Belts, Pulleys & Chain Drives for Robotics: The Ultimate Guide

Between a motor and the thing it moves, something carries the torque across a gap. Sometimes that gap is a few millimeters and you bolt the load straight to the rotor. Usually it is not. The motor lives where there is room and cooling; the joint lives where the geometry demands; and a flexible element spans the two. That element is a belt, a cable, or a chain, and the choice quietly sets your robot's backlash, its reflected inertia, its noise, and how often a technician stands over it with a tension gauge.

Robotics leans on these drives harder than most machinery because two of their properties are worth real money in a moving robot. First, they let you put the heavy motor near the base and drive a distal joint remotely, so the arm or leg swings light. A cable-driven finger has almost no actuator mass in the finger itself. Second, a belt or cable stage is cheap, quiet, and forgiving of the small misalignments that a rigid gear train punishes. The cost is compliance you have to design around and tension you have to maintain.

This guide treats the three families as one design space. We cover timing-belt geometry (GT2, HTD, AT, and why the tooth profile matters), pulleys and the ratio they set, tension and the backlash-versus-life tradeoff, the sizing math that actually sizes a belt (width, tooth shear, tension, reflected stiffness), capstan and cable/tendon drives that let dexterous hands and some legs move mass remotely, chain drives where the load is brutal and dirty, and the honest decision of when a belt beats a gearbox and when it does not.

The take: A toothed belt is the default robotics transmission for light-to-medium loads over a short span: cheap, quiet, near-zero backlash if you tension it right, and it decouples motor mass from joint mass. Reach for a cable or tendon drive when you need to move a distal joint with almost no local actuator inertia, as in dexterous hands and some legs. Reach for chain when the load is heavy, dirty, and slow. Reach for a gearbox when you need high ratio and torsional stiffness in a compact package, and for direct drive when backlash and bandwidth must be perfect and you can pay in torque and heat. Size the belt by tooth shear and tension, not by "it looks strong enough."

Companion reading: gearboxes (harmonic & cycloidal), linear motion systems, robot actuators, end-effectors & grippers, and legged & quadruped robot hardware.

Table of contents

1. Key takeaways
2. Why flexible drives earn their place
3. Timing belt geometry: pitch, profile, construction
4. Pulleys, ratio, and the belt-drive kinematics
5. Tension, backlash, and belt stiffness
6. Sizing a timing belt: the real math
7. Capstan and cable/tendon drives
8. Chain drives
9. The tradeoff: belt vs gear vs cable vs direct drive
10. A worked sizing example
11. Integration, failure modes, and maintenance
12. How to choose
13. Frequently asked questions

Why flexible drives earn their place

A rotary servo or BLDC motor makes torque at a shaft that spins fast. Robot joints usually want the opposite: more torque, less speed, and often at a location the motor cannot physically occupy. Three jobs have to happen between the two.

• Move the power across a distance. The motor sits where there is room, mass budget, and cooling. The joint sits where the kinematics put it. A belt, cable, or chain carries torque across that span with a fraction of the mass a rigid shaft-and-coupling chain would need.
• Change the ratio. A single belt or chain stage gives a modest reduction (typically up to about 5:1 to 8:1 in one clean stage) by running a small pulley into a large one. That trims speed and multiplies torque without a gearbox.
• Decouple inertia. This is the robotics-specific reason. Put the motor at the base and drive a distal joint through a belt or cable, and the distal link swings without carrying its own actuator. Lower moving inertia means faster acceleration for the same motor and less energy thrown around on every move.

That last point is why a flexible drive is often the right answer even when a gearbox would also work. A quadruped leg that belts its knee from a hip-mounted motor swings a light shank; a dexterous hand that tendons its fingers from forearm motors has fingertips light enough to be safe and fast. The robot actuators guide treats the actuator as a whole; here we open up the transmission stage inside it.

Rule of thumb: If the joint is far from where the motor wants to live, or the distal inertia is hurting your dynamics, a belt or cable stage often beats a gearbox before you have even looked at ratio. Remote mounting and low reflected inertia are the flexible drive's home turf.

