Doc,

the damper shown at your link is made by MED. It's a new unit and it isn't even on their website yet. I spoke with MED and the unit is intended for use on 1275 engines, not the smaller engines.

what you say is incorrect, Mr. Rosenthal. Damage to a crank from torsional vibration happens mostly when the clutch is not in use. It can happen every time the engine runs through the RPM range where there is what Vizard refers to as a "critical vibration period". He provides some useful information on page 390 of the thrid edition of his book "Tuning the A-series Engine". He also explains elsewhere in the book, the need to balance the damper along with the crankshaft. The diaphragm and backplate are located by three sets of straps which do not allow the diaphragm and backplate to move once the bolts securing the straps are done up. Since the diaphragm is one of the crudest parts in the engine and the one most likely to be miles out of balance, it is essential to balance the diaphragm along with the rest of the crank, damper and backplate.

An engine does not have to be out of balance at all for torsional vibration to exist. Torsional vibration is a natural result of combustion in each of the cylinders and the downward moving rod and piston placing Torsional(twisting) loads on the crankshaft. The mass of the flywheel resists the crank's need to rotate in response to the piston and rod movements.

The heavier the flywheel, the more it resists the crank's need to rotate and torsional vibrations are set up as the crank is "wound up" and then released, like a spring. The damper is installed in an attempt to reduce the torsional wind/unwind movements and in so doing reduce the stress on the crank.

An excellent article on crankshaft torsional vibration is at: http://www.dinancars.com/bmw/technial-info/the-dangers-of-power-pulleys-and-understanding-the-harmonic-damper

Yep, that's the one I got from MED (although I got the complete pulley/damper/long bolt) - Very nicely made.

mini7bpy,,, you quote very good articles,,, but i believe what David R was leading to was the fact that those 3x straps that attach the diaph to the backing plate actually do "Have to" swing in an arc whilst opperating in both directions (engage & dissengage),,, therfor the whole diaph & backing plate mass actually "Does" move "out of sync" in relation to the flywheel & cranks assy.

please understand that your statement --->

"""The diaphragm and backplate are located by three sets of straps which do not allow the diaphragm and backplate to move once the bolts securing the straps are done up. """"

is actually quite incorrect,,, the straps must swing in an arc,,, all be it only slight, however it still Must swing in order for the dissengagment to take place,,,& yes it is a very crude way about it, hence that`s "One" of the reasons why the factory opted for a re-design when going to the later Verto set up.

But stilll any drama at one end of the crank directly relates to drama all along the entire crank & can quite often be confused to dramas resultant at the other end,,, some preople claim problems "Caused" by the dampener but are often more caused by poor flywheel assy (&/or) balance,,,, poor crank condtition (as in previous crank fatigue or excessive bearing wear) etc... & that then translates to the dampener copping the abuse & often being blamed for the original cause,,, yet it`s most often the other way round & the broken dampener is just another part of the result of another underlying issue that has caused the whole failure.

_________________
No offence intended here but--> anyone writing a book about minis 30 years ago may not have experienced such worn or stuffed-with components as we are finding these days.

You should put your heart & soul into everything you do.

miniman,

the phenomena you describe is trivial compared with the impact of a diaphragm that is either out-of-balance as a part OR has been mounted(using the straps) slightly off-center with respect to the flywheel/backplate assembly. The bolt holes in the straps are enough larger than the bolts that a fair bit of movement is allowed for the diaphragm lump until the bolts are cinched down. Because the diaphragm is a movable target, care must be taken when locating it before it is cinched down by the straps and their bolts. The diaphragm must be returned to its "as balanced" location after any time that the diaphragm's bolts are slackened or removed as when the lutch disc is replaced. Failure to do this may result in a BADLY out of balance crank assembly.

I'm not making this up. I have played around with a crankshaft/flywheel/backplate/diaphragm assembly on a dynamic balancing machine and I know the difference between what you described and what I saw. They are orders of magnitude different. The amount of dislocation caused by the straps moving(as you described) is absolutely minute compared with the effect caused by the diaphragm(even one which is truly balanced by itself) moving within the range of motion allowed by slackening the strap bolts slightly.

It's all a rather crude mess of an arrangement which is not helped by the relative crudeness of the diaphragms which are often badly out-of-balance themselves. A diaphragm is a heavy lump, crudely formed, and it must not only be balanced by itself, but it must also be precisely located before balancing with the crank/flywheel/backplate assembly as well as AFTER balancing has occurred as when either replacing the diaphragm or changing the clutch disc.

