Caja de cambios de mezcladora interna de goma: Lee el perfil de carga por lotes, No solo kW de motor

An internal mixer gearbox is often specified from motor power. The motor is, di, 400 kW, the rotor speed is defined by the process, and the gearbox is ordered to match the motor. On paper the drive train is consistent.

Then the unit goes into service and the gearbox develops a problem that a power-matched unit should not have. The reason is that motor power is a steady-state quantity and an internal mixer is not a steady-state machine. What the gearbox experiences is a sequence of load states within each batch, repeated for years, and the peak of that sequence is what the gears and bearings actually have to survive.

A Batch Has Stages, and Each Stage Loads Differently

An internal mixer is a batch machine. Material enters, is masticated and mixed under pressure, and is discharged, and then the sequence repeats. The machine is built for this: a mixing chamber with two counter-rotating rotors, a ram or floating weight that applies pressure from above, a discharge door below, and temperature control and instrumentation throughout.

Within a single cycle the resistance the rotors see is not constant.

At the start of a batch, the chamber takes cold or partially warmed compound. Viscosity is high, the material does not yet coat the rotors or the chamber walls, and the rotors are working against a mass that resists being sheared. This is commonly the highest-torque part of the cycle, though not universally so: on some compounds and some loading sequences the peak lands mid-cycle, or at the moment the ram comes down. The point is not where the peak sits but that there is a peak and it has to be found per recipe.

As mixing proceeds, the compound heats, viscosity falls, and the torque required to shear it drops. The ram may be raised and lowered to control the working of the batch. Fill factor, the proportion of the chamber volume occupied by compound, changes the load directly: too little and the rotors do not develop shear, too much and the torque rises beyond the drive's capability.

At discharge, the door opens and the batch leaves. For a short period the machine is running with little or no load, and then the cycle restarts.

So the sentence "the gearbox sees 400 kW" is a statement about one operating condition, at best. The gearbox is subjected to a repeating load profile whose peak magnitude, Sincronización, Duración, and frequency depend on the actual mixing process. These conditions must be established from the machine's operating requirements or recorded duty rather than assumed from rated motor power.

The Load Classification Already Says This

There is a piece of established engineering reference material that settles the question of whether a mixer is a steady load.

Mechanical design handbooks classify machinery by load characteristics rather than by rated power, and use that classification as the starting point for drive selection. In those classifications, rubber mixing machinery including internal mixers sits in a category well above steady duty, in the same band as the other machines whose resistance to turning rises and falls within each working cycle. Rubber extruders that run continuously appear in a lighter category again. The exact category name and its load factor differ between handbooks and editions, so the number should be taken from the handbook the plant actually uses, not from a summary of it.

The point that survives across those references is the one that matters here. A rubber internal mixer is not a machine whose duty can be described by its rated power, and a drive selected against steady power is being selected against a condition the machine spends most of its cycle away from.

Once that is accepted, the specification question changes from "what is the motor power" a "what does the load profile look like."

Two Rotor Types, Two Load Distributions

The next distinction matters because it determines whether the two output shafts carry equal load, and that determines how the gearbox's torque capacity should be understood.

Tangential, or non-intermeshing, rotors. The rotor flights do not pass through each other; the rotors' swept paths are separate, and there is a gap between them. Mixing occurs in the high-shear region between each rotor's tip and the chamber wall, with distributive mixing depending on material transferring between the two rotors and circulating through the chamber.

A defining property of tangential machines is that the two rotors can run at different speeds. That has a direct consequence for the drive: the two rotors are not identically loaded at the same instant, and the torque split between the two output shafts is not necessarily even.

Intermeshing rotors. The rotors are conjugate and rotate toward each other, so their flights intermesh and the rotor gap remains essentially constant through rotation. The mixing action comes from the shearing in the inter-rotor region as well as at the walls. The geometry fixes the rotors' relative position continuously, which constrains how independently they can behave, but it does not by itself tell you what the instantaneous torque split is. That is a question about the specific rotor design, the compound, and the fill, and it is answered on the machine rather than inferred from the rotor type.

