Ask the Expert: 5/20/20 Edition

From time to time I’ll be doing some brief Ask the Expert questions and answers. These questions will be on topics that might be very familiar to you, especially gear design, but which you may find of interest in some way. You might think of it as a combination of Ask the Expert and Back to Gear Basics. Hopefully you will find this to be of value not only to you, but your co-workers/employees as well.
As the saying goes — you don’t know what you don’t know.
Chuck Schultz

No advice on staying sane in an insane world this week; we have important gear stuff to keep us busy! Many thanks to those who responded to our plea for something interesting to write about. Feel free to submit more questions.

Two questions about helix angle:

1. I want to design a pair of helical gear according to the power and shaft speed but I don’t know what helix angle is suitable. Could you help me with this problem? In fact, I want to know how we should choose the helix angle in different situation.

2. Why can’t we grind above + or – 45 degree helix angles?

For such a fundamental element of gear design, helix angle has been subject to much misinformation. Let’s unpack a bit of history first. As an industry, we have made more “zero-angle” helical gears, AKA — spur gears — than anything else. They are simple to make and early manufacturing methods needed that because the ability to cast or machine a tooth flank at an angle was not widespread.

Without getting into tooth generation theory, the evolution of the helical gear hinged on improving both the machines and the cutting tools. And when the need for increased power capacity became apparent, the herringbone gear was the initial “solution.” Our international readers are probably more familiar with Citroen automobiles than Americans; that chevron on their logo is a herringbone in honor of their founder’s early patent on that gear type. I do not know if patent concerns were involved with the eventual adoption of 30 degrees as the “standard” helix angle for herringbones. [One small aside — not all machines produce a 30 degree angle. One brand is ever so close to 30 degrees but not quite. Do not attempt to run parts made on two different brands of machine. A customer found that out the hard way!]

With herringbones, the helix angle is a function of the guides which control the rotation of the pinion like shaper cutter. To change the helix angle, you spend a shift changing the guides, resulting in very accurate helix angles but not a big selection to choose from. I start with herringbones because they had a long-lasting effect on our thinking about helix angle. Machines with 15, 20, 23, 30, and 45 degree helix angles were in common use as companies differentiated their products. Tool makers responded with the appropriate cutting tools.

Hobbing machines with the ability to produce a helix angle require a differential and/or a change gear mechanism to rotate the work table at a relative speed to the cutter. Fat books of change gear ratios were kept at hand to figure out what gears to put in that mechanism; often you could only get close to the desired number. As a legacy of the herringbone days, hobs are still sold with transverse dimensions to match that 15, 20, 23, 30, and 45 degree helix angle model in addition to the ubiquitous “spur gear” hob.

Modern electronic machine tools have freed us from the tyranny of the guide and the change gear. To answer the second question first: your machine designer recognized that very few helical gears have a helix angle over 45 degrees. A different class of machines — thread or worm grinders — are used for the small number of gears designed with very high helix angles.

Some engineers have been taught that any helix angle over 20 degrees should be avoided; others have seen the plethora of “herringbone” angles and followed a different path.

As our understanding and experience with single helical gears has grown, we recognize the effect the “thrust” force and inherent “overturning” moment has on bearings and other components. Provided your bearings and fits can accommodate the forces from the teeth, any helix angle will “work.” Other considerations are related to tooth meshing action. The face width and the helix angle figure in to the face contact ratio; to obtain “true” helical action, the face contact ratio must be greater than one. Rating calculation methods penalize designs that do not meet that threshold. Very low helix angle set [generally less than 10 degrees] can suffer from lubrication problems in high-speed applications because of the velocity of the oil as it is squeezed axially through the mesh.

I try to use helix angles between eleven and thirty degrees in new designs. The exact angle depends upon the cutting tools available and the bearing arrangement. Companies adopt policies on helix angle selection to speed the design process, but sometimes these policies do not reflect current capabilities or technical understanding. If you run into a policy “road block” it might be time to re-examine the reasons for the “old family recipe.”

About Charles D. Schultz 678 Articles
Charles D. Schultz is President of Beyta Gear Service and one of Gear Technology's technical editors.