Author Topic: Design of steel frame mount and vibration isolation  (Read 80390 times)

Guy_Incognito

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Design of steel frame mount and vibration isolation
« on: September 27, 2006, 03:11:42 AM »
Alright, I've started this topic with the goal of designing a steel frame, with isolators of some sort to prevent vibration from being sent from the engine to the Rest Of The World.

First up :
Please leave all the arguments about why in the other threads about the place. By all means, if you've got something that to say about how to properly isolate the engine with resilient mounts, speak up. If you wish to discuss alternatives to the steel frame/resilient mount, or wish to debate general engine life running on mounts, take it elsewhere.
Specific cases of engine longevity issues (eg. damage due to undampened resonance at particular frequencies) are welcomed, as long as you are willing to help the attempt to come up with a solution. A solution that doesn't involve concrete.  :D


Might be handy to skim through a vibration isolation primer to get a quick idea of the concepts. It's at :
http://www.wtc.net.cn/Primer_Vibr_Isol.pdf

Ok, to start off:

You've got an engine that's been relatively well balanced. It doesn't chase you around the shed when its bolted to a couple of wooden railway sleepers. Much.

Pretend that you live in a treehouse, or a small boat. You need the engine, there's no alternative to a 6/1 and an extra 2 tons of concrete is not physically possible.

Also pretend that your engine is mounted next to and powers an extremely sensitive set of scales that you need to be as wobble free as possible.

Don't poo-pooh these two requirements, I've been in a situation very close to it  :D

Let's discuss the general specs and requirements:

- A listeroid 6/1 running at 600-650RPM with two main forcing frequencies, a 5Hz power pulse from firing once every two revolutions and a 10Hz pulse from flywheel imbalance/reciprocating forces.

- It weighs approximately 500kg, once you take into account a decent frame and possibly a generator as well.

- You want the maximum amount of vibration isolation.

- You'd prefer if it didn't oscillate a great deal while sitting on those mounts. I'll arbitrarily set a limit of 10mm worst-case. More is briefly allowable on spin-up/down.

From all this, we should be able to design a 'standard' resilient mount that reduces vibration well and can be made relatively easily with reasonably available bits. It should have enough info about it that you can make an informed choice as to substitutes or alterations.

So, what would you use as the resilient mount? Frame design? I've got a pretty good general idea - see my "6/1 and generator setup" thread, amongst all the arguments. I'm interested to hear what other people would use. Sucesses, failures, etc.
Outline your resilient mount setup - what it's made of, how well it works. If you have an idea of why it worked or not, let us know and we'll attempt to back it up with the proper calcs as to why it was good or bad.


mobile_bob

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Re: Design of steel frame mount and vibration isolation
« Reply #1 on: September 27, 2006, 03:56:05 AM »
boy this should prove to be very interesting :)

some observations
before the resilient mounts are discussed the following will have to be discussed and determined

1. we will have to start with the engine mounting area of the frame, it will have to be very rigid, it will have to be viewed as a part of the engine case itself and as such add structural support to the engine crankcase as its primary purpose

2. the math is going to have to be done to determine how to move the torque, vibrations, and centers out to the extensions of the center support.

3. mounts for auxillary components, such as generators will have to be of secondary importance and not a primary concern of the design, this is not to say that one should not provide room to mount them sufficiently


primary members should be rigid to up and down forces, twisting will be taken care of in other ways

the box section should incorporate round crossmember, to aid in rigidity and stiffen the side rails from twist.

the center section where the engine is to be bolted will have to be plated over the rails and over the round members, both top and bottom, forming a torque box. then bolt bosses run thru the plates and preferably thru the round members and welded in.  the engine will then need to be securely bolted down with careful attention to shimming any "soft" foot.

an example might be a structure made up of two channel irons of perhaps 6" cross section, with 4 round crossmember welded in place, two under the motor mounts and two at the ends, and the whole decked top and bottom with 1/4" plate
and to make it more dead and resistant to harmonics, fill it with concrete!

then pick your poison on resilient mounts.
my preference would be to use conical  rubber dock bumpers, as the various frequencies would seem to me to work the rubber in differing cross sections.

just my thinking, perhaps others may follow

yes this should be fun...

Guy are you there? :)

bob g
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Guy_Incognito

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Re: Design of steel frame mount and vibration isolation
« Reply #2 on: September 27, 2006, 04:20:32 AM »
Looks like you're onto it already bob!

"an example might be a structure made up of two channel irons of perhaps 6" cross section, with 4 round crossmember welded in place, two under the motor mounts and two at the ends, and the whole decked top and bottom with 1/4" plate
and to make it more dead and resistant to harmonics, fill it with concrete!"

