A mixer or agitator nearly always needs the same thing from its drive: very low output speed, very high continuous torque, and a load that grows as the product thickens. The gearbox for mixer and agitator duty is therefore selected on torque and on shaft loading, not on horsepower. This guide works through the four numbers that decide the frame - impeller torque, output speed, overhung and bending load, and the service factor that viscosity forces you to carry - and then shows how mounting style follows from the vessel geometry.

Why an Agitator Drive Is a Torque Decision, Not a Power Decision

Motor power tells you how fast energy enters the vessel. It does not tell you what the gearbox has to survive. Two mixers can run from the same 15 kW motor and impose entirely different loads on the reducer, because the gearbox sees whatever torque the impeller needs to move the fluid at the speed you asked for.

T = 9550 × P / n

Where T is output torque in newton-metres, P is the absorbed shaft power in kilowatts at the impeller, and n is the impeller speed in r/min. The motor's rated power only caps this number; the actual absorbed power depends on the impeller geometry, the fluid density and the rotational speed, and in turbulent regime it scales roughly with the cube of speed. Halve the impeller speed and the absorbed power can fall by a factor of eight - which is exactly why agitator drives are always specified at the slow speed the process needs, with the gearbox doing all of that reduction in one unit.

That is also why the selection conversation normally starts with the mixing duty, not the motor. A drive selected on a nameplate power figure alone is the single most common way an agitator gearbox is mis-sized in practice.

The Speed Range an Agitator Actually Uses

Impeller speed separates mixing duties into bands, and the band determines how much reduction the gearbox must supply. A four-pole motor at 1450 r/min driving an impeller at 60 r/min needs a ratio of about 24; driving one at 15 r/min needs about 97.

At the top of that range a single gearbox is often no longer enough, and the drive becomes a combination - a helical or helical-worm unit feeding a planetary or a final worm stage. The important engineering point is that efficiency degrades as the reduction is stacked up: a worm stage alone can lose 20 to 35 percent of the input power as heat, and that heat has to leave the housing. On a slow-turning, heavily loaded agitator drive the thermal rating of the gearbox, not its mechanical rating, frequently becomes the limiting factor.

Bending Load and Why It Often Decides the Frame

An agitator is a cantilever. The impeller and shaft hang below the gearbox, and the bending moment they create is transmitted into the gearbox output bearings. That load is not a torque and it is not covered by the torque rating.

The practical consequences are specific:

• Shaft length matters more than impeller mass. Bending moment is force times lever arm, so a shaft extended to reach the bottom of a tall vessel multiplies the load on the output bearing by the same factor.
• Side-entering mixers load the seal and bearing in one direction permanently. The shaft is horizontal and the impeller weight acts as a constant radial load, so radial capacity is a hard selection criterion rather than a margin.
• Top-entering drives see oscillating load. Impeller imbalance and vortex-induced forces reverse direction each revolution, producing a fatigue load that reduces bearing life even though the mean load looks modest.

The standard mitigation is a separate support or steady bearing at or near the impeller, or a base bearing that takes the shaft load so the gearbox only transmits torque. When the shaft cannot be supported - short vessel, existing installation - the gearbox selection has to be driven by radial load capacity and by the resulting bearing life, and that calculation should be done before the frame is committed.

Viscosity, Starting Torque and the Service Factor You Actually Need

Viscosity is what separates agitator sizing from almost every other gearbox application, because it makes the load grow in two directions at once.

First, viscous drag rises as the product thickens. A batch that starts water-thin and ends as a paste can present an absorbed power at the end of the batch that is several times the value at the beginning, and the gearbox has to carry that peak continuously, not just for a moment.

Second, viscosity multiplies starting torque. A high-viscosity or settled product can take three to five times the running torque to break loose, and that is a cold-start condition that recurs every batch.

Conventional practice is to express both effects through a service factor applied to the calculated running torque:

T_selected ≥ T_running × SF

Two practical notes on that table. A service factor applied to torque is not the same as a service factor applied to power, and mixing the two is a common source of a frame that looks adequate on paper and overheats in service. And the factor protects the gearbox; it does not protect the motor, which needs its own check against the starting and peak torque.

The gearbox service factor explained article works through the torque-versus-power distinction in detail if you want the full reasoning.

Mounting: Foot, Flange or Shaft-Mounted

On an agitator the mounting decision usually follows the vessel, not the gearbox.

Top-entering drives are normally foot-mounted on a support structure or flange-mounted to the vessel top plate, which means the gearbox feet or flange must align with a mounting face that often already exists. Where the vessel top is small, a foot-mounted unit on a pedestal leaves more room for the shaft seal than a flange-mounted unit sitting directly on the nozzle.

