How to select a servo motor

Five specifications determine the choice, and they are evaluated in a fixed order: torque, speed, supply voltage, control interface and mechanical envelope. The sequence matters because the first fixes the case size and the last can disqualify an otherwise correct unit. The range covers 3–980 kg·cm across four control interfaces and five case formats. The difficulty is less in finding a candidate than in establishing the torque class the load requires.

Every figure below is the published specification of the model named, linked to that model’s own page. No figure is an average, and none depends on how many models are listed.

Servo torque classes: start with the class, not the model

3 – 20

Micro and slim-wing formats. Weight and thickness bind before torque.

S1009D 3 · S2007D 7 · S2221F 22

21.8 – 62

The 20 × 40 mm case. All four interfaces at 25, 35 and 48.

S8035D 40 · BLS8145D 48 · S8060D 62

75 – 85

Same case, heavier gear stack, 88 g.

BLS8175D 75 · BLS8185D 85

130 – 980

Case-mounted actuator. Case and spline change with the load.

S9100D 130 · S9350D 390 · S9750D 980

The classes are defined by us rather than by a published standard. What places a load in one is the arithmetic in step 01.

01 — Torque Load × arm × margin Sets the case size; later choices cannot compensate.

Servo torque: load × arm length × safety factor

Servo torque is quoted in kg·cm, defined as one kilogram suspended one centimetre from the output spline. Multiply the mass at the end of the arm by the distance from the spline to its centre of mass, then apply a safety factor. Where the arm carries its own weight — a boom, a leg, a camera mast — measure to the centre of mass of the complete assembly rather than to its tip.

T = m × L × S kg·cm

m mass at the end of the arm, kg · L spline to centre of mass, cm · S safety factor
S = 1.5 for a static hold, 2.0–3.0 where the load accelerates, reverses or takes shock.

Five worked cases, each resolving to a different torque class

Worked torque calculation: load, arm length, static torque, torque with a 1.5 safety factor, resulting torque class and the model supplied in that class
Load, kg	Arm, cm	Static	× 1.5	Class	Model at that class
0.10	6	0.6	0.9	3	S1009D 3 kg·cm
0.40	8	3.2	4.8	7	S2007D 7 kg·cm
2.40	10	24	36	40	S8035D 40 kg·cm
8.00	18	144	216	250	S9200D 250 kg·cm
20.0	22	440	660	700	S9450D 700 kg·cm

Swipe the table sideways.

Stall torque is not continuous torque. The published figure is the peak the output reaches at rated voltage immediately before stall. It is the correct basis for sizing and the wrong basis for a joint that holds position for hours. Continuous rating depends on duty cycle, ambient temperature and mounting, so request a derating curve for the application rather than applying a fixed percentage to stall torque. The technical guide on gearing and duty cycle covers how continuous rating is derived.

02 — Speed Seconds per 60° Measured at no load, at a stated voltage.

Servo speed in seconds per 60°: a no-load ceiling

Transit speed is quoted as the time to travel 60° at no load, at a stated voltage. The time increases under load, and it increases more steeply as the load approaches stall torque. Two servos of equal torque can differ by a factor of two, so use the published figure rather than inferring speed from the torque rating.

Fastest models in the range, at the voltage stated on each model page

Transit speed in seconds per 60 degrees, with the torque and voltage each figure is quoted at
Model	s / 60°	kg·cm	Note
BLS8120H	0.03	20 @ 12 V	fastest in the range
S1009D	0.07	3 @ 7.4 V	micro, 7.7 g
S8335DW	0.07	35 @ 8.4 V	vs 0.10 s at the same torque
BLS8125D	0.07	25 @ 8.4 V	—
BLS348	0.09	48 @ 8.4 V	vs 0.106 s at the same torque
BLS8185D	0.11	85 @ 16.8 V	heaviest standard case
S9350D	0.16	390 @ 30 V	actuator

Swipe the table sideways.

