zero-backlashvfowormdrive

Zero-Backlash VFO Worm Drive

Build Your Own High Quality VFO Drive Mechanism

Good VFO mechanical drive parts are almost impossible to find, and when available they are usually quite expensive. Because of this I set out to design and build an easily reproducible zero-backlash drive mechanism. The basic concept is a worm drive arrangement where all mating parts are spring loaded to eliminate any backlash or slippage.

• The driven gear is made from a 3/8 inch (9.5 mm) thick nylon (Delrin) disk. I cut this with a hole saw mounted in my drill press, but it could be cut with a small bladed hand saw. This disk is trued on a drill press or lathe and a "V" groove turned into the outer edge. Purpose of the "V" groove is to make most of the threads on the sides of this V, and thus allow for wear over time to deepen those threads into the V. This allows the fit, and thus accuracy, to actually improve with use. Threads for this driven gear are cut with a section of threaded rod that has slots cut in it to make it into a threading die. A temporary bracket was made to hold the driven gear against the thread cutting worm while the worm was rotated in my drill press. This takes a while, but going slow will result in a much smoother set of threads.
• The "worm" or driving gear is made from conventional threaded rod, or a bolt with sufficient length threads. For the prototype I used 1/4 inch (6.2 mm) rod threaded at a relatively coarse 20 TPI (Turns Per Inch).
• The driven gear is fastened to and rotates on the shaft of a potentiometer or variable capacitor. The mounting plate for this capacitor or potentiometer pivots on a single bolt and is held against the driving worm by spring tension.
• A nylon-insert locking nut adjusts end-play in the driving worm part by slightly squeezing the chassis sides to eliminate any longitudinal play in that shaft.

NOTE:

Since building this original drive unit, I have duplicated it several times using both wood and plastic for the driven gear. Inexpensive 1/4 inch diameter threaded rod is available in 8 foot lengths from most DIY or hardware stores. Go to the electrical section and look for "hanger rod" which is used to suspend conduit from ceilings.

This picture shows the basic drive layout with nylon-insert nuts adjusting slack out of the worm itself by slightly compressing the chassis sides.

This is the back side, showing the potentiometer that will be used to adjust a varactor tuned VFO. The pot is mounted to a small square of aluminium plate that swivels on the screw shown in the right side of this photo. A spring pulls the pot and the driven gear against the worm, which eliminates any backlash in the drive.

In this picture you can see the spring that holds the driven and driving gears in tight contact.

That long set-screw in the nylon gear serves an additional purpose. At extremes of rotation it hits against the worm and separates the driven and driving gears, thus it serves as a clutching mechanism at approximately the 270 degree rotation points. No, it was not originally planned that way, but it is a useful feature for potentiometer tuned VFO mechanisms. A second screw could be strategically place to limit rotation to 180 degrees if this mechanism were turning a variable capacitor. The problem with worm drives is that their torque is so high that they can damage other components if they do not have positive rotation stops.