Whoosh. (that was the sound of the point being missed.)

You use a machine that looks like a fancy wheel balancing machine (but a lot more elaborate) You spin it and find the heavy side of the damper and then remove a little weight (usually with a drill or grinder or mill) and then spin it again. Repeat till it is balanced. Quite easy if you have access to the machine.

Some dampers are supposed to be ready to fit straight out of the box and some are supposed to be balanced before use.
I find that they all need doing.

hee hee

-----------------------

Mini7boy,,, In an attempt to not to sound condecending or patronising at all,,, nor meant in any way,,, but building mini engines my whole life,,, balancing them all (one thing at a time & assemble spin add spin add spin etc etc, done it for my whole life) ,,, i`m pretty sure i got the point of balancing the entire rotating mass(s) & "Centering" the clutch assy(s) about 35 plus years ago & yes,,, you`re obviously quite correct in what you have said,,,

But it wasn`t Davids point,,, nor was it mine,,, & as you also suggest that the "Imbalance" whilst dissengaging the clutch is "Absolutely Minute" or "Trivial" (as you have put it),,, well it actually is a rather major factor,,, the "swing" or "Arc" that is created by the use of the 3x sprap pairs, (flexing) whilst opperating the clutch is "rotating" that big heavy """diaph & back plate assembly""" away from its "Balanced" position,,,

that was Davids point

& my point to you was only to enlighten you that "That" was Davids point

I`m fully aware of (& i totally believe that David is also) what happens while spinning the clothes line (full of wet clothes) & then trying to wobble the pole that holds it up
cheers mate

_________________
No offence intended here but--> anyone writing a book about minis 30 years ago may not have experienced such worn or stuffed-with components as we are finding these days.

You should put your heart & soul into everything you do.

Mini7boy for own satisfaction just try this little experiment .
1 assembly a clutch back plate, clutch plate and pressure plate to a flywheel.
2 set up a dial indicator as per the pics.
3 then use a screw driver under one of the straps opposite the dial indicator and try to bend the strap sideways.
The whole back plate and pressure plate will move off centerline.





Now to put this experiment into something you can understand.
1 the mass of the back plate and pressure plate is 3.55kgs.
2 the distance the assembly is moving off center is .15mm[not much distance]
3 the engine is rotating at 6000 rpm and the clutch is used.

Now if the assembly move even .15mm from the original balanced position you now have a out of balance force affecting the rotating mass.
With this case the out of balance force can be calculated . It is a centrifugal force as the thing is rotating.
CF =mass x radians[squared] xdist of movement
radians =rpm x2 [3.142] divided by 60
IE 6000 x 2[3.142] devdide by 60
=200x 3.142 rad/sec this give you the number of radians

therefore CF =3.55[kgs] x[200 x3.142[{squared} x.00015 meters
=3.55[kgs] x 394,886.56 x .00015 meters
=210.27 newtons
Now add this force to the torsional vibration calculations at the critical node points and this is why crankshafts in mini engines get a lot more stress than a conventional crank/ clutch assembly.

I agree that the MOST critical aspect is to get the initial postion of the clutch assembly perfectly positioned. If you add the above force to an assembly that is not set up correctly then it will lead to premature crank failure.

_________________
Research is the difference between speculation and investment. Anyone who copys some one else will always be second
www.minisprintgt.com

yeah,,,well,,, that was exactly what i was going to say

_________________
No offence intended here but--> anyone writing a book about minis 30 years ago may not have experienced such worn or stuffed-with components as we are finding these days.

You should put your heart & soul into everything you do.