The torque split must be established from the specific machine and gearbox design. Rotor type alone does not show that the load is even or uneven, and an inferred split does not predict which component will fail. Use per-shaft measurements or a validated load-sharing model, together with the manufacturer's ratings, before drawing conclusions about gearbox capacity.

What Motor Current Tells You and What It Does Not

Motor current is the measurement most plants have, and it is genuinely useful. It tracks the current the drive draws, which follows the load closely enough to show the shape of the cycle, the repeatability from batch to batch, and abnormal events in the form of spikes. A trend of rising peak current over months is real evidence of a developing condition, whether that is rotor wear, chamber wear, or a change in compound.

Current is not the power itself, and the difference is not a technicality. Alternating current drawn by an induction motor is related to the power it delivers through the supply voltage, the power factor, and the motor's efficiency, and each of those moves with load and with temperature. Two batches drawing the same current at different power factors are not delivering the same power to the rotors.

What motor current cannot do is divide the load between the two rotors. The reading is one total covering both shafts plus the losses, and no amount of analysis recovers the split from it. The split is a different quantity from the total, and the total does not contain it: two rotors can carry sixty and forty percent of the same sum, or fifty and fifty, and the log looks identical either way.

So the boundary is worth stating plainly. A current log gives a usable indication of load trend, which is enough to answer whether the load is steady or varying and whether it has changed since last year. It does not give power, and it does not give torque, and the combination of current with rotor speed does not supply either: power has to come from a measurement that includes voltage and power factor, and torque follows from measured power and speed rather than from the current that happened to accompany them. What a fast-enough log does give is the shape of the cycle, the peak, and the duration of the peak, which is why the record matters more than the gauge. What no analysis of current can give is how the load divides between the two shafts, and that is one of the quantities a gearbox assessment turns on. The rest can be got from the drive with more or less work. This one cannot be got at all.

The practical consequence is that a gearbox assessment based on motor current is an assessment based on a total, not on the quantity the gears and bearings experience. It is a starting point, and it should be labeled as one.

Converting the Process Into a Load Specification

The productive exercise is to build a load record from what the plant already has, and to add the measurements that are missing.

Two entries on that list are the ones that change the specification. Peak torque means little without the duration of the peak, because a gearbox can survive a short overload that it could not survive continuously, and a drive selected on the mean is being selected against a load it does not see most of the time. Fill factor is the variable under the plant's control, and the one that most directly moves the torque.

The record those rows build is what an internal mixer gearbox assessment is made against. How much of it already exists is a question about the individual plant rather than about batch machines in general. Some installations log current and ram pressure continuously from the control system and can produce most of the table without adding an instrument. Others record almost none of it, and in that case the profile has to be established by a measurement campaign planned by qualified personnel, with the sampling rate, the duration, and the operating conditions agreed in advance so that the data means something. What should not happen either way is a specification written on the assumption that the record exists when it does not, or on a single spot reading taken in place of it.

The Thermal Side That Runs in Parallel

There is a second duty that a batch profile imposes and that a power calculation misses.

The gearbox heats during each torque peak and cools between batches. Over a shift, its thermal state is the result of a series of heating events and cooling periods, not a steady condition. If the cycle is short and the peaks are close together, the unit does not return to its baseline temperature between batches, and the operating temperature climbs until it reaches an equilibrium that is higher than a steady-load calculation would predict.

That matters because lubricant life, seal life, and bearing clearance all depend on operating temperature. A gearbox with adequate mechanical capacity that settles at a running temperature well above what the sizing calculation assumed is running its oil and its seals outside the conditions the sizing was written for, and the consequences tend to present as separate maintenance problems that nobody connects back to the duty cycle. How much life is affected in a particular unit is answered by the oil and seal suppliers against their own products and the measured temperature, not by a general rule.

