Electric Motors for Gearboxes

When choosing an electric motor for a gearbox, three things must match: the gearbox input flange and the motor flange, the gearbox input bore and the motor shaft, and the gearbox ratio and the motor speed. Once these three agree, what remains is the power and service factor check. DRG Motor does not sell gearboxes; what we deliver is the three-phase motor for the gearbox you own or the one your machine builder specified.

Motors with suitable frames and speeds are listed on our Buy Electric Motors Online page. Pages dedicated to worm, helical, parallel shaft, bevel helical and planetary gearboxes are also being prepared.

Gearbox types and what changes on the motor side

The gearbox type does not define the motor on its own, but it tells you where to look:

Worm gearbox

Because there is sliding between the worm and the wheel, losses are clearly higher than in toothed gear types and they grow with the ratio. In one manufacturer's catalogue, for the same gearbox size, dynamic efficiency drops from 0.89 to 0.52 as the ratio rises from 7.5 to 100. In the same catalogue the smallest sizes start with frame 56 and 63 motor flanges, and the largest size goes up to frame 160.

Helical (coaxial) gearbox

Input and output are on the same axis and there is little sliding between the gears. One manufacturer's catalogue gives an average efficiency of 0.97 for one stage, 0.96 for two stages and 0.94 for three stages. The motor power is therefore chosen close to the power needed at the output; the ratio, output torque and service factor are what really decide.

Parallel shaft, bevel helical and planetary gearboxes

Most parallel shaft gearboxes have a hollow output shaft slid onto the driven shaft, with the housing held by a torque arm; they are common on conveyors and elevators. In large industrial types the motor sits on its own base with feet and is connected to the free input shaft with a coupling. A bevel helical gearbox turns the output through 90° and is close to the helical type in gearing efficiency: one manufacturer gives up to 0.98 (one stage), 0.97 (two) and 0.96 (three stages) for helical, parallel shaft and bevel helical units. Planetary gearboxes belong to two separate worlds: heavy-duty units on mixers, drums and cranes accept a standard asynchronous motor via IEC flange or coupling, while precision servo planetary heads need servo motors, which DRG does not make or sell.

Input type: IEC flange or free input shaft?

IEC motor flange input

The input cover of the gearbox carries a recess machined for the motor flange and a keyed bore that receives the motor shaft. The motor sits directly in this recess with no coupling in between. Catalogues and nameplates describe this input by frame size and flange type together, such as "71 B14", "90 B5" or "100/112 B5". If the input is B5 the motor needs a large flange; if it is B14 it needs a small face flange. Even on the same frame, one cannot replace the other.

Free input shaft

Here the input side has a protruding shaft instead of a motor recess. The motor is connected to it with a coupling, a belt and pulley, or an intermediate bell housing. A flange is not required; a foot-mounted B3 motor sits on a common base. In this layout alignment is the most important step of the installation.

B5 large flange and B14 small flange

B14 small flanges are common on worm and small helical gearboxes, while B5 dominates on larger frames. The DRG catalogue offers a B14 mounting option, but no B14 flange dimension table is published. For B14 orders we confirm the flange dimensions against your gearbox input at the time of order. B5 dimensions are given below.

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DRG B5 flange dimensions: frames 63–160

Frames 63–160 are the range used most often in belt and gearbox drives. Values are taken from the B5 large flange table in the DRG catalogue, in mm.

100L and 112M, as well as 132S and 132M, share the same B5 flange and shaft; this is why many gearbox catalogues list these inputs on one line as "100/112" and "132". If the IEC input flange of the gearbox and the flange and shaft diameter of the motor belong to the same frame size, they generally fit; still, compare P, M, N and D with the gearbox's own catalogue. Full drawings are on our technical drawing page.

Information to prepare before ordering

Nameplate of the old motor

• Power (kW) and rated speed (rpm)
• Voltage and connection: 230/400 V or 400/690 V
• Frame size and mounting code (B5, B14, B34, V1, etc.)
• Cable entry and terminal box position

Nameplate of the gearbox

• Type and size, gearbox ratio (i)
• Input designation (for example "90 B14") and input bore diameter
• Output torque and, if given, service factor
• Mounting position (B3, V5, V6, etc.)

If the nameplate is unreadable: measure

With a caliper, measure the flange outside diameter (P), the bolt circle (M), the spigot diameter (N), the hole diameter and number of holes, and the input bore diameter with its key width. A useful clue: on a B5 flange the holes are clearance holes and the bolt is tightened with a nut from behind; on a B14 flange the holes are threaded and the bolt screws straight into the flange.