Timing belt geometry: pitch, profile, construction

A timing belt (synchronous belt) has teeth that mesh with matching grooves on the pulley, so there is no slip and the input and output stay phase-locked. That phase lock is what lets a belt carry position, which a flat or V-belt cannot. The geometry breaks into three choices.

Pitch

Pitch is the distance between adjacent teeth along the belt, measured tooth center to tooth center. It names the belt series and sets the load per tooth.

Series	Pitch	Profile	Where it lives in robotics
MXL	2.03 mm	Trapezoidal	Legacy small drives, light instruments
GT2	2 mm	Curvilinear (rounded)	3D printers, small arms, light automation; the hobby and light-robotics default
GT3 / 3M	3 mm	Curvilinear	Small-to-medium arms, gantries
HTD-5M	5 mm	Deep curvilinear	Medium joints, gantries, AGV wheel drives
HTD-8M	8 mm	Deep curvilinear	Larger joints, heavy gantries
AT5 / AT10	5 / 10 mm	Trapezoidal, high-stiffness	Industrial linear axes, stiff robot joints

Bigger pitch means a bigger, deeper tooth that carries more load and resists ratcheting (jumping a tooth under overload), at the cost of a larger minimum pulley and a slightly rougher ride. Smaller pitch runs quieter and wraps smaller pulleys, which matters when a distal joint needs a compact output pulley.

Tooth profile

Profile is the tooth's cross-section shape, and it decides how load spreads across the mesh and how much backlash and stress the tooth sees.

• Trapezoidal (MXL, XL, L, and the classic timing profile): straight-sided teeth. Simple and cheap, but stress concentrates at the tooth root and the mesh ratchets earlier under load. Fine for light, low-torque work.
• Curvilinear (HTD, then GT2/GT3): rounded teeth that seat deeper and spread contact stress, so they carry more load with less tooth deformation and less ratcheting. GT (Gates GT / Powergrip GT) refined HTD to also cut backlash by fitting the tooth more precisely in the groove. This is the mainstream robotics choice.
• Modified trapezoidal, steel-corded (AT5, AT10, AT20): a tall, stiff tooth on a steel-corded body. Very high stiffness and load, used where a belt axis must be nearly as rigid as a screw. The tradeoff is a larger minimum pulley and less compliance to absorb shock.

Construction

A timing belt is a composite. The teeth and body are molded elastomer (neoprene or polyurethane), a nylon fabric facing covers the tooth to cut wear and friction, and a set of helically wound tensile cords runs along the pitchline carrying essentially all the tension. Cord material sets the belt's stiffness and stretch:

• Fiberglass cord: the common general-purpose choice, stable length, moderate stiffness.
• Steel cord: highest stiffness and lowest stretch, standard on AT belts and stiff linear axes.
• Aramid (Kevlar) cord: high strength and shock tolerance, used where the belt sees jerk or reversing loads.

The tensile cord is the load path. When you compute belt stiffness, you use the cord's effective modulus, not the rubber's. When you route a belt around too small a pulley, it is the cord you fatigue by over-bending.

Pulleys, ratio, and the belt-drive kinematics

A timing pulley (sprocket, in belt terms) has grooves matching the belt pitch. Its size is named by tooth count, and the tooth count is what you compute with, because the belt meshes on teeth.

The pitch diameter (the effective diameter at the belt's tensile cord, where the kinematics actually happen) is:

d_pitch = pitch * N / pi
   pitch = belt pitch (mm), N = pulley tooth count

Example: a 20-tooth GT2 pulley
   d_pitch = 2 * 20 / pi = 12.73 mm

Note the pitch diameter is not the outside diameter you measure with calipers; the OD sits slightly below the pitchline because the cord rides near the tooth tips. Use tooth counts and pitch diameters for ratio and speed, never the caliper OD.

