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Author Topic: Generator vibration and torque pulses  (Read 24041 times)

rcavictim

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Re: Generator vibration and torque pulses
« Reply #15 on: October 11, 2007, 02:25:31 AM »
I don`t know why so many people come on here and ask for technical suggestions, then when they get them they completely ignore them and end up doing it their own way anyhow and are usually disappointed with unsatisfactory results.
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hotater

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Re: Generator vibration and torque pulses
« Reply #16 on: October 11, 2007, 02:58:20 AM »
jens---  If you have movement at the dog house then *somewhere* is the 'pivot point' of that vibration.  the pivot is probably at a solid place and somewhere not as solid is moving.  Figure out that point of rotation and install a brace to cancel it out.

Remember that troops crossing a bridge don't march....WEIRD things happen with the strongest of structure under rythmic loads.
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.

hotater

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Re: Generator vibration and torque pulses
« Reply #17 on: October 11, 2007, 03:41:42 AM »
Well maybe I misunderstand what you have. 

If the generator moves AT ALL, it is moved by action of the engine.  It shouldn't.  Something is to flimsey to hold it.  Find out what that is and weld a brace to it.

I think the genhead is mounted to  springboard and any movement in the engine is transferred and magnified until you can see it.

IF you're saying EVERYTHING that's connected together is moving the SAME amount.....then add weight.
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dkwflight

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Re: Generator vibration and torque pulses
« Reply #18 on: October 11, 2007, 03:56:07 AM »
Hi I think you need to get rid of the rubber mounts. Probably the frame is not stiff enough.

I  built my frame of 6"x6" timber. I don't see the kind of movement you describe. My frame is sitting on bricks in sand. The viberation of the engine running compacts the moist sand to a high density. The movement I see at the engine is fairly small and mainly due the a flex in the timber. I get belt slap from torque pulses when under load. If the belt is loose enough I can hear a short duration squeal when the belt slips as the engine fires.
The idea of mounting to a large mass of concrete has a lot of merit. Consider bolting your steel frame to a block of concrete.
Dennis
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Still in devlopment for 24/7 operation, 77 hours running time

ronmar

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Re: Generator vibration and torque pulses
« Reply #19 on: October 11, 2007, 04:06:47 AM »
Jens
 Â  Your choice of welding the cross tubes on top of the long end rails is still one area where you are going to experience flex.  Triangles make rigidity, open squares do not.  It is a simple fact that cannot be avoided.

Do you have a packing box like movers use?  They arrive stacked flat and are assembled by first opening the 4 sided tube into a square,  then folding the top and bottom flaps in and tapeing them in place.  If you do, unfold the top and bottom so you again have the box tube, then push on opposite corners and see what happens.  Lay it on it's side and set some weight on it and see what happens(especially when you push the weight in the axis perpendicular to the tube).  Now fold and tape the top and bottom flaps shut and repeat these two simple tests.  With the ends folded, you have material between the opposing corners and basically have formed 2 triangles that stiffen the structure.  Your tube configuration is like the unfolded packing box, and allows movement of your engine and generator in the fore-aft axis...  There is no material between opposing corners.  This is one of the more valuable lessons tought by an erector set, that the lego generation typically misses out on.

I think you will find quite a bit more rigidity if you take a sawsall and cut the ends off of those 4 cross tubes, drop them down in between the side rails, and weld them all the way around.  Due to the wall thickness, it is still not what I would call ideal, but it should be significantly stiffer in the fore-aft axis when assembled that way(mine sure is:).  You could also cut and fit triangular gussetts to all the tubes on both sides and ends.  You could also cap the ends with a long piece of heavy flat strap that is also welded to the side rails, but I think placing the cross tubes down between the side rails would be easier and take less additional material.  

Look at it this way, You are not satisfied with it now, so you really have nothing to loose(except time and weld rod/wire) by cutting it up and experimenting with the steel you already have.

Good Luck

Ron  
« Last Edit: October 11, 2007, 04:15:12 AM by ronmar »
PS 6/1 - ST-5.

