washplant off was fine. Weighing things with the washplant on resulted in variations of about 50 milligrams, which was bigger than some of the things I was trying to weigh! Eventually after mucking around with lead sheets, rubber mounts, springs, etc, we went with wire-rope isolators under the balance, which solved it except for when everything got in sync. Seeing as when it did that it was about one big *thump* every second, there wasn't really much we could do there. For all the other time though, it went from a 50 milligram jiggle down to a 0.3 milligram jiggle... which improved the results we got no end.

All the frequencies are the same then as they are in our challenge now - in fact some were lower (and therefore worse).
I used the same methodology then as I'm doing now, except then I had the advantage of a hired vibration analyser widget which highlighted all the frequency peaks. As we (should!) only have two large peaks at 5/10Hz, the problem here is a bit simpler.

mobile_bob

  • Administrator
  • Hero Member
  • *****
  • Posts: 2940
    • View Profile
Re: Design of steel frame mount and vibration isolation
« Reply #20 on: September 29, 2006, 07:37:29 AM »
this is all assuming the engine to be centrally located within the subframe, which it cannot be.

it will be offset to allow for the generator head, or..
even if you centrally locate the engine there will be more weight on the generator side, unless you load the subframe on the opposing side an equal amount.

this is where the simple equations become more complex, that is unequal distribution of not only dead weight, but all other factors as well.

you may end up with mounts that are of differing densities based on position, which would not be unusual, but kind of hard to calculate without some extra steps, i would think

still working with some math might get you in the ballpark, but you will have to figure more variables it would seem.

also i would think that 1200 lbs overall is quite on the light side,

engine, 750#
st head 300#
pulleys and belts 100#
subframe, at a minimum of 500# with ballast

total weight,, 1550# is probably more the reality as a minimum

engine offset 1/3  would give approx 1000 # divided by two, or 500# per on the engine end and 250# per on the genhead end as a very rough guestimate

then whatever this ratio of offset is, will certainly play a role in magnitude of frequency if not an altered frequency, between engine end and genhead end.

and this all assumes some other wierd effects from the armonics 60hz at 1800rpm of the genhead, and any other harmonics added from the belt drives

then there are the changing frequency of the vibration as the engine speeds up and slows down that will have to be addressed with presumably bump stops or some other form of limitation

i suppose a person could do all the math, but he will need all the inputs first, weight distribution, placements etc. to do so.

me i would probably do some math to get close and figure to hopefully get in the ballpark, then trial and error to tune to fit.

bob g
otherpower.com, microcogen.info, practicalmachinist.com
(useful forums), utterpower.com for all sorts of diy info

Guy_Incognito

  • Sr. Member
  • ****
  • Posts: 255
  • Just a guy, incognito.
    • View Profile
Re: Design of steel frame mount and vibration isolation
« Reply #21 on: September 29, 2006, 01:28:59 PM »
" you may end up with mounts that are of differing densities based on position, which would not be unusual, but kind of hard to calculate without some extra steps, i would think"

Aha! You're right about the distribution of weight of course. But this is the beauty of airmounts. You inflate them to a particular height to match the load that you want to carry, and they hold their spring rate relatively well across a wide pressure range. So the two nearest the engine can be inflated to, say, 60psi , and the other two near the generator to 40psi to level up the frame. Those mounts I pointed out are particularly interesting as they can carry up to 600lbs each. So if more mass is needed, you can boost the pressure to keep the operating height correct. As long as the spring rate stays pretty much the same across the 4 mounts, things should be ok.

