Geared Motor or Bare Gear Unit: Which to Order and Why

A geared motor is one of our gear reducers with an AC motor flanged directly onto it and shipped as a single assembly; a bare gear unit is the same gearbox with a plain input shaft, waiting for a motor you supply. Our R, F, K and S series are each offered both ways, and the catalogues say so in one repeated sentence without ever explaining which way to choose. This guide answers that: which series can be motorised and over what power band, what the motor fixes permanently at the factory, what the motor adds to the order line, and the cases where a bare unit is the better buy. Which series to use in the first place is a separate question, and the gear reducer selection guide answers it — this article assumes you already know whether you need an in-line, parallel-shaft or right-angle unit.

Key definitions

• Bare gear unit: the gearbox alone, with a solid input shaft. You provide and align the motor and the coupling.
• Geared motor: the same gearbox with an AC motor mounted directly on it through a motor flange, supplied as one unit.
• Input power band: the range of motor powers a frame size is listed for in the selection table. It is a limit, not a suggestion.
• Permissible output torque T2: the torque a frame is rated for continuously at a given ratio.
• Pole count: the motor's pole number, written -4p in the designation. A 4-pole motor is taken as 1,450 r/min at 50 Hz in our selection examples.
• Terminal-box orientation: which side the motor's terminal box faces, at 0, 90, 180 or 270 degrees. It exists in the designation only because a motor is part of the unit.
• Mounting position M1 to M6: the orientation the unit is installed in. It sets the oil fill and the positions of the breather, oil sight glass and drain plug.
• Service factors f1, f2, f3: the driven-machine, prime-mover and starting multipliers applied to required torque. f2 is the one the motor decision touches.

Which series can be ordered motorised

Series	Shaft arrangement	Frames	Input or motor power	Permissible output torque	Offered as a geared motor?
R	coaxial (in-line) helical	13 (R17–R167)	0.12–160 kW	87–17,700 N·m	Yes — bare unit or geared motor with a directly flanged AC motor
F	parallel-shaft helical	10 (37–157)	0.12–200 kW	196–16,800 N·m	Yes — the motor is mounted directly on the gear unit through a motor flange
K	right-angle helical-bevel	12 (K37–K187)	0.18–200 kW	156–49,400 N·m	Yes — bare unit or geared motor
S	right-angle helical-worm	7 (S37–S97)	0.12–22 kW	93–3,990 N·m	Yes — bare unit or geared motor
P	2- or 3-stage planetary with power splitting	sizes 9–36	motor power 0.4–1,293 kW	up to 2,600,000 N·m	No — not described as a geared motor; a motor bell housing, motor bracket or motor swing base is listed as an add-on piece
H/B	modular helical (H) or bevel-helical (B), 1–4 stages	sizes 1–22	motor power 2.52–5,366 kW	up to 470,000 N·m	No — tables are built around 750, 1,000 and 1,500 r/min inputs
ZDY / DBY	cylindrical and bevel-cylindrical, hardened teeth	ZDY 80–560, ZLY 112–710	nominal input speed 750 / 1,000 / 1,500 r/min	—	No — not described as a geared motor

Two readings matter before you shortlist. The motorised option lives in the four modular series and stops where they stop — above 200 kW of motor power, or above about 49,400 N·m, the catalogue hands you over to P, H/B or ZDY, and those are specified by the motor power they are driven at rather than sold with a motor attached. On the P series the motor is accommodated by named add-on pieces — a bell housing on the input, a motor bracket, a motor swing base — which is a different commercial arrangement from a flanged geared motor even though the end result also arrives as one skid.

And a motor is not always part of the question. The T series bevel gearbox has no motor field in its designation at all: it is driven from a shaft line that already exists, and its ratios run from 1:1 upward precisely because it is often there to change direction rather than speed. The SWL screw jack is the same case from the other end — it is driven through its worm shaft, with a maximum input speed of 1,500 r/min, a duty limit of 20% ED over a 30-minute cycle and a starting torque that can reach 200% of running torque, so the motor that drives it is specified against those limits and not from a frame table.

What the motor fixes at the factory

The reason this is a decision and not a packaging preference is that ordering motorised freezes four things that are awkward or impossible to change on site.

1. The mounting position, M1 to M6. Each of the six positions has its own oil quantity and its own locations for the breather, oil sight glass and drain plug, set when the unit is built. Remounting the unit in a different position later leaves it reading its level at the wrong point and venting from the wrong place — the point made at length in our gear reducer maintenance guide.
2. The output-shaft side. A, B or A+B for a double-ended shaft. With the motor already on the unit, the geometry of the whole drive is decided, so getting the side wrong means turning the machine, not turning the gearbox.
3. The terminal-box orientation. 0, 90, 180 or 270 degrees. It is the cheapest field on the order line and the one most often left blank; a terminal box facing a wall or a walkway is a cable route nobody budgeted for.
4. The motor power and pole count. Both are part of the designation, selected per the motor column of the service-factor table. Fitting a different motor afterwards means a different flange joint and a unit that no longer matches its own nameplate.