Ratio and speed follow directly:

ratio i = N_driven / N_driver          (>1 is a reduction)
output speed = input speed / i
output torque = input torque * i * eta   (eta ~ 0.95-0.98 for a good timing belt)

Belt linear speed v = omega_driver * d_pitch_driver / 2
Center distance and belt length are coupled: for two pulleys,
   L_belt ~ 2C + (pi/2)(d1 + d2) + (d2 - d1)^2 / (4C)
   C = center distance

A single clean belt stage gives a useful reduction up to roughly 5:1, sometimes 8:1 if the small pulley still keeps enough teeth in mesh. Past that you either stack two stages or move to a gearbox. The constraint that bites first is teeth in mesh: the small (driver) pulley must wrap enough teeth to share the load. As the pulley shrinks or the ratio grows, the driver wraps fewer teeth, each tooth carries more, and the belt ratchets. Keep at least about 6 teeth in mesh on the loaded pulley, and honor the belt series' minimum pulley tooth count (often 10 to 20 depending on pitch and cord).

Rule of thumb: Belt-drive ratio is set by tooth counts and limited by teeth-in-mesh on the small pulley, not by the center distance. If you need more than about 5:1 to 8:1 cleanly, stage it or use a gearbox.

Tension, backlash, and belt stiffness

Tension is the single most consequential thing you set on a belt drive, and it is where most belt problems are born.

Why tension matters

A timing belt does not rely on friction to carry torque, but it does need enough pretension that the belt stays seated in the pulley grooves and the slack span does not go loose and let the mesh disengage. Under torque, one span of the belt tightens (the tight side) and the other loosens (the slack side); the difference is the effective tension that carries the load:

T_tight - T_slack = T_effective = torque / (d_pitch / 2)

You pretension so that even at peak torque the slack side stays under tension (does not go to zero and flap). Too little pretension and the belt jumps teeth (ratchets) or the mesh rattles; too much and you crush the pulley bearings and shorten belt and bearing life. Manufacturers give a target static tension in newtons or a target span natural frequency you measure with a tension meter or a phone app that listens to the plucked span.

Span frequency method: f = (1 / 2L) * sqrt(T / mu_linear)
   f = fundamental frequency of the plucked free span (Hz)
   L = free span length (m)
   T = span tension (N)
   mu_linear = belt mass per unit length (kg/m)
Solve for T at the target f the maker specifies.

Backlash versus compliance

A well-tensioned timing belt with a curvilinear profile has near-zero backlash; the teeth seat with little clearance and the belt carries position faithfully in both directions. What it does have is elastic compliance: the belt is a spring. Do not confuse the two. Backlash is dead lash you feel on reversal; compliance is a spring that stores and returns energy. Backlash you eliminate with a tight-meshing profile and correct tension; compliance you design around in the control loop.

Belt stiffness and resonance

Each loaded span acts as a spring with stiffness set by the cord:

k_span = A * E_spec / L
   A * E_spec = the belt's specific tensile stiffness (N per unit strain),
                given by the maker per unit width (multiply by belt width)
   L = span length (m)

The two spans between a carriage and its pulleys act as springs in parallel, so the effective stiffness is k = k_span1 + k_span2, worst at mid-span where both are long. That stiffness with the moving inertia sets a mechanical resonance:

f_n = (1 / 2pi) * sqrt(k / m)     (linear axis)
f_n = (1 / 2pi) * sqrt(k_torsional / J)   (rotary joint via belt)

On a long belt axis this resonance can land as low as 20 to 80 Hz, and your position-loop bandwidth has to sit safely below it or the axis rings. This is the real ceiling on a belt drive's dynamic performance and the reason a stiff joint that needs high bandwidth often wants a gearbox or direct drive instead. When the transmission is this compliant, closing the servo loop on a load-side encoder rather than the motor encoder is often the difference between a stable axis and a singing one.