Side-entering drives are almost always flange-mounted, because the gearbox flange doubles as the mechanical seal housing and forms the pressure boundary. Here the process connection, not the drive train, fixes the interface.

Where the impeller shaft is short and stiff, a shaft-mounted reducer on a hollow output shaft removes the coupling entirely: the unit slides onto the shaft and torque reacts through a torque arm. This is common on smaller portable mixers and on agitators inside vessels where a separate bearing pedestal would intrude on the process volume. The trade-off is that the gearbox now rides on the shaft and depends on the shaft stiffness to keep its bearings aligned, so the shaft must be sized for it.

Heat also deserves a mention at this point. Slow, high-torque agitator drives generate their losses continuously at low output speed, where fan cooling on the gearbox is ineffective because the input shaft is the only thing turning fast. A unit that is thermally marginal in a still, hot room will run hotter than the same unit in open air, and the remedy is usually a larger frame or an oil cooler rather than a different gear type.

A Worked Selection Example

Take a top-entering agitator in a 4 m vessel handling a product whose viscosity rises from 500 to 12,000 cP over the batch. The process requires 45 r/min and the vendor's mixing calculation gives an absorbed shaft power of 11 kW at the end of the batch.

The running torque is T = 9550 × 11 / 45 = 2335 Nm. With a high-viscosity duty factor of 1.8 the selection target becomes 2335 × 1.8 = 4203 Nm, and the ratio from a 1450 r/min motor is 1450 / 45 = 32. The frame is then chosen as the smallest one whose rated output torque at i = 32 exceeds 4203 Nm, whose thermal rating covers 11 kW of continuous loss under the actual ambient, and whose output bearing can carry the bending moment of the unsupported shaft length. Only when all three checks pass is the frame fixed.

You can run the torque and ratio steps against the catalogue range with the online selection system, which returns the frames that satisfy them and lets you compare mounting options before requesting a drawing. For a size 50 frame worked example in a comparable continuous duty, the M3PHF50 hollow shaft industrial gearbox shows how the shaft-mounted arrangement is specified when the impeller shaft can carry the unit directly.

Documentation to Send with an Agitator Enquiry

Viscous-duty selection stalls most often because the enquiry arrives without the inputs that decide it. The list that removes the back-and-forth is short:

• Vessel diameter, height and product level
• Impeller type, diameter, number of impellers and blade pitch
• Product viscosity range across the batch, plus density
• Required impeller speed and whether it varies
• Shaft length from the gearbox mounting face to the lowest impeller
• Whether the shaft is supported by a base or steady bearing
• Ambient conditions and any washdown or hazardous-area requirement

With those inputs the torque, service factor, thermal check and radial load check can all be run, and the answer is a frame that will still be correct in the third year of operation rather than one that merely starts.

Frequently Asked Questions

Can I use a standard helical gearbox for an agitator?

Yes for low-viscosity duties where the impeller speed is moderate and the shaft is supported. As viscosity rises and output speed falls, the required ratio pushes the selection towards helical-worm or bevel-helical units, and the thermal rating becomes the controlling limit because the losses accumulate in a slowly turning, poorly ventilated housing.

Why does my agitator gearbox run hot?

Agitator drives run at low output speed, so the input shaft - and any fan mounted on it - is the only part turning quickly. Losses generated continuously in the gearing have to leave through the housing surface, and a unit selected on mechanical torque alone can be thermally marginal. The fixes are a larger frame, better lubrication, or an oil cooler, in that order of preference.

How do I handle the high starting torque of a thick product?

Cover it with the service factor rather than relying on the motor, and confirm that the motor itself can develop enough breakaway torque. A settled or cold product can demand three to five times the running torque, and the gearbox must be rated for the peak it will see at every batch start, not the average of the batch.

Is a shaft-mounted reducer suitable for a top-entering agitator?

It is suitable when the impeller shaft is short and stiff enough to hold the gearbox aligned, and when the vessel has no room for a separate bearing pedestal. Where the shaft is long or the bending load is high, a foot-mounted or flange-mounted unit with its own output bearings, plus a support bearing near the impeller, is the more reliable arrangement.

What information speeds up a quotation?

Impeller diameter and type, viscosity range, density, required speed, shaft length, whether the shaft is supported, and the ambient conditions. With those, the duty point can be converted into torque, ratio and radial load and a specific frame quoted in writing rather than a range of options.

Next Step

Agitator selection rewards doing the torque and bending checks early. Put your impeller duty into the online selection system to shortlist frames, then send the shaft geometry, viscosity range and mounting constraints - the frame can be confirmed against all three before any order is placed.