With regards to passing through the resonant points:
My cunning plan is to set the frame up so that at the correct mount height of 8", there is only about 1/2" clearance from  some bump stops on the bottom of the frame to the floor. Actually it'll be set while running loaded to have the minimum clearance after you take into account the "normal" amount of jiggling, which was why I'm trying to suss out roughly how much jiggling might occur....
Once that height's been set, the frame will hopefully (!) just bounce a little on the bump stops as you spin up. With the airmount resonance around 1.3Hz, bouncing will likely occur as you pass through 78 RPM for any flywheel imbalance and about 160RPM for the reciprocating imbalance. Hopefully at those two relatively low speeds, the forces involved aren't large enough to cause issue, seeing it's a velocity-squared thing. Hopefully  :D

"total weight,, 1550# is probably more the reality as a minimum"

You reckon it'll wind up that heavy? I was working on someone's basic estimate of weight, but anyway, the heavier the better and the mounts are good for about 2800 lbs in total at a stretch. And I work in metric units generally , so I wouldn't know a thousand pounds if it dropped on me like a ton of bricks.  ;)

Regarding unusual harmonics  - This is of some concern for stresses in the engine and frame, but not much concern for the mounts and jiggling - if you can stop a 5Hz pulse reasonably well, you can block anything higher with ease. Hopefully the loose sand/gravel/ball bearings/lead shot or even....... concrete will dampen whatever crops up.  There's that word again - "hopefully"! I hate using that word! I'd rather say, "well, all the math says odds are good it'll turn out to within 10% of what we wanted." That's a much more positive statement.  ;)

I do have access to a relatively decent accelerometer if I ask nicely, so I might be able to find out about any higher harmonics for certain. Surely though we can come up with something on the cheap so that those without the gear can use some sort of simple method (like the coffee-cup-of-water) to see if there's anything nasty lurking in the higher frequencies. Then they can take action before they get to some disastrous harmonic-related crankshaft-snapping event.  :)

listeroidsusa

  • Guest
Re: Design of steel frame mount and vibration isolation
« Reply #22 on: September 29, 2006, 02:29:56 PM »
It seems to me that everyone is trying to make something simple extremely complicated. I use a 4" channel frame with 3 4" channel crosspieces with the frames I build. The key here is to build it straight and level so the engine bolts down solidly. On mine I simply used a triangular mounting system since a 3 leg "stool" is stable and sometimes with the variables of engine, base, and floor the 4 point mounting is able to rock slightly. ANY available motion will be amplified. On the 3 point mounts, 2 at the engine and one on the generator end, a glass of water can easily be balanced on the fuel tank. Before, with it solidly mounted to the concrete floor,(15" thick) the anchor bolts would tend to loosen over time and the engine would shift slightly when it fired. With the base sitting on 3 simple pads of 1/4" rubber flat belting there is absolutely no movement or vibration after even a year of running.

Imagine the toy that has the line of steel balls hanging on strings. When you pull one ball back and let it hit the other balls the force is transmitted through the other balls to the end ball which then swings out and returns the energy back to the first ball through the stationary balls. Now take this toy, put the balls in motion and put a wide rubber band between any ball and see how the transfer of the impact is eliminated as the rubber absorbs the kinetic energy. This is the same principle that my mount works on. Many people have seen the engine running  in my shop and it is completely motionless. It works and that's good enough for me. I too, can quote formulas, and mony other ways to theoretically explain this, but why?, IT WORKS!

Mike Montieth
Listeroids/USA

Guy_Incognito

  • Sr. Member
  • ****
  • Posts: 255
  • Just a guy, incognito.
    • View Profile
Re: Design of steel frame mount and vibration isolation
« Reply #23 on: September 30, 2006, 12:59:04 AM »
True Mike - what you want is what works for you.

The problem seems to be that everyone has had vastly different experiences with engines and their balance and their mounting.
So just as Lister said, "Ah, the hell with it, just mount it to a two-ton block'o'crete  :D ", we're heading towards a frame design that is guaranteed to work under all conditions. Which is a good thing - as long as you're not too worried about fabrication cost - but it leads to overkill in some situations.

My personal goal is minimum ground vibration - absolute minimum. It will be mounted in an area that's basically a tree-hugging-hippy paradise   :) so, whilst they all understand the need for generators, they'd rather that they were not seen and not heard and not felt. Compounding the problem is my house is mounted on long poles, which might make an excellent resonator at the frequencies we're talking about. Further compounding the problem is the soil stability on the slope where it's going, and the 6 months of the year where the ground is saturated.

