FREQUENTLY ASKED QUESTIONS

How Accurate are Dial Indicator Readings? Quite accurate in the unlikely case that none of the below mentioned problems are present: Sagging indicator brackets Sticking/jumping dial hands Low resolution equals rounding losses Reading errors: ± sign error, parallax error, mirror image Play in mechanical linkages Tilted dial indicator equals offset error Axial shaft play equals gap error Numerous refinements and variations of measurements have been developed to combat the many sources of error. But even the best of these refinements are exceeded in accuracy and ease of use by other methods of alignment (i.e., laser alignment).

Rim & Face, Reverse Indicator and Rim & Direct Angle.  What is Best? This question is of importance because there are now a number of alignment systems on the market, using different methods. To answer, ask yourself the question: What do I want to accomplish? You want to bring offset and angularity within tolerance. Rim & Face measures the offset through the RIM reading and measures the angularity through a face (distance measurement) reading which can easily be inaccurate if the diameter at which it is measured is small such as 4" or less, or even wrong if the shaft floats in or out during the measurement. Reverse Indicator takes two rim readings which establish the offset and compute angularity from these offsets. It is not affected by moderate shaft float but only accurate if the distance between the measuring devices exceeds 4". Also, accuracy is dependent on measuring resolution which in case of laser systems should be at least 5 microns if you expect to see improvements over normal dial indicator readings. Rim & Direct Angle: Since offset and angularity is what you were out to measure, obviously a system measuring the RIM and DIRECT ANGLE is best, particularly if it also has a measuring resolution of 1 micron (.00004") (OPTALIGN PLUS, ROTALIGN ULTRA, etc.).

Resolution and other matters... Have you ever been forced to check for soft foot when your brackets were less than 10 inches apart and the MTBM’s supports were several feet distant? If you answer yes to that question or if you have ever made foot corrections under similar circumstances, then resolution matters to you. Resolution: The smallest detectable increment of measurement. Synonyms: sensitivity, fineness For laser shaft alignment systems mounted on a pair of shafts, resolution is the smallest movement between the two shafts that the electronics can detect. On all systems Ludeca sells, this value is 1 micron (.00004") or better. Linearity: The closeness of a calibration curve to a straight line. Having output directly proportional to input. Synonyms: Straightness, direct proportion. For a laser shaft alignment system, linearity is easy to evaluate. A graph of the known position versus the displayed value should be a straight line. If it is, we say the system is linear. Our products are linear to one percent. Repeatability: The ability of an instrument to reproduce displayed values when the same input is applied to it consecutively under the same conditions. Synonyms: Reproducibility. Repeatability is expressed as the difference between two or more sets of measured values when given identical input positions of the sensor(s). Accuracy: The ratio of the error in a measurement to the ideal, or expected, value. Synonyms: deviation, error, how close a value is to being correct. Accuracy is how close a measuring system comes to measuring the absolutely correct value. For specific measurements or specifications, the term error is used and is expressed either as a percent of full scale or as an absolute value. Accuracy is adversely affected by non-linearity, non-repeatability, and poor resolution.

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