Why is there a speed accuracy range quoted?

Answer: 

We generally quote the speed accuracy as a percentage of the indicated speed. For test machines running at very slow speeds, we also add an extra accuracy tolerance based on an absolute value. These speed accuracy statements are considered to be part of the original manufacturer’s specification.

Example:

  1. Test machine with an accuracy of +/- 0.2% of indicated speed is set to run at 200 mm/min. The allowable speed tolerance is therefore “+/- 0.2% of 200 mm/min”. This equates to +/- 0.4mm/min. So the machine will be running between 199.6 – 200.4 mm/min.
  2. Test machine with an accuracy of +/- 0.2% of indicated speed is set to run at 2 mm/min. The allowable speed tolerance is “+/- 0.2% of 2 mm/min or ±20 µ/min (whichever is greater)”. This equates to = +/- 0.004mm/min (= 4 µ/min) and since 20 µ/min is greater, this will be considered as the dominant tolerance. So the machine will be running between 1.98 – 2.02 mm/min.

However, machine wear can be expected over time and may potentially adversely affect both speed and displacement measurement. Machine wear is dependent on factors such as the frequency of usage, harsh operating environments, and the types of test performed (e.g., sudden breaks of stiff materials may cause energy recoil which affects mechanical parts etc.).  A full overhaul of the test frame may be required to bring the test system back to its original manufacturer’s specification.