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Author Topic: concrete vs resilient mounting  (Read 53070 times)

Guy_Incognito

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Re: concrete vs resilient mounting
« Reply #60 on: October 08, 2006, 04:24:22 AM »
Quinnf,

It's hard to tell without really pinning down some numbers. Those numbers I picked were quite random and it was only chance that I ended up (and realised while typing) that there was a resonance for the power pulses at that particular point.

The thing is, the concrete block is very well-damped, so resonance peaks are muted as well. But because it's well-damped, vibration forces are easily transferred past the springy subsoil. A few PDFs that smokey posted in the frame thread had quite a lot of detail in it about building a mount to go in differing types of subsoil and their natural frequencies with the loading from the concrete.

Quote from: smokey
Just went to google and found that ASME paper here:

http://www.slideruleera.net/FoundationsForCompressors.pdf

In looking for that one I found 2 more that are related

"Don't Gamble on Machinery Foundations" by William Kauffmann, 2 pages - 1973
http://www.slideruleera.net/MachineryFoundations.pdf

"Basic Vibration & Vibration Isolation Theory" by Unisor Machinery Installation Systems, 11 pages - mid 1980s
http://www.slideruleera.net/BasicVibrationTheory.pdf

Quite interesting reading there , actually.


phaedrus

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Re: concrete vs resilient mounting
« Reply #61 on: October 08, 2006, 04:34:29 AM »
OK Guy Ico, long reply, sri. (ref reply # 58)

That seems to me to be a Good Question. It is probably a matter of extent, of degree, more than anything else. If I were to undertake to do that (make a resilient mount) I’d want to start with a well-balanced engine, then I would be concerned with several aspects (I have seen too many screw-ups to be other than cautious).

First: The job has got to be safe. The increased motion of the engine (that you cite) would, I expect, introduce a new and unknown but larger pattern and amplitude of vibration in the engine structure(s). These engines are pretty stiff, so that might well be dangerous.

In particular I would worry about cast iron structures in dynamic tension  (such as flywheels) being subjected to tensile transients that might exceed safe limits - in other words this possibility alone would be very serious and discourage me from the trial. Cast iron is marvelous stuff, but when it lets go in a flywheel it can kill – I have seen this and believe me it is not just theory – it is bloody, expensive, and conflict-producing (like legal and otherwise) Iron flywheels (and pieces) can go through concrete walls. Nevertheless I might explore damping elastic pad mounts – you can call these “resilient”, but they, the ones we use in industrial application, are specifically designed to damp vibration. A good industrial supply catalogue will have some guidelines. In any event I would consider the possibility of the engine becoming unbalanced while on such pads – it could then cause great damage. Therefore any such mounting ought to have the pads “caged” and also have vibration sensing emergency trip shutdown provisions. This is common in industry, but it costs money and adds complexity. A trade-off.

Second: transients in the crank web introduced by increased engine motion might well create a cycle-to-failure period replacing the infinite life design of the crank. One would not know if they had created such a “clock” until the “bell rang”, e.g. the crank snapped.  This “clock” might tick for thousands of hours.  Think of the dynamic motion of the crank as designed, and then add the mass of those wheels bouncing…flexing the web. Might do some bearing damage too, and the two aspects might interact…  I understand (but am not certain) that Lister cranks are forged steel – if so you’ve got a spinning (with transient torsions from the piston already) bar-bell on a stiff steel spring, do you really want to bounce it? I’d want to keep that business as simple and quiet as possible.

Third: Based on experience with diesels I assure you that what appear to be trivial matters – using a brass fitting rather than a steel one in, for example, a oil pressure sensing tube, can (and this has happened to me) create a time-to-failure booby-trap. Once I set myself up this way for nearly 100 GM truck diesels to lose oil pressure, each at almost the exact same hours – the disaster took something like a year to present itself…very embarrassing. The point is that any deviation from tried and true practice might, (it’s really impractical to predict); result in a “casualty”, as people in the power-generation racket like to put it. In the case of a Lister type I’d worry about fuel leaks, broken brackets, oil leaks, and sump-sloshing causing the engine to either under or over lubricate. I’d worry about the governor linkages…  And, in particular, I’d want to consider what would happen if the engine locked-up…  Would it then roll right over in the (or on the) mounts? I have seen a 900 MW steam turbine rotor fail suddenly and transfer a large portion of its kinetic energy to the case. It cracked the case, 9 inch thick steel, for something like 20 feet. Almost rolled right off the top of the turbine building…

All told, I would undertake to mount a Lister type with modern damping elastic pads only in a most conservative and cautious way. I’d like to take measurements of the vibration patters of properly mounted (on concrete) engines of the same type and then examine those patterns on a “portable” mount (something like a Lister bolted to a plate bolted to railroad sleepers set in sand), – using an oscilloscope and any tools, (like a vibration meter from coleparmer) I could get my hands on to define the situation better. (That comparison ought to imply something about acceptable ranges.) I would ballast the engine base as heavily as is practicable (probably with a mix of concrete and scrap) and then work toward finding mounting pads that resulted in a pattern and amplitude of vibration that was as close to the factory design(s) as I could get. I’d cage the pads. I’d add a vibration trip emergency shutdown system, smoke alarm, over speed trip – all the bells and whistles. Then I’d cross my fingers and keep a close eye on things. I am sure it can be safely and properly done, but I am also sure that it must be done right. It’s not a casual undertaking.

