Author Topic: concrete vs resilient mounting  (Read 44246 times)

Guy_Incognito

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Re: concrete vs resilient mounting
« Reply #75 on: October 09, 2006, 12:38:31 PM »
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I don't know your age but I do know you are young and naieve, risk assement ain't about saving lives, cos first you need to quantify the value of life and limb in dollars in that particular market, risk assesment is exactly what I said it is, we ain't culpable m'lud cos we did this here risk assessment,see.... one day you will learn enough to see the truth in this.

You old cynic. I will agree to disagree on this, otherwise we'll be here all week.

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re your question, you asked the question a dozen time, and I answered it a dozen time, your method is wrong, it is bad practice, it is whistling in the dark.

So, doing some theoretical work before going out there and doing it in practice is bad? How many people do you kill when you're off trying something for the first time? Surely you try to get some idea of what's going to happen beforehand? And the principles of vibration isolation - while complex, granted - are hardy some inscrutable black art. Ok, maybe the fine-tuning is.The "get it to the stage where it likely won't kill anyone" part isn't.

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Lots of anaolgies based upon assumptions skipped

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Risk assessment my ass, a PROPER risk assessment will include bloody idiots like you who think they can calculate everything and take shortcuts and make assumptions

Why, I believe we agree here! See the final words of my previous post. But I will disagree with you when you think I am the one taking a shortcut. I could go out to the shed and bang up a frame and off I go into the sunset, until 10 years later under some particular load and speed, it all falls apart. I'm the one taking the long way round , testing my patience with people who make assumptions on my intentions and want to slot me into some box that I don't fit into. I don't care what your opinions are of number crunchers. Number crunching serves a purpose when you're off in the great unknown trying to work out a solution that won't kill anyone. Or I suppose I could simply agree with you, go test a frame and kill someone  by accident. ("M'lud! I was experimenting with some rubber mounts and it reached resonance, flung off it's flywheels and killed someone. Yes, I know, resilient suspension designs have been proven for decades, m'lud. Yes I know that there's formulae that will put me in the right ballpark without all this dangerous experimenting, but... A foolish act, m'lud? Guy_Fawkes recommended it! Surely he is the authority on risk and covering his ass...")

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I was the one who found JC's head, still in his safety helmet, and had to fish it out from under where it was jammed, I was the one who had to go tell his missus what happened, and it was real quick, so you can't tell her the canadian ex logger was a real sharp edged bastard with his two eggs side by each but he was smart and didn't trust the assholes he was working with because he knew he didn't know it all and he knew they didn't have the same attitude  as him, so he lived and JC died, because JC listened to their bullshit and attended the safety meeting and planning meeting and didn't realise everyone was doing what everyone always does, signing off on bullshit and assumptions, because nobody had the guts to stand on their own convictions and say "we don't have the kit on site to do this job safely".

All arguments aside - you poor bastard. I'm truly sorry you had to go through that.


What I am trying to do is pin down the details further, instead of just going off and making a frame that I assume will be fine, but in reality will wind up killing someone. Which I think is the kind of assumption that you're railing against. I think. I'm happy with ballpark figures and large margins. All I want is indicative loads and resonances to make sure the thing I build is strong enough that I can test further without it all going pear-shaped.

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But, you won't give up your spreadsheets and pulled out of your ass calculations, because without them what have you got? You don't want the hard way, you don't want the experience and mental discipline, you want quick answers, you don't care that they are wrong, you just want quick.

Wrong guy_fawkes, terribly wrong.

I don't want quick. As I mentioned before,  If I wanted quick, I wouldn't be mucking about with spreadsheets and dealing with crap from you.  So I put my spreadsheet away, then what? You don't seem to know squat about resilient mounts. But you think I should go out and start dicking about trying to do something, ignoring all the numbers (because they're based on figures that are ill-defined)? I'd be out there making a nice-looking frame, which would likely be overstressed and fail terribly. I don't want quick, I want the right answer. Seeing that your answer is "Don't use a resilient mount", and you cannot tell me any good reason why not except "That's The Way It's Always Been Done", I will now hereby slot you into the box that says "Part of the Problem."

I'll happily give up my spreadsheet if someone has a nice, resilient mount design that does the job and isolates that thumping great big bit of iron from the rest of the world. Oh wait, no-ones got that yet? Well, back to the spreadsheet. There'll probably be plenty of tweaking and tuning involved in the real-world, and I'm genuinely interested in whether the spreadsheet lands me in the right ballpark , or whether it's miles off.

