A short, math-based guide so you never buy a servo that is either overkill or underpowered.
Torque needed ≈ load weight (kg) × arm length (cm) × 2. The ×2 covers dynamic loads (acceleration, impact) and a healthy safety margin. A joint holding a 1.5kg arm with a 20cm reach needs roughly 1.5 × 20 × 2 = 60 kg·cm — which is exactly the class of 45-70 kg·cm servos we sell.
Servo torque is always rated "at" a voltage. The same servo can deliver 45 kg·cm at 8.4V but only ~35 kg·cm at 6.0V. When comparing servos, always compare at the same voltage — it is the single most common spec-sheet trap in the hobby and education market.
Plastic gears are cheap and quiet but strip under repeated load. Metal (steel) gears handle sustained torque and survive crashes — at the cost of a little noise and weight. For anything above 20 kg·cm duty, metal is the pragmatic choice.
Digital servos respond faster, hold position better and centre more accurately — essential for robot arms and wheeled platforms that need precise steering. Analogue is fine for simple RC. If your project has a microcontroller and a budget, go digital.
Outdoor robots, lawn bots and anything that sees dew, rain or cleaning spray need an IP-rated servo. We offer a waterproof option on our high-torque line — ask for it in the quote.
Our 45 kg·cm digital servo with full metal gears is the workhorse we spec for robot joints — MOQ from 2 pcs. Get a quote and tell us your arm length and payload; we will confirm the right torque class.
Rule of thumb: take the load arm length (in cm) × weight (in kg) of what the joint must hold, then double it for dynamic load and safety margin. A robot arm holding a 2kg payload at a 10cm arm needs roughly 20 kg·cm × 2 = 40 kg·cm of torque.
Torque scales roughly with voltage for hobby servos: a servo rated 45 kg·cm at 8.4V may deliver only ~35 kg·cm at 6.0V. Always compare ratings at the same voltage — this is the #1 spec-sheet trap in servos.
Digital servos have faster response, higher holding torque and better centering, at higher current draw and cost. For robot joints and precision builds, digital is worth it; for simple RC applications, analogue is fine.