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What is repeatability, really?

Repeatability is defined as the ability to obtain the same results in multiple measurements under the same conditions. Repeatability is one of the most important things to consider when purchasing any type of measurement instrument since non-repeatability renders any measurement useless. It is no different with a laser shaft alignment system. No matter how easy a tool claims to be if it’s not repeatable then it is not easy to use, you will get frustrated and you have good reason. Repeatability should be achievable by any person who picks up the tool and at any rotational position around the coupling. A good laser alignment system should not have to measure the exact same place in order to achieve repeatable answers. If you cannot repeat readings at different rotational positions, how would you ever know which one is correct?

The Thirty/Thirty Balancing Rule

When performing a field balance job, many technicians will select a trial weight and then rush through the trial runs without paying attention to the readings they are taking. It is a good idea to look at both the amplitude and phase during each of the balancing runs. Upon taking the initial measurement take note of the phase and amplitude, in each trial run, you should observe a change in phase of at least 30ᵒ or 30% change in the amplitude of the plane in which the trial weight has been placed. If you do not get enough change in either phase or amplitude, then your trial weight is not large enough to get the proper reading. You must select a new trial weight and re-run the trial runs. This rule is also valid when performing a shop balance job with a soft bearing balancing machine since they also balance using the trial weight method of balancing.


Does my Instrument Really Need to be Calibrated?

If you use any type of precision tools, they should definitely have their calibration checked periodically. Usually the manufacturer will state a recommended calibration interval and sometimes the interval is specified in an ISO or other certification to which your company conforms. In any case it is always advisable to check the calibration on some interval to be sure that you are actually achieving the results that the instrument is capable of achieving.

We sell laser alignment tools manufactured by Pruftechnik. In the case of our laser alignment systems, it is recommended that the calibration be checked every two years. While it is true that the vast majority of the systems sent in for calibration have absolutely nothing wrong with them and require nothing more than a calibration check, I have seen many times that when a unit fails calibration the end user will comment that they were just using the tool the day before they sent it in and it was working fine. Normally when there is a problem with the calibration, the tool will be very difficult to use, but that is not always the case.

You can look on the back of the systems receiver for a round sticker that has a year in the center and a little arrow pointing to a month on the outside edge. This year and month are the expiration for the current calibration and the date that a new calibration check is suggested. If your unit is due for a check you should have it done at your earliest convenience.