Worm Gearset Design -- Pressure Angle & Tooth Bending Strength

[matts05]

Good evening,

I am working the conceptual design and analysis of a gearbox utilizing a single-enveloping worm gearset. My company will be outsourcing detailed design/analysis/manufacturing, however I am on the hook for recommending basic design parameters (diametral pitch, gear teeth, worm starts, pressure angle etc) which meet our particular load requirements for preliminary sizing, weight estimation, packaging etc. I have approached this by analyzing various gearsets from commercial catalogs using classical methods from Dudley, Shigley, AGMA 6022, and AGMA 6034, and have downselected to a few candidates.

(1) My first question: It seems like for the range of pitches I am considering, all of the catalog single start worms utilize a pressure angle of 14.5 deg, however the analysis of my particular application tends to favor higher pressure angles due to increased tooth strength and a tendency towards a self-locking condition. The only reason I have been looking at commercial catalogs is just so that I narrow my options down to a handful of meshes which "work". Since we will be using custom gears in the end anyway, is there any reason to stick with a 14.5 deg pressure angle? As in, is the pressure angle fixed to any other parameters as it seems to be in the catalogs I have seen, or am I free to change it in a custom design without screwing up anything else about the mesh?

(2) For my second question, how have you analyzed the worm gear tooth for static and/or fatigue loading, and have you taken advantage of load sharing between multiple teeth? It seems like the modified Lewis bending equation presented in Shigley 9th ed tends to be used, however it seems to apply the full tangential worm gear load to a single tooth. A table of worm gear strength values in Dudley's Handbook of Practical Gear Design 3rd ed claims to use a "contact ratio" of 1.5, and ISO 14521 seems to knock its calculated tooth shear stress down by a 0.5 "contact factor". Are there any other resources out there regarding load sharing between worm gear teeth in regards to static analysis?

Thank you,
Matt

 

[mfgenggear]

Hi Matts05

it is common in aerospace to to use 20 Deg. Pressure angle, how ever it depends on the the torque and rpm.
fundamentals are used same as a helical gear set , purchase the latest ANSI AGMA 908 & 2001 power ratings.

my recommendation is contact a consulting engineering firm because data (Number of teeth, diametral pitch, and pressure angle)
could change upon their recommendations. work concurrently with them. so part of the constraints will be max size of the housing. as with standard gears the pinion & gear pitch diameters(center distance)
then from that the diametral pitch and pressure angle to prevent tooth bending failure, scuffing, contact stress.
as well as the material , heat and more. follow the AGMA recommendations. use spread sheets with the gear formulas.
you could design a gear set then have them verify it (check ) and you will learn.

 

[mfgenggear]

also the gear ratio input and out put, HP input, rpm input, and output. contact ratio.

 

[Andreas Jung]

You are doing the great work, really appreciate!

Lastly, I am also facing some issues wo develop my design regarding to gearbox after last a deep internet research I have get the exact solutions from "Rehfuss". They have expertise in custom-designed functional products development. I hope you will get the satisfied solutions from them.

 

[spigor]

(1) The historical reason to use the 14.5° reference profile angle (often called pressure angle P.A.) instead of 15° P.A. is because sin(14.5°) is close to .25. It used to be an important factor before the machine computing age.
Small P.As are beneficial in the worm milling, because the generated radial forces are smaller and tool's tip is wider than in case of bigger angles.
Also, due to the somewhat smaller radial forces, the 14.5° P.A. should contribute to little better efficiency, less worm bending under load and bigger contact ratio than in case of a 20° P.A. gearset.
These reasons seem to have led the 14.5° angle to be the standard P.A. for worm gears in the US, while in Europe, the 20° P.A. seems to be standard.

(2) In case of a worm gearset, the actual contact ratio (i.e. how many teeth are in contact, sharing the load) depends a lot on the gearset's geometry and the manufacturing technology and errors. There's a good info in

https://core.ac.uk/download/pdf/153776393.pdf

(

https://www.eng-tips.com/viewthread.cfm?qid=482179,

3DDave). A contact ratio of 1.5 seems to be a conservative approach, a contact factor of 0.5 (ratio 2.0) seems to be more realistic, especially if the gears were given some time to "wear in" under limited load. Anyway, without a proper calculation it's just guessing.