I was thinking along the same lines with something like thick wall 3" box section for everything (only because I have some) and 3/8" plate on both sides under the engine (again, it's at hand). And perhaps a moderate amount of loose sand to assist in damping. I was also looking at raising the mount points at each end of the frame up to near-crankshaft height - that is, a U-shaped frame - mainly as the low-Hz airspring mounts I was looking at run at a 8" height, but also to help a little with any CG issues. There'd be small bump stops under the frame for any excessive movement when ramping through the resonant point of the mounts. 

The airsprings allow for some ideas with portability as they can deflate a few inches, so you could , for example , affix wheels to the frame proper and then deflate the mounts so the frame lowers onto the wheels. The relatively lightweight base plate for the mounts could then be pulled up off the ground a smidge with a simple cam-action lever+chain on each end.  The whole lot is then able to be wheeled about to a new position, the base plate lowered again and the airmounts reinflated to take the weight again.

Anyway, I'm posting this at work and lunch is over, so I'd better go look busy.  ;)

GIII

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Re: Design of steel frame mount and vibration isolation
« Reply #3 on: September 27, 2006, 04:31:25 AM »
I know of an engine that was mounted on relatively soft mounts that gyrated crazily, several inches, during spool up and coast down.  Perhaps some dampers of shock absorbers would be useful with soft mounts.

George

Tom

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Re: Design of steel frame mount and vibration isolation
« Reply #4 on: September 27, 2006, 04:55:09 AM »
My frame is currently 2 lengths 4" channel iron welded to 4 2x2 bars of 1/4 box tube. I plan to use urethane Scout II body mounts to isolate the frame from a 3" channel iron sub frame mounted to a concrete slab/block.

You can't tell from the pictures, but the mounts are 2 piece simply go through a hole to clamp it down. My plan is to place both channels leg down. Weld studs on the 3" base, place large puck over stud. Place large hole in main frame over puck and put the small puck and washer on top. The system has a steel tube down the center that the acts as a spacer and allow the body/main frame to move.

I know this is lousy description, but I think it will work well and not cost to much as these kits can be had for $75.
Tom
2004 Ashwamegh 6/1 #217 - ST5 just over 3k hours.

mobile_bob

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Re: Design of steel frame mount and vibration isolation
« Reply #5 on: September 27, 2006, 05:59:40 AM »
remember guys,, the round section (pipe) crossmembers are of paramount importance.

the use of round crossmember will keep the two side rails parallel to each other and will make for a unit that is very resistant to twist, i cannot stress the importance of this component.

the use of any other type of crossmember while keeping the rails equal distant to one another will not mitigate the twisting component, no other cross section is as effective. do not use channel iron, or box tubeing they will twist under stress.
use the largest round tube (pipe) that will fit and still allow the top and bottom plates to lay flat on the side rails.

the round crossmember keep the rails from twisting, the top and bottom plates keep the rails from flexing in the middle.

the addition of concrete or even sand will aid in deadening the harmonics, which in my opinion is also important

bob g
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Guy_Incognito

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Re: Design of steel frame mount and vibration isolation
« Reply #6 on: September 27, 2006, 08:17:52 AM »
Bob - Well, if we're going to build a frame proper, then tube it is  :)

GIII - If you have relatively soft mounts for isolation, you're right, you probably will have a whole lot of bucking about. But you can lessen this to a fair degree by using bump stops on the frame that are just outside the normal vibration limit. They generally won't even get bumped that much when spinning up and down. At a particular RPM, there's a resonance that only takes a few pulses for things to really buck about, just like pushing someone on a swing. If you give them a small push at the right time, they can fly high. But put a bump stop just at the limit of motion of the swingback from that first push and you'll never get them any higher and there won't be too much bumping going on either.

Tom - How stiff are those mounts? I presume that , as body mounts, they were originally spec'd to isolate higher frequencies (eg road/tyre hum or engine/transmission vibration). It'd be an unlucky coincidence, but they might flex at the 5/10Hz resonant frequencies, giving you lots of motion. You might be able to get around this by loosening or tightening the mounts a bit though to tune it out if necessary. I'd be interested to see how they go.

From what I've read, if you want true isolation (as opposed to damping, like weighting with a concrete block, or placing it in sand/gravel), you need a mount that has a natural resonance of at least 1/3 the frequency of the vibrations that your are trying to isolate. For, for example, if those Scout body mounts were tuned to isolate you from common V8 engine vibration at say, 1500rpm, they'd have to be naturally resonant at around 30 Hz. Putting those mounts on a engine with 5/10Hz pulses and you'll find that the pulses are too 'slow' for the mounts to catch and isolate and they'll continue outwards past the mounts. So in that case, they'd dampen the movement of your engine (so it doesn't buck around), but they'd pass the vibration on.