Of course, an experiment to see what happens is always useful, and given plenty of time and money I guess it’d be interesting to see just how loose a mounting set up can be and still run without trouble for, say, 50,000 hours. We might all be surprised and pleased to find that the Lister type is so forgiving that it matters little. I suppose we shall just have to wait and see...people are mounting 'em pretty freely.  Best, Phaedrus
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dkwflight

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Re: concrete vs resilient mounting
« Reply #62 on: October 08, 2006, 04:40:56 AM »
Hi
Cageing the mounts sounds good. I seem to remember some Chevy motor mounts would go bad after a lot of years and oil on the rubber, would let go and the tourque would roll the engine to the right and bind the throttle at full.
You had to be on the ball and shut the engine off before you got into trouble.
Dennis
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Guy_Incognito

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Re: concrete vs resilient mounting
« Reply #63 on: October 08, 2006, 07:30:54 AM »
phaedrus,

All good points to consider. Caging or otherwise restricting mount movement is always a good idea. In the design I'm looking at with airmounts, they'll be set to "fly" just above the normal range of jiggle. Any movement outside the normal range of motion will be caught with a fair-sized bump stop. All the misc parts that are able to be restricted will be - the injector lines and fuel lines are a good example of things that will have a few more restraints of them.

For flexing of crankshafts,etc ... this is a point that I have difficulty grasping. Someone point out to me where I've gone wrong with the following example:

Say we have a crankshaft, and a flywheel on the end of it and the whole shebang's rotating. It's imbalanced somehow, by forces acting on it, by a weight being in the wrong spot on the flywheel or crankshaft,whatever. There's no bearings getting in the way, it's just a crankshaft and flywheel spinning by itself.

The whole thing will want to rotate about it's centre of mass (or apparent centre of mass if a force is acting on it). If it can freely rotate about it's centre of mass, the net bending force on the flywheel and crankshaft will be zero and nothing will flex. Basically, the centreline of the crankshaft/flywheel will go around the centre of mass in a small circle.

If I restrict it's movement a little bit, say with a bearing that's mounted to a resilient mount, then the system cannot freely rotate around the centre of mass and the force generated is proportional to the angular velocity squared and the distance of the shaft from the centre of mass. The force is essentially dependant on how flexy the mount is - if the mount allows the bearing (and thus, the shaft) to move towards the centre of mass a little bit, then the less force is applied.

If I now restrict the shaft with a rigid bearing between the flywheel and imbalance, the distance between the centre of mass and the centre of rotation is now at a maximum and thus the forces are at a maximum. Basically the fixed bearing acts as a pivot for the imbalance forces to lever against.

Can anyone help me out here? It's really starting to bug me.  :D

mobile_bob

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Re: concrete vs resilient mounting
« Reply #64 on: October 08, 2006, 03:51:42 PM »
seems reasonable to me,,, but then again i just woke up :)

perhaps someone will analize each of your examples and give a well reasoned answer has to why you are wrong.

i for one would like to sit down and listen to that reasoning

so whoever takes it upon himself to explain this, please try and do a complete job of it, and not just pick around the edges.

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hotater

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Re: concrete vs resilient mounting
« Reply #65 on: October 08, 2006, 05:09:04 PM »
I'm with Mobile Bob....I'm receptive to a complete explaination.

Guy Inc.  brings up some good points, but I have trouble with the 'rotating around it's mass'.....or more likely, I don't have the theoretical part of that ingrained enough to understand it.

In  *my* head I can see the crankshaft and flywheels spinning 'in space, without bearings' as ONLY happening with perfect balance.
    My experiment at about ten years old of making an aluminum spinning 'top' and then drilling a small hole in one side to better hold the starting string that ruined it's ability to spin convinced me of that a long time ago.

My *observation* with my engine has been-- If given ANY room to move around, that space will gradually increase until there is a point of failure somewhere in the hold-down system.

With TRBs big enough for a dump truck and a crankshaft bigger than a V-8 and cast iron doesn't flex all that much...where is the movement EXCEPT for the movement of the entire engine in reference to the world?  If we stop THAT movement and tie the engine TO the world, aren't we then using the power generated by the engine for benificial work instead of losing some to 'vibration'?

I have a Mini-Petter pumpset that produces a pound more pressure with the (solid concrete) mounting bolts tightened than when slightly loose (from shrinkage of the 6 x 6 wood 'pad') between engine and concrete.  Is it because the engine is 'stronger' or is it because of turbulence caused by vibration in the pumping part?

  If all the movement is confined to the rotating mass and the reciprocating piston AND the design of the (extremely) large parts are to prevent any internal flexing or movement from the forces encountered, there is no effective stress to  adjacent parts....as long as everything is stable.    Stress applied without movement has no energy,  right?

