The gear ratio defines the number of turns of the hand wheel required to move the pointer once around the dial face. The ideal gear ratio provides unique indication of position over the adjustment range while maximizing the resolution of the scale.

Say, for example, the indicator is used to display a roller gap. Let's assume the rollers are adjusted using a 1/2 - 20 leadscrew and the maximum range is 2.6 inches.  The hand wheel indicator is mounted to the leadscrew.  What should the ratio be in order for 2.6 inches of movement to turn the dial one full rotation?  Here’s what you need to do to figure it out:

1.  Determine the relevant unit of measurement:  inches, millimeters, feet, etc.  In this case, it’s inches.

2.  Establish the RANGE, in this case 2.6 inches.

3.  Determine the PITCH of the leadscrew in the same unit of measurement.  In this case, it happens to be .05 inches per turn (1 inch / 20 threads per inch -- 1 ÷ 20 equals .05).

4.  Divide the RANGE by the PITCH and that gives you the ratio:  2.6 ÷ 0.05 = 52.

5.  52 is not a standard ratio available for these indicators.  You should, therefore, order the next larger available standard ratio, in this case, 60.  The full range of movement would then fall slightly short of one complete 360° turn of the pointer on the dial face (360° x 52/60, or 312° to be exact).  Another option would be to order a custom dial face with a special scale which covered only 312° of the dial face. This dial face could be numbered 0 to 2.6 inches rather than 0 to 52 turns.  A final option would be to order a custom ratio of 52 and custom dial face that is 360°. This dial face could be numbered 0 to 2.6 inches rather than 0 to 52 turns.

6.  The ratio is specified in the part number, as in S50/1-N-60-I-1-S-ON-N-C1.