Electric Motor and Gearbox Matching for the Cement Screw of a Concrete Batching Plant

In a concrete batching plant, the screw conveyor under the cement silo carries material to the weighing hopper and may start and stop hundreds of times a day. When its motor fails, most operators first read the power on the nameplate and look for the same kW. Yet in a cement screw the power is not the only value that defines the motor: the inclination of the screw, the input side of the gearbox, the frequent starts caused by the weighing cycle, the dusty and open environment under the silo and the first start from a tube that stayed full overnight must all be considered together. In this article we go step by step through how to match the motor and the gearbox and which information should be collected before a replacement.

A cement screw is not an ordinary conveyor

The screw under the silo takes cement from the conical silo outlet and delivers it to the weighing hopper above. That is why in most plants the screw is not horizontal but installed at a certain angle. As the inclination increases, the flight not only moves the material forward but also lifts it; cement sliding back inside the tube creates additional friction. This is why, of two screws with the same diameter and length, the inclined one needs more drive power. When replacing an old motor there is usually no problem if the angle of the screw has not changed; but if the plant has been relocated, the screw extended or the angle made steeper, the previous nameplate value may no longer be a correct reference.

The cement itself also changes the behaviour. Freshly flowing, aerated cement from the silo flows almost like a liquid; cement that has waited for a long time and compacted forms lumps inside the tube. In humid weather, crusting at the tube inlet can also be seen. When choosing the motor, these two extremes, free flow and compacted fill, need to be considered separately.

Gearbox input flange and the existing motor frame

In most cement screws the motor is flanged directly onto the input side of the gearbox. In this arrangement the mounting interface must match before the motor power does. The gearbox input flange is machined for a specific IEC frame size; the motor shaft diameter, keyway and flange type must fit this input. A large flange with through holes (B5) and a small flange with tapped holes (B14) cannot replace each other.

In practice, the following order is followed:

• Note the IEC frame code (e.g. 132S, 132M, 160M) and the mounting arrangement on the old motor's nameplate.
• Check on the gearbox nameplate or in its catalogue which frame the input side is designed for.
• If you are thinking of changing the motor power, check whether the new power is in the same IEC frame; if the frame gets larger, the flange and shaft may no longer fit.
• Do not exceed the torque limit the gearbox can transmit; a more powerful motor may overload the gearbox.

At HEM Motor, B3, B5, B14, B34, B35 and vertical V1/V3/V5/V6/V15 mounting arrangements are offered according to the frame; the flange size is confirmed with the order. Suitability is not confirmed just by looking at B5 or B14 in a model name. You can compare dimensions in the technical drawings and the product catalogue, and see the mounting arrangements FAQ for the options. General information about motors to be coupled to gearboxes is on the electric motors for gearboxes page.

Weighing cycle: how do frequent starts and stops affect the motor?

A cement screw is not a machine that runs continuously. When the scale approaches the target value the screw stops, and it runs again for the next batch. In some control systems the screw is switched on and off at short intervals in the final part for accurate dosing. At every start the motor draws several times its rated current and the winding heats up. As starts become more frequent, the motor has less time to cool down.

For this reason, the following information is important in a replacement request:

• Roughly how many batches are made per hour and how many times the screw starts and stops in each batch
• Whether short pulse operation is used in the final dosing
• Whether the motor is started direct-on-line, with star-delta, with a soft starter or with a frequency inverter

Star-delta starting reduces the starting current but also reduces the starting torque; on a full, inclined screw this may prevent the motor from starting in the star position. A frequency inverter allows soft starting and speed adjustment. Our three-phase series are suitable for speed control with a frequency inverter; for long operation at low speed with full torque an external fan may be required, and if the cable between drive and motor is long a dv/dt or sine filter is considered; these conditions are stated with the order. Details are on the inverter FAQ page. When the thermal relay trips frequently, the protection setting also needs to be reviewed; for this you can read our motor protection article.