War story: A team belts an arm's wrist from a forearm motor, tensions it "by feel" on the loose side to reduce bearing load, and the wrist holds position fine at rest but overshoots and buzzes on fast moves. The mesh was ratcheting one tooth under peak torque, then re-seating: the position kept jumping by exactly one belt tooth. Nobody saw it because at rest the belt looked fine. The fix was to pretension to the maker's specified span frequency and go up one pitch size so peak torque no longer approached the ratchet limit. Set tension to spec with a meter; do not guess.

Sizing a timing belt: the real math

Sizing a belt means choosing pitch, width, and pulley sizes so the belt survives tooth shear, cord fatigue, and pulley bending over the design life. Four checks, in order.

1. Effective tension from torque

T_effective = 2 * T_motor * i * eta / d_pitch_output    (N, at the output pulley)
   or at the driver:  T_effective = 2 * T_motor / d_pitch_driver

Include dynamic peaks: a joint that accelerates a load sees more than its static torque. Size to the worst point in the duty cycle, not the average.

2. Tooth shear (does the belt ratchet?)

The load has to pass through the teeth actually in mesh on the small pulley. Each tooth can carry a rated shear force per unit width. The check:

T_effective <= F_tooth_rated * width * (teeth_in_mesh) * mesh_factor
   F_tooth_rated = allowable tooth load per mm width per tooth (from maker)
   teeth_in_mesh = floor(N_driver * wrap_angle / 360)
   mesh_factor   = derate when fewer than ~6 teeth are engaged

If this fails, you widen the belt, go up a pitch, or increase the small-pulley tooth count. Ratcheting is the belt-drive failure you design against first.

3. Tensile-cord rating and service factor

The belt as a whole has an allowable working tension set by the cord. Apply a service factor for shock, reversing loads, and duty:

T_tight = T_effective + T_slack <= T_allow / SF
   SF ~ 1.5 (smooth, steady) to 2.5+ (reversing, shock, high duty)

4. Pulley diameter and belt life

Small pulleys over-bend the cord on every pass and fatigue it. Honor the minimum tooth count for the pitch and cord, and remember belt life falls fast below it. Rated life also assumes the maker's tension window; under- or over-tension both cut it.

Width selection

Belt widths are standard (for GT2, common widths are 6, 9, 15 mm; HTD and AT run wider). Pick the width that passes the tooth-shear and cord checks with the service factor, then round up to the next standard width. Wider belt buys load capacity linearly, at the cost of a wider pulley and a bit more mass.

The output of this process is a concrete part: a pitch, a width, two tooth counts, a center distance, a belt length, and a target tension. If any check fails on a small distal pulley, that is the signal to consider a cable drive or a gearbox instead.

Capstan and cable/tendon drives

When you want a distal joint to move with almost no local actuator mass, you route a cable (a tendon) from a base-mounted motor to the joint. Cable drives are how dexterous hands get light fingers and how some legs and arms keep distal inertia tiny. Two mechanisms matter: the capstan and the routed tendon.

The capstan and the Euler-Eytelwein relation

Wrap a cable several turns around a driven drum (the capstan) and the friction between cable and drum lets a small holding tension resist a large load tension. The governing law is the belt-friction or capstan equation, worked out by Euler and Eytelwein:

T_load / T_hold = e^(mu * theta)
   mu    = friction coefficient between cable and drum
   theta = total wrap angle in radians

The ratio grows exponentially with wrap. With mu = 0.2 and three full wraps (theta = 6*pi), e^(0.2 * 18.85) = e^3.77 ~ 43. A small motor holding tension controls a load 40 times larger. Robotics uses capstans two ways. A friction capstan relies on this exponential grip and is simple but can creep under sustained load. A positive-drive capstan clamps or terminates the cable to the drum so there is no slip at all, giving a zero-backlash, high-stiffness, perfectly back-drivable reduction. Capstan stages are prized on force-controlled and haptic joints exactly because they are backlash-free and transparent (low friction, easy to back-drive), which a gear train never is.