Now that MY confusion is added to the rest, I feel better.   ;)
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GuyFawkes

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Re: concrete vs resilient mounting
« Reply #66 on: October 08, 2006, 11:15:39 PM »

For flexing of crankshafts,etc ... this is a point that I have difficulty grasping. Someone point out to me where I've gone wrong with the following example:

Say we have a crankshaft, and a flywheel on the end of it and the whole shebang's rotating. It's imbalanced somehow, by forces acting on it, by a weight being in the wrong spot on the flywheel or crankshaft,whatever. There's no bearings getting in the way, it's just a crankshaft and flywheel spinning by itself.

The whole thing will want to rotate about it's centre of mass (or apparent centre of mass if a force is acting on it). If it can freely rotate about it's centre of mass, the net bending force on the flywheel and crankshaft will be zero and nothing will flex. Basically, the centreline of the crankshaft/flywheel will go around the centre of mass in a small circle.

You say you have a degree in physics, yet you appear to have forgotten such basics and modulus of elasticity.

Metal bends and twists under torque, it has to, and if it bends and twists then all by itself it is imposing precsessive 9as in gyroscopic precession) forces upon itself, so it is undergoing cyclic fatugue.

I have repeatedly asked you and other to do the maths, nobody will.

Why do you think early spoked flywheels were spiral spoked, yes the material was not as good as later cast, but by definition unless the entire flywheel was NOT accelerating and decellerating with each stroke as a theoretical solid disk with no elasticity, but WAS twisting radially with the torque, spiral spokes would not have been necessary.

Better materials didn't mean those forces disappeared, it just meant they were eclipsed by improving youngs modulus shear etc etc etc
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Guy_Incognito

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Re: concrete vs resilient mounting
« Reply #67 on: October 09, 2006, 01:05:06 AM »
Ah, Guy_F - sometimes the simplest things(?) elude me.

I am aware of young's modulus. You don't need it often when you majored in optics unfortunately. Neither do you have to understand crankshaft flex in a 90-year old engine design that's being mounted in such a fashion that there is no 'fixed' point in it.

The old adage is, "on the internet, no-one knows you're a dog". So until someone can give me a hard answer, with numbers and pretty vector diagrams (preferably) so that I can follow the same steps and reach the same conclusion, all claims of "it's perfectly obvious" are moot. If it's obvious, one should be able to explain it in a few lines of text. So is it?

With the brief clarity of a cup of tea this morning, what I am thinking is that, say, with a rigid bearing between flywheel and crankshaft, any forces on either end will have a bending moment around the immobile bearing. Allow that bearing to move in the direction of the force a little and the forces at the other end of the now moveable pivot are altered. Everything still flexes, just where and how much is the question here, is it not?

Can you do the math Guy_F? I've posted this question about crankshaft forces before up in the thread and I'll throw it out to the crowd again. Can anyone explain fully?

If you can, then set it out and let us (well, me) know once and for all. If you can't .... then at least we've still got heated words to back up our arguments,eh?
« Last Edit: October 09, 2006, 01:08:58 AM by Guy_Incognito »

mobile_bob

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Re: concrete vs resilient mounting
« Reply #68 on: October 09, 2006, 01:24:05 AM »
hey, i am no engineer,,, and i am still waiting :)

i understand physic's, and am fair with computing and modeling

so i think i can follow along  with the explanation.

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

bob g
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GuyFawkes

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Re: concrete vs resilient mounting
« Reply #69 on: October 09, 2006, 03:01:30 AM »
Ah, Guy_F - sometimes the simplest things(?) elude me.

I am aware of young's modulus. You don't need it often when you majored in optics unfortunately. Neither do you have to understand crankshaft flex in a 90-year old engine design that's being mounted in such a fashion that there is no 'fixed' point in it.

The old adage is, "on the internet, no-one knows you're a dog". So until someone can give me a hard answer, with numbers and pretty vector diagrams (preferably) so that I can follow the same steps and reach the same conclusion, all claims of "it's perfectly obvious" are moot. If it's obvious, one should be able to explain it in a few lines of text. So is it?

With the brief clarity of a cup of tea this morning, what I am thinking is that, say, with a rigid bearing between flywheel and crankshaft, any forces on either end will have a bending moment around the immobile bearing. Allow that bearing to move in the direction of the force a little and the forces at the other end of the now moveable pivot are altered. Everything still flexes, just where and how much is the question here, is it not?,

Can you do the math Guy_F? I've posted this question about crankshaft forces before up in the thread and I'll throw it out to the crowd again. Can anyone explain fully?

If you can, then set it out and let us (well, me) know once and for all. If you can't .... then at least we've still got heated words to back up our arguments,eh?

Funny how you can get a degree in optics and never encounter a lens being deformed by the shock loading of transmission of laser pulses. Youngs is the rate of change of stress with strain, and in optics is VERY commonly used as the operating principle of polariscopes, polarimiters, etc.

It's not even my field, but I know this. Strange.

