Time synchronous averaging - MATLAB I have a time domain signal that corresponds to a vibration signal of a machine. I have a second signal that corresponds to a tachometer signal (There is a pulse every one revolution of the shaft). 
I'd like to use time synchronous averaging under MATLAB. I'd like to take all blocks of time from the time domain signal whose durations is the duration of one revolution of the signal and average them out.

The problem I have is that the speed of the shaft is slightly fluctuating. I've set it to 900RPM but it goes from 895RPM to 904RPM during the acquisition and so the duration of one revolution fluctuates as well (some miliseconds).
My question is would averaging blocks of signals of different durations be a problem? And if so, how should I deal with this?
The answer to your question "is it a problem" is really up to you to determine. Sometimes the simple things work well enough.

There is a technique called Dynamic Time Warping:

https://en.wikipedia.org/wiki/Dynamic_time_warping

and Matlab has a function dtw

https://www.mathworks.com/help/signal/ref/dtw.html

which can use to figure out if it is a problem.

I suggest you pick a reference interval, that would be the closest to the average shaft interval and use that to warp the rest against.