At least, that's what I think would happen , but I've only had a crash course in this stuff over the last week.  :D


Andre Blanchard

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Re: Design of steel frame mount and vibration isolation
« Reply #7 on: September 27, 2006, 03:56:11 PM »
Mount engine on a very rigid base.
Put a counter shaft on each side of the engine.
Gear (sprockets, timing belt, etc.) the shafts to the crank with a ratio of -1.
Put balance weights and a flywheel on each of the shafts and modify the balance weight on the crank.

« Last Edit: September 27, 2006, 04:02:20 PM by Andre Blanchard »
______________
Andre' B

xyzer

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Re: Design of steel frame mount and vibration isolation
« Reply #8 on: September 27, 2006, 04:01:22 PM »
Hmmm.....I don't know if it would not mess up the scales on a boat if it was a flimsy wood deck. But this is my frame...all heavy angle iron you can see my rubber mounts if you look close. BALANCE is the most important issue IMO! I have found you can number these things to death and you will get hit in the face with other problems....Mine works good...it looks different than my Cadd drawing changes came about in the evolution. The mounts were a McMaster Carr item.  



No ton-o-concrete....I was shakin more than the Listeroid!........http://video.google.com/videoplay?docid=-9194771268947755783
« Last Edit: September 27, 2006, 06:08:41 PM by xyzer »
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Eco Diesel

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Re: Design of steel frame mount and vibration isolation
« Reply #9 on: September 27, 2006, 04:35:04 PM »
I like where this thread is heading because if portability is to be at all considered the ton of concrete just doesn't fit.Although I do believe 100% what Guy F. says about why concrete mounting  is the way to go! I have just recently started on my skid frame/mount for a 6/1 to use with alternator and water pump. I have started off with 2 frame rails that are 6"x6"x1/4" wall tubing. These rails are cut at each end on an angle and enclosed to allow dragging the skid forwards or backwards. I used 3" x1/4"wall round tubing to hold each frame rail parallel. (glad to hear I used the proper stuff round that is) The frame rails were drilled at each end to allow the round tubing to pass all the way through both sides of the 6x6 and welded on both sides of the 6x6. Now, sitting on top of the 6x6 rails are 2 - 3"x3"x1/4" wall square tubing cross members for which the engine will sit on. These 3x3 members were machined so as to not just sit on top of the rails but are shaped to drop in between the fraim rails and snuggly fit onto the inside radiuses of the 6x6 tubing. I am hoping this will help strengthen the whole assembly and to help with weld fatigue from dragging it across a field over and over. I am designing the frame this way because I want to be able to either drag to a different location or to be able to mount it semi permanently. This 6x6 tube sections of the skid frame will then be filled with either steel shot (same stuff I use in my dead blow hammers or that is used in blasting rail tankers or to use scrapp punchings from cnc punching of holes etc) I am hoping this will not only take the ringing and vibration out of the frame but to as well make it heavy as hell. It should end up being a dampened frame if all goes as planned. When this skid frame is to be semi permanently mounted it will be placed upon 3 - 3"x3"x 1/4" wall tubings that spaced evenly along the length of the skid frame and be held down/in place by large U bolts. These 3 base members will be bolted to a 7" thick 25mpa floor with 3/8" thick combine pickup belting (works great for mounting large compressors) I can't say yet whether this will work well for sure or not but will post results of this design I am building.Great forum here.

Troy

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Re: Design of steel frame mount and vibration isolation
« Reply #10 on: September 28, 2006, 12:30:52 AM »
The steel bit of any engine mount is the easy bit! The rubber bit is what will cause you headaches (just ask the Challenger crew).
If you really want to play with numbers, see if you can find a (surprisingly thin) book called "Engineering Design With Natural Rubber" which was put out by the Malaysian Rubber Producers Research Association. Despite it's name, this outfit was based in the UK. It's changed it's name (and function?) since I knew it 25 years ago. Haven't seen the book for 25 years but I remember that the equation(s) to calculate what you're trying to do had about 20 variables, several of which were "fudge factors".
Choice of material is important too. The urethane mounts mentioned have their advantages (Fuel/oil resistance being an important one), but may have issues at big amplitudes with heat build up. Conversely, the hysteresis which causes this provides damping which may or may not be a good thing.
This is going to have to be an empiracle exercise and I wish you the best of luck!

Cheers,

Brian

hotater

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Re: Design of steel frame mount and vibration isolation
« Reply #11 on: September 28, 2006, 12:52:46 AM »
Beware the extreme top heavy nature of the Lister. It'll take some wide track skids to drag it across farm ground.    :o
7200 hrs on 6-1/5Kw, FuKing Listeroid,
Currently running PS-Kit 6-1/5Kw...and some MPs and Chanfas and diesel snowplows and trucks and stuff.

