Gear WB: Backlash resulting in bad profile

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abdullah
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Gear WB: Backlash resulting in bad profile

Postby abdullah » Tue Aug 18, 2020 2:02 pm

Not Part Involute Gear, but Loo's Gear WB.

Hi

I am trying to make an inner gear (ring gear of a planetary gear). I would like to add some backlash so that I can 3d print or mill it.

As there is no ring gear, I am trying to cut a cylinder with the corresponding gear.

I am not certain that adding backlash can help me, because I would need a negative backlash (I am not sure if the reverse backlash could help). But I am running another problem before I can do that test.

As soon as I insert a positive backlash value, I get a bad gear profile:
Screenshot_20200818_155442.png
Screenshot_20200818_155442.png (33.96 KiB) Viewed 710 times
Any clue why this is happening?

OS: Ubuntu 18.04.4 LTS (MATE/mate)
Word size of OS: 64-bit
Word size of FreeCAD: 64-bit
Version: 0.19.
Build type: Release
Branch: unknown
Hash: 7566bd1343708e5c0b8fe093f27f84e045ec266e
Python version: 3.6.9
Qt version: 5.9.5
Coin version: 4.0.0a
OCC version: 7.3.0

Edit: Correct the spelling of backlash.
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GearsNoBackslash.FCStd
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Last edited by abdullah on Wed Aug 19, 2020 12:14 pm, edited 1 time in total.
abdullah
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Re: Gear WB: Backslash resulting in bad profile

Postby abdullah » Tue Aug 18, 2020 2:29 pm

As a follow up. It is not true that I cannot set the backlash parameter. I cannot set 0.1 mm, but I can set 0.01 mm. However, I am not sure if the backlash is properly calculated, it looks to me more than 10 um when I put 10 um. Maybe it is that I just do not understand it well.

Anyone with a sound understanding of gear can check whether the software is providing what it should?
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papyblaise
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Re: Gear WB: Backslash resulting in bad profile

Postby papyblaise » Tue Aug 18, 2020 2:31 pm

Hi , I don't know what is your problem because at home it's Ok
Attachments
gear.PNG
gear.PNG (22.63 KiB) Viewed 700 times
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EduBambu
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Re: Gear WB: Backslash resulting in bad profile

Postby EduBambu » Tue Aug 18, 2020 3:07 pm

Good Morning!
I imagine you want something like that;

Engre01.png
Engre01.png (155.43 KiB) Viewed 691 times

I did as indicated, a circle with a diameter larger than the gear; I built a gear of sufficient diameter to surround the satellites, and subtracted it from the larger diameter disk.

The detail is that I used a backlash value in microns and turned it backlash in reverse.
Engre02.png
Engre02.png (175.8 KiB) Viewed 691 times

For the satellite gears I used 5 microns, and for the crown with internal teeth, I used 10 microns.

I used the reverse backlash only on the outer crown.

Now with the reverse backlash increased to 15 microns! The increase in satellite clearance with the crown is almost 30 microns.

Engre03.png
Engre03.png (160.36 KiB) Viewed 691 times

Gear data: number of teeth of the satellites = 16
number of crown teeth: 48
Module; 1
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looo
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Re: Gear WB: Backslash resulting in bad profile

Postby looo » Tue Aug 18, 2020 3:24 pm

Looking at the formula I guess we somehow changed to angular backlash but kept the property-type (length). You should be able to get the correct amount of backlash by using:

Code: Select all

<backlash in mm> / dw / 2
Image

I guess it's best to rename this property to polar-backlash or angular backlash.
chrisb
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Re: Gear WB: Backslash resulting in bad profile

Postby chrisb » Tue Aug 18, 2020 4:05 pm

EduBambu wrote:
Tue Aug 18, 2020 3:07 pm
I built a gear of sufficient diameter to surround the satellites, and subtracted it from the larger diameter disk.
This may work, but internal gears are different from a subtracted external gear.
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looo
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Re: Gear WB: Backslash resulting in bad profile

Postby looo » Tue Aug 18, 2020 4:37 pm

chrisb wrote:
Tue Aug 18, 2020 4:05 pm
EduBambu wrote:
Tue Aug 18, 2020 3:07 pm
I built a gear of sufficient diameter to surround the satellites, and subtracted it from the larger diameter disk.
This may work, but internal gears are different from a subtracted external gear.
The profile should be the same. So from a kinematic point of view this shouldn't be a problem. Using a negative clearance (-0.25 or smaller) and 0.25 for the head-property should work. Additional fillets would be nice to have, but are not yet available.
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Re: Gear WB: Backslash resulting in bad profile

Postby chrisb » Tue Aug 18, 2020 5:03 pm

looo wrote:
Tue Aug 18, 2020 4:37 pm
The profile should be the same.
Thanks for the information. I was mislead by the different fillets, which are probably used to minimize fatigue failure.
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EduBambu
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Re: Gear WB: Backslash resulting in bad profile

Postby EduBambu » Wed Aug 19, 2020 3:04 am

According to the technical literature at https://qtcgears.com/tools/catalogs/PDF_Q420/Tech.pdf, there are 4 measures referred to as "backlash" on a spur cylindrical gear.

Engre07.png
https://qtcgears.com/tools/catalogs/PDF_Q420/Tech.pdf :acesso em 18/08/2020 às 23:50
Engre07.png (82.62 KiB) Viewed 610 times


None of them has the numerical value indicated in the combined FreeCAD view. Except when it is "zero".

I tested 3 gear combinations and with one of the gears I chose the backlash value, in the combined view, of 0.010 mm (10 microns).

Engre04.png
Engrenagem 16 dentes módulo 1, backlash informado na "combo view" 0,010 mm
Engre04.png (171.01 KiB) Viewed 610 times

All gears were made with 16 teeth, and modules of 1, 5 and 10 mm.

I measured the gaps directly on the gears, on the circumference of the primitive diameter. For that I cut the gears in the primitive diameter.

Engre05.png
Engrenagem 16 dentes módulo 10, backlash informado na "combo view" 0,010 mm
Engre05.png (146.52 KiB) Viewed 610 times

The gear on the left had the backlash changed, the gear on the right was designed with the backlash equal to "zero".

Engre06.png
Engrenagem 16 dentes módulo 5, backlash informado na "combo view" 0,010 mm
Engre06.png (168.11 KiB) Viewed 610 times

Apparently, the backlash value obtained on the changed gear is equal to; J (real) = J (inf.) X m x Z / 4, this by gear.

Where;
J (real) = Backlash measured in gear
J (inf) = Baclash reported in combined view
m = gear module
Z = number of teeth


Therefore, in case you need a specific backlash value, the following function should be used:

J (inf) = J (real) / (m x Z / 4)

This is a resource that I am using to meet my needs and produce the backlash that interests me. I don't know if it is the most correct way to proceed.

Note: I did the same test with 20 teeth gears, to define how the number of teeth influenced the final backlash value.
chrisb
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Re: Gear WB: Backslash resulting in bad profile

Postby chrisb » Wed Aug 19, 2020 5:15 am

Can you please edit the first topic and change the subject to Backlash without the s in the middle. Future searchers will be thankful.
A Sketcher Lecture with in-depth information is available in English, auf deutsch, en français, en español.