Can I do the math (for an engine), you ask. Yes I can.

I've just spent about 3 hours tonight lecturing a (very promising) student.

People come in two flavours, those that prefer to be able to ask a question and get an answer, and those that prefer to to learn. The latter are in the minority.

The easy way to differentiate between them are the book learners and the thinkers. Book learners are always quick to do things like reach for spreadsheets and start banging made up (as opposed to tabulated from instruments over time) numbers into the first formula they found that appears to fit the bill.

Thinkers know everything that ever was and ever will be is subject to change, and sometimes some of those changes will be unexpected. A book learner will tell you he has calculated that slings and chains and load gives a safety factor of seven, a thinker will still arrange things so there is a safety sling and make sure nobody is standing in the path of anything, including the slings. A friend of mine lost his head, literally, when a hawser with a safety factor of about six parted, three other were hospitalised with serious injuries, the only one who walked away unscathed was a thinker, a canadian logger, who ducked for the cover he made damn sure he was standing next to the instant he saw the moisture being wrung out of the hawser.

Of course, if the book learners had KNOWN that the magnetic resonance (hall effect) testing of the wire rope was giving erroneous results because nobody included the effects of the big welder on the instrument head they might have calculated a different safety factor, and if the book learners had bothered to figure the rapid temperature rise from frozen on the drum to being played out under full tension they might have lowered it a bit more, and if the book learners had known that they had used the wrong equation (or rather, left out two important ones from the over all calculation) they might have lowered it a bit more.

Inquests suck. Misadventure my ass.

That wasn't the first, or the last, time in my life I have said out loud to other people "These assholes are going to kill someone one day" and lived to be proven right, me, I'm too small to stop it, I shouted, I've blown whistles, I've threatened to quit, I've quit, it never makes any difference because the one thing that is never in short supply is some asshole with book learning who is ready to sign off on it.

I've had pressure, deadlines and downtime and seriously big bucks for every hour that shit wasn't up, but money and metal can always be replaced, kill someone and you have had all they ever were and ever will be and nothing you ever do will dent that.

Only difference big bucks at stake makes is there is enough in the kitty for blone bints with big tits and an open bat tab to schmooze the PR machine when it all goes wrong.

You got a spreadsheet for taking a mans hand off so he never works again? What does that work out at in dollars?

I know you got fancy tables for risk assesment, that's what they are called anyway, but the purpose isn't to mitigate risk, it is to wash your hands when the numbers come up, hey, we did a risk assessment, so we ain't culpable.

Kipling said "when arf your bullets fly wide in the ditch", things ain't changed

The Young British Soldier

When the 'arf-made recruity goes out to the East
'E acts like a babe an' 'e drinks like a beast,
An' 'e wonders because 'e is frequent deceased
Ere 'e's fit for to serve as a soldier.
Serve, serve, serve as a soldier,
Serve, serve, serve as a soldier,
Serve, serve, serve as a soldier,
So-oldier OF the Queen!
   
Now all you recruities what's drafted to-day,
You shut up your rag-box an' 'ark to my lay,
An' I'll sing you a soldier as far as I may:
A soldier what's fit for a soldier.
Fit, fit, fit for a soldier . . .    
First mind you steer clear o' the grog-sellers' huts,
For they sell you Fixed Bay'nets that rots out your guts --
Ay, drink that 'ud eat the live steel from your butts --
An' it's bad for the young British soldier.
Bad, bad, bad for the soldier . . .    
When the cholera comes -- as it will past a doubt --
Keep out of the wet and don't go on the shout,
For the sickness gets in as the liquor dies out,
An' it crumples the young British soldier.
Crum-, crum-, crumples the soldier . . .    

 But the worst o' your foes is the sun over'ead:
You must wear your 'elmet for all that is said:
If 'e finds you uncovered 'e'll knock you down dead,
An' you'll die like a fool of a soldier.
Fool, fool, fool of a soldier . . .    
If you're cast for fatigue by a sergeant unkind,
Don't grouse like a woman nor crack on nor blind;
Be handy and civil, and then you will find
That it's beer for the young British soldier.
Beer, beer, beer for the soldier . . .    
Now, if you must marry, take care she is old --
A troop-sergeant's widow's the nicest I'm told,
For beauty won't help if your rations is cold,
Nor love ain't enough for a soldier.
'Nough, 'nough, 'nough for a soldier . . .    
If the wife should go wrong with a comrade, be loath
To shoot when you catch 'em -- you'll swing, on my oath! --
Make 'im take 'er and keep 'er: that's Hell for them both,
An' you're shut o' the curse of a soldier.
Curse, curse, curse of a soldier . . .    
When first under fire an' you're wishful to duck,
Don't look nor take 'eed at the man that is struck,
Be thankful you're livin', and trust to your luck
And march to your front like a soldier.
Front, front, front like a soldier . . .    
When 'arf of your bullets fly wide in the ditch,
Don't call your Martini a cross-eyed old bitch;
She's human as you are -- you treat her as sich,
An' she'll fight for the young British soldier.
Fight, fight, fight for the soldier . . .    