A bare unit leaves one and four open and still fixes two and three, because mounting position and shaft side are properties of the gear unit itself.

The motor half of the type designation

Our units are ordered by a designation of the printed form YF K A 67 B Y-4 -4p i9.66 M1 B 180 2: enterprise code, series letter, mounting structure, frame size, motor type, motor power and poles, ratio or output speed, mounting position, output-shaft side, terminal-box position and additions. On a motorised order the fields that only exist because of the motor are the motor type, the -4p pole count and the 180 terminal box. On a bare order those fields fall away and the ratio or required output speed has to carry more weight, because the supplier no longer learns your intended motor speed from the motor itself — state the input speed you will drive the unit at.

Field	Bare gear unit	Geared motor
Series, mounting structure, frame size	required	required
Ratio i or required output speed	required — and state the input speed	required
Mounting position M1–M6	required	required
Output-shaft side A / B / A+B	required	required
Motor power and pole count	not applicable	required
Terminal-box orientation 0 / 90 / 180 / 270°	not applicable	required
Rotation direction viewed from the output shaft	required on K series	required on K series

The K-series line in that table is the one that catches people. K units bear load in one direction only, and the catalogue asks for the rotation direction at order so the spiral-bevel set is loaded the right way. That requirement does not go away because a motor is attached; it is a property of the gearing.

Where the motor enters the sizing

The sizing method itself does not change: the requirement is T_N ≥ T2 × f1 × f2 × f3, with f1 from the driven-machine class and the daily operating hours, f2 from the prime mover and f3 from the number of starts per hour. The motor touches the middle factor. f2 exists because the catalogue ratings assume a particular kind of prime mover, and a drive that is turned by something else is read off a different row of the table. If your drive is not a standard AC motor on a flange — an engine, a turbine, a line shaft — f2 is not the value a geared-motor selection would have used, and the figure has to come from the series catalogue rather than from habit. Tell us the prime mover and we will take f2 from the table for you.

Two further sizing rules are worth stating because they are the most common mistakes on motorised orders:

• The frame's input-power band is a limit. Each frame size is published with both a power band and a permissible output torque at each ratio, and both have to hold. An R47 is listed at 305 N·m and an R57 at 440 N·m; a bigger motor on an R47 does not buy the R57's torque. When the torque is short, the selection moves up a frame.
• The power band narrows as the frames grow. R17 is listed at 0.12 to 0.75 kW and R167 at 11 to 160 kW; K37 at 0.18 to 3 kW and K187 at 18.5 to 200 kW. A motor that sits at the bottom of a large frame's band is a legitimate selection when the torque demands that frame, but it is worth confirming rather than assuming.

When a bare gear unit is the right answer

• The drive has to hold its load. The S series catalogue makes no self-locking claim, so a hoist, an incline conveyor or a gate that must stay put when power is lost needs a brake motor or a mechanical holding device specified for it. That decision belongs to the motor half of the drive, and it is discussed in full in our worm gear reducer selection guide.
• You already own the motor. A unit coming off an existing machine, or a motor held as plant stock, is the clearest case for a bare gear unit — send us the motor's power and pole count and we will confirm the frame it suits.
• The plant standardises on one motor supplier. Where spares policy fixes the motor, the gear unit is the part to buy from us and the motor the part to keep common.
• The duty is above the four modular series. Over roughly 200 kW or 49,400 N·m the choice is P, H/B or ZDY, and those are driven units by design: you supply the motor and we supply the mounting arrangement.

Everything else points at the motorised unit. One flange joint, one nameplate, no alignment job between motor and gear unit, and a single supplier responsible for the fit — and on the right-angle and parallel-shaft series, a hollow output shaft on the driven shaft removes the second coupling as well.

Our recommendation

For a new drive inside 0.12 to 200 kW and 49,400 N·m, order the geared motor and put the four fixed fields on the order line: mounting position M1 to M6, output-shaft side, terminal-box orientation, motor power and poles. Order the bare gear unit when a motor already exists, when a brake or holding device decides the motor, or when plant spares policy does. Do not use motor size to buy torque a frame is not rated for, and do not assume f2 without naming the prime mover.

Send the output speed, the torque or absorbed power, the load character, the daily running hours, the starts per hour, the prime mover and whether you want the motor fitted through the enquiry form. Our Reducer Sales department and engineers return a frame size, a complete type designation with the motor fields filled in, the permissible torque against it and a dimension drawing; the series catalogues behind every figure here are on each page under gear reducers, including the R series catalogue. Yanfeng Group Co., Ltd. has built reducers and plastic weaving machinery in Pingyang, Wenzhou since 1987, and the full range is summarised in our industrial gear reducer series overview. Office hours are Monday to Saturday 08:00–17:30 and Sunday 09:00–17:30, China Standard Time (UTC+8).