The most obvious difference between a spur gear and a helical gear is the presence of that “helix” angle. Civilians will call them “straight-cut” and “crooked-cut” — like those are acceptable technical terms. We “experts” cringe at such talk but then resist letting go of our own long-held “beliefs” on gear design — despite good evidence to the contrary.
One of the most contentious areas of disagreement in gear design is how much helix angle is the right amount. To quote my old friend Ray Drago (firstname.lastname@example.org): “It depends. Technically, any angle other than zero can be used — which is why there are so many different opinions on the topic.
Once you have a helix angle you will get a thrust force and an overturning moment. The laws of physics must be obeyed and the gear system designer must have a way to handle those forces in the bearings and mounting. Before the advent of rolling-element bearings, various types of bushings were used. Today we have a variety of rolling-element bearings to select from, and with differing capacities for multi-axis force reaction. We’ll discuss bearing arrangements in a future blog.
I mention thrust forces here because the need to eliminate these loads — while still enjoying the advantages of the helical gear— lead to the development of the herringbone gear. If you pair a right-hand helix with a left-hand helix, the forces cancel each other so that the amount of angle really does not become an issue. Early on, however, the 30° angle became “standard” in the most common gear cutting machines.
There was nothing magic about the 30° angle; the machines used a guide to rotate the cutter as it passed across the part. And custom guides could be made for other angles, if needed. Changing guides is a major undertaking though, so most machines were left in the “standard” configuration.
When you dig into the underlying mathematics of helical gears you develop an appreciation for the relationship between tooth size, helix angle, face width, and face contact ratio. Your homework assignment is to look through your reference books for the face contact ratio formula.