Compare within a single torque class. At 35 kg·cm, the S8335DW is quoted at 0.07 s against 0.10 s for the BLS8135D. At 48 kg·cm the spread is 0.09 s for the BLS348 against 0.106 s for the BLS8145D. Transit speed is a function of the motor, the gear ratio and the supply voltage; it is not a function of the torque rating.

03 — Voltage Scales with the supply Quote every figure at a stated voltage.

Supply voltage: torque and speed both scale with it

Every torque and speed figure is quoted at a specific voltage, and both change when the supply changes. The S8035D is a single model with three published operating points, taken from its own datasheet.

Supply 8.4 V

40.0kg·cm

0.088 s/60° · stall 5.2 A

Supply 7.4 V

34.0kg·cm

0.104 s/60° · stall 4.5 A

Supply 6.0 V

28.0kg·cm

0.131 s/60° · stall 3.7 A

That is −15% torque at 7.4 V and −30% at 6.0 V, with the time per 60° increasing by 18% and 49% over the same range. Operation below rated voltage is not a proportional derating: it removes torque and speed faster than it removes current, which is why an undersized supply makes a correctly sized servo appear underpowered.

Supply levels present in the range, each with a model specified at that level

Supply voltage ranges in the catalogue, typical battery packs for each, and a model specified at that voltage
Supply	Typical pack	Where it appears
4.8–6.0 V	4–5 cell NiMH, BEC at 6 V	S4020D, 21.8 kg·cm at 6.0 V
6.0–8.4 V	2S LiPo, 6 V BEC	most of the standard and coreless range
7.4–12.0 V	3S LiPo, 12 V DC bus	BLS8120H, 20 kg·cm at 12 V
8.4–16.8 V	4S LiPo	BLS8185D, 85 kg·cm at 16.8 V
24–30 V	24 V or 30 V DC bus	S9350F / S9450F industrial actuators

Swipe the table sideways.

Do not exceed the top of the stated range to recover torque. Stall current rises with voltage, and so does the heat that has to be dissipated through the case.

04 — Interface PWM, TTL, RS485, CAN Deferred until torque and speed are fixed.

Control interface: PWM vs TTL vs RS485 vs CAN

PWM uses one signal wire per servo, with no addressing and no feedback. A bus is addressed, returns position and temperature, and places the whole machine on two wires. A twelve-axis arm built on PWM will be rewired; a single control surface on a bus pays for a protocol it never uses.

Comparison of PWM, TTL, RS485/Modbus and CAN servo control interfaces across wiring, addressing, feedback and integration cost
Property	PWM	TTL	RS485 / Modbus	CAN
Wires per axis	3 — power, ground, signal	3, daisy-chained	2, daisy-chained	2, daisy-chained
Addressing	none; one signal pin per servo	ID per servo	ID per servo	ID per servo
Feedback	none	position, temperature	position, temperature, load	position, temperature, load
Typical fit	1–3 axes, direct from a receiver	4–12 axes on short runs	long runs in noisy cabinets	vehicle and motion controllers
Cost of changing later	re-terminate every axis	new controller	new controller	new controller and harness

The mechanical assembly is identical across all four interfaces. Only the controller and the firmware differ, which is why the interface can be deferred until torque and speed are fixed. The servo motor FAQ covers protocol behaviour in more detail.

One mechanical core, four interfaces, identical torque and speed

Model availability by torque class and control interface, from 25 to 75 kg.cm
Torque	PWM	TTL	RS485	CAN
25 kg·cm	BLS8125D	BLS8125TTL	RS8125D	BLS8125DC
35 kg·cm	BLS8135D	BLS8135TTL	RS8135D	BLS8135DC
48 kg·cm	BLS8145D	BLS8145TTL	RS8145D	BLS8145DC
75 kg·cm	BLS8175D	not produced	RS8175D	BLS8175DC

Sequence the decision: torque and speed first, interface last. The single gap in the matrix is the 75 kg·cm TTL variant, which is not produced; at that torque the RS485 and CAN units share the same mechanics. Transit speed is not identical across the four either — 0.11 s on the PWM unit against 0.08 s on the bus versions — so verify on the model page rather than assuming equivalence.

