Hollow planetary gearbox design - need advice 2

[Kikk0]

Hi,

We are currently designing a planetary gearbox with the following characteristics:

The gearbox need five stages to gain the desired torque.

The input is an electric motor with nominal output of 5Nm up to 3300 rpm.

The gearbox wil basically be operated by rotating a nut on a bolt with a elastomer packer element in between to be compressed axially to expand radially in a tubing.

The torque needed will be from 0-4000Nm. for most of the setting process the maximum torque will be about 2kNm. when 2kNm is applied we almost have volume filled the packer element so that the additional 2kNm to gain 4kNm will be gained in about half a turn of the nut.



module 2
ring gear teeth 42

the last stage has
sun: 18teeth and ring 12 teeth 5 planets

to design the gearbox with the space available results in safety factors below 1 for both tooth bending and surface durability on the last stage. BUT, the load of 4kNm is a peak load where the last stage are more or less static when the torque reaches 4kNm.

According to MIT calc using 1570MPa Rm steel the surface safety factor is 0,56 and the bending is 0,93 (for 1hour lifetime)

How does that "static" condition affect the safety factors. since most of the cycle will be between 0-2kNm (where there are up to 40 rotations of the nut)

the MIT calc predicts 1,03 SH and 3,09 SF at 1300Nm.

Since it is "safe" at 1300Nm will it manage 4kNm peak "static"?

thanks

 

[MikeHalloran]

Sure. Once. Then you throw it away.

How does that fit into your economic model?




Mike Halloran
Pembroke Pines, FL, USA

 

[gearcutter]

This is the problem when in-experienced people are give software to go and design a system in which they have very little understanding of the subject.
I’ve heard others use the term; “garbage in, garbage out”.
I would highly recommend that you engage the services of a reputable gear consultant.
You will find contact details for several on the AGMA’s website.


Ron Volmershausen
Brunkerville Engineering
Newcastle Australia

http://www.aussieweb.com.au/email.aspx?id=1194181

 

[Kikk0]

It's always a first time for everyone...

I will try the AGMA website for contact details

Is there any good literature that could be recommended on the subject?

 

[gearcutter]

"It's always a first time for everyone..."
I couldn't agree with you more but at least have someone on-hand that knows what they are doing so as they may guide you through the process.
It's not sound engineering practice to solely rely on software.......you need to back it up with experience.
Do a Google search specific to your needs. There is heaps of info out there.

There are several good books;
Gear Handbook by Dudley
Analytical Mechanics of Gears by Buckingham
Gear Geometry & Applied Theory by Litvin
Practical Gear Design by Dudley
Dudley's Gear Handbook by Townsend
Spur Gears by Buckingham
Fundamentals of Gear Design by Drago

Ron Volmershausen
Brunkerville Engineering
Newcastle Australia

http://www.aussieweb.com.au/email.aspx?id=1194181

 

[mfgenggear]

Gearcutter

I agree it's like playing Russian roulette. :>)

Mfgenggear

 

[TVP]

Kikk0,

There are two major sources of theoretical/empirical design standards for gears on which software like MITCalc, Romax, etc. are based: AGMA and ISO. MITCalc uses the ISO 6336 series of standards. At a minimum, I suggest you review these standards in order to understand the basis of the calculations. The industry magazines Gear Technology and Gear Solutions have articles in their archives specific to ISO 6336 that would be worth reading.

 

[Kikk0]

Thanks for the replies above, I understand that designing a gearbox is a complex job requiring both theoretical and practical knowledge on the subject. We will ally with the machine shop to manufacture the

regarding the operation of the gearbox it is very similar to these tools:

http://torcgun.com/tools-a-equipment/jgun-single-speed

where a nut or bolt is being screwed down and torqued up. in the final screwing process there is a high torque buildup vs rotation of the bolt or nut.

we require 100 screwdowns and torque up to 2kNm and 20 break loos torque of up to 4kNm in reverse direction.
we need a 10mm hole through the center of the gearbox (through all the sun gears)

Will my main concern be the tooth bending parameter since there is more or less static condition in the last stage when breaking loose the nut?

Is surface contact stress more an issue when dealing with high loads and constant rotations?

Obviously I would need rather wide gears in the last stage. is there any rule of thumb on the gear width to it's diameter (sun and planetary gear)?

I will review the standards and proper literature suggested here.

 

[tbuelna]

Kikk0,

Your gear analysis will need to consider both tooth bending and contact. Just like any other gear design, you will need to select your gear sizes/geometries, materials, and heat treatments based on the loads and number of fatigue cycles they will be subject to in service. These values will be different for each gear and each stage. AGMA publications have some excellent data for guidance on material selection.

As for the question about rule of thumb regarding gear aspect ratio, with spur gears a basic rule of thumb is to keep the aspect ratio (L/D) below 1.0 . However, this rule also assumes good load distribution across the gear face, and this is usually difficult to achieve with the planet gears of an epicyclic. So it would be wise to do a careful structural analysis of your planet carriers, or use a conservative FoS on your gear calculations.

Hope that helps.
Terry