July 14, 2022
I rambled on about herringbone gear in my last posting but still have more to share on these once commonplace gears. Today we’ll cover the use of continuous tooth herringbones in oil pumps. That lack of a groove between the helices traps fluid as the gears rotate in mesh and forces the lubricant out both sides of the mesh. Clever pump designers quickly realized that they could use this phenomenon to pressurize and move fluids even when those fluids had very high viscosities. All that was needed was an accurately made set of gears in a close fitting enclosure. Accurately made, in this application, meant more than just holding the backlash within a certain range. To be a good pump gear, the parts had to have a tighter tolerance on the outside diameter, on the tooth depth, and the apex position plus they needed a better surface finish. The later required more attention in the lapping process that was generally used to insure smooth operation of the gear mesh. The outside diameter was addressed during blank preparation and cutter sharpening was monitored to make sure the tools were discarded once they were out of specification. That left apex location. All double helical gears have a tendency toward “apex wander”, where slight variation in the axial positions of the apexes results in the shafts moving side to side. This is why gearboxes that use double helical designs only fix the axial position of one shaft; the rest must “float” to allow for the apex wander. Failure to permit the float causes unequal load sharing and almost impossible to resolve noise and vibration problems. Keep that in mind if you are debugging a new machine with noise and vibration problems. Pump gears cannot tolerate more than a tiny bit of apex wander. The solution was to make the face width wider on one part, lap a set in a special fixture, and use dial indicators mounted to that fixture to determine how much stock to take off each end face to get the apex wander within specification. Thousands of pump gears were being made and these special fixtures were found in every shop that made them. Over time, herringbone pump gears have been replaced with other types of pumps and this important technique –along with those special fixtures- has disappeared from corporate memory. But now it has been shared with you. It might come in handy in sorting out those misbehaving double helical gearboxes.