From: v087mxgb@ubvmsb.cc.buffalo.edu (Shawn E Thompson)
Newsgroups: rec.autos.tech
Subject: Re: Vibration (drivetrain modification) - expert (imNSho) opinion
Keywords: auto, repair, vibration, modification
Date: 11 Sep 92 00:01:00 GMT
Organization: University at Buffalo
News-Software: VAX/VMS VNEWS 1.41

In article <1992Sep10.160714.3616@porthos.cc.bellcore.com>, jimo@navaho.uucp (25656-o'keefe) writes...
> 

Jim (& all interested parties)


interesting story. 


I used to design torsional vibration dampers for crankshafts
and driveshafts/drivetrains. 


I have specific knowledge of specific applications that
I would probably be more professional *not* to disclose.


Anyway, the problem between the 2 driveshaft designs 
is the added spring-mass effect of an additional 
U-joint and torsional mass. 

See, U-joints induce a sinusoidal impulse frequency
(freq=1/(2*angualr velocity), that causes torsional
vibration. This is due to the 180 degree symmetry of
the non-uniform load transfer between the input and
output shafts across the joint. Having *2* U-joints
mounted 90 degrees out of phase cancels the effect this
has on output angular velocity and makes it constant
(and equal to input angular velocity). However, at the
shaft (between the two joints) there is still a sinusoidally
varying angular velocity (hence angular acceleration and
torque). This causes torsional vibration of the shaft.

Torsional vibration is the twisting of the
shaft (in addition to its rotational speed).

Now, generally this vibration is usually relatively
small and the energy is consumed in the hysteresis 
damping of the driveshaft components and the coulomb
damping in the system's friction.

Now, I have to digress to the engine to make a point:

In a IC engine the crankshaft experiences this same
effect, but on a much larger scale, from the movement
of the piston/rod masses and the firing pressure of
the combustion cycle.

In this kind of system , this effect can cause catastophic
torsional stress failure of the crankshaft and related
components. One of two things are used to solve this:

1. In gasoline engines (high volume passenger cars, etc)
it is more economical to add a spring-mass to the system
to move the critical resonant frequency(-ies) out of the
normal operating range of the engine (higher *usually*).
This is what your harmonic balancer does!

2. In Diesel engines and high-compression gasoline engines
where higher firing pressures cause a much greater manifestation
of this phenomenon, a viscous or friction torsional vibration
damper is added to the system to actually convert the torque
energy into heat.


Back to drivetrains:



In a drivetrain, this effect is much more subtle (although it
may not seem it in your van). These can be solved the same ways.
Your mechanic changed the spring-mass characteristics of your
system (effectively stiffening your drive-train torsionally
and raising the natural (resonant) frequencies out of the
operating range). This *may* work, but it could also raise
a lower order harmonic into your operating range.

Also, be careful about replacing a driveshaft. The auto industry
is extremely price sensitive, so they would *not* add a more
expensive two-piece driveshaft design without good reason.

You may introduce excessive side-loading on the system and
possibly acclerate wear on the bearings. I dunno, its hard 
to tell without all the design details.

Although this doesn't help you, I must tell you, returning 
to a dealer cannot help you if this is a design problem....


Best bet is to add a harmonic driveshaft balancer to your
stock driveshaft. Problem is radial height is at a premium under
a vehicle, so getting an effective one is challenging.

These are manufactured and available. They can bolt over the shaft
or at the joints, or some other convenient location...

Check around for one of these in particular. You may have to 
experiment with the various sizes (masses/spring rates of the
rubber interface) until you get the optimum setup.



Good Luck



Shawn Thompson
(ex-automotive-industry-engineer-now-temporarily-a-microscope-
engineer-where-the-heck-did-that-connection-come-from?)
Newsgroups: rec.autos.tech