Vernier (Reduction) Dial Drives

Vernier dial drives have been made in a number of styles. Here are some examples from my collection over the years.

Vernier TO General Radio Type 502G Vernier Dial. It's 2 3/4" diameter, and marked 0-100 over 270 degrees. Probably sold with a bushing for the pinch roller shaft, but missing now. Sold new for $1.25, later raised to $1.50.

Vernier TO Vernier TO Vernier TO Vernier TO

Here's one style, made by Jackson Brothers, in England. That's a quarter in the foreground, for comparison. The dial or pointer is mounted with a standard 3/8" nut. This style is not found on their current web site. About 10:1 reduction ratio. Open with a spanner wrench on the front in the two slots or by grabbing outer edge of large mounting ring with pliers while gripping back end of case in vice using only enough pressure to hold, not to deform case.

Vernier TO Vernier TO Jackson Brothers Type 5620 Adjustable Torque Ball Drive. The screws and springs allow adjustment of desired tension, and disassembly for cleaning or lubrication. The dial or pointer is attached with two small screws. 6:1 reduction ratio.

Vernier TO Vernier TO Jackson Brothers Type 4511DAF Ball Drive. These are very common, often found stacked in dual ratio knob sets. About 6:1 reduction ratio. While not intended to be user servicable, some have reported opening them for cleaning and relubrication, and adjusting tension by bending the assembly tabs.

Vernier TO Vernier TO Vernier TO Vernier TO Vernier TO

This style, manufacturer unknown, I first saw as a teenager in the 1950's. It can be completely disassembled for adjustment of desired tension or cleaning and lubrication. It can be assembled with mounting ears forward or to the rear. About 5:1 reduction ratio.

Vernier TO Vernier TO Vernier TO National Company Velvet Vernier dial. National made a number of different dials based on their drive reduction mechanism, some with calibrated knob skirts, as shown, some with pointers, paper scales marked by the user, and frames. About 5:1 reduction ratio.

Vernier TO Vernier TO Here's another unknown, removed from a homebrew audio oscillator with mostly Japanese parts, so I presume it's from Japan. Complete with cracked glass, but missing any sort of calibrated scale or pointer. Nice black crackle paint on the bezel, which is about 1/2" (1.27 CM) deep, and more professional looking than the National dials with frames. About 8.5:1 reduction ratio.


Vernier TO Here's a variable capacitor with the reduction drive built in. This has a 5:1 reduction ratio.


Vernier TO Here's an example of a pointer used with the flat flange of a Jackson Brothers Ball Drive.

Vernier TO Vernier TO A pair of variable capacitors with built in reduction gears as well as an additional ball drive. The reduction gears provide about a 2:1 reduction. The small variable cap requires 6 turns of the shaft to move the VC through 180°. A dial attached to the ball drive flange would have the calibration spread over about 360°. The ball drive on the dual gang VC is not the usual 6:1 reduction; it only takes about 3½ to turn the VC through 180°.


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Copyright © 2003 John Kolb Last revised July 5, 2003