Guy_Incognito

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Re: Design of steel frame mount and vibration isolation
« Reply #12 on: September 28, 2006, 05:54:25 AM »
Well it sounds like a everyone's got some good ideas on how to make it work, and it seems that we're all trying something same-but-different, so we'll have a good idea of what works and what won't.

I'll try and do mine by the numbers as much as possible with the airmounts - the biggest issue I'll find with them is the lack of any damping, so I'll have to take extra consideration with engine balance. I'll do a reasonably large frame in the style of Bob's suggestion, with sand as a damper for any higher harmonics that might crop up. The positive side of them is the 90-95+ percent isolation, so there'll be practically no vibration transfer to the ground. If I went back to my original generator setup posts, and xyzer's unbalanced engine, we worked out that there was a 300 pound force going up and down 10 times a second at 650RPM. With a solid steel/concrete mount, that force is transferred directly to the ground and dissapated. With the isolators in there, it'd be a 12 to 15 pound force instead. Hell of a difference. But the flipside of that is, there'd be a whole lot of jigging of the engine going on instead - roughly about 3 inches of movement at that imbalance. But it will allow me to tune for the best balance pretty easily!

A question for the crowd:

As my intended design will have little or no damping in the mounts, it will be pretty mobile due to torque and imbalance forces. This is why I hope some of the energy there will continually shuffle the loose sand around a bit as opposed to bouncing the frame to ever higher heights  ;).

If I also had a relatively long frame (that is, a fair gap between engine and generator, approximately double that in Hotater's pic, for example), will the mass of the generator at the other end act as a inertial damper to the torque forces from engine firing? As long as the frame's relatively stiff, (and belt issues aside) it should have an reducing effect by simply being a weight out there at the end of a lever, correct?



Guy_Incognito

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Re: Design of steel frame mount and vibration isolation
« Reply #13 on: September 28, 2006, 08:33:33 AM »
I'll jump ahead a little bit here and quickly look at the amount of bouncing about caused by an imbalance at speed. If there's too much bounce with my airspring mounts, I need to look at alternative mounts and frame design.

Somebody check my math please:

We start off with an equation to determine the dynamic amplitude of something on a resilient mount when there's an eccentric weight attached rotating at a certain RPM. I snitched this one from an example in a pdf here : http://www.sdrl.uc.edu/ucme662/Vibs1_review_notes.pdf

                mee . ω 2
X (ω ) =  --------------------------
               k −ω2 . m + j.ω.c

Look at this mess! Ok, we'll try and work out the bits we need to solve it:

m = 500kg , the mass of the listeroid engine + frame
mee = the mass.distance of the eccentric weight , in xyzer's case, about 1kg at a radius of 0.298 metres, so 0.298kg.m
ω = the rotating speed of the eccentric weight in radians/sec = 2.pi.(650RPM/60s) = 68rad/s
k = the spring constant of the airmounts I'm using = 58lb/in = 258N/in = 10157N/m
c = the damping factor and is pretty much zero for airsprings, so this bit can be left out.

This condenses to:

 0.298kgm . 68rad/s2
------------------------------
10157N/m - 500kg . (68rad/s2)

which is:

1377 kgm/s2
-------------
-2301843kg/s2

Which *finally* gives 0.000598m or 0.598mm deflection at 650RPM.

Seems to be awfully small  ??? . But it might be right , as there's only a tenth of a second for the imbalance force to start moving the frame before it moves in the other direction.

Quick! Is there a mechanical engineer in the house!?  ;D

xyzer

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Re: Design of steel frame mount and vibration isolation
« Reply #14 on: September 28, 2006, 05:10:14 PM »
Oh what a tangeled web we weave.... ;D.....The problem with trying to mathamaticaly design your system is all the variables....what is the balance...or out of balance of an unknown Listeroid...IMHO..there are many out of balance conditions and combiniations of the balance condition....a perfectly externaly balanced 6/1 will most likely have a 50% balance...all can be balanced except for 50% of the reciprocating mass...that is on a balanced engine....now get a who knows what listeroid and the games begin!...where is the rotating mass heavy or light? are the flywheels the same?...heck no! One could be perfect...the other way off...there are more combinations off inbalance to cause many different contortions of expelled energy.....I built my sub frame with the gen head mounted...started it and saw what I had... :P...not good...externially added weight to the rim till I felt it was the best I could do...Now I could see what I was dealing with. Then I built my base frame to accomodate my rubber isolators. I could see how much movement or vibration I had to deal with...Not a bit of math involved. Now if It was a perfectly balanced Listeroid...a totally different issue...Guy I can't help you with the math to many variables...But I can say when you design it leave room for changes in design! There will be some...you can bet on it! It is an EVOLUTION! IMHO     
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