When shakin' their bustles like ladies so fine,
The guns o' the enemy wheel into line,
Shoot low at the limbers an' don't mind the shine,
For noise never startles the soldier.
Start-, start-, startles the soldier . . .    
If your officer's dead and the sergeants look white,
Remember it's ruin to run from a fight:
So take open order, lie down, and sit tight,
And wait for supports like a soldier.
Wait, wait, wait like a soldier . . .    
When you're wounded and left on Afghanistan's plains,
And the women come out to cut up what remains,
Jest roll to your rifle and blow out your brains
An' go to your Gawd like a soldier.
Go, go, go like a soldier,
Go, go, go like a soldier,
Go, go, go like a soldier,
So-oldier OF the Queen!
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Guy_Incognito

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Re: concrete vs resilient mounting
« Reply #70 on: October 09, 2006, 03:46:42 AM »
Quote
Funny how you can get a degree in optics and never encounter a lens being deformed by the shock loading of transmission of laser pulses. Youngs is the rate of change of stress with strain, and in optics is VERY commonly used as the operating principle of polariscopes, polarimiters, etc.

Sigh. Yes, I know the definition of young's modulus, and it's application in optics is how I'm aware of it. And to be honest, I've never dealt with deformation of lenses by high-energy laser pulses in practice. Or, to put it another way, I've never come across an application that needs to have that factored in. It's most likely out there.

Quote
Can I do the math (for an engine), you ask. Yes I can.

Good. Care to share? Or are you of some deviant personality style that knows the answers, but will not help a person find them? Don't let me reinvent the wheel here. If you care about the safety issues as much as you claim, you'll let me know. Drop a hint or something. I'm all ears.

Quote
People come in two flavours, those that prefer to be able to ask a question and get an answer, and those that prefer to to learn. The latter are in the minority.
he easy way to differentiate between them are the book learners and the thinkers. Book learners are always quick to do things like reach for spreadsheets and start banging made up (as opposed to tabulated from instruments over time) numbers into the first formula they found that appears to fit the bill.

(.......Lots of drivel deleted about accidents and lucky/unlucky people Guy_F knows, but don't really advance his point further than the above statement...... and a poem too! Niiice.)


I don't think it's quite as clear cut as you make out, but anyway....
So.... let me get this straight. The people that take the time to at least attempt to do the calculations before going ahead and getting themselves in trouble aren't the thinkers? And the people that go ahead, get in trouble, but are saved by some other safety feature are the one's in the right? Is this assuming that all non-thinking people that do the calculations aren't going to have that extra safety feature? Perhaps some people are like that, but I'm not one to blindly trust the numbers, even though I quote them a lot. Numbers are a useful guide, and even to be within an order of magnitude is enough to get a handle on it to factor in some level of safety.  I'm pretty sure that in all the posts here, I've never advocated going out there and just fiddling around after you've put together some half-arsed numbers. Hmmm. I think I even mentioned a 10X factor of safety for the forces. And posted that if real-world results differ while you're trialling it , you need to stop and find out why. But what do I know, I don't fit into your definition of a thinker.

Quote
I know you got fancy tables for risk assesment, that's what they are called anyway, but the purpose isn't to mitigate risk, it is to wash your hands when the numbers come up, hey, we did a risk assessment, so we ain't culpable.

I will only say this once to you Guy_F:

Risk assessment is not about ass-covering. It's about people not getting hurt. Period.

The calculations that I do to at least try and determine forces involved are a part of that risk assessment. They could be hideously wrong. But they are my best effort at reducing the risks involved to a manageable amount.
If you truly believed all your words you bleat about safety, you would gladly share your knowledge and numbers with me.

I've asked you publically and PM'd you for any more detail regarding those engine loading experiments. If someone has the numbers, I'd like to know, so I can at least try and calculate a factor of safety. 

Good thing we're not talking in a bar here Guy_f, after your statement about risk management, we'd be fighting in the street at this point.

Added, after calming down somewhat: The reason I get worked up about your poo-poohing of risk assessment is that it keeps me alive in my job where many hazards are present. Yes, it's a bonus for my employers that they don't have to pay out for all the severed limbs of course - and in these legal times a risk assessment is not a CYA anymore if someone gets hurt. Your examples given are good examples for the case for proper risk assessment - the fact that the one guy that was uninjured took the time to assess the risks and make sure cover was present indicates that if the others had done the same, they'd be uninjured or alive now.

