[Supras] Balance shafts

[berniek at technicaldevelop.com]

Answer to Supras-Bounces on behalf of Mike Long:

   
    The subject of balance shafts is entirely different than the 
function of torsional vibration dampers.  You cannot feel torsional 
vibration.  It is a back and forth motion WITHIN he crankshaft set up as 
a spring-mass resonance like a watch balance wheel with a spring so 
stiff that the balance wheel oscillates thousands of times a second.  A 
bearing needs the journal to turn to set up a wedge of oil (hydrodynamic 
wedge) which lifts the journal off the bearing shell.  If the journal 
(particularly the front main and sometimes the connecting rod) of a 
crankshaft torsionally oscillates almost destructively, the hydrodynamic 
wedge cannot form due to very quick reversals in the journal vs. bearing 
speeds.  That is the death of bearing inserts Jeff was referring to with 
harmonics (the shaft can oscillate in a fundamental frequency mode or on 
harmonics of the fundamental (just like guitar strings when you pluck 
with a light touch left finger at a node on a string, or like the 
harmonic series in a trumpet or Saxophone with the octave key open.


Engine balance shafts are a totally different matter and relate to 
shaking of the engine.  A four cylinder engine is in PRIMARY balance 
because pistons 1 and 4 are moving in directions opposite to those of 
pistons 2 and 3.  Yet, the opposing piston velocities are not equal.  
The reason is the fact that near BDC, the connecting rod swings to an 
extent WITH the crankpin, so piston motion per degree of crank rotation 
is not the same as near TDC, where the change in piston motion is more 
abrubt because the large end of the connecting rod APPROACHES AND LEAVES 
the crankpin rotation more quickly.  If you draw a sketch of an engine 
with long stroke and very short connecting rods this becomes obvious.  
This is much of the story of connecting rod angularity.  So a 4 cylinder 
engine is not in SECONDARY   balance.  It has a vertical shaking force 
at twice the crankshaft speed which the balance shafts are supposed to 
subdue.  There are off-center masses on each which counter rotate to 
equalize the vertical shaking force of the engine itself.  If you remove 
the oil fill cap from an inline 4 when it is running, you will hear a 
humming sound from air rushing in and out.  That again is because of the 
same effect:  The crankcase volume is NOT constant ALL of the time due 
to the, so the hum will occur at a frequency twice the RPM of the 
crankshaft.  


An inline 6 IS in primary and secondary balance, which can be 
demonstrated.  Crankshaft counterweighting is used to reduce the 
longitudinal stresses within the block (and in the main bearings) which 
would be present if counterweighting were not present. 


Hope this helps and is not too technical.


Bernie