When should I use belts instead of chain?

Belts are recommended over chains for drivetrains, although new teams may find belts more challenging . Belts are generally lighter and more compact than chains . They are also more efficient and quieter compared to chains .

Belts are suitable when rotational accuracy is important, as they can transmit power with virtually no slop when properly tensioned . This precision is why engines use timing belts to synchronize shafts .

However, belts have some disadvantages compared to chains:

Belts are not customizable in length; they come in predetermined closed loops, making them less suitable for prototyping where length changes are frequent .
Belts can be wider than chains, which might be a consideration for space constraints .
While individual belts can be expensive, cost savings can be realized with pulleys, which can be 3D printed .

When using belts, proper tensioning and belt wrap are crucial to prevent skipping or falling off the pulley . Belts should have at least 90 degrees of contact with the pulley, with 180 degrees or more being best practice .

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Rigging > Belt or chain driven elevators

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Almost ubiquitous in FRC®, belt and chain elevators have existed seemingly forever. The basic premise is to have sprockets or pulleys mounted at the top and bottom of the elevator to the robot superstructure. Then, the belt or chain is hard bolted to the elevator near the bottom sprocket or pulley. When the chain or belt is driven, the elevator will move up and down. It is possible for elevators to have multi stage designs, but powering them will be more complex.

(Figure: 7236 Recharged Green, Rover Ruckus)

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Timing Belt > Belt Wrap > Best practices for wrap

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(Figure: Ethan Doak, goBILDA)

(Figure: 8103 Null Robotics, offseason prototype, properly done belt wrap with tensioners)

Pulleys can be made at home. Pulleys can be 3D printed for most situations, allowing you to cut costs and create unique tooth counts easily.
Belts are very strong. They're reinforced with fiberglass cords that are incredibly hard to break, giving belts immense strength. (If you break a belt, it's most likely because it was out of alignment or tensioned far too tightly.)
When tensioned correctly, there is absolutely no slop. Engines use timing belt for a reason - because it's the best possible solution for them to perfectly synchronize their shafts. There's nothing that matches the rotational accuracy of a properly tensioned belt.
Belts are efficient and quiet. Compared to the loud shredding sound of a chain run, belt runs are dead silent, and they're more efficient than chains (although this makes zero practical impact in the robotics use case).

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Timing Belt > Disadvantages:

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Belts aren't customizable. You buy a belt of a specific length and you're stuck with that length until you buy another one. This isn't too bad if you're planning out your robot properly, but chain will work better for prototypes where the chain length will be changing often.
Belts can be wider than alternatives (especially chain). This probably won't have much of an impact, but belt can often be wider than other power transmission methods, so it may not always fit.
Belts can be expensive (but you'll save money with pulleys). While you can buy chain 10 feet at a time, you'll most likely be buying each belt brand new. While this can get expensive, you'll be saving money on pulleys.

(Figure: 7236 Recharged Green, Rover Ruckus)

(Figure: 8417 Lectric Legends, Rover Ruckus)

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Timing Belt

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When you think of a belt, you're probably thinking of a very important men's fashion accessory. However, there's another type of belt, and it's way more relevant to robotics - the timing belt. If you've ever tinkered with the insides of a car before, you probably recognize timing belts as an important component designed to keep everything under the hood in sync.

Term: Timing Belt — Timing belts use a series of small, wide teeth to engage a pulley with a number of matching grooves. They earn their name because they can be very precise, transmitting power with virtually no slop and ensuring a snug connection between shafts.

(Figure: Timing belts and a pulley)

While a timing belt may complete a similar objective to chain, its characteristics and strengths are very different. Timing belts are lighter and more compact than chains, but they lack the customizability of their bulkier brother - belts come in a closed loop of predetermined length, and there's no changing that length on the fly.

Like chain, belt is identified by its pitch - common pitches found on FTC® robots include HTD 5mm, HTD 3mm, and GT2 3mm.

When using timing belts, correct tension is very important. There are two main ways to get your tension right. The first is easy - goBILDA and Actobotics already have belts integrated into their hole patterns. You can buy correctly sized belt directly from each vendor, and your tension will be perfect as soon as the belt is installed. As your designs gain complexity, so will your belt runs - maybe there are more than 2 pulleys, and maybe your pulleys are all different sizes. To compensate for this, the second way to ensure tension is to use a dynamic tensioner, similar to those found in complex chain runs. To design for these tensioners, we recommend planning more complex belt runs in CAD before building them in real life.

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Drivetrains > Drivetrain selection

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Warning: It is not recommended for teams to use spur gearboxes on their drivetrain. Instead, use planetary gearboxes, as they are less prone to shock loads and breakage.

Traction/Pushing Power: While this feature is often overemphasized, it is still very important. Pushing power describes a drivetrain's ability to endure defense/engage in defense. In addition, traction will be important if the drivetrain must traverse obstacles or some sort of terrain.

Many factors affect the pushing power of a drivetrain, including wheel type, motor gearing, and overall weight of the robot.

Note: If you already have a very agile drivetrain with experienced drivers, a team can avoid defense instead of having to fend it off or engage in it.

Powering the drivetrain: Generally, there are four options for power transmission: direct drive, chain, gear, and belt. More detail about each option can be found in the power transmission section.

Teams should stay away from direct drive, as gearboxes are prone to breaking, especially under shock loads (for example, if the wheel is hit by another robot, or the wheel slams into the wall).

We recommend belted drivetrains, but realize that belt is a difficult option for new teams. Chain and gears are also great options - chain requires a bit more forethought, as 1+ tensioner per side is required to maintain correct tension in the chain.

CAD or a detailed sketch is generally recommended with chain in order to visualize the chain run (where the chain will be placed). Gears are slightly simpler, and can be a fantastic and easy option, especially when using extrusion. We would advise to stay away from TETRIX gears, and use the gears from other kits such as the REV delrin gears (with hex hub strengthener) or REV aluminum gears.

See the /docs/common-mechanisms/power-transmission/index section for more details.

An important step is determining what you want out of your drivetrain.

Do you want speed?
Pushing power?
Ability to get over terrain?
Do you need to strafe?

All of these questions should be answered before choosing a drivetrain.

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Timing Belt > Belt Wrap

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Belt should, at the very least, have 90 degrees of contact with the pulley. The best practice is to have 180 degrees or more of contact, as it is very unlikely to fall off with proper tensioning. Belt skipping, especially on drivetrains or arms, is very possible without proper belt wrap or tensioning. When tensioning belt, be sure to not undertension or overtension it. Undertensioning belt can result in the belt falling off the pulley or belt skipping, where the belt can skip along the pulley. Overtensioning belt often results in the motor burning out, or less seriously, a loss of efficiency. Push along the belt, and if it moves slightly without significant resistance, chances are you've done it correctly. If it's too tight, then the belt will barely move under a gentle press.

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