Now , my goal is overkill for some people. Some are just putting a bit of rubber in - like you. Some are using bushings and what-not. What I personally want is something that's got a bit of engineering behind it, so I can get some plausible numbers for isolation and shaking, put some decent hours on it and not have something crack a weld or tear loose. Just like all the ME's who design pump mounts, centrifuges and vibratory screens.

But having said all that, the more people who tell us about their mounts, what they did and how they work, the better! We might be able to trim down on the overkill a little bit.

Haven't had much issue with flexing or cracking on your frames? How many hours do you reckon they've done in total? All sizes of engines on them? Not being prickly about it - just want to know  :)

gpkull

  • Full Member
  • ***
  • Posts: 204
    • View Profile
Re: Design of steel frame mount and vibration isolation
« Reply #24 on: September 30, 2006, 02:24:53 AM »
i made my subframe out  of 2X4X1/4 rect tubing. had it on HD casters. tryied removing the casters and replacing w/ vibration dampners . they look like the bumpers at loading docks so you dont tear up their concrete while backing up. each step of the way got better i havent mentoned all of them . better i mean smoother. i pored my block and will report my results soon. i realize that the ground will still have the thump even though the engine is attached to 2500 lbs. of crete. i spoke vibration to a freind they said put some tires under it. i think this would only sqash tires and make top very unstable. i amm going to bolt car tires w/ rims around my project. i figure the crete will satisfy the roid and the aired up tires will soak up most if not all the thump.   will post asap it just seems that for every step forward its two back .


                                                                      happy listering

                                                                                   

mobile_bob

  • Administrator
  • Hero Member
  • *****
  • Posts: 2940
    • View Profile
Re: Design of steel frame mount and vibration isolation
« Reply #25 on: September 30, 2006, 02:25:40 AM »
i follow several reasonings here with this topic

basically there are many outcomes or needs that are specific to each individual and their equipment.

it should be assumed that to start with one should have a sufficiently rigid and safe subframe to start with.
so the discussion basically comes back to how to calculate what mounts to use.

personally i don't like to calculate anymore than i have to, mainly because of a few unknown parameters, or consequences of a design. and i don't have the computational energy or the needed test equipment to get to a predetermined optimum system. i suspect i am not in the minority on this point,,  so....

i personally like the empirical testing to get to optimum for my use, but the problem is with cost of perhaps several different resilient mounts, air mounts would simplify the need for various mounts but add complexity and parts to fail that i personally would not invite. i realize others may accept this as being ok by them.

what i am thinking is what kind of resilient mount can i use that is cheap, easily modified, or altered to tune to optimize my needs.

what i come up with as a possible candidate is dock bumpers, particularly the rectangular units that have a pyrimidal (sp, my spelling sucks).

i could start with a 4 X10 bumper that is 3 inches tall, assuming this dimension is more than is needed to support the weight on a particular corner.

so for example if the weight distribution is 2/3 toward the engine end and 1/3 to the genhead end i would need to cut off 3.33 inches of the length of the genhead bumpers to start with.

next i could run the engine and determine how stiff the suspension is, presumably it would be too stiff, so i could simply remove a half inch in length of each , in steps , retest and continue until i reached the point that the supports would be too soft, then back up a half inch with new bumpers.

the use of pyrimidal shape allows for a progressive rate, so it would stand to reason that while running at speed and load the movement would be small, and as the engine spun up or down and began to transmit more movement the shape of the bumper would allow for stiffer support and limit movement.

these dock bumpers are widely available, cheap as dirt in most cases, and easy to work with. some come with cast in bolt studs other with cast in nuts, some with just holes, or drill to suit.

thru empirical testing i think i could come up with a good result, cheaper and with less calculation.

are they as sexy as air mounts? nope,,, but pretty simple, and that has an elegance all its own.

admittedly this is only one way of many, and not necessarily what i would suggest others to do.

but that is how i am going to do it, and i am stickin to my story :)

bob g
otherpower.com, microcogen.info, practicalmachinist.com
(useful forums), utterpower.com for all sorts of diy info

Guy_Incognito

  • Sr. Member
  • ****
  • Posts: 255
  • Just a guy, incognito.
    • View Profile
Re: Design of steel frame mount and vibration isolation
« Reply #26 on: September 30, 2006, 03:54:47 AM »
Hmmm. Those dock mounts actually sound like a pretty good sort of catch-all setup.

