Author Topic: Low buck vibration isolation mount  (Read 18912 times)

biobill

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Low buck vibration isolation mount
« on: November 07, 2006, 11:37:14 PM »
  I'm thrilled. Just test ran a 6/1 on an experimental mount in my basement. Couldn't detect any vibration transmitted to the floor. A glass of water placed 4" from mount had one, and only one, tiny ripple at start up and shut down. Deflection at head seems reasonable for exhaust and plumbing longevity.
  Mount consists of 1600lb highly reinforced concrete riding on 10X20 truck tires which have been cut and reassembled in an apropriate shape. Provisions were made to adjust rigidity in case my guesstimates were way off but so far so good. Still lots of plumbing to do before extended testing. Have photos but they're on film. Will have to upgrade my technical skill before i can post them or get some help from a local enthusiast. Right Geno?
                                             Bill
Off grid since 1990
6/1 Metro DI living in basement, cogen
6/1 Metro IDI running barn & biodiesel processer
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Geno

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Re: Low buck vibration isolation mount
« Reply #1 on: November 08, 2006, 12:21:40 AM »
Will have to upgrade my technical skill before i can post them or get some help from a local enthusiast. Right Geno?
                                             Bill

You got it.
Geno

Guy_Incognito

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Re: Low buck vibration isolation mount
« Reply #2 on: November 08, 2006, 12:36:36 AM »
I'd be interested to see how much static deflection in lbs-inch you've got there -  how much do you weigh and how much does it deflect in inches when you hop on the frame?

So all up you've got about 2500 lbs on it with engine,etc?

biobill

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Re: Low buck vibration isolation mount
« Reply #3 on: November 08, 2006, 07:49:54 AM »
Guy
 I'm not an engineer so you'd have to give me some details on how to determine the static deflection. The mount is oval shaped, 39"X63". It will move more if I jump on an end than in the middle. Clue me in and I'll be happy to measure, might even learn something. I'm afraid that I used more gut feeling than science in it's construction.

 What I did was cut cross section samples from identical tires. These are old bias ply and are probably stiffer in cross section than a radial. The cross section of the tread is thick on the edge and thinner in the center. When compressed the center tends to bulge out. Stood my 200lbs on different sized sections with the bead suported and checked the preload bulge. A piece about 20" long seemed just about right and the math was easy, even for me. 10lb/in, tire circumferance 130", two tires- 2600lb. Right in the ballpark. I wasn't looking for dead nuts accuracy because I had a plan for tuning to the load. If the mount was too stiff, overall or in spots I figured I could soften it up by boring holes in the tire tread. If it's too soft, or if it fatigues, I have seven pipes spread around the mount, that go from the tire cavity to the surface of the concrete. Through these I could introduce shreaded rubber, sand, packing peanuts ::) or whatever to different parts of the cushion.

                                                                       Bill
« Last Edit: November 08, 2006, 07:52:21 AM by biobill »
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Guy_Incognito

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Re: Low buck vibration isolation mount
« Reply #4 on: November 08, 2006, 11:15:11 AM »
Nothing tricky, just measure the height of the engine/frame, put your weight on it and measure the height again with the extra weight.

Eg. engine and frame sits at 20" - you add another 200lbs on top and it moves down 1/4".
So if it moves 1/4" with 200lbs on it, its spring rate at that particular height is 4*200 = 800lbs per inch.

In theory, you can work backwards from that with the mass of the engine/frame to figure out at what speed it might jiggle excessively and how much force gets transmitted through your mounts to the ground throughout the rev range.

I'd like to see some photos too if you can get them online somehow  :)


biobill

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Re: Low buck vibration isolation mount
« Reply #5 on: November 08, 2006, 06:03:21 PM »
   Guy
 Looks like it only moves about 1/16" and that's with me on the end nearest the measurement. Can't get to the center as it's already occupied. And I'm not moving that monster again.
  I expect this number to change. When I was pouring the concrete I used shreaded rubber (garden mulch, wonderful stuff) covered with a membrane to fill the voids with the expectation that it would sift into the hollows and not carry any load.
  I'm not sure that thinking of the tires as springs is helpful though, as they allow and resist movement in all planes. Seems to me that they act more like a spring/dampener combination. Also, due to the shape of the tread cross section they are quite progressive.

Quote from: Guy_Incognito
  In theory you can work backwards from that with the mass of of the engine/frame to figure out at what speed it might giggle excessively...