BruceM

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Re: Generator vibration and torque pulses
« Reply #20 on: October 11, 2007, 04:34:48 AM »
Once more I'll suggest that you try Mr X's simple wheel chauking method.  Balancing the wheels alone I woud expect to have accomplished nothing.  The manufacturers don't control in the slightest the weights of the conrods and pistons. The Mr X method will greatly help to compensate for this problem.

If the Mr. X method doesn't do the job adequately, then I'd check to see if your two pistons and conrods, wrist pins, are grossly mismatched.  I can't comment on the steel frame- mine is wood, mounted on rubber pads.  But I can now set a glass of water on my valve cover with the engine under 70% load, no problem.  The twin should be better if well balanced, I think.  (I just have a 6/1)

Bruce M


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Re: Generator vibration and torque pulses
« Reply #21 on: October 11, 2007, 05:01:49 AM »
Sitting on wood should be OK, at least to get it way better than it is now.  It's not like you're lacking in movement!

Chalk the wheel at about axle height- some  of us will admit to using a 5 gallon plastic pail as the brace for holding the felt tiped pen to the wheel.  :) 

I found that "Sculpey" plastic modeling clay was a good temporary weight for testing, and that a few blobs of 4 oz were a good test weights.  The clay goes on the inside of the rim, of course.  I ended up with about 3.5 lbs of lead weights.  When I got to the fine balancing, I quit marking and just tested. Your feet and a plastic container of water on the valve cover will suffice for the fine tuning.  Just stop marking once you feel it's not making any difference, and just try moving your trial weight "around the clock". 

Bruce M 

hotater

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Re: Generator vibration and torque pulses
« Reply #22 on: October 11, 2007, 05:41:34 AM »
jens---

Balance the wheel that shows the most motion....that's the one that needs it next.
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ronmar

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Re: Generator vibration and torque pulses
« Reply #23 on: October 11, 2007, 06:01:56 AM »
Jens
 Â  The engine is not causing the vibration, only a small portion of it's mass is.  Most of the engines mass is dampening the vibration forces. The resultant movement of that large mass can be very powerfull.  I think if you had a real vibration problem, you would probably be afraid to run it as it would be trying to walk across the floor.  The movements you describe sound harmonic in nature.  In your current configuration, imagine the top of the 2 horizontal box tubes under your engine as one plane or plate.  The top of the 2 tubes under the generator are another plane or plate.  The top of the longer side rails is a single plane at a lower level.  With un-triangulated tubes, these upper and lower planes are basically joined by vertically mounted leaf springs(side walls of the box tube) and no shock absorbers.  By removing this spring and flex, as you noted, you will directly couple that much more mass to the engine to further dampen the resultant movement from an out of balance condition.    In addition to the end gussets, you might also add some vertical plates(shear webs) perpendicular to the tubes out in the middle.  This will help to stiffen/couple the upper and lower planes.

I guess what I am trying to say is that untill you stiffen the frame, you will have a hard time determining what movement is from engine vibration, what is caused by engine torque(movement allowed by flex) and what is driven from stored energy in the flexing frame.

Search back thru the balance threads, I know I, along with others have described how to performed the Mr-X method.  One thing I found in my experiments was that the results were less confusing when you isolated forces.  I took my initial measurements with the engine coasting and the decompression engaged.  This was difficult, even with 2 people.  I had my best results spinning up the engine with head and crankcase cover removed, using an electric motor.  No power or compression torque effects, only pure spinning out of balance mass energy being applied.  As Bruce mentions, a 5 gallon pail makes a handy marker stabelizer:).  I also used a 1" dial indicator measuring th movement of the whole frame.  I clamped it to a heavy bar of steel setting on the ground for fine tuneing.  The needle arc in thousandths of an inch showing fore-aft movement is a little more subjective than how a water glass looks or how it feels to my hand.  I also did this final testing with the frame setting up on 3/4" pipe rollers,  as I reached a point where the out of balance forces had difficulty overcomming the mass and the friction with the concrete garage floor.  When I was done, with my frame setting on 3/4" pipe rollers so it moved easilly in the fore-aft axis, I had around .005" of movement with the electric spinup.  While still on rollers, I re-assembled and ran the engine.  With the engine running, I had I think .015" fore-aft movement.  This increase in movement was from torque and the only way to remove that was to not generate electricity.  So the whole assembly needs to have enough rigidity and mass/friction to not move while generating it.  With the frame back on the garage floor and powering a 3KW electrical load, the movement is bareley perceptible.

