Torsional Vibration Analysis is a method by which torsional vibrations can be detected within any rotary system.

What is torsional vibration? Torsional vibration is the angular velocity variation in any rotating component. Let's say we have a printing unit gear train made up of several gears driving from one to another. No gear is perfect, so each gear has errors in it that cause it to drive the gear next to it imperfectly. Now if all of the gears have errors, and each one drives the next, you can see that the rotational speed of the last gear in the train could be far from perfect. The motion of that last gear is influenced by of all of the gears that drive it. What's worse is that even downstream gears can have an effect. It is not uncommon for bent vibrator shafts or worn out vibrator gearboxes to show up in print as register problems.

Torsional Vibration Analysis allows the engineer to look at only one gear in the train, preferably the last one, and sort out all of the gears that are driving it to find out the contribution of each one. So, if a 17" gear somewhere low in the gear train has a lot of pitchline runout (the main cause of torsional vibration in printing presses), TVA will find it. Now, if there is more than one 17" gear in the gear train, TVA can't distinguish between the two, but it does narrow down the choices of where to look for the problem.

The components of a typical TVA system are:

a sensor, usually a magnetic pickup looking at the tops of the gear teeth, but could be an encoder, or any other device capable of generating a pulse train,

a recording device, usually a multi-channel analog tape recorder,

a torsional converter, a device that filters out the carrier frequency of the individual pulses and outputs the useful information to...

a spectrum analyzer.


The spectrum analyzer sorts the incoming signal into its component waveforms and displays them by frequency and magnitude. Each gear has its respective frequency for each speed tested, and the magnitude indicates the level of error contributed from that frequency or gear size.

How it all comes together

Typically, TVA test data is recorded on all units in the press simultaneously as a matter of convenience. Then each unit is analyzed and the TVA data is plotted in a waterfall fashion showing the successive test speeds ascending on the vertical axis of a graph. The result is a TVA graph for each unit that can be scrutinized individually to determine problem areas.

However, there is a huge limitation with standard TVA testing.


Any comparison from unit to unit can only result in assumptions of what is actually happening between the two units. Because TVA cannot consider the phase relationship of the components, direct comparisons of individual units cannot be safely made.

Imagine this: I have a 6 color press that is exhibiting register movement in units 2 and 3. I run a TVA on the print units and see that the both units show that the 12" gear in the gear train has a .007" torsional vibration. This would typically be dismissed as an acceptable amount of torsional vibration.