How to Select a Gear Reducer: R, F, K and S Series Explained

Choosing a gear reducer comes down to four numbers and one drawing: the output torque the machine needs, the output speed, the ratio that gives it from the motor speed, the service factors that cover the real duty, and the mounting position. This guide walks through that method as we apply it in our own selection tables, then gives the ranges of every series we manufacture so you can shortlist a frame size before you contact us. We have made helical geared motors and industrial gearboxes in Wenzhou since 1987; all figures below are from our series catalogues, which are downloadable from each series page.

Key definitions

• Gear reducer (gear unit): a gearbox that reduces the motor's speed and multiplies its torque by the ratio i.
• Geared motor: a gear reducer with the electric motor flanged directly to it, sold as one unit.
• Ratio i: input speed divided by output speed; a 1,450 r/min motor through i = 29 gives 50 r/min.
• Permissible output torque T2: the torque the gear unit is rated for continuously at a given ratio; the number in the selection table.
• Service factor: a multiplier that scales the required torque up for shock loads, the type of motor and frequent starting.
• Mounting position M1 to M6: the orientation of the unit; it decides oil fill and the plug positions.

The four families of helical geared motor

Series	Shaft arrangement	Frame sizes	Input power	Ratio range	Permissible torque	Typical use
R series	Coaxial (in-line) helical	R17–R167 (13)	0.12–160 kW	≈3.4–290	87–17,700 N·m	Conveyors, mixers, machine drives where motor and shaft are in line
F series	Parallel-shaft helical	F37–F157	0.12–200 kW	≈3.8–282	196–16,800 N·m	Shaft-mounted drives on rollers and screws, narrow spaces
K series	Right-angle helical-bevel	K37–K187 (12)	0.18–200 kW	≈5.4–197	156–49,400 N·m	Heavy right-angle drives with high efficiency
S series	Right-angle helical-worm	S37–S97 (7)	0.12–22 kW	≈6.8–290	93–3,990 N·m	Compact, quiet, high-ratio right-angle drives

All four share the same construction points: a three-layer high-density polymer paint finish that seals out dust and tolerates high temperature, precision-ground alloy-steel gears for meshing accuracy, and high-grade ball bearings. Each is available foot-mounted, flange-mounted, with a hollow output shaft, and in the combinations the type designation key lists.

Above these torques, two further families take over:

Series	Construction	Motor power	Output speed	Output torque
P series planetary	2- or 3-stage planetary with power splitting, sizes 9–36	0.4–1,293 kW	0.19–60 r/min	up to 2,600,000 N·m
H/B series industrial gearbox	Modular 1- to 4-stage helical (H) or bevel-helical (B), sizes 1–22	2.52–5,366 kW	1.7–1,200 r/min	up to 470,000 N·m

Selection in six steps

1. Fix the output speed n2. From the machine: belt speed and pulley diameter, screw pitch and feed rate, or the drum speed you need. This gives the ratio: i = motor speed ÷ n2. For a 4-pole motor use 1,450 r/min at 50 Hz.
2. Find the torque the machine really needs, T1. From the load and the radius it acts on, or from the absorbed power: T1 (N·m) = 9,550 × P (kW) ÷ n2 (r/min).
3. Apply the service factors. Required torque T2 ≥ T1 × f1 × f2 × f3, where f1 comes from the driven-machine class (uniform, moderate shock, heavy shock) and daily operating hours, f2 from the prime mover, and f3 from starts per hour. The tables are in every series catalogue; for a uniformly loaded conveyor running 16 hours a day with an electric motor and under ten starts per hour the product is typically 1.2 to 1.4.
4. Choose the shaft arrangement, which is the series letter: in-line (R), parallel (F), right-angle (K or S). Choose S over K when the ratio is high, the power is small and noise matters; choose K when efficiency at high power matters.
5. Read the frame size from the selection table: the smallest size whose permissible torque at your ratio is at least T2, checking that the motor power you need is offered on that frame.
6. State the mounting position (M1–M6), output-shaft side and terminal-box orientation on the order. The oil quantity, breather, oil sight glass and drain plug are set for that position at the factory.

A worked example

A belt conveyor needs 45 r/min at the drum and absorbs 4.2 kW; it runs two shifts with an electric motor, uniform load, few starts. T1 = 9,550 × 4.2 ÷ 45 = 891 N·m. With f1 × f2 × f3 = 1.25, T2 = 1,114 N·m. Ratio from a 1,450 r/min motor: i ≈ 32. The smallest in-line frame that offers about 1,100 N·m at i ≈ 32 with a 5.5 kW motor is in the R87 range, so the shortlist is an R87 geared motor at 5.5 kW, 4-pole, in the mounting position the conveyor frame allows. We confirm the exact ratio and the permissible torque from the selection table before quoting.

Ordering: how a type designation is read

Our units are ordered by a designation of the form YF K A 67 B Y-4 p i9.66 M1 B 180° 2: enterprise code, series letter, mounting structure (foot, flange, hollow shaft and their combinations), frame size, motor type, motor power and poles, ratio or output speed, mounting position M1 to M6, output-shaft side A/B/A+B, terminal-box position, and any additions. Writing the designation this way removes almost every ambiguity from an international order. The full key and the M1–M6 drawings are in the catalogue PDF on each series page; a single-stage variant (RX/RXF) covers ratios of about 1.3 to 6.6 where a two-stage unit would be oversized.

Non-standard and replacement units

Because the frame sizes and ratios follow the industry-standard designation, an existing R, F, K or S unit from another maker can usually be replaced by ours in the same frame size and ratio; send the nameplate photo. For applications that do not fit a catalogue unit, our engineering department designs non-standard reducers, and the modular H/B and P series are configured per application from the standard stages.

Send the six numbers above, or simply the machine and its duty, through the enquiry form and our engineers will return a selection and a dimension drawing. All series are made in our Pingyang factory, ISO 9001:2015 certified, with a one-year free maintenance period.