From nameplate to motor: a worked example

Suppose the gearbox nameplate shows a ratio i = 25 and an input of "100/112 B5". The motor must have a 100L or 112M frame, a 28×60 shaft and a 250 / 215 / 180 flange. If a 4-pole 3 kW motor is enough, DRG-100L2-4 (1450 rpm, 19.8 Nm) has the right frame for this input.

• Output speed: 1450 / 25 = 58 rpm.
• On a two-stage helical gearbox (average efficiency 0.96) the theoretical output torque is 19.8 × 25 × 0.96 ≈ 475 Nm.
• On a worm gearbox with the same ratio (catalogue size 075, dynamic efficiency 0.81 at i = 25): 19.8 × 25 × 0.81 ≈ 401 Nm.

With the same motor and the same ratio, the output torque differs by about 74 Nm depending on the gearbox type. The resulting torque must not exceed the rated torque of the gearbox or the service factor of the application; compare it with the gearbox catalogue. For your own figures you can use the Gearbox Output Speed and Torque Calculator. Planning a new motor speed on an existing unit? See our motor-only swap guide first.

Starting, braking, service factor and mounting position

Starting torque

The gearbox multiplies torque by the ratio, and the starting torque of the motor is passed to the output in the same proportion. Our 4-pole motors up to 15 kW start with 2.0 to 2.4 times their rated torque (Ts/Tn, DRG catalogue). Star-delta starting reduces the starting torque to one third; on a 400 V supply this only works with our 400/690 V wound motors, i.e. from 4 kW upwards.

Need for a brake

Hoisting, inclined conveying or holding a load in position calls for a brake motor. We do not offer brake motors in the DRG range, so please raise this need with us before ordering. Relying on the self-locking of a worm gearbox is not a braking solution.

Service factor

The service factor is a gearbox selection matter: it is set by the type of load, the daily operating hours and the number of starts per hour. Service factor tables with worked cases are in our gearbox selection article.

Mounting position

When the gearbox stands on its feet horizontally, the motor runs horizontally; in layouts with a vertical output the motor may end up vertical. Besides B3, B5, B14, B34 and B35, our catalogue lists vertical versions (V1, V3, V5, V6, V15 according to frame); write vertical use into the order note.

DRG electric motors: models suited to gearbox inputs

DRG Motor is a manufacturer, supplier and seller of electric motors. We build three-phase motors at 1000/1500/3000 rpm: aluminium housings cover 0.25–11 kW, cast iron housings 0.55–355 kW, with IP55 protection and a choice of 230/400 V or 400/690 V windings. A selection of the 4-pole and 6-pole models used most in gearbox drives:

The B5, B35 or B14 mounting option is stated in the order note.

Live price table

The prices below are those of the electric motors to be fitted to the gearbox and come live from our shop data:

DRG MOTOR motors are guaranteed against material and manufacturing defects for 2 years from the delivery date (warranty certificate).

Motors in this power range

• DRG-90L-4 – 1.5 kW – 1445 rpm – 90L cast iron
• DRG-100L2-4 – 3 kW – 1450 rpm – 100L cast iron
• DRG-112M-4 – 4 kW – 1450 rpm – 112M cast iron
• DRG-132M-4 – 7.5 kW – 1460 rpm – 132M cast iron
• DRG-160M-4 – 11 kW – 1465 rpm – 160M cast iron

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Frequently asked questions

What does the price of a gearbox motor depend on?

Nothing on this page prices a gearbox: what you see is the cost of the motor alone. They depend on power, number of poles, frame material and mounting type; current values appear in the live table above.

Can a 6-pole motor be fitted to a gearbox?

Yes, if the flange and shaft match. However, at the same power the frame may be larger, and the output speed drops by about one third while the torque rises; check that this torque does not exceed the gearbox rating.

Which dimensions should I give when ordering a B14 flange motor?

Flange outside diameter, bolt circle, spigot diameter, number of holes and thread size, plus shaft diameter and length. We confirm the B14 dimensions with these values at the time of order.

Is the fit between a motor and a gearbox bought separately guaranteed?

Only a comparison of dimensions shows the exact fit. If the IEC input and the motor belong to the same frame size they generally fit; we recommend comparing flange, spigot and shaft dimensions between the gearbox catalogue and our motor drawing.

A photo of the gearbox label plus the old motor's plate is usually all we need to name a matching motor: phone +90 (542) 666 11 11, e-mail [email protected], contact page. For all models and the gearbox motor price list, see Buy Electric Motors Online.