The key - as you state  - is easy tuneability. Whether it's by adding mass or air or trimming mounts, everyone's going to need something slightly different.

I've got a chart here somewhere of static deflection vs frequency response for rubber mounts - this'll tell you at least what kind of general mount you'll need and what not to try.

I'll post it over at coppermine if I can find it.

Edit:

http://listerengine.com/coppermine/albums/userpics/10075/isolating_p1.jpg
http://listerengine.com/coppermine/albums/userpics/10075/isolating_p2.jpg
http://listerengine.com/coppermine/albums/userpics/10075/isolating_p3.jpg
« Last Edit: September 30, 2006, 05:02:52 AM by Guy_Incognito »

gpkull

  • Full Member
  • ***
  • Posts: 204
    • View Profile
Re: Design of steel frame mount and vibration isolation
« Reply #27 on: September 30, 2006, 05:50:35 PM »
bob  the  bumpers are what i tried i think 3$ each.   3x6 on the big end  2.75 x5.5 on the small end standing 3.5 inches tall. yes they have holes drilled alredy. my experiment was a hit in the dark. dont know what density or any thing else. they did dampen . a step in the right direction but not even close to desired outcome. i feel that the approach was good but was still lacking mass to asorb the thump. i now have a macth set of tire chucks one for each wheel       carry on smartly

Guy_Incognito

  • Sr. Member
  • ****
  • Posts: 255
  • Just a guy, incognito.
    • View Profile
Re: Design of steel frame mount and vibration isolation
« Reply #28 on: October 01, 2006, 12:55:38 AM »
Aired up tires might be the go, gkpull.
If you go this way, can you give some feedback as follows?

Looking at the chart here :
http://listerengine.com/coppermine/albums/userpics/10075/isolating_p3.jpg
Static loaded deflection of your mount is on the x-axis, shaking frequency is in Hz on the Y axis.

So looking at the chart - in theory, if you inflate (or deflate) your tyres to a point where they drop at least 30mm when weight's on them, you'll get 50% isolation of a 5Hz thump. In theory  ;)

If it bucks around all over the place with the tires, you need to deflate them a bit more. If it gets worse, you're probably approaching resonance - deflate them a lot more and try again. Or if you've got tires with tubes, you could start from flat and inflate until you get something comfortable.



Guy_Incognito

  • Sr. Member
  • ****
  • Posts: 255
  • Just a guy, incognito.
    • View Profile
Re: Design of steel frame mount and vibration isolation
« Reply #29 on: October 02, 2006, 11:01:12 AM »
For those of you that have balanced your engines or have got some bits lying about -
Can anyone tell me the typical weight of a 6/1 piston and gudgeon pin?
If you balanced it, what mass you used as a bobweight? The bobweight is generally half the mass of the conrod/cap/bearings,etc isn't it? But not the gudgeon pin and piston?

Going to plug it into an equation to get an idea of the amount of motion of the engine and frame at speed caused by a 50% balance factor. Have searched around a bit, but I don't think my keywords are right. Basically it will condense down to the mass of the piston+gudgeon pin+ 1/2 bobweight (if correctly balanced) going up and down with a stroke of the listeroid.

In some lecture notes that I've got, there's an example that's pretty much exactly what we have - a 500kg single cylinder engine, speed 600RPM, resilient mounts, 10kg of reciprocating weight, a 200mm stroke. The resulting dynamic movement they get with their particular type of mount is 2.2mm.

If I alter that to a 136mm stroke , at 650RPM, with the mounts I have, I get a movement of 1.4mm.

But all that depends on the correct mass of the piston and gudgeon pin, so anyone got one handy?
Just after a general figure to I can get an idea of ballpark movement from reciprocating bits.

The only real 'known unknown' after this one will be reaction from torque of from the power pulses.