Cutting Timing Pulley Teeth on a Lathe

[ArtF]

Dan:

I think I had the belt thickness as double .. Next version fixes it..

Thx
Art

[Dan]

Great! Thanks Art!

Dan

[ArtF]

Dan:

Ive tentatively changed the GT2-2 and GT2-3 pulleys so they match the numbers specified. Its tentative
because Ive checked the numbers, and I could swear the originals make better sense, but dont agree
with the pulley specs from a couple sites.

  The pitch circle for pulleys should be in the center of the belt. The outside diamter then , is the pitch diam
subtracting the belt thickness. ( They call it PD - 2H in spec, or PitchDiam - (2 times the half height of a belt).

  If I use PD - .5Belt, I get your numbers and numbers that match a couple pulley makers I checked, this seems
illogical to me and a mistaken application of spec.. but Ill assume its me and release it so it matches the specs
as you found them to be. Its online now, so feel free to yell if they look wrong..

Thx
Art




   
 

[kit]

This thread set me thinking - Surely the only sensible value for 'pitch diameter' is the distance the belt travels during one revolution divided by Pi? This equals the pitch of the belt multiplied by the number of teeth on the pulley. Regular readers probably consider this as blatantly obvious, but it isn't specifically stated in the thread, so I had to check.

Anyway, I found these useful tables for pulley outside diameters which may be of interest if you haven't got them already. Results are in both millimetres and inches to suit your geography.

Thanks for setting me off on a journey of discovery.

Kit

[Dan]

Art,

Does it mean that I was wrong treating the "outside diameter" shown by Gearotic as the pulley blank diameter?

Dan

[ArtF]

Dan:

No, it means gearotic gave you the wrong number.

Let me show you an example.. .

A GT2-2 20 tooth...

  Pitch Diam is the  (Pitch*Teeth)/PI; so..

  40 / PI = 12.73mm , Gearotic has always reported this properly..

In Pulleys, this pitch is considered to be in the middle of the belt, so the actually
pulley is smaller. Usually by a spec called 2HH ( which stands for 2 times half height..).
I have tables of these HH values as they relate to the matching belt. The value I had for this
gt2-2 is 1.016. This would mean the pitch line is .508 upwards from bottom of belt and that the belt
is 1.016mm thick ( generally, though is only ensured that the pitch is 1/2 of that above the gear..)

  If we subtract 1.016 from 12.73 we get a outside radius of 11.714.

  Thats the value Gearotic was putting out..

Now if you check for example, a 2mm pulley of 20teeth here,
http://www.cmtco.com/catalog/2mgt2.html

you will find the pitch diam is .501" or 12.73mm , so we're good there of course..
Outside diam though is 0.481 or 12.217mm.

  This is .503mm off. Gearotic was reporting too small an outside diam by exactly
1/2 of the 2hh value that I had. Im suspecting my 2hh value table for those 2 was off
and that the spec showed me the 2hh as 1hh.

So I have updated the valuse so the proper outside dia is put out. Im not sure where
the error came from, but its seems safe to make the change. So the numbers you get
now for blank diameter shoudl now agree with industry.

Thanks for bringing this too me, I do like to try to make it all as accurate as possible..

  The only thing that bothered me was my 1.016mm for the thickness of the belt sounded
more reasonable than .5008mm as a belt thickness.... but..spec is spec and perhaps the
gt2's are special.. but it does mean the pitchline is .25mm above the outside of the gear..
seems small to be..

  but.. numbers are numbers, and they always look better to me when I see agreement
in industrial sites.. so lets make it match the makers.. tentatively.. lol


Art

[kit]

Maybe I should have included the link in my last post! The drawing speaks a thousand words as usual and there are links to tables for various sized belts.

http://www.pfeiferindustries.com/timing ... -l-en.html

Kit

[Dan]

Thanks for explaining it Art and for the fix. And Kit, thanks for the drawing.

I just measured the thickness of a GT2-2 belt and it is around 0.5mm.

Art, did you also address the tooth profile error?

Dan

[ArtF]

Dan:

  No, the profile is much more complex, specs can vary and the ones I have dictate the shape. Unlike a belt thickness ( Thx for that, the .5mm thickness
matches the fix perfectly.), the shape is determined by a list of radial curves and centers, so variation between manufacturers can be for marketing and legal
l reasons. I will review them though in case Im screwing them up somewhere..

Art

[Roger]

SDP-SI have an excellent screed on tooth belt dimensions.
For GT2 the rules are as follows:
A toothed pulley with N teeth has a pitch circle diameter (PD) of 2.00*N. (The pitch on the belts is 2.00 mm)
The radial distance between the PD circle and the Outside Diameter (OD) circle of the pulley is 0.010" or 0.254 mm, by definition. (So the difference in diameters is 0.508 mm.)

The actual GT2 profile is in the Gates patent, but they have managed to draw the diagram in a reasonably incomprehensible form. This is of course entirely accidental ...  ;)

If you search through the SDP-SI web site you will find that you can download a DXF for any of their stock pulleys. Fine - but with a tiny bit of knowledge you can open that DXF up to see what the profile looks like, in terms of arcs etc. How nice! However, be aware that the DXF drawings are NOT an exact representation of the Gates profile. The DXF is several microns wrong in several places.

If you search through the web you will find Chinese GT2 belts and pulleys. Well, they CALL them GT2, but they are not licensed by Gates afaik, and their profiles are a bit more off. You may also find their idea of the profile as well.

Other good brands have profiles which are very similar to the GT2, but they have to call them something else because of the patents etc which Gates holds.

Now, some background to explain all this. The GT2 profile is very similar to the HTD profile, but the HTD patent expired. So Gates developed and patented the GT2 almost entirely as a successor to the HTD, rather than for any really serious technical reason. The GT2 patent has now also expired - so Gates now have the GT3 patent ...  Fortunately, even Gates admit the GT2 and GT3 are compatible. In short, the differences are small.

The big advance is the curved profile on the belt which snugs into the pulley, as opposed to the loose and rattly profile of the old XL-type belts. Since the rounded teeth engage so much better, the rounded tooth belts are both quieter and transmit more power. And the GT-style has almost zero backlash, which is a lot better than the XL ever were.

I have been cutting GT2 pulleys in various sizes, with good success, to upgrade my CNC. They replace the old XL belts it came with.

Cheers
Roger

[ArtF]

Thx Roger:

I appreciate the data. It does match with the spec I have in the program now, Im not sure where
the double belt thickness came from.. but in a table of all those pulley types it
may have inadvertantly become transposed with another. .

PD seems good now. :)

Art