Dust, rain and sun under the silo

The area under the silo is usually open, and the motor is exposed to rain, sun and the temperature difference between day and night. Cement dust also penetrates even the finest gaps. HEM Motor's three-phase models on sale are offered with IP55 protection and class F insulation; still, paying attention to a few points extends the motor's life:

• A layer of cement building up on the cooling fins makes heat dissipation harder; it should be cleaned regularly.
• The front of the fan cover should not be blocked by dust or bag residue.
• On an inclined screw the motor also sits at an angle; where condensate water will collect and the orientation of the terminal box should be discussed with the order.
• In installations exposed to direct rain, a simple canopy or protective cover helps; this cover must not block the cooling air.

As for frame material, a cast iron frame is frequently preferred on plant sites where vibration and impact are heavy. To evaluate it together with the other drives in the plant, you can look at the concrete plant electric motors page.

First start in a tube full of compacted cement

After a weekend or a long breakdown, the screw tube may remain full of cement. During this time the cement loses its air and compacts around the flight. At the first start in the morning, the resistance the motor faces is clearly higher than the load in the normal cycle. In this case increasing the motor power is not the only solution, and it often strains the gearbox, the coupling or the screw. Measures applied in operation include:

• Emptying the screw before stopping it for a long standstill
• Running the aeration system at the silo outlet before starting
• If the screw has remained full, starting gradually and monitoring the motor current
• If the starting problem repeats, reviewing the starting method and the gearbox ratio together

Example models

In cement screws, 4-pole motors are usually coupled to the gearbox input. The models below are given as examples; the correct power is determined by the inclination, length and capacity of the screw and the gearbox ratio:

• 132S-4 cast iron frame: 5.5 kW (7.5 HP), 4 poles, 1,460 rpm rated speed, IEC 132S, 400/690 V (Δ/Y), 50 Hz, rated torque 36 Nm.
• 132M-4 cast iron frame: 7.5 kW (10 HP), 4 poles, 1,460 rpm, IEC 132M, 400/690 V (Δ/Y), 50 Hz, rated torque 49.1 Nm.
• 160M-4 cast iron frame: 11 kW (15 HP), 4 poles, 1,465 rpm, IEC 160M, 400/690 V (Δ/Y), 50 Hz, rated torque 71.7 Nm.

Note that the 5.5 kW and 7.5 kW models belong to the 132 frame family, while the 11 kW model is in a 160M frame. If your gearbox is machined for a 132 frame, moving to 11 kW may break the flange and shaft fit. Current prices are on the product pages.

Hypothetical example: request for a more powerful motor on a 132-frame gearbox

In a hypothetical plant, a 5.5 kW, 4-pole motor is used on an inclined cement screw and the gearbox input suits a 132 frame. The operator sees the thermal relay tripping at morning starts in winter and wants to upgrade the motor to 11 kW. During the review, however, it turns out that the screw is left full every evening, the motor is started with star-delta and the fins are covered with cement. Since the 11 kW motor is in a 160M frame, it will not fit the gearbox input directly. In this example, leaving the screw empty, cleaning the fins and reviewing the starting method are considered first; if a power increase is really needed, the gearbox torque limit and the 7.5 kW option in the same frame are discussed. This scenario is only an illustration and does not belong to a real plant.

Conclusion and information needed for an order

The right motor for a cement screw is one that fits the input side of the gearbox, can handle the start frequency of the weighing cycle and withstands the conditions under the silo. It is not possible to state an exact power by looking at the machine name; for a replacement or a new installation, send the following information together:

• Power, speed and a nameplate photo of the existing motor
• IEC frame code, mounting arrangement and gearbox input flange information
• Supply voltage and starting method
• Inclination and length of the screw and the number of batches per hour
• Installation environment (open air, dust, rain) and whether full starts occur

You can review suitable models and current prices on the buy electric motors page. To ask for the right model using your motor's nameplate data, send the nameplate photo or reach us via our contact page.