Tendon-routed joints

A tendon runs from a motor-driven pulley or capstan at the base, through guides or sheaths, to the joint it actuates. Key design facts:

• Tendons pull, they cannot push. A single tendon actuates one direction. To drive a joint both ways you either run an antagonistic pair (two tendons, like biological flexor and extensor muscles) or return the joint with a spring against one tendon. Antagonistic pairs also let you set joint stiffness by co-contraction, tensioning both at once, which soft and dexterous robots exploit.
• Pretension is mandatory. A tendon must stay in tension through the whole range or it goes slack, loses position, and can jump its pulley. You pretension the routing and account for the tension in the bearing and structure loads.
• Routing friction and stretch are the enemies. Every guide, sheath, or pulley the tendon crosses adds friction (again the capstan relation, now working against you) and hysteresis, so commanded tension at the motor is not the tension at the joint. Cable stretch adds compliance and position error. High-quality hands minimize direction changes and use low-friction sheaths (PTFE-lined) or open pulley routing.
• Cable choice: coated steel wire rope (7x7 or 7x19 construction) for stiffness and life, or high-modulus polymer (Dyneema/Spectra) for light weight, low friction, and quiet operation, at the cost of creep and a shorter fatigue life over small pulleys.

Cable drives appear in the Shadow Dexterous Hand and many research hands (tendons from forearm motors), in some tendon-driven arms and continuum robots, and in legs where a base-mounted motor drives a distal joint to keep the swinging mass low. The legged robot hardware guide covers where designers pick cables over belts for exactly this inertia reason.

Rule of thumb: Use a cable/tendon drive when the distal joint's local actuator inertia is the problem and you can route the cable cleanly. Budget for pretension, routing friction, and stretch from the start; they are the difference between a crisp hand and a mushy one.

Chain drives

A roller chain runs over toothed sprockets and carries torque through link engagement. In robotics it shows up where the load is heavy, the environment is dirty, and precision is secondary: tracked mobile bases, the drive from a gearmotor to a heavy wheel or track, some heavy slow joints, and machinery integrated into a cell.

What chain buys:

• High load and shock tolerance. Steel links carry far more force per unit width than a belt and shrug off jerk loads that would ratchet a belt.
• Contamination tolerance. Chain runs in dirt, grit, and outdoor conditions that would abrade a belt, which is why it dominates on tracked vehicles and agricultural machinery.
• Long center distances and multiple sprockets on one chain loop, useful for driving several shafts.

What chain costs:

• Backlash and chordal action. Chain has inherent slack (backlash) and a speed ripple called chordal action, because the chain wraps the sprocket as a polygon, not a circle. That makes it a poor choice for smooth or precise motion.
• Wear and elongation. Chain "stretches" as the pin-bushing joints wear, so it needs a tensioner and periodic adjustment or replacement.
• Lubrication and noise. Chain needs lubrication and runs louder than a belt.

For most robot joints a belt or gearbox is the better answer. Chain earns its place when the duty is heavy, slow, dirty, and cost-sensitive, and precision does not matter.

The tradeoff: belt vs gear vs cable vs direct drive

The honest comparison across the four ways to get torque from a motor to a joint:

Property	Timing belt	Gearbox	Cable / tendon	Chain	Direct drive
Backlash	Near-zero (tensioned)	Low to moderate (zero for harmonic)	Near-zero (positive capstan)	Moderate to high	Zero
Torsional stiffness	Low to moderate	High	Low to moderate	Moderate	Highest
Ratio per stage	~1:1 to 8:1	3:1 to 300:1	~1:1 to ~50:1 (capstan)	~1:1 to 7:1	1:1
Remote mounting	Excellent	Poor	Excellent	Good	None
Reflected inertia at joint	Low	Higher (motor + gear)	Lowest	Moderate	Motor rotor only
Efficiency	~95-98%	50-90% (type-dependent)	~85-95% (routing-dependent)	~95-98%	~100% mech
Noise	Low	Moderate	Very low	High	Very low
Back-drivability	Good	Poor (high ratio) to good	Excellent	Good	Excellent
Contamination tolerance	Moderate	Sealed = high	Low (routing sensitive)	High	High
Cost	Low	Moderate to high	Moderate	Low	High (torque motor)
Maintenance	Re-tension, replace belt	Low (sealed)	Re-tension, replace cable	Lube, tension, replace	Low

