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On a plastic raw material mixer, the motor and gearbox are selected as a package: the gearbox determines the speed and torque that the mixer shaft needs, and the motor must have enough power to deliver this torque at the gearbox input, even when the mixer is full. Selection starts not from the machine's name but from the mixer type, the shaft speed and the torque required with a full vessel. Understanding how the load builds up in tanks where granules, masterbatch and powder additives are mixed together determines the life of both the motor and the gearbox.
The mixer type determines the load from the outset
In the plastics sector the name "mixer" is used for very different machines. Before deciding on the motor and gearbox, be clear about which one you are working with.
- Vertical screw mixer: a screw in the centre of a conical or cylindrical tank picks up granules from the bottom, carries them upwards and drops them from the top. The screw speed is relatively low; the motor is usually connected through a gearbox or belt. Torque varies with the tank fill level and the bulk density of the granules.
- Horizontal ribbon mixer: ribbons rotating in a horizontal trough move the material in two directions. The shaft turns slowly and torque is high; there is almost always a geared drive.
- Drum or rotating vessel mixer: the vessel itself rotates; the load comes from the unbalanced weight of the vessel and material. A slow, steady motion is required at the gearbox output.
- High-speed heating-cooling mixer (PVC dry blend): the blades rotate at a very high tip speed and heat the mixture by friction. Two-speed motors or special drives are often used on these machines.
The two-speed drives in the last group require a special motor; they may fall outside the scope of the standard series in HEM Motor's 2026 catalogue, so ask us with the nameplate and application details. This article focuses on geared mixers in the first three groups.
Gearbox selection: output speed, torque and service factor
Three values determine the gearbox. Discussing motor power before these are clear may later end in gear or bearing failure.
Output speed
The mixer shaft speed is a value set by the machine builder according to mixing quality and time. The gearbox ratio is found by dividing the rated speed of the motor by this output speed. For 4-pole motors the rated speed is around 1,450–1,460 rpm, and the ratio is calculated accordingly.
Output torque
The torque required at the mixer shaft comes from the resistance of the material in the vessel. Gearbox catalogues give the permitted output torque for each ratio. The required torque must stay below this value.
Service factor
Gearbox manufacturers recommend a service factor according to load type and daily operating time. Mixers are considered machines with a uniform but fluctuating load; a tank running a few hours a day and a tank turning through every shift do not have the same factor. The gearbox's own catalogue rating is compared with this factor when selecting it.
On the motor side, what matters is that the gearbox input flange fits the motor's IEC frame. Mounting arrangements (B3, B5, B14, B34, B35 and vertical V1/V3/V5/V6/V15) are offered according to the frame and the flange size is confirmed with the order. On a vertical screw mixer the motor usually works on top of the tank, shaft down; in that case the vertical mounting arrangement must be stated in the order.
Calculating motor power from torque
The relationship between power, torque and speed is: power (kW) = torque (Nm) × speed (rpm) / 9,550. This relationship lets you move from the torque at the mixer shaft to the motor power.
The values below are a hypothetical example. Assume that a torque of 1,400 Nm is measured at the shaft of a horizontal ribbon mixer with a full vessel and that the shaft turns at 25 rpm. The shaft power is 1,400 × 25 / 9,550 ≈ 3.7 kW. Assuming a gearbox efficiency of 0.9, about 4.1 kW is needed at the motor shaft. In this case a 4 kW motor is at its limit; a higher fill level, a dense additive or wet material can exceed it. Such a calculation makes a 5.5 kW motor the safer choice, but the gearbox output torque limit must also be checked against this power.
| What changes on the mixer | Effect on motor and gearbox |
|---|---|
| Vessel fill level increases | Shaft torque increases; motor current rises |
| Powder or wet additive is added | Material resistance increases; starting becomes harder |
| Mixing time is extended | The motor stays under load longer and heats up more |
| A higher shaft speed is wanted | The gearbox ratio changes; motor power is recalculated |
If your gearbox ratio and the required power are known, review suitable HEM Motor models and current prices; the motors for gearboxes page also gives an overview of geared drives.
