Monday, October 14, 2019

Using Flanged Bearings as Pulleys

Flanged Pulleys


You can buy flanged pulleys for GT2 (and other) belts ready made with ball bearings press fit into them, but I usually try not to use those because the bearings are very tiny and I don't trust them to last very long.  Also, if the tooth side of the belt is contacting the pulley, there will be few teeth in contact with the belt and that may lead to a zipping noise at high speeds and possibly to defects in prints if the belt is in a 3D printer.

A toothed GT2 pulley with 5 mm bore.
A smooth pulley with 3 mm bore.  This is the type I used in The Spice Must Flow sand table.

I used eight of those small pulleys in The Spice Must Flow sand table, mainly because I needed the large flanges to help keep the very long belts on the pulleys.  I solved the zipping noise problem by twisting the belts so the smooth sides of the belts would contact the pulleys.

In UMMD I used stacked F608zz bearings as pulleys in all three axes.  I prefer this type of construction because the bearings are much larger and more likely to last longer under the load of the belt tension.  The problem is that the flanges of those pulleys are small, so they don't work so well for long belt runs.

One of the stacked F608zz bearings pulleys I used in UMMD.  The belt runs were relatively short so the bearing flanges were enough to keep the belt on the pulleys.


A Printable Solution


I have been looking at yet another redesign of the sand table mechanism and thinking I'd like the replace the tiny pulleys with larger ball bearings, but I had to figure out a way to get large enough flanges that would keep the belts on the pulleys.

6 mm wide GT2 belts are used in a lot of 3D printers and in The Spice Must Flow sand table.  If you're going to stack flanged bearings to use as pulleys, you need to know some critical dimensions.  The commonly used F608zz (as I used in UMMD) is a 22 mm diameter x 7 mm wide x 8 mm bore pulley.  However, the flange uses up about 1.5 mm of that 7 mm width, so stacking two F608zz bearings will have enough width to accommodate up to about 11 mm wide belt. This works well for 9 and 10 mm wide belts as long as the runs aren't too long.

The diameter of the flange is 25 mm and the thickness of the flange is 1.5 mm, so the flat surface for the belt is 5.5 mm wide.

Dimensions of an F608zz bearing.
Flanged bearing are available in many sizes and the same concept can be applied to any of them.  You can get the dimensions with a quick web search for something like "F608zz bearing dimensions" and then select "images".

What if you want to use a relatively large bearing for reliability but need a bigger flange to accommodate longer belt runs?  This is the problem I faced in redesigning the sand table mechanism to operate more reliably and quietly.

This is what I came up with:

3D printed flange adapters fit on the bearings.  This will take only a few minutes to print.

I want to use a printed flange that fits over an existing bearing. The bearing's flange will stop the printed flange from coming off the bearing.  Stacking a pair of these looks like this:

Stacked F608zz bearings with printed flanges (green) to extend the existing, small flanges.  The space between the flanges is now 9.8 mm, but can be adjusted by appropriate tweaks to the dimensions of the printed parts.
I'd probably apply a bit of superglue to bond the printed flanges to the bearings, though it probably isn't strictly necessary.  In this configuration, the belt will contact the bearing's steel surface.  This type of design can be applied to any size flanged bearings to make wide flanged pulleys.

Here's another idea:

This would print in 3 pieces without any support material- the two flanges (green) and the central tube (red).

Like the printed flanges, the tube that separates them can be made any size needed.  If the two bearings are clamped together (and they should be) by the mounting system, the flanges should stay in place without any need for glue, though it might be a good idea to bond the three printed pieces to each other.

When you print any of these, you need to print the parts so they fit closely on the bearings, and use a random seam location setting so that the layer start and end points will be distributed around the surface of the parts to provide best concentric fit.  Using concentric infill for the top and bottom layers would probably also be best. You may want to print a few of them at a time or add a sacrificial object to the bed to allow the prints to cool a little between layers for best print quality.

It might be a good idea to print a gauge with holes that match the size of the bearings you're using +/- 0.3 mm or so in 0.05 mm steps.  Then you can push a bearing into the holes in the gauge to see which printed hole size will fit best.  I used holes that were 0.3 mm larger than the bearing dimensions and they came out fitting well but not tightly.  I could probably get a slightly tighter fit by using holes that are 0.25 mm larger diameter than the bearings.

The Result


Assembled pulley with wide flanges printed in PETG.


The pieces, all print solid.

Assembled pulley.


It remains to be seen how long the flanges will last if a belt is rubbing against them.  It will depend on the material- both belt and printed parts, and the tension on the belt, and maybe the belt speed, too.  I'm not sure I'd use pulleys like these in a high accuracy/precision 3D printer, but in a sand table, why not?

I'll be updating the sand table design and expect to be using this sort of assembly to replace the small pulleys it currently uses.  I'll post an update when that happens.

The Fusion360 CAD file for the F608zz bearing version of this pulley is located here.