And in any risk assessment you try and include everything - "So what would happen if it *did* break?" is a part of that, particularly if you can remember to say "Do I trust the numbers that we came up with for the strength of that thing?". If you don't continually keep doing that kind of thing, you end up with accidents.
« Last Edit: October 09, 2006, 06:40:54 AM by Guy_Incognito »

mobile_bob

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Re: concrete vs resilient mounting
« Reply #71 on: October 09, 2006, 04:05:47 AM »
Damn Guy i can't help but like ya! if anything you sure are entertaining :)

i follow your rational on the use of formulae and spread sheets to develop or support the development of anything where there are many variables , some of which either are not known, or cannot be known and assumed.

assumptions get you into trouble everytime, and working with unknowns are just as dangerous, working with variables that cannot be known is an exercise in futility, in my experience.

i have been doing a butt load of reading on this subject, basically to get a pattern of thought from many sources.
there is much written on the difficulty in modeling this sort of thing (engine mounting) so empirical testing seems to be the norm and the most expedient in getting the thing done, that is if it can be done.

where i can see math, equations, calculations etc being useful is if i knew for sure all the variables and even then i would use  the answer as a starting point and would expect to hopefully be in the ballpark, but i would also accept straight up that the answer might be way off of the reality.

having said that i do have a question, and it is a genuine question, that has bothered me over the run of this whole topic.

first i would like to preface the criteria

1.  an engine that is running on a hard/rigid/dense block such as concrete, the shock that is delivered to the crankshaft via the conrod, travels thru the main brgs, thru the case, thru the mounts and to the block. with every action there is an equal and opposite reaction, and it would appear that a significant amount of the shock is sent right back up the chain to the crankshaft. obviously the lister design is strong enough that this is of no concern.

2. an engine that is resilient mounted either on rubber or on a block of concrete that is sitting on wet clay, will have some movement, this movement appears to do two things,  one it absorbs a significant amount  of the shock sent down the crank as in #1 above, but .. two, it also allows the flywheels to change plane, and impart forces back onto the crankshaft from gyro forces simply stated

which of these two are you most concerned with? i would assume from thinking about this and what you have said it would be #2 as #1 has not been an issue with the original engine.

if it is #2 as i assume, is the real issue the crankshaft is not sufficiently heavy in cross section or design to carry 300# of flywheels and be allowed to move at all which would force it to counter or lever against gyro forces?

just thinking

bob g
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Guy_Incognito

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Re: concrete vs resilient mounting
« Reply #72 on: October 09, 2006, 07:29:29 AM »
Dammit Bob, I didn't want to have to start thinking!  :)

Aye, assumptions can bite you in the ass. But sometimes assumptions can help you appreciate the forces involved. If , by a few assumptions, you work out that the forces are "large" , then you can take steps to take your large forces into account.

Anyway, regarding your questions:

A couple of things :

 - One would presume that the forces exerted by the counterbalance in each flywheel are quite large at speed. This isn't just your normal "imbalance" that we have from poor manufacturing, its the entire balance weight needed to offset the piston/conrod mass. somebody did work out what it should have been, a couple of pounds at that distance? It's a pretty big force at 650RPM. It appears the crank can sustain that sort of force for a goodly period of time - that is, not many listers or listeroids of questionable balance have broken crankshafts. And listers of good balance that have "normal" counterweight forces have sustained 100,000 hours without broken cranks.

I'll try and step through it one more time.... maybe I should put it graphically with a picture.

A single cylinder listeroid operating is out of balance, by definition. So the centre of mass is offset from the centre of rotation a little bit. Not much really, when you take into account all that mass from the flywheels, but it is offset. Applied forces shift the apparent centre of motion as well, basically acting as a weight on an arm.

So , to reiterate, if you simply had a crankshaft and two flywheels spinning in the air, they wouldn't spin directly on the centreline of the crankshaft, the crankshaft centerline would actually spin in a little circle, due to the centre of mass being offset from the centre of rotation - the whole thing's unbalanced. Like an unbalanced washing machine, the whole thing wants to spin around it's centre of mass.

Now , if I slapped a pair of bearings on the crankshaft, and let them spin around with the rest of it, the only force on the bearings would be the force needed to move the bearings around the centre of mass. Not much force really, bearings don't weigh that much.

Go to the whole other extreme now - fix those bearings solid and make everything rotate around them - and the force on the bearings is now something else entirely.

It can be condensed to the entire mass of the two flywheels and crankshaft rotating on an arm that is the distance from centre of mass to the centre of rotation. There will be bending forces on the crankshaft where the counterweights on the flywheels - and the forces from the conrod - try and move the fixed bearings causing a bit of flex.

So from there to the middle ground : If the bearings are allowed to oscillate a little - basically "chase" the actual centre of mass of the entire mass of "Flywheel - crankshaft - Flywheel", then the closer they are at getting to the centre of mass - which the whole thing wants to rotate around -  the less bending force is exerted on the crankshaft.

Can anyone follow this, or should I just lay off the drugs?

With the Gyro forces .... isn't that only generally applicable when you try and twist the axis of rotation at 90 degrees to the spinning object? (eg, viewed from the top of a listeroid, twisting left and right). Simple motion up/down/left/right in the same plane doesn't generate it, correct?