Ron
PS 6/1 - ST-5.

clytle374

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Re: Generator vibration and torque pulses
« Reply #24 on: October 11, 2007, 06:05:55 AM »
Jens-
The top of the dog house is moving toward/away from the engine, right?  Or side to side? 
Remember you can't balance out the torque pulses,  the mass of the flywheels is very high.  When the engine starts compressing it twists the case one way, and when it fires it twists the other.  Might try cutting some wood 2x4s to fit in between your upper square boxes, and hammer shims in to tighten them as much as possible.  See which one falls out first, should show you weakest point.

To clarify 2x4 on end, parallel to the base rails, shims on the ends of the 2x4s.  Might tie all the upper boxes together. I bet no matter how tight you get the ones between the engine and generator, they fall out before the engine gets up to speed. 

Andre Blanchard

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Re: Generator vibration and torque pulses
« Reply #25 on: October 11, 2007, 02:48:37 PM »
Here is what I see.

______________
Andre' B

sailawayrb

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Re: Generator vibration and torque pulses
« Reply #26 on: October 11, 2007, 05:26:39 PM »
There are probably an infinite number of ways of doing this successfully.  However, there are a couple of fundamentals that you must do.

1) The structure between the engine and generator must be sufficiently rigid.

2) The weight of the structure that is between the rubber isolation mounts and the engine/generator should be as heavy as practical for your situation/objectives.  The reason is inherent to F=MA.  Larger Mass results in less Acceleration (and associated stand Displacement resulting from engine vibratory Force) that the rubber isolation mounts will ultimately see and then have to attenuate.

3)  You have to know the vibration frequency that you want attenuate and the total weight of the stand (i.e., engine + generator + structure weights).  Only then can you determine the number and characteristics of the rubber isolation mounts that you will need to use to achieve the desired vibration attenuation.  Calculation information can be found:  http://www.karman.com/selectvibro.cfm

Here's some photos of what my stand looks like.  There are 8 total rubber isolation mounts.  I can place glass of wine on floor a foot away from running engine and there isn't even the slightest ripple.  My stand now sits in corner of my garage with about 1/8" clearance on three sides.  Wheels can be installed/uninstalled as required to allow moving the stand to more optimal location for maintenance/overhaul.



« Last Edit: October 11, 2007, 10:19:09 PM by sailawayrb »

mobile_bob

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Re: Generator vibration and torque pulses
« Reply #27 on: October 11, 2007, 05:54:51 PM »
i would add one more critical component to the equation

that being placement of the isolators,

in my opinion they are in the wrong place as constructed

the ones under the engine need to be further out by maybe as much as a foot

and wider by nearly the same,,

this from what i can see in the pictures


where they are located they have very poor mechanical advantage or control of the vibrational and torque action applied to them

i spent a hell of alot of time argueing with GuyF about the possibility of using resilient mounts,,, and it took me a long time
to understand his concerns.

that being

if you want to build a resilient mount system, you must engineer it, if you don't your results will range from possibly good if you are extremely lucky
to absolutely dangerous and everywhere in between.

you setup is clearly somewhere between the two extremes.

time now to go back and engineer the system,, and see if what you have can be modified or if you will need to scrap it and start over again.

if i might suggest go back and look at how the oem lister was mounted to the block of concrete,, particularly the early version where ~625 lbs of concrete
was used. print out the drawings and use a straight edge to see how the vector lines work out

after doing so you will see where those rubber mounts should be placed.

this all assumes that the steel subframe assembly is very rigid!  any flex and all bets are off, all your engineering will be off
because the flexing frame becomes part of the resilient nature of the mounting,, and there is no easy way to calculate
the action of a flexing frame and its contribution to your problem.

as much as i hate to admit it GuyF in many ways was and is correct, what looks good or appears to be intuitive often times just don't work!

your frame assembly while looking good is an example of what he was illustrating in 10k words or more :)

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

GuyFawkes

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Re: Generator vibration and torque pulses
« Reply #28 on: October 11, 2007, 06:02:39 PM »
Everyone always tries to re-invent the wheel, and it's always the same reasons, they want to do the job a wheel does, but they don't fancy using an actual wheel...........

