How a Servo Works: The Motor Wants to Spin. The Pot Says When.
Part of the Crawler Tools & Guides series.
A servo looks like a sealed brick with a horn on it. Radio says left, horn goes left. That's the user manual most of us got. Then you stall one on a ledge, it buzzes like it's mad, and somebody says you needed more kg. Maybe. Or you needed to know what was actually spinning in there.
TL;DR
- A servo is a tiny closed loop. The radio says where the horn should be. A sensor on the output (usually a pot, sometimes a Hall) says where it is. The board compares those two and yells at a motor until they match.
- The motor is fast and weak. That's fine. The gears are why 25kg lives in a 40mm cube.
- When the pot catches up, the motor stops. Holding a line against a rock isn't magic. It's a leftover error and a little current that never quite goes to zero.
- Digital vs analog is how often the board checks in, not a different motor. Metal vs plastic is what breaks when you stall it, not how it turns.
- This will not pick a servo for you. It will make the spec sheet less of a rumor.
That's the whole post. The rest is the why.
The loop
Your receiver (or a gyro, or a crawler gyro box) sends a pulse. Longer pulse, more angle. Shorter pulse, the other way. The board reads that as "go here."
On the output shaft there's a sensor. Most of the time that's a potentiometer: old school analog voltage divider. As the horn turns, the voltage changes. That voltage is "we're here." Some nicer digitals swap the pot for a Hall sensor. Same job, no wiper dragging on a track.
Want vs is. If they agree, nothing spins. If they don't, the board drives the motor one way or the other until they do. That's the whole trick. Everything else in the case is in service of that comparison.
Motor: fast, weak, eager
The can still wants to spin at a zillion RPM, and it still couldn't turn a tire by itself. That's not a defect. You wouldn't want a motor that strong in a box that size. You'd cook it, and the horn would slap. Torque at the horn is a gear problem. The motor just makes speed.
Value cans (RTR, Amazon, the $20 aisle) are usually brushed. Fine. They work. Brushes wear, they get noisy, they still crawl. Brushless is the quality end of the same job: no brushes to trash, usually more punch for the size. Everything I stock is brushless. I think one of them is coreless, which is not the same thing. Coreless is still a brushed motor with a lighter rotor. Marketing loves to mash those words together.
Gears: where the kg actually lives
Same story as the drivetrain, just smaller. Motor spins fast. A stack of gears slows that down and multiplies torque. That's how a motor you could pinch between two fingers moves a 2.2 on a 13lb truck.
Plastic gears are quiet and cheap and they round off when you stall them. Metal gears take the hit and send it somewhere else (the case, the spline, the next tooth). Neither one changes the loop. They just decide what fails first when the horn is asked to do something the truck already lost.
The kg number on the box is stall torque, at a voltage, on a good day, often with a question mark. Useful for ranking. Not a promise the horn will hold a 45° lean at that number forever.
The pot (and the Hall): the adult in the room
The motor would happily freewheel until the gears exploded. The sensor is what keeps it honest. It's tied to the output, so when the horn moves, "we're here" moves with it.
A pot is a wiper on a track. Cheap, proven, and the usual reason a servo starts hunting at center: dirt, wear, a cracked contact. The board is still comparing. It's just comparing against garbage. Pot gearsets also have a peg that physically stops the output so you can't spin the wiper off the end of the track. Electronics stop it at the angle you asked. The peg is the last-ditch "don't wreck the pot."
A Hall sensor reads a magnet instead. No wiper, so it doesn't sand itself down, and it cares less about a little water. No peg required, which is why some of them can be programmed for 360 / winch. Not rare on nicer digitals. Not common on the cheap ones. The Lil Spinner is the one in my lineup that does it that way. I never even marked it on the listing.
Either way, you don't want slop between the sensor and the spline. If the horn can move and the sensor doesn't, the board thinks it's done. The truck disagrees.
Why it stops (and why it buzzes)
Commanded angle matches pot angle. Error is ~zero. Motor current drops. Horn sits there.
Put that same horn against a rock it can't move. The command still says 45°. The pot is stuck at 30°. Error stays. Current stays. That's the buzz. Same neighborhood as stalling a drive motor, just in a smaller can with worse airflow, usually inside a body.
A bigger servo can take more of that before something gives. A better mount can keep the servo from twisting the axle while it tries. Neither one invents extra torque from the sky. They just survive the error longer.
Digital vs analog, without the brochure
Analog boards mostly wake up when a pulse arrives, shove the motor, go back to sleep. Digital boards remember the last command and keep working the loop a lot faster. That's the snap. That's also the idle buzz some people hate.
It is not a different motor. It is not automatically more kg. A cheap digital servo is still a cheap motor and cheap gears with a busier brain. A good analog one will crawl all day and not ask you to program anything. Pick the loop speed you want. Then look at the gears and the voltage it was rated at.
What this is not
This is not a shopping list. We sell servos. You already knew that. The loop above is the same in a $20 can and a $200 can. Price buys better gears, better pots, better bearings, a board that doesn't brown out when the BEC dips. It does not buy a different physics.
Voltage still matters. A servo rated 8.4V on a 6V BEC is a weaker servo. A 6V servo on an unregulated 3S BEC is a former servo. Same as the gear down, volt up post: the number on the box assumes the volts on the label.
More setup math and calculators live on the Crawler Tools & Guides page, including the Gearing Calculator for drivetrain numbers and the Crawler Geometry Tool for the links.