GuyFawkes

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Re: concrete vs resilient mounting
« Reply #73 on: October 09, 2006, 10:47:22 AM »
Damn Guy i can't help but like ya! if anything you sure are entertaining :)

i follow your rational on the use of formulae and spread sheets to develop or support the development of anything where there are many variables , some of which either are not known, or cannot be known and assumed.

assumptions get you into trouble everytime, and working with unknowns are just as dangerous, working with variables that cannot be known is an exercise in futility, in my experience.

i have been doing a butt load of reading on this subject, basically to get a pattern of thought from many sources.
there is much written on the difficulty in modeling this sort of thing (engine mounting) so empirical testing seems to be the norm and the most expedient in getting the thing done, that is if it can be done.

where i can see math, equations, calculations etc being useful is if i knew for sure all the variables and even then i would use  the answer as a starting point and would expect to hopefully be in the ballpark, but i would also accept straight up that the answer might be way off of the reality.

having said that i do have a question, and it is a genuine question, that has bothered me over the run of this whole topic.

first i would like to preface the criteria

1.  an engine that is running on a hard/rigid/dense block such as concrete, the shock that is delivered to the crankshaft via the conrod, travels thru the main brgs, thru the case, thru the mounts and to the block. with every action there is an equal and opposite reaction, and it would appear that a significant amount of the shock is sent right back up the chain to the crankshaft. obviously the lister design is strong enough that this is of no concern.

2. an engine that is resilient mounted either on rubber or on a block of concrete that is sitting on wet clay, will have some movement, this movement appears to do two things,  one it absorbs a significant amount  of the shock sent down the crank as in #1 above, but .. two, it also allows the flywheels to change plane, and impart forces back onto the crankshaft from gyro forces simply stated

which of these two are you most concerned with? i would assume from thinking about this and what you have said it would be #2 as #1 has not been an issue with the original engine.

if it is #2 as i assume, is the real issue the crankshaft is not sufficiently heavy in cross section or design to carry 300# of flywheels and be allowed to move at all which would force it to counter or lever against gyro forces?

just thinking

bob g

Assumptions will kill you bob, you know this, you state this, then you ask a series of questions based on an assumption.

if there are 100 factors and you think there are 99 or only know the answers for 99 then you do NOT have an approximately accurate answer, you have something that is LESS use than a number pulled out of your ass, because you don't bet your life on a number pulled out of your ass.

incognito is talking about building a test rig, but he ain't building a test rig, he is just talking about it, he is building a real rig, the difference between a ricardo variable compression test engine and dynamomenter and a CS, the test rig serves no purpose but to test, and no matter what breaks it has belt and braces, and it only ever gets you basis starting data.

you ask the question that the lister way and incognitos way are both basically isolastic mounts.

yes they are, in theory.

in practice there are two huge differences.

1/ the lister way has mass, change that and you change EVERYTHING

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.

The difference is one is a large and massive system, and the forces at work are distributed weakly over large masses and through large contiguous volumes of material, and the other system focuses all that effort and work at extreme density and flux through very small masses and through very small volumes of material.

5 pounds weight will support your back nicely when applied to a cushion, the same weight wil drive a sharpened spike right through your body.

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3Kw 130 VDC Dynamo to be added. (compressor + hyd pump)
Original Lister D, megasquirt multifuel project, compressor and truck alternator.
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GuyFawkes

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Re: concrete vs resilient mounting
« Reply #74 on: October 09, 2006, 11:32:28 AM »
Quote
Funny how you can get a degree in optics and never encounter a lens being deformed by the shock loading of transmission of laser pulses. Youngs is the rate of change of stress with strain, and in optics is VERY commonly used as the operating principle of polariscopes, polarimiters, etc.

Sigh. Yes, I know the definition of young's modulus, and it's application in optics is how I'm aware of it. And to be honest, I've never dealt with deformation of lenses by high-energy laser pulses in practice. Or, to put it another way, I've never come across an application that needs to have that factored in. It's most likely out there.

Quote
Can I do the math (for an engine), you ask. Yes I can.

Good. Care to share? Or are you of some deviant personality style that knows the answers, but will not help a person find them? Don't let me reinvent the wheel here. If you care about the safety issues as much as you claim, you'll let me know. Drop a hint or something. I'm all ears.

Quote
People come in two flavours, those that prefer to be able to ask a question and get an answer, and those that prefer to to learn. The latter are in the minority.
he easy way to differentiate between them are the book learners and the thinkers. Book learners are always quick to do things like reach for spreadsheets and start banging made up (as opposed to tabulated from instruments over time) numbers into the first formula they found that appears to fit the bill.

(.......Lots of drivel deleted about accidents and lucky/unlucky people Guy_F knows, but don't really advance his point further than the above statement...... and a poem too! Niiice.)


I don't think it's quite as clear cut as you make out, but anyway....
So.... let me get this straight. The people that take the time to at least attempt to do the calculations before going ahead and getting themselves in trouble aren't the thinkers? And the people that go ahead, get in trouble, but are saved by some other safety feature are the one's in the right? Is this assuming that all non-thinking people that do the calculations aren't going to have that extra safety feature? Perhaps some people are like that, but I'm not one to blindly trust the numbers, even though I quote them a lot. Numbers are a useful guide, and even to be within an order of magnitude is enough to get a handle on it to factor in some level of safety.  I'm pretty sure that in all the posts here, I've never advocated going out there and just fiddling around after you've put together some half-arsed numbers. Hmmm. I think I even mentioned a 10X factor of safety for the forces. And posted that if real-world results differ while you're trialling it , you need to stop and find out why. But what do I know, I don't fit into your definition of a thinker.