Oh, and you say you can calculate the forces involved? And you say that you've done testwork with engine forces under load? Still haven't seen any details, Guy_F. If you want to convince me, don't bother with the long stories. Give me hard facts, design tips or  numbers for the problem at hand. Otherwise you're part of the problem, not part of the solution and you'll have no more time from me.

mobile_bob

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Re: concrete vs resilient mounting
« Reply #76 on: October 09, 2006, 03:59:43 PM »
GF:

i will have to respectfully disagree on your assessment of my use of assumptions as being dangerous,

assumptions are like hypothesis, for the purpose of asking a question, quite apart from assumptions used to design and build something.

if you recall i asked a couple of questions!

so far i am not getting a direct answer, but rather a tangent.

perhaps i am not making my question clear, or it is getting caught up in all the verbage.

so let me restate,

you have made reference as to your concern for the crankshaft breaking if not solidly mounted as lister prescribed, so

is it because the crankshaft is of insufficient strength to put up with forces imparted back to the crankshaft by means of fighting against the flywheel gyro effects, or by some other means?   

please explain

bob g
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phaedrus

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Re: concrete vs resilient mounting
« Reply #77 on: October 09, 2006, 04:43:21 PM »
About crankshaft flex – and the change in that which would occur in a (more) resilient mounted engine…

I’d suggest that one imagine a stationary crank and flywheel set up with bearings that allow some significant angular rotation on the main-journal axis. Ordinary bearings do this. For clarity imagine that the rod journal is at TDC. Now imagine that the bearing housing-carrier set is accelerated along the axis of the cylinder. The mass of the flywheels will cause the crank to bend – mostly in the rod journal and web region – that’s where the force is concentrated.  (If one placed an inside mike on the rotational axis under the rod journal they could measure the deflection – and we do this in large stationary engines to assure proper as-found alignment.)  Now reverse the acceleration. Repeat. The engine is bouncing and the crank follows, bending first one way and then the other. In practice, with the engine actually rotating, this flexation would occur, I suppose, 1300 times per minute at 650 revs. It is not a uniform sinusoidal motion however, because the power flow into the system follows a four-stroke pattern with the piston power stroke creating a one-in-four event. In practice the motion is somewhat rotational, but for simplicity…

Now consider that the crank is in reality a torsional spring (it is).  By allowing the axial bending moment to increase relative to the as-built torsional deflection (by resilient mounting) we have changed the loci of stress in the crank web, and I expect, increased it too. That is unknown territory. One might even create a critical oscillation.

All this exists whether the engine is mounted on 6000 pounds of concrete or floating in space – but the amount of flex and the pattern of flex changes.

In practice I suspect quite strongly that the effect of resilient mounting is similar to what would occur with a misaligned rigid shaft coupling. The effect, if large enough, will cause excessive web deflection and a time-to-failure “clock”.

Additional outboard bearings, properly supported, would mitigate this effect. That’s a complex and expensive undertaking and I would not wish to attempt it, but if done right it would surely eliminate most of the web deflection increase that (more) resilient mounting creates. (As a matter of fact in Kawasaki racing kart engines an additional outboard main bearing is a more or less standard racing modification  - ‘cause otherwise they snap cranks.)

In another line of thought I'd like to applaud the seveal of you all who have emphasised the importance of safety - "doanwanna git anybuddy kilt 'r hurt" - what's life about anyhow if not to have a good time? Can't do that if we hurt people.

Best, Phaedrus
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xyzer

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Re: concrete vs resilient mounting
« Reply #78 on: October 09, 2006, 04:49:58 PM »
So after some research I see the Indians started putting the TRB in the 6/1....With out the revision approval of LISTER. All this rubber or not to rubber debate going on, and we arn't following the directions of Lister will cause them to explode. My question is with all these invisable forces going on don't the TRB apply latteral forces to the case? I seems they would be directly related to the rotating plus firing pulses...Just a thought!....they seem to handle this modification without any problems. Sure is a different load applied than with straight bushings.
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mobile_bob

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Re: concrete vs resilient mounting
« Reply #79 on: October 09, 2006, 05:51:19 PM »
Guy:
sorry i had to go make some money, now i am back

"2/ even if we assume the two systems are identical in theory, you are missing a fundamental point because you aren't using your brain or your eyes, you aren't thinking, you are being mesmerised by bullshit."


i don't assume the two systems are identical in theory, rather they exhibit some of the same behaviors, based on differing variables

at this point you are correct i am thinking, and not useing my eyes and brain as i do not have a sytem in place to observe and measure

as far as being mesmerised by bullshit, i assure you i am not.


you like to site master ricardo and his work, do you not suppose he did a lot of thinking before he built any test rig, beit a variable compression test engine or whatever?
further, do you not suppose it is likely even after building the test engine and getting some data, he applied it into alot more thought before building anything?

i am not talking about a redesign or suggesting anything,,, i am simply asking a question

what evidence makes you believe the crankshaft is in danger of breaking in a resilient mounted engine?

do the portable lister units have a higher rate of crankshaft failure?