  When I was running this engine, Metro 6/1 DI, on a pallet it was quite animated during start up and shutdown. Not very smooth while at opperating speeds either compared to my new engine. It would definately benifit from some balancing. On this mount though it's much, much smoother. Could only detect vibes from run up/down by a single ripple in a glass of water. while running the water is absolutely still. Hard to wiggle twice your weight in concrete I guess. The frame is really lame. Poor design, sloppy welds, skimpy materials and out of true. It came with the genset and I'll replace it as soon as time permits, or it breaks. Needs close watching.
  Pic's of construction details are still in my disposable camera. Will get developed next time I head down to the "bright lights". Geno indicated that he would help me with getting them posted.
Off grid since 1990
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Guy_Incognito

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Re: Low buck vibration isolation mount
« Reply #6 on: November 08, 2006, 10:04:11 PM »
Quote
I'm not sure that thinking of the tires as springs is helpful though, as they allow and resist movement in all planes. Seems to me that they act more like a spring/dampener combination. Also, due to the shape of the tread cross section they are quite progressive.

Yeah, the dampening alters the natural frequency a little, but it's still generally pretty close ( within about 50rpm with the mass of a typical lister setup). So it moved about 1/16" , mostly with the extra weight on one end? About 3500 lbs per inch is a pretty good sort of ballpark figure then. I put in an average damping factor for rubber(ish) mounts of about 0.2. As for progressiveness, that why I asked you to measure its movement when it's already loaded with the engine and concrete - as long as you use a reasonably sized weight at that point, you can get an ok idea of the spring rate at that particular ride height.

Plugging those numbers into a spreadsheet, I get jiggly points of about 110 RPM (firing pulses, probably most noticeable on startup) / and 220 RPM (imbalance pulses, probably most noticeable when spinning down on decomp.) and at operating speed about an 80% reduction in forces transmitted to the ground, which is pretty good.

Sounds like a pretty good setup and - as with pretty much everything - if it works for you, well, that's generally all thats necessary  ;)

biobill

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Re: Low buck vibration isolation mount
« Reply #7 on: November 09, 2006, 12:53:41 PM »
OK, it's starting to sink in. My minds been spinning(slow, like the 'roid) wondering how the hell this guy halfway around the world can predict, pretty darn accurately as near as I can tell without a tach, the behavior of my set and mount. He's never seen it, has virtually no details on it's construction, doesn't know the state of balance in the engine, and yet, he nailed it. Black magic?
  As I was playing guitar last night it finally clicked. It's about resonance and harmonics. Frequency, just like you said only a little lower than the musical range. Did I get it?
  Guy, if you've got the time, I'd love to know what to expect as the static deflection declines as I expect it to when the shreaded rubber stops carrying it's load.

                                                     Thanks, Bill
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Guy_Incognito

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Re: Low buck vibration isolation mount
« Reply #8 on: November 10, 2006, 02:01:12 AM »
Quote
Did I get it?

You've pretty much got it. It's just at lower frequencies than what you see on your guitar. Unfortunately it's also at much lower frequencies than what you see in "normal" engines, so there's no perfect mounts on the shelf ready to go for us like there is with your average V8 diesel.

Resilient mounts have certainly been hotly debated here. Have at look at this thread ,which is the one I started :
http://listerengine.com/smf/index.php?topic=1097.0

Particularly, this post:
http://listerengine.com/smf/index.php?topic=1097.msg15117#msg15117

There's a few more threads over in General discussion as well.

My personal opinion is that the theory applies quite well to much bigger and badder things than a humble lister. It's also handy to at least get ballpark numbers before going to all the time and expense of trying something that doesn't work. What you don't want to do is to go to all the time/effort/money and wind up with a spring/mass combo that gets tuned to exactly 650RPM. That would be bad.

With your setup, you'll find that the resonant points will drift upwards a little as the rubber settles. Your resonant points are at a low enough RPM that it won't affect you to any great extent as you pass through that RPM pretty quickly when spinning up or down. You'll get a bit less vibration isolation at running speed, but probably not enough to worry you.