Quote
I know you got fancy tables for risk assesment, that's what they are called anyway, but the purpose isn't to mitigate risk, it is to wash your hands when the numbers come up, hey, we did a risk assessment, so we ain't culpable.

I will only say this once to you Guy_F:

Risk assessment is not about ass-covering. It's about people not getting hurt. Period.

The calculations that I do to at least try and determine forces involved are a part of that risk assessment. They could be hideously wrong. But they are my best effort at reducing the risks involved to a manageable amount.
If you truly believed all your words you bleat about safety, you would gladly share your knowledge and numbers with me.

I've asked you publically and PM'd you for any more detail regarding those engine loading experiments. If someone has the numbers, I'd like to know, so I can at least try and calculate a factor of safety. 

Good thing we're not talking in a bar here Guy_f, after your statement about risk management, we'd be fighting in the street at this point.

Added, after calming down somewhat: The reason I get worked up about your poo-poohing of risk assessment is that it keeps me alive in my job where many hazards are present. Yes, it's a bonus for my employers that they don't have to pay out for all the severed limbs of course - and in these legal times a risk assessment is not a CYA anymore if someone gets hurt. Your examples given are good examples for the case for proper risk assessment - the fact that the one guy that was uninjured took the time to assess the risks and make sure cover was present indicates that if the others had done the same, they'd be uninjured or alive now.

And in any risk assessment you try and include everything - "So what would happen if it *did* break?" is a part of that, particularly if you can remember to say "Do I trust the numbers that we came up with for the strength of that thing?". If you don't continually keep doing that kind of thing, you end up with accidents.

you should do a risk assesment before inviting people out into the street.

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.

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.

your problem is you lack the experience and mental discipline to see the problem clearly,

you are exactly analogous to those people, and most of them have ridden bikes for years, who know for a fact because it is obvious because it is common sense because they thought it through without any mental discipline or experience, that you turn the handlebars on a motorcycle right if you want to turn right, the reality is you don't even need to turn them, you just apply force, precession will do the rest, and you apply the force to turn the bars left if you want to turn right, more force for a sharper turn, that force causes the precessive forces and those forces cause the system to lean one way or another and that causes changes in wheel geometry and thats what makes the bike turn.

only way to teach people this is send them out on a wide straight empty road, put your left hand across on the right bar and try and go straight, you learn or suffer a great deal of pain.

pain is a good tutor.

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 and walk away from it, the most dangerous part of a motor vehicle is the nut behind the steering wheel. He assumes he is a good driver and in control and nothing unexpected is going to happen.

like I said, inquests suck.

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".

Things never used to be done like that, it never used to be that assholes got to keep asking the same question until they got the answer they liked.

Many years ago (true story) in sheffield there was a large furnace that was used to pour a speciality steel, various buy outs went through and the new management spent millions on a computer system to control the pour and sacked the old guy who used to decide when to pour, quality and consistency plummeted, they spent more millions on more computers and consultants and made incremental improvements, but still nowhere near the quality and consistency they used to have, so one day one wag decided to call the old boy they sacked in, he comes in, free or charge, for one day and tells them when to pour, suddenly quality and consistency was back up to old levels.

The bean counters (like you) who were convinced that if an old man could do it they could do it with enough number crunching tried again, more millions and more time, and improved it some, but it was still shit compared to what the old man used to do, day in and day out.

The had accidents too, pouring hot steel is dangerous, get it slightly off and you get fountains of molten metal that fly further than you were expecting.

Eventually they went back to the old man, and management was there when he came back in for the day.

He made them sit there and listen to the old days, when sheffield also had many small glassworks, before assholes like them got into management and closed them down, and one old boy in one of the glassworks made him up this small rod of coloured glass that he used to hold in his fist, so nobody saw it and assumed be was just squinting and using his hand as a shield from the heat, but he was watching for a colour change in the melt through this glass rod, and when he saw it he'd tell them to pour.

So they got their answer, a piece of glass and an old mans skill and experience got a result that they simply could not hope to duplicate with computers and numbers, so even though they had the answer to their production, quality and safety problems handed to them on a plate for free they couldn't use it, and said it was useless to them.

If you go to that site today there is a shopping mall and cinema multiplex there. The old man is long dead and only his memory lives on in people like me who re-tell the story now and again.

The answer to the questions you keep asking me is tied up in that story, but you won't see that, you aren't equipped to see that, and so you, like that steel company, won't ever get an answer that is any use to you, and like the steel company you won't accept that that represents a failing on your part, and not an inadequate answer.

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.
--
Original Lister CS 6/1 Start-o-matic 2.5 Kw (radiator conversion)
3Kw 130 VDC Dynamo to be added. (compressor + hyd pump)
Original Lister D, megasquirt multifuel project, compressor and truck alternator.
Current status - project / standby, Fuel, good old pump diesel.