Read it as a decision aid, not gospel; every cell depends on the specific part and how you close the loop. The shape holds:

• Need low cost, low noise, remote mounting, modest ratio, and near-zero backlash? Timing belt. The robotics default for light-to-medium joints and gantries.
• Need high ratio and high torsional stiffness in a compact volume? Gearbox: planetary for general work, harmonic or cycloidal for high ratio and low backlash at the joint.
• Need the distal joint to have almost no actuator inertia, with backlash-free back-drivable feel? Cable/tendon or capstan drive: dexterous hands, haptics, some legs.
• Need to move heavy, dirty, slow loads cheaply? Chain.
• Need zero backlash and maximum bandwidth and you can pay in torque, heat, and cost? Direct drive, no transmission at all.

A worked sizing example

Size a belt stage for a robot arm's elbow joint driven remotely from an upper-arm-mounted servo.

Requirements. Joint peak torque 12 Nm, joint speed up to 3 rad/s, reduction from a servo that makes 0.6 Nm continuous and 1.8 N*m peak at up to 3000 rpm (314 rad/s). We want the servo on the upper arm and the elbow driven by a belt so the forearm swings light.

Step 1: ratio. Needed reduction i = joint_torque / (servo_torque * eta). Using peak: i >= 12 / (1.8 * 0.97) = 6.9. Pick i = 7 with tooth counts N_driver = 16, N_driven = 112? That is a large output pulley. A cleaner path is a two-stage or a single stage of i = 7 with N_driver = 18, N_driven = 126. The output pulley gets big, which is the signal that we are near the single-stage belt limit. Keep i = 7, driver 18 teeth, driven 126 teeth, and check teeth in mesh.

Step 2: pick pitch and pulley. Choose HTD-5M (5 mm pitch) for the load level. Driver pitch diameter: d = 5 * 18 / pi = 28.6 mm. Output speed = 314 / 7 = 44.9 rad/s, well above the 3 rad/s needed, so torque, not speed, is the binding constraint. Good.

Step 3: effective tension. At the driver, T_effective = 2 * T_motor_peak / d_pitch_driver = 2 * 1.8 / 0.0286 = 126 N.

Step 4: tooth shear. Wrap angle on the small pulley is roughly 180 degrees (a two-pulley layout), so teeth in mesh = 18 * 180/360 = 9 teeth, comfortably above the 6-tooth minimum, so no mesh derate. Suppose HTD-5M allows about 8 N per mm width per tooth working load (illustrative; use the maker's table). With 9 teeth in mesh, a 15 mm belt carries 8 * 15 * 9 = 1080 N of tooth capacity against a 126 N demand: a wide margin, so tooth shear is not binding. A 9 mm belt (8 * 9 * 9 = 648 N) still passes with margin. Choose 9 mm and keep the service factor in reserve.

Step 5: cord and service factor. Apply SF = 2 for a reversing joint. T_tight = T_effective + T_slack; with a sensible pretension keeping the slack side around 30 to 50% of tight, T_tight is on the order of 180 to 220 N, far below an HTD-5M 9 mm belt's cord rating (hundreds of N). Passes.

Step 6: resonance sanity check. With the forearm inertia reflected through the belt and the span stiffness of an HTD-5M 9 mm belt over, say, a 250 mm center distance, the joint resonance lands well above the servo's practical bandwidth (a few tens of Hz), so the loop is stable with a motor-side encoder. If the forearm were heavier or the span longer, we would move feedback to the joint.

Result. HTD-5M, 9 mm wide, 18-tooth driver, 126-tooth driven, i = 7, center distance chosen for a standard belt length, tensioned to the maker's span-frequency spec. The large output pulley is the one uncomfortable part; if packaging cannot fit a 126-tooth 5M pulley (about 200 mm pitch diameter), that is the cue to split into two belt stages or use a compact gearbox for the reduction and a short belt just to relocate the motor.

