Torque (kg·cm)

The single most important number. It’s the force at a 1cm arm — so a 20 kg·cm servo lifts ~20kg at 1cm, but only ~10kg at 2cm. Size it to the actual load and lever length of your joint, with margin.

Gear Material

Plastic/nylon gears are quiet and cheap but strip under shock loads. Metal (brass/steel) gears survive crashes, high torque and stalls — essential for robot arms, crawlers and anything that takes a knock.

Rotation Range

Most standard servos sweep ~180° (some 270°) and hold position. A continuous-rotation servo spins 360° like a geared motor with speed/direction control — use it for wheels, not for precise angles.

Analog vs Digital

Digital servos refresh the motor far faster than analog — giving crisper holding, better precision and higher torque, at the cost of more current. Analog is fine for learning and light hobby use.

Voltage & Current

Higher voltage (6V, 7.4V) means more torque and speed — but watch stall current. Power servos from a dedicated 5–7.4V supply with a shared ground, never straight from a microcontroller’s regulator.

The SG90 is the servo almost everyone starts with. At just 9 grams and a couple of dollars each (in multi-packs), it’s the perfect first actuator for learning PWM control on an Arduino, ESP32 or Pico. It has enough torque for light grippers, small pan-tilt mounts, robot claws and flaps — and if you burn one out, you’ve lost pocket change. Just don’t ask its plastic gears to do heavy lifting.

The MG90S is the drop-in upgrade to the SG90: same tiny footprint and pinout, but with metal gears and a bit more torque. That gearset makes a real difference in small robot arms, hexapod legs and camera gimbals where plastic gears would strip. It runs a touch warmer and costs a little more, but for any micro build that actually takes load, it’s the smarter buy.

When 20 kg·cm isn’t enough, the ANNIMOS DS3235 steps up to a coreless motor, stainless-steel gears and around 35 kg·cm of torque (more at 7.4–8.4V). It’s built for 1/8 and 1/10 crawlers, large robot arms, animatronics and steering under real load, with an IP66-sealed case for outdoor and dusty environments. It’s heavier and thirstier than the DS3218 — you’ll want a proper battery/BEC — but nothing else here moves this much weight so precisely.

The FEETECH FS90R is a micro continuous-rotation servo — it spins fully forward or backward with speed set by the PWM signal, rather than holding an angle. That makes it the easiest way to add driven wheels to a small robot without a separate motor driver: just wire it to a pin and use the Servo library. It’s the go-to for line-followers, micro:bit buggies and tiny two-wheel bots. Pop-on wheels are widely available to match.

The FeeTech STS3215 is a different class of servo: a smart serial-bus servo with a 12-bit magnetic encoder that reports back its exact position, speed, load, temperature and voltage over a single daisy-chained TTL bus. That feedback is why it’s become the standard actuator for open-source robot arms like the LeRobot SO-ARM100/101. Instead of blind PWM, you address each servo by ID and read real angles — ideal for repeatable, closed-loop robotics. It’s pricier and needs a bus driver board, but nothing else here is this capable for arm builds.

If you’re tired of clone-quality lottery, the Hitec HS-311 is a genuine name-brand standard servo backed by a real manufacturer and a two-year warranty. It’s an honest analog workhorse: ~3.7 kg·cm, reliable three-pole motor, durable resin gears and a universal connector that fits every board and bracket. It won’t win torque contests, but for classroom kits, trainer aircraft and dependable general-purpose motion, its consistency is worth the small premium over no-name servos.

Building a robot arm or crawler?

Get the ANNIMOS DS3218 — ~20 kg·cm of digital metal-gear torque with a wide 270° range.

Prototyping on a budget?

Grab a multi-pack of MG996R metal-gear servos — the best torque-per-dollar in hobby robotics.

Just learning servos?

Start with cheap SG90 micros (or metal-gear MG90S for load) and practise PWM safely.

Moving serious weight?

The coreless ANNIMOS DS3235 delivers ~35 kg·cm with steel gears and an IP66 case.

Need driven wheels?

Use continuous-rotation FEETECH FS90R servos — wheels without a separate motor driver.

Want closed-loop feedback?

The FeeTech STS3215 serial-bus servo reports real position/load — perfect for robot arms.

What’s the difference between an analog and a digital servo?

Both take the same PWM signal, but a digital servo’s internal chip pulses the motor several hundred times per second instead of ~50Hz. That gives it faster response, tighter holding torque and better precision at the end-stops — you feel it “lock” into position. The trade-off is higher current draw and a bit more cost. For learning and light hobby work, analog (SG90, MG996R, HS-311) is perfectly fine; for robot joints that must hold accurately under load, digital (DS3218, DS3235) is worth it.

How much torque do I actually need?

Torque is quoted in kg·cm — the load a servo can hold at a 1cm arm. Because it scales with lever length, a joint 4cm out only gets a quarter of the rated figure. Estimate your load and its distance from the shaft, then pick a servo with 2–3× headroom so it isn’t stalling. Light flaps and small grippers are happy with an SG90/MG90S; a two-link robot arm usually wants MG996R-class or a DS3218; heavy crawlers and big arms need the DS3235.

Can I power a servo directly from my Arduino or ESP32?

Only for the smallest servos, briefly. Servos draw large current spikes when they start moving or stall — easily hundreds of milliamps to several amps — which will crash a board powered from USB or its onboard regulator. Use a separate 5–6V (or 7.4V for high-torque digital) supply sized for the stall current, connect the servo’s power to that, and tie the supply ground to your microcontroller’s ground. Only the signal wire goes to the MCU pin. A capacitor across the servo supply also helps tame the spikes.

What’s the difference between a standard and a continuous-rotation servo?

A standard servo moves to and holds a specific angle (typically within ~180°) — you command a position. A continuous-rotation servo (like the FS90R) has its feedback pot disconnected, so the same signal instead sets speed and direction and it spins freely 360°. Use standard servos for arms, steering and pan-tilt; use continuous-rotation servos as simple driven wheels. You can’t get precise angle control from a continuous servo, and a standard servo can’t be a wheel without modification.

What is a smart / serial-bus servo and do I need one?

A smart servo like the FeeTech STS3215 replaces one-way PWM with a two-way digital bus: you address each servo by ID over a shared TTL line and it reports back position, speed, load, temperature and voltage from a built-in encoder. That lets you daisy-chain many servos on a few wires and run true closed-loop control — which is why they power open-source robot arms like the SO-ARM100/101. You don’t need one for simple projects, but for a multi-joint arm where repeatability and feedback matter, they’re transformative.