Help with Herringbone gears...

[nhendrickx]

Hi,

I'm working on my thesis and I need a way to 'connect' the axis of a pendulum to the axis of an encoder...

Since there is no space to mount the encoder on the same axis as the pendulum I figured the best way to make them perpendicular would be gears.

But, because the pendulum only makes very small movements (<20° from center) I thougt it would be best to use herringbone gears for there increased contact surface and at the same time I could 'multiply' the movement of the pendulum to get a better reading on the encoder.

I'm using Creo parametric 2.0 (student edition)

Everthing is set to mm...

These gears/files are intended to use for 3D printing

The only problem is that I don't have very much experience using Creo, I've certainly never worked with relations of parameters before.

I've managed to make a gear as in this tutorial: Helical gear in ProE - YouTube

But when I try to change the gear to the specifications I need, it fails to regenerate.

I hope that someone can either explain to me how to correctly scale this down or otherwise point me in a direction where I could find some pre drawn gears or a generator.

Otherwise if someone would be so kind as to draw some for me;

The smallest gear fits on to an axis of D=6mm, and can be as small as it can be made to fit on this axis, the larger gear can then be scaled up accordingly to get a transfer ratio of 2 or 3.

As stated before, I'm using Creo parametric 2.0 (student edition)

and the parts are intended for 3D printing (so the teeth can't be too fine)

Thanks in advance,

Niels

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[JonathanHodgson]

Hi Niels,

What are the parameters of the gear(s) you are trying to make?  (And, to save us watching a video for 18 minutes, what are the parameters used in the video?)

[nhendrickx]

Parameters from the video:

Module = 3

Z = 20

Width = 30

Theta = 10

Relations:

pitch=m*z

addendum=m*(z+2)

dedendum=m*(z-2.5)

base=pitch*cos(20)

alpha=360/z/4

Involute curve equation:

x=base/2*(cos(t*90)+Pi/2*t*sin(t*90))

y=base/2*(sin(t*90)-Pi/2*t*cos(t*90))

z=0

It is my understanding that as long as the module and theta stay the same, the gears will fit together...

As for the parameters of the gears I'm trying to make...

I tried making the number of teeth on the gear 10 so I would have a 2/1 ratio between the original gear from te video and the new one, but as I said before it wouldn't regenerate.

I also tried changing the module, but this wouldn't regenerate either.

It doesn't really matter to me what the size of the gears are, i was going to scale the gear down as small as I could to fit on the 6mm diameter axis and then scale the larger gear to fit the ratio.

[JonathanHodgson]

I think the geometry of the gears is falling apart.

Is theta the pressure angle?  Our gear design software won't even make the original gear with a pressure angle of 10° - the tooth profile just sort of turns inside-out.  20° works OKish, although the base circle is already within the working flank of the other gear:

but if you reduce one gear to 10 teeth. the geometry fails again:

From a gear design perspective, fewer than 15T rarely works well - if you want a 2:1 ratio then I'd suggest going for at least 17:34, or try 19:38 if that doesn't work, and then adjusting the module to get the size you want.  You should really use a full-fillet root to reduce the stress concentration (a pot drive is low-torque but 3D-printed parts are weak), and you may then need to adjust the addendum and dedendum offsets (the +2 and -2.5) to ensure the tip of each gear doesn't clash with the root fillet of the other.

I also can't see from your parameters what the helix angle is, which will affect the resulting profile - the above designs assume spur gears (which should be fine for your pot drive if the involute is done right - helical or herringbone gears are mostly used to reduce noise).

Here's 17:34, 1.5 module, 20°PA, 38.25 centre distance (you might actually want to run maybe 38.5-38.75 to ensure some backlash) and 1.0 addendum (+2) with 1.3 dedendum (-2.6), and a full fillet root:

[JonathanHodgson]

Alternatively, there's an old adage in modified cars: "Never build what you can buy".

Have you tried looking for off-the-shelf gears?

http://www.hpcgears.com/

The accuracy (and hence the accuracy of movement transferred) will be far better than a 3D printer can achieve.

[Patriot_1776]

Isn't the pressure angle for ("stub"?) gears 14deg?

[JonathanHodgson]

In my work, pressure angle is just a design variable - we have a number of 'standard' values but they're all higher than 14°.  Working (as opposed to hob) pressure angles can often approach 30° for motorsport spur gears, although mainstream automotive helical gears are typically somewhat lower.

That may well be a standard somewhere, though!

[Patriot_1776]

I just seem to remember the standard 2 being 14deg pressure angle, and 20deg P/A.  But, I'm old and that was a long time ago..... 

[Patriot_1776]

Well, I was close, but no cigar.  It was 14.5deg P/A:

Pressure angle - Wikipedia, the free encyclopedia

[rrich-2]

you can get great gear models in native Creo/wildfire from Stock Drive Components - Small Mechanical Components: Precision Gears, Gear Assemblies, Timing Belts, Timing Belt Pulleys and Couplings - SDP/SI  when you download the files there are parameters and relations in file that you can modify.  I have had great luck in modifying the number of teeth and such.

Ron

[DenisJaunin]

Hello Niels,

I attached a model of TPC but it is not in the student version.

Kind regards.

Denis.

[nhendrickx]

Thanks for all the replies...

-As much as I'd love to use something like 17/34 I just don't have the room for it in the setup...

-The reason I haven't gone for simply buying the gears is because the setup for the pendulum is already mostly finished, this means the pendulum axis is 7 mm and the     encoder axis is 6 mm diameter, the smallest i could find is a gear with an OD of 12 mm for an inner bore of 6, which i would need to double to get to 2/1 ratio...

Once again simply don't have the room for it.

However If i could make a smaller version and 3D print it I could simply leave a section of the gear out to create room, since the pendulum would only move 20° or so in each direction.

- The reason I wanted to do this with helical gears is because they have a larger contact surface, which would be better for the transfer of these small angles that constantly change direction I would think.

I guess I will need to try it with regular spur gears, or simply try to find a way to put the encoder in line with the pendulum axis and lose the multiplication factor.

Thanks,

Niels

[dschenken]

How about thread and pulleys? OTOH, a linkage could also be used with any non-linearity taken out by software.

[JonathanHodgson]

"I don't have room for 17/34" doesn't make sense - just make the module smaller... (and for the same pitch diameters, i.e. same ratio and centre distance, a higher tooth count will reduce the OD of the gears as the teeth will be shallower).

Having said that, depending on the accuracy of movement you need, I suspect that the resolution of a 3D printer may cause problems and I'm not sure whether going to helical gears will help.  I like the suggestion of the thread and pulleys, or a linkage.

What is your centre distance (how far apart are the shafts) and what package limitations do you have around them?