What's new

Need accurate timing belt profiles

R-man

Senior Member
I have a situation where a timing belt follows a serpentine path with toothed pulleys, and also plain rollers on the smooth side of the belts. To calculate the belt length passing over the plain rollers I need to know how far the pitch line is from the smooth side of the belt. For the FHT-1 1 mm pitch belts that I have been using so far I have an accurate profile that shows the pitch line.

Now I am going to be working with Gates GT2/GT3 2mm pitch belts. I have found a few profile diangrams on the internet, some of which indicate the pitch line. BUT there are significant differences between them! So not so helpful. There are other companies with very similar belts with names like 2GT and 3GT which is compounding the confusion. The Gates website and design guide do not show profiles.

Is there a resource that has 'official' tooth profiles of timing belts? Or even just an official profile of the GT2/GT3 2mm pitch belts? Any leads greatly appreciated.
 

NateLiqGrav

Alibre Super User
I'm not sure you intended this to be in the scripts section.
That being said - the correct way to do this is to allow extra belt length for install and then have an adjustment mechanism to tension the belt after install.
 

R-man

Senior Member
I'm not sure you intended this to be in the scripts section.
That being said - the correct way to do this is to allow extra belt length for install and then have an adjustment mechanism to tension the belt after install.
Thanks for your reply. The problem is that there is not much room for the adjustment mechanism so its limited to about +/- 0.6 mm. My approach is to adjust by changing the diameters of the rollers to bring the length required to the middle of my adjustment range. And to do that I need to know the distance of the pitch line from the smooth side of the belt.
 

R-man

Senior Member
Because all of my queries are usually related to scripts I posted here eithout thinking. Next time I'll think before I post.
 

Ken226

Senior Member
The manufacturing materials and methods involved with rubber belts, hoses, tires etc don't allow for holding to close tolerances. The variation will be even greater if you want it to work with belts from other brands.

The design will have to allow for some variance. One of the most compact adjuster designs I've seen is the valve train timing belts on Ducati motorcycles. It is a roller mounted on an eccentric axle. It allows for about 25mm of adjustment around 180° of eccentric rotation.

Perhaps your design could allow for one or more similar compact adjusters. A single eccentric adjuster with it's axle 5mm off center would allow 10mm of adjustment.

2 adjusters with the axles mounted 5mm off center would give you 20 millimeters of total adjustment
 

Attachments

  • 45120291a.jpg
    45120291a.jpg
    74.5 KB · Views: 15
Last edited:

R-man

Senior Member
The manufacturing materials and methods involved with rubber belts, hoses, tires etc don't allow for holding to close tolerances. The variation will be even greater if you want it to work with belts from other brands.

The design will have to allow for some variance. One of the most compact adjuster designs I've seen is the valve train timing belts on Ducati motorcycles. It is a roller mounted on an eccentric axle. It allows for about 25mm of adjustment around 180° of eccentric rotation.

Perhaps your design could allow for one or more similar compact adjusters. A single eccentric adjuster with it's axle 5mm off center would allow 10mm of adjustment.

2 adjusters with the axles mounted 5mm off center would give you 20 millimeters of total adjustment
Thanks Ken226 for pointing out that type of adjustment mechanism. I came up with a similar idea independently (after much trial and error) using a 1/4 x 3/8 x 1/8 (mm 6.35 x 9.525 x 3.175) bearing that has a delrin core with a 2.5mm hole offset by 1.75mm. There are limits on its motion but it is enough to cover the +/- 0.5mm tolerance of the 324mm FHT-1 belt.

Re: Attached a step file: The large 48 tooth pulley (with belt) has a radius of 8.1 mm. The whole system exists within an 18.0mm vertical range, which limits the motion of the adjustment roller. The size of the largest smooth roller (contacting toothed side of belt) should, by 'rule of thumb', be at least the equivalent of a 40 tooth pulley . The sizes of the other smooth rollers are adjusted to create a 324mm path, with considerations for clearance. The first 9 pulleys/rollers are spaced 12.7mm. I have removed flanges etc to better communicate the geometry.

It seems that the 1mm pitch belts are being phased out and the next size up is the GT2-2mm pitch belt. I'm in the process of redesigning to use these belts.
 

Attachments

  • Roto1.stp
    2.4 MB · Views: 7
Last edited:
Top