May 31st, Jim Snyder wrote:
>
> You mention that an owner could balance their connecting rods themselves.
> I assume that the important issue is that the rods all be the same weight.
> How close to equal do they need to be? What would be an acceptable
> difference between heaviest and lightest?
>
>
Thanks for your excellent question, Jim!
Two remarks:
1. Nothing in real life is perfect. This also applies to balancing.
One thing you should think about first is how accurately a crankshaft
should be balanced so that one one hand, everyone should agree
that it has been done good enough, on the other hand you do not spend
too much time in trying to obtain 'absolute zero' as a result.
This issue was addressed AFAIK first in the 1960s in Germany where
a group of mechanical engineers sat together to discuss this topic
and developed an engineering guide line on this subject.
(VDI 2060, for those of you who understand German).
Later this was adapted (with only minor changes) as an international
engineering standard through the International Standard Organisation
(a United Nations organisation, actually).
This standard is known as "ISO 1940.1, Balance quality of rotating
rigid bodies". In most cases this standard works just fine.
To make a very long story short (?! really ?) e.g. for a crankshaft
like that of an MGB we consider it to be acceptable if, after balancing,
only 1 g (.03 oz) is remaining [at a radius of, say, 50 mm, or 2 in, in
each plane].
This is called the "maximum allowable residual unbalance".
This (theoretical) radius of 50 mm where you grind or drill away any
excess material to improve mass distribution is also more or less
(half the stroke =) the center line of the crank journals ("wrist pins",
I don't know if this is the correct American word but at least that is
how Harley Davidson owners call them).
It follows that the difference in weight between the con rods should
be in the same order of magnitude. 1 g or 2 g difference (.03 / .06 oz)
is about the maximum that should be allowed.
(BTW a MGB conrod weighs about 1 kg, that's slightly more than 2 lbs).
2. Actually it is a bit more complicated than just that.
It is not _just_ the _overall_ weight of the connecting rod
but also the way in which the mass is distributed.
You can think of a conrod as being split up in two parts;
1. the part that goes up and down, with the piston,
piston rings etc. ("reciprocating").
'The small end'
2. the part that rotates with the crank journal
("oscillating").
' The big end'
Ideally, masses of small ends and big ends of the con rods should
(also) be the same for all four of them.
I'm not an ASCII graphics artist so I can not draw for you how it should
be done but any good book on engine tuning should be able to show you,
at least in theory, how you can weigh the small end portion separately.
In real life these measurements invariably have some errors but
the tolerable margin on the distribution (small end mass vs. big end mass)
is not so critical as the margin for the total mass.
So, yes, you CAN balance connecting rods yourself!
You just need a reasonably accurate scale (see if you can borrow one).
We use a max. 3000 g scale with a resolution of 0.1 g for this.
BTW the figures that I have given above are _our_ standards,
It is likely that at the factory, even on a good day,
they were not _that_ accurate.
Hope again that this has been of some interest to you
(I could go on for hours....)
Have a nice weekend!
Bert Palte
Holland, Europe
|