Integration, failure modes, and maintenance

Mounting and alignment

Belts need parallel shafts and coplanar pulleys. Angular or parallel misalignment makes the belt track to one edge, ride up the flange, and wear fast or throw off. At least one pulley should have flanges to keep the belt centered; on a two-pulley drive, flange the larger or the driven pulley. Provide a way to set center distance for tensioning: a slotted motor mount, an idler, or a movable bearing block. An idler also increases wrap on a small pulley (more teeth in mesh) at the cost of an extra bend in the belt; a toothed idler on the tooth side is gentler than a flat idler on the back.

Failure modes

• Ratcheting (tooth jump): too little tension, too few teeth in mesh, or overload. The belt skips a tooth and loses position by one pitch. Fix with correct tension, larger small-pulley tooth count, wider or larger-pitch belt.
• Tensile-cord failure: cracking or breaking of the cords from over-bending on too-small a pulley, over-tension, or fatigue. Honor minimum pulley size and the tension window.
• Tooth wear and shear: chunking or wearing of teeth from overload, misalignment, or contamination. Widen or step up pitch, fix alignment.
• Edge wear and tracking off: misalignment or a missing flange. Realign, add a flange.
• Cable failures (tendon drives): fraying at pulleys (bend fatigue), stretch and loss of pretension, and slip on friction capstans. Use larger pulleys relative to cable diameter, positive-drive capstans, and a re-tension schedule.
• Chain failures: elongation from pin-bushing wear, stiff links from poor lube, and sprocket tooth wear. Lubricate, tension, replace at the wear limit.

Maintenance

Belts are low but not zero maintenance. Re-tension after the first hours of run-in (new belts seat and lose a little tension), then on a schedule; check tracking and tooth condition; replace on a life or condition basis. Cable drives need periodic re-tensioning and inspection for fraying. Chains need lubrication, tensioning, and eventual replacement. Sealed gearboxes win the maintenance comparison, which is one reason a joint that must run untouched for years sometimes chooses a gearbox even where a belt would work mechanically.

Rule of thumb: Design a tension adjustment into every belt and cable drive from the start. A drive you cannot re-tension is a drive you will replace early. Run-in re-tensioning alone prevents a large fraction of "belt problems."

How to choose

A short procedure that lands you on the right drive.

1. Locate the motor and the joint. If they cannot be co-located, or distal inertia is a problem, you are in flexible-drive territory. If they can be co-located and you need high ratio and stiffness, look at a gearbox or an integrated actuator first.
2. Estimate ratio. Up to about 5:1 to 8:1 per stage is belt or single-cable-stage territory. Higher ratios push you to staged belts, a gearbox, or a capstan with a large wrap.
3. Judge the inertia and bandwidth need. Low distal inertia with modest bandwidth: belt or cable. High bandwidth and stiffness: gearbox or direct drive. Force control and back-drivable feel: capstan or cable.
4. Judge the environment and duty. Clean and light: belt. Dirty, heavy, slow: chain. Sealed and maintenance-free for years: gearbox.
5. Pick pitch and profile (for a belt) from the load: GT2/GT3 for light-to-medium, HTD-5M/8M for medium-to-heavy, AT for stiffness. Then run the sizing math: effective tension, tooth shear, cord and service factor, pulley minimums, and a resonance check.
6. Decide feedback. Motor-side encoder when the belt or cable is stiff enough that the resonance sits well above your bandwidth; a load-side encoder when the transmission is compliant or you need accuracy past the belt's stretch.
7. Design the tensioner and the maintenance plan before you build. Slotted mounts or idlers, a target tension from the maker, a run-in re-tension, and a replacement interval.

Follow that order and you avoid the classic mistakes: the belt tensioned by feel that ratchets under peak load, the single belt stage stretched to a ratio that leaves too few teeth in mesh, the cable drive whose routing friction eats half the commanded force, and the chain chosen for a joint that needed smooth precise motion.