do the engines used in other non stationary applications have a higher rate of failure?

are we to conclude the reasoning that a lister/oid should not be resiliently mounted because of breaking crankshafts as a supportable
conclusion, or an assertion, or theory , or a feeling?

my feeling is yes there might be a higher failure rate, but i have no documentation to support the feeling, do you?

my theory might also be that a resilient mounted engine imparts stresses on the crankshaft that are not prevalent in a concrete mounted engine,
and these stresses should shorten the life of the crankshaft, but i have no documentation to support this theory,, do you?

i could assert the same as above, but have no documentation to support my assertion,  do you ?

so how am i to conclude that there is significant risk of failure?

i would suspect that a poorly balanced engine would have as much or more effect on failure of  a crankshaft, whatever way it is mounted, but
i don't have documentation to support that either.

Guy if you don't know, Just say so. no harm in that is there?

i will tell you straight up i dont know! that is why i continue to ask, perhaps someone might stumble across this subject and one day in the future come across the
answer.

a while back you stated that i am playing devils advocate on this subject, in some ways i probably am.
the primary reason i question and debate this thing with you and others is not to necessarily come to an answer as the primary reason for doing so.
rather it is the journey and the process that brings out other information, procedures, and thought that has a much wider application that may very well have
nothing to do with lister/oids.

bob g
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phaedrus

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Re: concrete vs resilient mounting
« Reply #80 on: October 09, 2006, 07:25:04 PM »
Good point about TRB interaction possibilities, yet plain journal bearings allow considerable axial deflection - they have to because of the oil clearance. That same clearance probaly provide cushioning and damping of transient accellerations perpindicular to the crank axis - the TRB types may well have higher crank stresses...
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xyzer

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Re: concrete vs resilient mounting
« Reply #81 on: October 09, 2006, 08:31:53 PM »
Good point about TRB interaction possibilities, yet plain journal bearings allow considerable axial deflection - they have to because of the oil clearance. That same clearance probaly provide cushioning and damping of transient accellerations perpindicular to the crank axis - the TRB types may well have higher crank stresses...

and case stresses....the taper bearings would try to spread the case under load...?!
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Doug

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Re: concrete vs resilient mounting
« Reply #82 on: October 15, 2006, 04:13:09 AM »
Maybe its the booze....

But this started getting interesting again....

Doug

More read more beer.....

Guy_Incognito

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Re: concrete vs resilient mounting
« Reply #83 on: October 15, 2006, 05:08:50 AM »
About crankshaft flex – and the change in that which would occur in a (more) resilient mounted engine…

I’d suggest that one imagine a stationary crank and flywheel set up with bearings that allow some significant angular rotation on the main-journal axis..... (etc)


Ok... I think I've got what you're on about there. But  - a query:

The resiliently mounted engine and bearings are moving from the applied forces on them. The forces are generated from the crankshaft/flyhweel rotation and combustion forces applied to them. The crank isn't following the engine when the engine jiggles, the engine is following the crank and flywheel about the place as they jiggle.  So if the engine *can* follow the crank/flywheel a little - as I postulated about it rotating around the apparent centre of mass - surely this reduces the axial deflection on the crankshaft as opposed to a rigid mount?

It does lag by a certain amount, so there'll be a point where the crankshaft zigs and the engine zags a little. It really depends on your damping forces and whether you're near resonance in your resilient mount as to whether it gets seriously out of phase.

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In another line of thought I'd like to applaud the seveal of you all who have emphasised the importance of safety - "doanwanna git anybuddy kilt 'r hurt" - what's life about anyhow if not to have a good time? Can't do that if we hurt people.

Too true. I'd rather we discuss it here until we're blue in the face, before we go out,experiment, and hurt someone as a result.

As for crankshaft/crankcase forces, this isn't a high-performance lightwieght motor where every last cubic mm of unnecessary metal is removed from the engine. From the stories of various hideously-imbalanced listeroids chasing people around the place, and no doubt hundreds of firmly-anchored-yet-very-unbalanced engines running fine, it's indicative that they're plenty strong enough to handle a heap of abuse. The clock's still ticking, but whether it goes off in 5000 hours or 50000 hours is the problem. 50000 hours, I could handle that. 5000 hours, well , personally I'd shrug philosophically and call it a learning experience.

Anyone know if there's much change between a 6/1 crank and the crank from a higher-powered single-cylinder variant?
« Last Edit: October 15, 2006, 05:44:30 AM by Guy_Incognito »

Firebrick

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Re: concrete vs resilient mounting
« Reply #84 on: October 15, 2006, 06:36:09 AM »
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i also understand what i have seen in the field, engines that are rigidly mounted to a subframe, that are meant to be
a moveable stationary engine (such as oil field mud pumps) break crankshafts quite easily if dropped a very few inches from the truck that is placing them ( i might add on the ground not concrete). whereas

the same engine mounted resiliently in a truck can sustain being dropped the same distance without breaking the crank.

no granted neither engine was running when the cranks broke and as such the failure was from shock loading.  but the principle seems to be the same. just happens much quicker instead of over time.

come on somebody educate me

I have seen a few cranks "sprung" from the engine being dropped in the factory.  Interestly enough, when they are spinning and are dropped they have no problems in this area.