The way the flywheel and clutch is set up I am surprised that they don't break the crank when you turn the steering.
Spin a bicycle wheel and turn the steering to see the gyro effect, then imagine a spinning overhanging flywheel and clutch...

mini7boy,
just a little addition to your discription of "torsional vibration" comments. A damper in no way assits with the balancing or the torsional vibration that is produced in a engine. It assists in leveling out the vibration frequency at specific rpm.
If you do the torsional vibration calculations on a engine you find that the frequency and magnitude varies with the rotational speed. The engine will have certain rpm where the vibration frequency and magnitude reach a maximum. These points are called "nodes".
Most of the big marine diesel engines have a circulating fluid system damper on each end of the engine. These absorb the vibrations and then convert the energy into heat which is the removed from the fluid.
These engines also have a complex measuring system built in that measures the torsional vibrations as it is running. One problem with large engines,30,000kw or bigger running between 200 to 400 rpm is that the frequency and magnitude can vary greatly if the cylinder firing pressure varies. This means that the node point or points are very close together and can be easily exceeded and some thing goes bang.
Yes it is good to make sure that every componet is perfectly aglined and balanced,but the point that I am trying to make is that any slight movement off center can cause a big problem at a specified rpm.
When you compare a normal clutch set-up and a mini set-up, a normal one is positively located ie dowlled to the flywheel which is dowlled to the crank. A mini is not.
You obviously have a fairly good understanding of what is occuring but look into it more and look at all the dynamics associated with it,not just one small part.

_________________
Research is the difference between speculation and investment. Anyone who copys some one else will always be second
www.minisprintgt.com

Mr. Rosenthal. I never said that a damper does any balancing. I know better even though some people refer to them as harmonic balancers. That said, they absolutely do work to dampen torsional vibrations that develop in the crank as the cylinders fire and impose twisting loads on the crank that are partially resisted by the inertia of the flywheel. The crank is thus twisted, however slightly, by these processes. Much like a shock absorber(misnamed) dampens suspension movements, a crank damper dampens these crank vibrations. The article whose link I provided makes this very clear. I don't have any more time to spend on this subject. It is a complex subject, poorly understood by most, and is further complicated by damper manufacturers providing all sorts of dampers, some with very little actual engineering behind them. Racing use really tests these dampers and some just aren't up to it. Hence, some of the dampers fail with use and cause more problems than the solve.

Mr. Rosenthal and Miniman, an interesting question for you.

Mr. Rosenthal said "When you compare a normal clutch set-up and a Mini set-up, a normal one is positively located ie dowelled to the flywheel which is dowelled to the crank. "

This is quite true, but it's equally true that virtually all auto manufacturers include torsional dampers on their crankshafts. Many of these engines have automatic transmissions and, of the others, almost none have a clutch arrangement like the Mini. An automatic Mini obviously covers both of these categories.

So, if as you claim, a damper is used on a manual shift engine because of clutch/crank out-of-balance conditions, then why do so many manufacturers include these dampers on those engines that do not have a Mini clutch setup and its problems that you mention?

It's simply because the real reason for using torsional dampers is to dampen crankshaft torsional vibrations that have nothing at all to do with the clutch setup like that used only on a Mini.

I don't deny that the clutch setup used on a Mini causes some interesting technical problems with respect to balance, but these problems are not the reason why Mini engines have torsional dampers installed on their crankshafts.

Once again, I will repeat that I NEVER suggested or stated that a torsional damper has ANYTHING to do with balancing, for a Mini engine or any other type of engine. The science is clear on why these dampers are necessary. If you look back through the bibliographies of many automotive engine design/engineering books, there are references to torsional vibration problems and dampers to address them in articles dating back to at least the 1930s.

I have attached a link to another article which makes this point perfectly clear, even in its title.

The title of this article is: Torsional Dampers Not Harmonic Balancers

http://www.grapeaperacing.com/tech/dampers.pdf

mini7boy

maybe you should have re- read what David said early in this thread------>



sorry mate,,, but it`s quite clear, simple & easy to understand that David has a very very good understanding of how things work

David & I have never ever stated that mini engines are the only engines that need vibration dampeners & niether did any of us suggest that it`s "ALL" to do with the minis sh!tty clutch design

please , just get back to the thread title topic, you really do seem to be beating your head against the wall mate

you can try to teach whatever you like, but if some of the kids in the class know more about the subject than yourself then you may loose some respect,,,

as Morris1100 stated earlier----> """"Whoosh. (that was the sound of the point being missed.)"""

I appreciate your input & your concern mini7boy,,, i really do,,, i think we all like to help out if & where we can,,, & i`m sure there are many many people out there who are actually learning from all this,,, which is obviously a good thing,,, but the term flogging a dead horse come to mind here mate.

_________________
No offence intended here but--> anyone writing a book about minis 30 years ago may not have experienced such worn or stuffed-with components as we are finding these days.

You should put your heart & soul into everything you do.