Last year or the year before I posted pictures of the top AND underside of the cast iron start-o-matic base, full of transverse and longitudinal runners creating a load of cells, maybe a few inches a side in dimension.

Just because you don't have access to a foundry & patterns doesn't mean you can't do the same STRUCTURAL and MECHANICAL job with a stick welder, power hacksaw and a bunch of steel section.

Looking at http://www.pacificsun.ca/~jens/pic/public/_MG_7642.jpg

It is ORDERS OF MAGNITUDE too weedy, best advice is throw it all away and start from scratch.

Nota Bene.

If welding steel to approximate a cast construction in terms of properties, jens frame is (sorry) a classic example of how not to do it, it is nothing but straight lines and right angles, you need gusset plates in ALL the corners in all three axes for each corner, same as a casting, and it doesn't hurt to use a bit a crane jib type triangulation too, to compensate for the different material properties of steel and cast iron. Other thing is spot and straight welds are a waste of time, only a matter of time until the fail, the important part of the run is where it wraps around each corner at the beginning and end of each run, leave out this and you lose at least 50% of the potential strength and stiffness.

6" RSJ is, I suggest, the absolute minimum steel section to use for construction on the frame, with plenty of gussets and fillet plates, and a high current root weld that penetrates visibly to the other side of the plate, and remember to work your way around as you weld so the welding doesn't pull it all out of square.....

the frame is as important as a car chassis or a motorcycle frame, cut ANY corners and you dramatically affect performance, a car with a good chassis and crap everything else, like a bike with a good frame and crap everything else, will nearly outperform a car with a crap chassis and top of the line everything else.

Plan three times, measure twice, and cut once, else you'll end up with a sturdy frame with crossmembers in all the wrong places and have to start adding and cutting to actually fit the engine and head etc, how about leaving room to pull out and fit a drip tray without disturbing the engine and head? how about anchoring fuel, electric and other connections to the frame so they can't get pulled or damaged, how about mounting an inline racor type fuel filter / water separator on the frame, or belt guards, or jacking points, plan all these before you cut your first piece of steel, much less start welding.

the frame as pictured in the link above is basically scrap, sooner that is accepted and a new one made the easier things will go.
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BruceM

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Re: Generator vibration and torque pulses
« Reply #29 on: October 11, 2007, 06:05:39 PM »
Jens,  My memory is badly impaired, but I do think I ended up with about 3 lbs (over a pound and a half on each wheel) opposite the external counterweights on my Metro 6/1. I can't remember the exact weights.  There is no internal balancing on these models, the amount of weight required is substantial. 

As Hotater suggests, start with the worst wheel.  I tested with clay, and then added about 2/3 of the indicated weight, then went to the second wheel, and repeat.

I did use a dial guage, but it's not necessary for the initial balance during chalking and is not that useful if dynamic balancing with the engine running.  I have no doubt that Ron's method of reving up the motor via electric motor works very well, with great precision,  and a dial guage would be handy for that method. By all means, use that method if you prefer.  I'm just saying that a "good enough for me" job can be done with the engine running at actual operating rpm.  Ron's method would be more scientific and lends it self to measurement as opposed to using your feet and a cup of water on the valve cover as indicators.

You don't need to get so sophisticated on the marking pen stabilizer-  but any sort of effective brace that lets you move the pen slowly to the wheel will suffice.

It is far easier and faster to do a pretty good balance one of these engnes with the Mr. X method than it is to explain it or defend it in a public forum.  Far, far easier.  Then you can address your frame and mount design issues as needed.




Bruce M