How much shorter is a question for the lubricant, and this article does not put a multiplier on it. Oxidation rate rises with temperature in a way that is well established in general terms, but the quantitative relationship belongs to the specific oil: it depends on base stock, additive package, and the oxidation test that was used to characterise it, and the lubricant supplier publishes the figures for its own product. A multiplier carried over from another oil, or from a general rule encountered in plant practice, is not a service-life prediction for this unit. The same applies to seals, where life is governed by the elastomer compound, the oil it runs in, and the temperature that combination tolerates, and where the seal supplier is the source for the limit. What can be said without a supplier figure is the direction and the ordering: a unit settling well above the temperature its sizing assumed is a unit whose oil and seals are being consumed faster than the sizing intended, and the gap between the assumed and the actual running temperature is worth recording as a number in its own right, so that the lubricant and seal suppliers have something specific to answer against.

Mixing chamber temperature control and the gearbox's own cooling are separate systems, but they share an environment: a mixer gearbox mounted close to a hot chamber is running in an ambient temperature well above the plant's air temperature, and its cooling capacity has to be evaluated against that ambient rather than against the room.

The Number That Has to Come From the Machine

For a new mixer or a gearbox replacement, the sequence that produces a usable specification runs in seven steps, and the order is the whole point.

1. Establish the cycle. Log current, Poder, ram pressure, and rotor speed through at least several full batches, at a sampling rate that captures the peak. Averages hide the quantity of interest.
2. Identify the peak and its duration. Par máximo, the rotor speed at which it occurs, and the fraction of the cycle it occupies.
3. Separate the stages. Inicio, Mezcla, and discharge loads, because the peak is not uniformly distributed and the thermal duty depends on the distribution.
4. Establish the torque split, which depends on rotor type. For a tangential machine, this cannot be assumed even.
5. Check fill factor and recipe consistency against the batch records, so that a load anomaly is correctly attributed to the process or to the machine.
6. Establish the thermal environment, including the ambient around the gearbox, not just the plant air temperature.
7. Only then match to a torque class, against the peak with its duration, the speed at which it occurs, and the thermal duty.

Step seven comes last because the torque class is an output, not an input. A plant that starts from a torque class and checks whether the machine fits inside it is a plant that will buy a drive sized to a condition the machine occupies for part of each batch cycle, and the failures that follow will all be attributed to gearbox quality.

The reason this matters more on a mixer than on a continuously running machine is the shape of the duty. A steady machine and its drive settle into an equilibrium, and a drive that is marginal announces itself quickly by running hot. A batch machine accumulates damage at the peak and pays for it at the bearing and the lubricant, years later, in a way that no single bad batch can be blamed for.

The One Question to Ask Before the Next Gearbox Is Ordered

The failure mode this article describes is not a gearbox undersized on power. It is a gearbox selected against a condition the machine occupies for part of each cycle and spends the rest of the cycle away from.

That is why the question worth asking before an order is not "what power rating" pero "what does the log look like." A logged cycle answers three things no nameplate can: how high the peak goes, at what rotor speed it occurs, and how long it lasts. Those three determine the mechanical and thermal duty, and the specification follows from them.

The fourth question, whether the two shafts are seeing the same load, is not answered by a logged cycle and cannot be. The total gives no way to split it. That one needs per-shaft measurement, or a load-sharing model confirmed against measurement on that machine, and until it has one, the split has to be treated as an open item rather than assumed even.

Internal mixer loads repeat with the batch cycle, which makes them suitable for systematic operating-duty analysis. Where reliable process and drive records exist, the peak conditions and their duration can be evaluated. Where those records are missing, establish the required load profile through an appropriate measurement plan rather than assuming it from the machine's rated motor power. A specification built from that evidence can be reviewed against the actual duty rather than a torque class quoted for a different machine.

The enquiry that follows from that is a short one to prepare, provided the plant has kept its records: the mixer and rotor specification, the batch timeline, measured power and motor speed, the load log, and the existing gearbox drawings. Send those and the transmission capacity can be checked against a measured duty rather than a nameplate figure, with the two-shaft split carried as an open item until it has evidence of its own.