05 — Envelope Case, spline, sealing A unit outside the envelope is excluded, not derated.

Case size and output spline: the mechanical envelope

Five case formats cover the range. Three checks follow the outer dimensions: the spline, because a 15T φ3.85 mm output will not accept a standard horn; the mounting ears, because a wing-mount flange is not a standard flange; and the sealing class, which is specified separately from size.

Micro

S1009D

23.5 × 9.0 × 19.5 mm, 7.7 g

15T φ3.85 mm spline, so a standard horn will not fit. 3 kg·cm at 7.4 V.

Slim wing

S2210F / S2221F / S2230F

10 / 12 / 15 mm case width

Thin enough for a wing bay: 11, 22 and 32 kg·cm. 25T φ4.96 mm spline on the 10 and 12 mm cases; the 15 mm one uses φ5.96 mm, so horns do not swap across.

Standard

S8035D

20 × 40 × 38.4 mm, 73 g

The common hobby footprint. 25T φ5.94 mm spline, so existing arms and horns carry over.

Low profile

BLS325 / RS325

short case, standard footprint

Same mounting pattern with less height above the deck. 25 kg·cm, 0.07 s.

Actuator

S9350D

68 × 35 × 120 mm, 690 g

From 130 kg·cm up the case is mounted, not bolted by ears, and all four interfaces exist again: PWM, TTL, RS485 and CAN. 25T φ11.8 mm spline on the 390 kg·cm unit.

Sealing is a separate specification. The S8335DW and its TTL variant are the only two models with a published IP67 rating; the S3240M is IP66, protected against jets but not immersion. Everything else is splash-resistant. If a product page does not carry a dimension drawing, ask us for one rather than scaling from a photograph.

Apply Seven reference points Reference points, not prescriptions; the load case decides.

Servo sizing by application: seven reference points

A six-axis arm and a camera gimbal are both described as a servo that holds position, and their torque requirements differ by two orders of magnitude. Use these rows as a cross-check on your own arithmetic, not as a substitute for it.

Typical servo torque class and control interface for seven applications, with an example model for each
Application	Class, kg·cm	Interface	Example	Why
RC control surface	10–25	PWM	S2221F	22 kg·cm in a 12 mm case
Camera gimbal	3–10	PWM or TTL	S2007D	7 kg·cm at 20 g
Robot arm joint	25–85	RS485 or CAN	RS8135D	35 kg·cm, addressed
AGV steering	100–250	RS485	S9100D	130 kg·cm, case-mounted
Pan-tilt head	20–50	PWM or RS485	BLS325	25 kg·cm, low profile
Humanoid or legged	25–85	CAN	BLS8145DC	48 kg·cm on a two-wire bus
Fixture or test rig	390–980	RS485 / Modbus	RS9750D	980 kg·cm

Swipe the table sideways.

Avoid Five mistakes Each passes a datasheet check and fails in service.

Five servo sizing mistakes that pass a datasheet check

Sizing on stall torque with no margin
A servo that only just holds the load has nothing left to accelerate it. The 1.5 factor is a floor, not a target.
Comparing two servos quoted at different voltages
40.0 kg·cm at 8.4 V and 28.0 kg·cm at 6.0 V can be the same servo. Normalise every candidate to your pack before you compare.
Picking the bus before the mechanics
At 25, 35 and 48 kg·cm the same core ships in all four interfaces. Choose torque and speed first; the bus is the last decision, not the first.
Forgetting that holding a load still draws current
Current turns into heat, and heat is what limits continuous torque. A joint that parks under load needs a duty cycle, not just a torque number.
Treating a bus servo as a drop-in replacement
Different wiring, different protocol, different firmware. Budget it as a small integration project.