Guy_Incognito

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Re: concrete vs resilient mounting
« Reply #85 on: October 18, 2006, 10:58:27 AM »
Hmmm. Forum software ate my last post, so abridged version follows:

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I have seen a few cranks "sprung" from the engine being dropped in the factory.  Interestly enough, when they are spinning and are dropped they have no problems in this area.

I would presume it's from the fact the angle that the crankshaft rotates spreads the decleration forces so they they are - on average - applied over an arc as opposed to a single direction. eg, if the crankshaft could rotate 180 degrees , then the maximum amount of bend at the end of deceleration would be at 90 degrees to the deceleration force applied.

Makes me wonder if this would be a bit of a saving factor when running a listeroid on a jiggly mount, but I still think we're looking at it from the wrong side of things. That is, the engine's not jiggling the flywheels, the flywheels are jiggling the engine. There are phase differences - at various RPM's around resonance, the flywheels will lead or lag the engine motion. But it's a case of the flywheels shoving the engine about - and those applied forces trying to move the engine (and flex the crank) are there regardless of whether the engine's fixed or not.


SCOTT

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Re: concrete vs resilient mounting
« Reply #86 on: October 26, 2006, 02:28:21 PM »
My quest for resilient mounts has hit a bit of a bump.  The engineer I was talking to referred me to http://www.acemount.com/  The engineer at Ace specked out 2 series “E” 50 durometer in series on the engine side and 2 series “E” 40 durometer in series under the gen head side of my frame.  These mounts were made by Firestone and cost $27 each so $27*8 = $216 not bad. 

I did a little more research and found that the Firestone patent had expired and there were manufacturers who sold copies of the original design, one of them was a company in Chicago that the Lister Petter engineer suggested I contact, I forget the name of the co.  They only sell in quantity, so they suggested I call Mcmaster. 

The mounts are available at Mcmaster (pg1253 style #6 , ½” diameter) for under $10 each.  I ordered 2  50A 800lb rating and 2 40A 375lb rating, thinking that if I used only one per corner it would be  “stiffer” than using 2 in series per corner.  This did not work; they are way too soft and allow far too much deflection.  I assume that having 2 in series would make this condition worse not better.  I have not spoken to the engineer at ACE about this yet.  It may be as simple as adding the 2 additional mounts per side.

I may try some higher rated mounts, like the 70A rated at 1425lbs.  I have read that some people have used conveyer belt under their frame, where is this available?  If there are people reading this that have their engines mounted on some kind of flexible material please let us know what you are using.  Please post as much detail as you can, composition of material, thickness of material, lbs rating if available, where purchased, and pictures would be great and a video clip of your running engine would be even better!

I will post any further progress good or bad.  This particular resilient mounting scheme is not as good as a solid mount to concrete.

Scott
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xyzer

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Re: concrete vs resilient mounting
« Reply #87 on: October 26, 2006, 03:28:57 PM »
Scott,
I used 5/side of McMaster Carr p/n 9376K131 rated for 125# ea
They are mounted  between the generator frame and the base frame. I get maybe 1/4" movement on start up and shut down. I have 4/side now and it works fine. I am going to add 1 more / side to see if it is better or worse. You can set it on concrete and it will stay put. There are 2 frames on my setup....one for the generator/motor and one that isolates the radiator fuel tank, and elec box. The mounts are between the 2. I have a 2x4 under each side of the outer frame to help it stick in place on slick surfaces. I will say that how well your engine is balanced is the secret!

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« Last Edit: October 26, 2006, 03:35:06 PM by xyzer »
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Guy_Incognito

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Re: concrete vs resilient mounting
« Reply #88 on: October 27, 2006, 12:11:19 PM »

I ordered 2  50A 800lb rating and 2 40A 375lb rating, thinking that if I used only one per corner it would be  “stiffer” than using 2 in series per corner.  This did not work; they are way too soft and allow far too much deflection.  I assume that having 2 in series would make this condition worse not better.  I have not spoken to the engineer at ACE about this yet.  It may be as simple as adding the 2 additional mounts per side.

Deflection when running, I take it? How much do they deflect when loaded and not running? Extra mounts in parallel as opposed to series is the same as getting a stiffer mount - the load is shared across two mounts instead of one, so the deflection is less. Of course, the stiffer the mount, the more engine forces are transferred through it.