Blog · 2026-08-15 · Belts
Every robot has to move power from where the motor is to where the work happens. Three mechanisms dominate, they fail in completely different ways, and choosing badly means rebuilding the mechanism you were most proud of.
A motor bolted directly to a wheel is the exception, not the rule. Usually the motor sits somewhere convenient, the load sits somewhere else, and something has to bridge the gap — while often changing the speed and torque along the way, as covered in Gear Ratios Explained. Timing belts, gears and chain all do this job. They are not interchangeable, and the choice is usually decided by two things people don't think about until it's too late: how much backlash you can tolerate, and how precisely you can hold two shafts parallel.
A toothed belt running over toothed pulleys transmits motion positively — no slip, unlike a flat or V-belt — while retaining the flexibility that makes belts pleasant to build with. The advantages compound: they're quiet, they need no lubrication, they tolerate imperfect alignment far better than gears, they can span a long distance cheaply, and they're compliant, absorbing shock loads that would chip a gear tooth. Efficiency is high, typically 95–98%.
The trade-offs are tension and stretch. A belt only works when correctly tensioned: too loose and it skips teeth under load, too tight and it eats motor bearings. Belts also stretch slightly under high torque, which means a belt-driven axis has a small amount of springiness — usually irrelevant, occasionally the thing that limits a high-gain control loop. And they need a fixed, accurate centre distance or a tensioner, which is what the Timing Belt & Pulley Calculator works out.
Belts are the default answer for 3D printer axes, camera sliders, robot arm joints, and any transmission where the motor needs to sit away from the load. If you don't have a specific reason to choose otherwise, choose a belt.
Meshing gears transmit power over a short distance with high rigidity and no stretch at all. Where belts are springy, gears are stiff — which matters for precise positioning under varying load, and for control loops where compliance shows up as oscillation. Gears also handle very high torque in a small space, and a gear train can achieve large reductions in stages.
What gears demand in return is precision. Centre distance must be correct to a fraction of a millimetre: too close and the teeth bind and howl; too far and they skip or wear rapidly at the tips. Shafts must be genuinely parallel. Get either wrong on a 3D-printed bracket and the mechanism will be noisy, inefficient and short-lived. Gears also introduce backlash — the small rotational play when reversing direction — which is inherent to the tooth clearance they need in order to mesh at all.
Choose gears when the shafts are close together, the load is high, rigidity matters, or the whole assembly must be compact — which is exactly why they live inside gearboxes rather than being exposed on most robots.
Roller chain is the heavy-duty option. It handles enormous torque, spans long distances, tolerates dust and knocks that would destroy a belt, and — usefully — its length can be adjusted by adding or removing links, so centre distance is far less critical. This is why chain persists on combat robots, large outdoor platforms and anything driving through debris.
It's also the messiest. Chain needs lubrication, and lubricated chain collects grit, which grinds the pins and stretches the chain over time. It's noisy, it's heavy, and a thrown chain at speed is genuinely dangerous. Chain also exhibits the "chordal effect" — the drive speed pulses slightly as each link engages the sprocket, more noticeably on small sprockets — which makes it a poor fit for smooth precision motion. Unless your robot is big, dirty, or being hit by other robots, chain is probably not your answer.
| Property | Timing belt | Gears | Chain |
|---|---|---|---|
| Efficiency | 95–98% | 95–99% per stage | ~95%, falls as it wears |
| Backlash | Very low | Present, needs care | Moderate, grows with wear |
| Rigidity | Springy under load | Very rigid | Moderate |
| Alignment tolerance | Forgiving | Very demanding | Forgiving |
| Centre distance | Fixed (or tensioner) | Critical | Adjustable by links |
| Noise | Quiet | Moderate to loud | Loud |
| Maintenance | Occasional tension check | Grease, rarely | Regular lubrication |
| Failure mode | Skips teeth, then shreds | Chips a tooth | Stretches, then throws |
Backlash is the dead zone when a transmission reverses direction: the driving element moves a little before the driven element responds. For a drivetrain it's mostly harmless. For a robot arm joint, a camera pan-tilt or any positioning axis that reverses, it's the thing that limits repeatability — and it multiplies through a gear train, so a three-stage gearbox has three stages of it.
Timing belts have very little backlash, which is one of their underrated advantages. Gears have inherent backlash, reducible with anti-backlash designs (split spring-loaded gears) or eliminated in specialised gearboxes like harmonic drives at considerable cost. In software, you can also always approach a target position from the same direction, which sidesteps backlash entirely at the cost of some extra motion.
Real machines mix mechanisms rather than picking one. The common pattern: a geared motor (gears inside, where they're precise and protected) driving a belt (out in the open, where its tolerance for misalignment and its quiet running are worth more). That combination gets high reduction compactly and then moves the power somewhere useful, which is why it appears on nearly every printer, plotter and light robot arm.
Whatever the combination, the ratio maths is the same throughout: each stage multiplies torque and divides speed by its ratio, and the overall ratio is the product of the stages. Work it out with the Gear Ratio Calculator, size the belt stage with the Belt & Pulley Calculator, and check the motor can supply the input torque using the Motor Sizing Calculator.
Printed timing pulleys work well — tooth loads are spread across many teeth and the belt is compliant. Printed spur gears work for light loads but wear quickly under continuous torque, and printed gears demand a precisely printed centre distance, which is exactly the thing printers get slightly wrong. Print pulleys freely; print gears with lower expectations.
Practically about 5:1, limited by tooth engagement on the small pulley — below six teeth in mesh, belts skip. Larger reductions want two stages or a gearbox, which the belt calculator's mesh warning will steer you toward.
Not on a schedule — on inspection. Look for cracked tooth roots, frayed edges, missing teeth or a belt that will no longer hold tension. A well-tensioned belt in a clean environment lasts years.
The selection is only half the job; the stage still has to survive the torque you put through it. Three checks apply across all three mechanisms. Check the small element first — the small pulley, the pinion gear, the small sprocket. It sees the most tooth passes per unit of travel, carries load on the fewest teeth, and wears out long before its larger partner. Undersizing here is the most common transmission failure in hobby builds.
Check the shaft and its fixing. A pulley or gear is only as good as its grip on the shaft: a grub screw seating on a round shaft rather than a machined flat will slip, and it will do so intermittently in a way that perfectly imitates a slipping belt or a skipping gear. Use the flat, add threadlocker, and prefer two grub screws at 90° for anything highly loaded.
Check the bearing loads. Belt tension, gear separation forces and chain pull all push the shafts apart, and that force lands on the motor's front bearing — often the weakest part of a cheap gearmotor. If a transmission is heavily loaded, supporting the shaft in its own bearing block rather than cantilevering it off the motor will add years to the motor's life.
Belt unless there's a reason not to; gears when it must be rigid and compact; chain when it must survive abuse. Once you've chosen belt, the next question is tension — which is where most belt problems actually originate, covered in How to Tension a Timing Belt.