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# Gear Face Width

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## Gear Face Width

(OP)
I am trying to design a planetary gear box that will have 5,000 in lbs of break away torque at the output shaft. The primary and secondary sets will have a 4:1 reduction ratio.
My problem is how do I determine the required input horspower of a 1700 RPM motor, and how do I calculate the face width. I was planning on using a Diameteral Pitch of 16 for the gears.
Replies continue below

### RE: Gear Face Width

Your input HP will be the same as your output HP, times the efficiency of the gear train.  Some planetaries I've used claim a 90% eff., but you may want to downgrade that to 880% or even 70% depending on the application, your production tolerances, etc.

Face widths will depend on the gear material and tooth forces.  Try Mark's Handbook, or maybe Machinery's Handbook.  Or even your local gear supplier, they may have standard gears that are torque rated.

Have you tried looking for complete standard planetary gearboxes or gear sets?

### RE: Gear Face Width

A rough guideline is 6 times the tooth
thickness unless you can assure alignment.

### RE: Gear Face Width

(OP)
Thanks for the input!
I havn't been able to find a standard box for my application.
The material I plan to use on the planetary, input and sun gears will be 8620 h/t'd to approx 50 RC. I will be using 4140 for the ring and output, also h/t'd to 50 RC. All at 20 degree pressure angle.

### RE: Gear Face Width

If I understand correctly, the overall ratio of your reducer is 16:1, so your output shaft will be turning at 1700/16 = 106 rpm.  Your output power is then 5,000 x 106/63,025 = 8.43 horsepower.  Efficiency will probably be about 90%, so your input requirement is 8.43/0.90 = 9.37 h.p.  You should probably use at least a 10 h.p. motor, and your gears should have a healthy safety factor beyond this.

What is it about your application that prevents using a catalog gearbox?

If you really have to make your own gearbox, you may do o.k. with 8620 and you may not.  8620 is not consistent in terms of Jominy hardenability.  If you're only making one gearbox, you'd be much better off with something you can count on, like 4340.

Calculating gear capacity is complex.  The normal procedure is to use ISO or AGMA standards.  It might be well worthwhile to engage a qualified gear designer.

### RE: Gear Face Width

(OP)
Philrock,
I think you are right about the gear designer. I didn't think this was going to get this complicated.
The reason for the non standard box is it needs to be hollow through the center. I havn't been able to find one yet.
Thanks for all the help, it's back to drawing board for me.

### RE: Gear Face Width

it has been interesting reading this thread and how it veered off the actual problem.
firstly idd has a problem of finding a gear box for 5000in lb torque, I am not from the inch world but as it is a torque rating it cannot be assumed to be the same horse power rating (philrock please note)
you would have to convert the torque to HP at the desired speed of operation.
secondly an interesting development was idd requires a hollow through the center... is it a requirement of a hollow throughout the center????? it may be difficult as planetaries usually have a small sun gear with splines or shafting and the hollow would have to be through that.
It may also be that the hollow is not right through the gear box!! and only at the end and that can be an east solution.
what is the application of the use? I would recommend a shaft mounted speed reducer if that may suit the application!!
look at our web site at
www.involutetools.com

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