Cheers,
Guy_I

biobill

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Re: Low buck vibration isolation mount
« Reply #9 on: November 10, 2006, 05:28:48 AM »
  Sure wish I'd read that before Guy. Talk about reinventing the wheel... I'd been avoiding the "mounting" diiscussions - afraid I might catch a stray round ;). You've done some good work that will be a benefit to many.
                                                                  Bill
Off grid since 1990
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biobill

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Re: Low buck vibration isolation mount
« Reply #10 on: November 13, 2006, 03:45:54 PM »
  Finally got some pictures.

 The first shows how the tires were assembled. Used 3/8" threaded rod, top and bottem to hold them together



  This shows the rubber roofing which seals the bottem of the tire in case I want to add something later to change the dampening characteristics. The tires will be bedded onto the rubber mat on the floor and held in place with 1/2" expanding anchors.




  The pieces leftover pieces of tire were cut to fit the holes and secured so the tread would stay perpendicular to the floor. then the hollows were filled with shreaded rubber and covered with membrane to support the concrete while it hardened. The form is 1/4" luan which bends quite easily when wet.



  The vertical pipes fit into holes drilled in the tires and allow me to add material in the future if needed, They also locate the concrete on the tires. There are two 5/8" threaded rods which could serve as jacking points in the future. I had been thinking shock mounts too, but they won't be necessary. There will be another layer of reinforcing added 6" above the one shown. The concrete acts as a bridge (with a jumping jack pounding it) in some respects and needs to be really strong.



  Twenty 80# bags later.  I used fiber reinforced mix with an extra scoop of portland per bag. The wood frame on top held my anchor bolts in their proper places. I had origionally planned to strip the entire frame out and fill the channels left in the concrete with lead to bed the frame on but ended up going on top of the wood. Seems fine so far.



  A day later. BTW, wvo makes a fine release agent in a pinch



 The engine hoist I'd hoped to use was not up to the task. So....1/2" at a time....



  Getting there. The fuel tank is suspended on a spring with index marks to show how full it is. I was surprised at how quiet it was inside, except for that damn idler gear. Outside, with no muffler, it sounds like a steam engine. Thats without a load, still have some plumbing and wireing to do before I can load it up.



 Side view



  Many thanks to Geno for the hours he spent edjucating me in computer operations and the use of his web site.

                                   Bill
Off grid since 1990
6/1 Metro DI living in basement, cogen
6/1 Metro IDI running barn & biodiesel processer
VW 1.6 diesels all over the place
Isuzu Boxtruck, Ford Backhoe, all running on biodiesel
Needs diesel lawnmower & chainsaw

rmchambers

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Re: Low buck vibration isolation mount
« Reply #11 on: November 13, 2006, 06:28:51 PM »
You know what they say about a picture being worth 1000 words?  well these pictures did it for me.

Thanks to you and Geno for getting them on the page, the rest of what you said makes a lot more sense now.  I had visions of the tires being cut and the flat pieces being sandwiched under the concrete block.

I'm interested to hear how the thing breaks in as it runs and if the transmission properties of the tires change as they get compressed.

Robert

mobile_bob

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Re: Low buck vibration isolation mount
« Reply #12 on: November 14, 2006, 06:21:13 AM »
 "I'd been avoiding the "mounting" diiscussions - afraid I might catch a stray round ."

what and miss all the fun?

as momma always said "its always fun till somebody loses an eye"

:)

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

biobill

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Re: Low buck vibration isolation mount
« Reply #13 on: November 18, 2006, 10:47:40 PM »
 ;D  Runnin'   making power and heat.
  I've got some mechanical noises to suppress but my son Liam, age 6, described them as "not bad". Vibration is a complete non issue. The cast iron radiator puts out a useful amount of heat upstairs and the long exhaust sheds quite a bit in the basement. A more efficient exhaust heat reclaimer is on TBDIF status
  Ran into a snag with my head (discussed:DIY, generators, indian head ) but I see a few transformers on Ebay that I hope will take care of that problem.
                                                             Bill
Off grid since 1990
6/1 Metro DI living in basement, cogen
6/1 Metro IDI running barn & biodiesel processer
VW 1.6 diesels all over the place
Isuzu Boxtruck, Ford Backhoe, all running on biodiesel
Needs diesel lawnmower & chainsaw

dkwflight

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Re: Low buck vibration isolation mount
« Reply #14 on: November 18, 2006, 11:13:04 PM »
Excelent job!
Dennis
28/2 powersolutions JKSon -20k gen head
Still in devlopment for 24/7 operation, 77 hours running time