Frequently asked questions

What does the pitch number in GT2 or HTD-5M actually mean? It is the tooth-to-tooth spacing along the belt: GT2 is 2 mm, GT3/3M is 3 mm, HTD-5M is 5 mm, AT10 is 10 mm. Larger pitch means a bigger, deeper tooth that carries more load per tooth and resists ratcheting, at the cost of a larger minimum pulley and a slightly rougher ride. Smaller pitch runs quieter and wraps smaller pulleys, which helps on compact distal joints.

Do timing belts have backlash? A properly tensioned timing belt with a curvilinear profile (GT, HTD) has near-zero backlash: the teeth seat with little clearance and carry position faithfully both directions. What it does have is elastic compliance, the belt is a spring, so it deflects under load and returns. Backlash you fix with tension and profile; compliance you design around in the control loop by keeping bandwidth below the belt resonance or by closing on a load-side encoder.

How do I set belt tension correctly? Use the maker's target, either a static tension in newtons measured with a tension gauge, or a target natural frequency of the plucked free span measured with a tension meter or a phone app. Pretension enough that the slack side stays under tension at peak torque (so the mesh never disengages), but not so much that you crush the pulley bearings. Re-tension after the first hours of run-in, then on a schedule.

When should I use a cable drive instead of a belt? When the distal joint's own actuator inertia is the problem and you want almost no moving mass at the joint, and you can route the cable cleanly. Dexterous hands, haptic devices, and some legs use tendons from base-mounted motors for exactly this reason. The price is pretension, routing friction and hysteresis, cable stretch, and the fact that a tendon can only pull, so you need an antagonistic pair or a return spring.

What is a capstan drive and why is it backlash-free? A capstan is a drum the cable wraps several turns around; friction lets a small holding tension resist a much larger load tension, growing exponentially with wrap angle (T_load/T_hold = e^(mu*theta)). A positive-drive capstan terminates the cable to the drum so there is no slip at all, giving a zero-backlash, high-stiffness, back-drivable reduction. That transparency is why capstans are favored on force-controlled and haptic joints.

How much reduction can one belt stage give? Up to roughly 5:1 to 8:1 cleanly. The limit is teeth in mesh on the small pulley: as the ratio grows or the driver shrinks, fewer teeth carry the load, each tooth sees more force, and the belt ratchets. Keep at least about 6 teeth engaged. For more reduction, stack two belt stages or use a gearbox.

Why does my belt keep jumping teeth under load? That is ratcheting, and it has three usual causes: too little tension (the slack side goes loose and the mesh disengages), too few teeth in mesh on the small pulley, or an overload beyond the tooth-shear capacity. Fix it by tensioning to the maker's spec, increasing the small-pulley tooth count or adding an idler for more wrap, or stepping up to a wider belt or a larger pitch.

Belt or gearbox for a robot joint? Belt when you want low cost, low noise, remote motor mounting, low distal inertia, and modest ratio with near-zero backlash. Gearbox when you need high ratio and high torsional stiffness in a compact volume, or a sealed maintenance-free unit. Many arms use both: a belt to relocate the motor and reduce inertia, feeding a compact gearbox at the joint for the ratio and stiffness.

What cable material should a tendon drive use? Coated steel wire rope (7x7 or 7x19 construction) for stiffness, strength, and fatigue life, or high-modulus polymer like Dyneema/Spectra for light weight, low friction, and quiet running. Steel wins on stiffness and life over small pulleys; polymer wins on weight and friction but creeps over time and fatigues faster on tight bends. Size the pulley diameter generously relative to the cable to keep bend fatigue low either way.

When is chain the right choice in a robot? When the load is heavy, the environment is dirty, the motion is slow, and precision does not matter: tracked mobile bases, heavy wheel or track drives, and machinery integrated into a cell. Chain tolerates shock and contamination that would destroy a belt, but it brings backlash, chordal-action speed ripple, wear-driven elongation, lubrication needs, and noise, so it is a poor pick for smooth precise joints.