Choosing Belts and Pulleys

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Choosing Belts and Pulleys

Traditionally RepRaps have been assembled using printed pulleys and T5 or XL belts. From an ideological standpoint, that makes sense, we should minimize the vitamin count wherever possible. Unfortunately, with the exception of the hot end and extruder, the pulleys and belts are probably the single most important parts when it comes to improving your print quality, and the commonly used parts are almost completely wrong for what we do.

What's wrong with the status quo?

There are a number of problems.

Depending on how well tuned your printer is, the quality of your printed pulleys will range from good to atrocious. Even the best quality pulleys will not be up to the quality of commercial pulleys.

More importantly, the type of belts we use are all wrong. The T in T2.5 and T5 stands for Timing. It is designed to synchronize rotating shafts. It is not designed for linear motion, so it does not account for backlash in a reversing pulley. XL is also designed as a timing belt, but is a SAE standard, not metric. __The two are not compatible!__ If you have XL belts and pulleys that were printed for T5 belts, they may seem to fit but you will never be able to get decent prints.

How do we fix the problem?

To get the best print quality, we want to use a pulley and belt that are designed for linear motion. The GT2 series of belts are designed specifically for that purpose. They use a rounded tooth profile that guarantees that the belt tooth fits smoothly and accurately in the pulley groove, so when you reverse the pulley direction, there is no room for the belt to move in the groove. You should see an almost immediate increase in print consistency, especially around small holes and details.

How do I choose a pulley and a belt?

Generally speaking, for best performance you want at least 6 teeth in contact with the pulley at any given time. That minimizes the chance of the belt slipping, and helps reduce backlash even further. In practice that means you want a minimum of a 12 tooth pulley, and I usually try to get at least 18 teeth.

Beyond that minimum, fewer teeth are generally better than more teeth, since a smaller pullet gives both more torque and more resolution. You get more torque because the longer your "arm", the less torque you have (Imagine the load is mounted on an arm the length of the radius of the pulley, the shorter that arm, the easier it is to lift the load), and you get higher resolution, since you have a fixed number of steps per revolution, and a smaller pulley moves a shorter linear distance per step.

Where do I get them?

There are two good sources that I know of for GT2 belts and pulleys. In the US, the best source is Stock Drive Products. They sell a huge variety of belts and pulleys that meet our needs. Here are some suitable pulleys:

Always call SDP for stock. Their website is always out of date.

Note, unfortunately, SDP does not carry a smaller diameter GT2 pulley with a 5mm bore, so to buy your pulleys here you need a larger pulley than in strictly ideal. These are fine, though. I use the 36 tooth pulleys on my printer and they work great.

I recommend GT2, 2mm pitch, but you can also use GT2 3mm pitch. A finer pitch is preferable for the type of movement we do, and the belts are cheaper, but if you find the 2mm belts or pulleys are out of stock, you can go to 3mm pitch without any significant downsides. If you do choose 3mm pitch pulleys, be sure to get 3mm pitch belts as well.

Another good source for GT2 pulleys and belts is Misumi.

They carry a huge variety of suitable pulleys, including several that are closer to out ideal diameter. In addition, Misumi has offices around the globe so they are a better choice for international customers (though some regions will only sell to businesses).

Their website does not make it easy to browse for belts and pulleys, but here are the relavent pages in their catalog:

Note: Misumi calls theirs 2GT (2mm pitch) and 3gt (3mm pitch). As far as I know they are compatible, but both offer the same benefits. Misumi's pulleys seem expensive, but they are metal pulleys. We don't really need metal pulleys for our needs, but they are fine.

References