Brief Six lines Enough to narrow the range to two or three models.

Servo selection checklist: the six lines we need

Most enquiries arrive as “which servo for my robot”, which cannot be answered without guessing. The six lines below are the minimum input required to return specific models.

Load:        2.4 kg at 100 mm from the spline
Motion:      90° in 0.35 s, 20 cycles per minute
Supply:      4S LiPo (14.8–16.8 V)
Interface:   RS485, 6 axes on one bus
Envelope:    ≤ 40 × 20 × 40 mm, ≤ 90 g
Environment: indoor, 0–40 °C, dust only

Copy the block into an email and replace the values. Where a line is still open, state that it is open — an declared unknown is more useful than an estimate.

Ask Eight recurring questions The questions that arrive with most quotations.

Servo motor selection FAQ

How do I calculate the servo torque a load requires?

Multiply the mass at the end of the arm in kilograms by the distance from the output spline to its centre of mass in centimetres, then multiply by a safety factor — 1.5 for a static hold, 2.0 to 3.0 where the load accelerates or takes shock. The result is in kg·cm, which is how servo torque is quoted. A 2.4 kg load at 10 cm gives 24 kg·cm static and 36 with margin, which lands in the 40 kg·cm class.

What is the difference between stall torque and continuous torque?

Stall torque is the peak the output reaches at full voltage just before it stops turning. Continuous torque is what it can hold indefinitely without overheating, and it depends on duty cycle, ambient temperature and mounting. We publish stall torque because it is reproducible from a datasheet; ask for a derating curve when the joint works continuously.

Can I run an 8.4 V servo at 6.0 V?

Yes, inside the stated range, but you lose torque. The S8035D produces 40.0 kg·cm at 8.4 V, 34.0 at 7.4 V and 28.0 at 6.0 V — 15% and 30% below the top of the range, with stall current falling from 5.2 A to 3.7 A alongside it.

PWM, RS485, CAN or TTL — which one should I choose?

PWM for one to three axes where you already have a receiver or a timer per axis. A bus as soon as you need addressing, position or temperature feedback, or more than about four axes. RS485 and Modbus suit long cable runs in a noisy cabinet; CAN suits a vehicle or motion controller that already has a CAN port; TTL is the cheapest way to chain several axes over short runs.

Is the same servo available in all four interfaces?

At 25, 35 and 48 kg·cm, yes: the same mechanical core is offered with PWM, TTL, RS485 and CAN at the same quoted speed. At 75 kg·cm there is no TTL variant, and the PWM unit is quoted at 0.11 s against 0.08 s for the bus versions — read the model page instead of assuming the four are identical.

What is the smallest and the largest servo you build?

The S1009D at 3 kg·cm and 7.7 g at the small end; the S9750D and its bus variants at 980 kg·cm at the large end, which are case-mounted industrial actuators rather than standard-size servos. The two ends differ by a factor of several hundred in torque, so the first question is which band you are in, not which model.

What if my torque lands between two classes?

Take the higher one. The price difference between adjacent classes is small next to the cost of a redesign, and running at 60–70% of stall torque leaves margin for friction, acceleration and a cold morning.

How do I convert kg·cm to N·m?

Multiply kg·cm by 0.098 to obtain N·m, and divide oz-in by 13.89 to obtain kg·cm. The catalogue is quoted in kg·cm and oz-in because that is the unit most buyers in this segment specify in, but both conversions are exact. A 40 kg·cm servo is 3.9 N·m; 555 oz-in is 40 kg·cm.

Request a servo recommendation

Send the six lines above, or the load and the arm length alone. You receive two or three candidate models, the voltage each figure is quoted at, and the interface options available in that torque class.

Send a load case → Browse the full range

Where to go next

Technical guideSuffixes, gearing, heat and duty cycle. Servo motor FAQTorque, protocols, IP ratings, MOQ. CertificationsWhat ships with the goods, by market. Ask an engineerSend a load case, get a model.