How Tuning Machines Work
Worm Gear: Background
The first recorded usage of a worm gear is from the First Punic Wars of the classical period. This is the “Archimedean Lever” used to launch King Herod’s larger ships. This is the utility of a worm gear - the ability to transfer large amounts of rotation force (torque) into a system with little effort. Likewise, worm gear ratios can be very high. Raising the gear ratio has a two fold effect of reducing both speed and friction. Finally, while the worm easily turns the gear and roller of a tuning machine with great ease, the gear cannot turn the worm. This is accomplished by ensuring that the lead angle of each worm thread is less than 4°.
In this video, Jorg Graf explains some more details behind the refinement of the tuning machine.
About Friction
According to Jorg Graf, the roller of the tuning machine is only responsible for about 3% of friction in the system. This fact stands in counterpoint to the guitarist’s focus on the roller. So using bearings as a buffer between peghead and roller will at most elimate 3% of the friction. Of course, those bearings are a peace of mind for a performer experiencing those squeaky tuning sessions that interrupt the silence between concert selections. The elimination of squeaking is worth much more to a guitarists than its real impact on tuning.
Gilbert Tuning Machines are one of the only high end machines available that completely forego the roller bearing. Owner, Greg Matonis, referred to them in one of our past conversations as, “gimmicky.” While the aesthetics of Gilbert tuners aren’t for everyone, the design illustrates what is most critical in the function of a worm gear drive, by making the fit of the worm into the gear serviceable. Not to say that there are a variety of relationships between the worm and its gear that work. The worm’s 90 degree alignment with the gear is most critical and as both worm and gear wear with repeated use, the Gilbert design allows for the adjustment of their fit. Additionally, one will find that the alignment of this gear / roller to the worm is so stable that it reveals any inconsistent distances between the roller holes on your peghead.
Notes on Installation
Because the alignment between worm and gear is so critical, a perfectly executed tuning machine can be rendered into a poorly functioning machine simply by improper installation. If the spacing of the roller holes into the peghead do not agree perfectly with the machine set being installed, then the worm / gear alignment will also be off, greatly reducing the performance of the machine with increased friction and wear. If you are a luthier, then pay special attention to the squareness of your headplate as it relates to the sides of your peghead and develop a system for ensuring that the angle of each hole is a reflection of its opposite side. This is much more difficult and critical than it may seem at first. In any case, if the guitar you own is notoriously difficult to tune, then first confirm that the roller holes in your peghead are spaced exactly the same as your tuner’s rollers. If not, I recommend having that fixed before buying new tuners.
Tuning machines can only function their best if the roller holes in your peghead reinforce the alignment of the worm and gear.