Gearbox for mixing applications

Innovative gear unit solutions form the backbone of a customised gearbox capable of absorbing the high bending and axial forces that occur in agitator/aerator applications.

Whether for promoting chemical reaction, homogenising product streams or blending stubborn ingredients, the mixing requirements of the general manufacturing sector, most particularly the chemical and food processing industries, are among the most arduous. In many cases, dense slurry is being mixed in vessels that can reach up to many thousands of litres. A critical element of the mixing system is the gearbox, which must withstand the high loads generated by the mixer.

For mixing applications requiring torques between 6 to 65 kNm, SEW-Eurodrive's MC Compact series of helical and helical-bevel gear units has recently been expanded with the introduction of the extended bearing distance (EBD) feature. Encompassing improvements to the bearings, shaft and housing, the EBD makes the MC Compact series suitable for mixer and agitator applications of 5 to 250 kW.

Resisting reactions

Agitators in heavy-duty applications contend with a number of reactionary forces. As the mixer pushes material outward, down and around inside the vessel, natural thrusts, or axial forces, result upwards along the axis of the mixer shaft. Simultaneously, as the blades of the mixer make contact with solid mass within the mixture, the shaft is pushed in changing directions creating an imbalance, or a radial load, perpendicular to the mixer shaft (Figure 1).

As the mixer shaft is securely connected to the gearbox shaft, these axial and radial loads are transferred to the gears and bearings. By bolting an additional bearing housing onto the existing standard bearing housing, the EBD increases by approximately 50% the distance between the upper and lower bearings, adding significant rigidity to the mixer shaft.

Two types of additional bearing housings are available, the 'Z' housing with additional support arms and the 'N' housing without support arms, depending on the requirements of the specific application (Figure 2). The options are further increased by the availability of three shaft variants, standard, heavy duty and a hollow type, for most of the eight MC Compact unit sizes. These, in turn, all impact on the choice of bearing size. The various combinations of these are selected to suit the individual mixing application, ranked by radial and axial load capabilities, from moderate to high.

Really high radial loads and moderate axial loads might require a size-seven gear unit with a heavy-duty shaft. On the other hand, a smaller application with moderate radial loads and high axial loads might only require a size-three unit with a normal-sized shaft.

Shaft size matters

When it comes to buying a mixer drive, there are two ways to go about it: specify the mixer shaft diameter or specify the axial and radial loads impacting on the shaft. Either way you need a certain shaft size to do the job. Shaft diameter is a key factor in the overall capability of the mixer. The mixer designer determines the shaft diameter from calculations based on product density, the depth that the blades enter the vessel, the dimensions of the mixer blades and so on.

Typically, drive solution providers opt for an over-sized gear unit in order to match the mixer shaft dimensions. This leads to an over-priced, overweight and over-engineered mixer drive. With the EBD concept, SEW-Eurodrive has been able to increase the gearbox shaft diameters across the board, for all MC Compact unit sizes. The EBD design allows for the selection of the smallest gear unit that can mechanically meet the load requirements and still retain the same shaft size. Bearing sizes have also been increased, resulting in a significantly longer bearing life.

Crucial in terms of meeting the mechanical load requirements, the gear unit comprises a single top bearing and two high-capacity lower bearings to accommodate the high mixer forces, both axial and radial. The lower bearings at the base of the gearbox shaft closest to the mixer shaft consist of two tapered roller bearings, secured face to face by a locknut. This way they act as a fixed bearing and absorb both axial and radial forces experienced by the shaft.

Using the 'Z'-type housing, the EBD Compact series can be flange mounted. The axial and radial forces absorbed by the paired tapered roller bearing arrangements are then transferred to the flange, thereby protecting the gear unit and internal gearings (Figure 3).

At the other end of the gearbox, a spherical roller bearing acts as a floating bearing to disperse any remaining axial loads impacting on the gears. The benefits of the EBD's bearing arrangements are significant. They lead to a high load capacity and low axial displacement, which makes for a more efficient and longer lasting mixer drive.

Minding the mixture

The very fact that these mixer drives are longer lasting means that lubrication of the bearings and gears is of primary importance. You need oil in every gearbox, just as you need oil in your car, or it just won't go. The issue here, in the general manufacturing sector, is to ensure that the lubricant never comes in contact with the mixture.

There are two options available for lubricating the moving parts of the gearbox while protecting the mixture from contamination by the gearbox lubricant. For the smaller gear units (up to 12 kNm), the EBD MC Compact incorporates an oil leakage indicator (Figure 4). This is an electronic sensor located below the tapered roller bearings, downwind from a set of double lip-seals. If any oil gets through the lip-seal, the sensor triggers an alarm on the operator's panel. This warns the operator that oil leakage is imminent.

For larger gear units (16 to 65 kNm), a drywell arrangement keeps the bearing housing free of oil (Figure 5). The drywell arrangement eliminates the possibility of oil leaking from the shaft or bearing housing. It consists of an oil reservoir in the main gear housing above the extended bearing housing. This has a defined level below which the oil is kept and a sealing arrangement to keep lubricant out of the extended housing. An oil lubrication pump driven by one of the shafts pumps the oil onto the upper bearings and gears. The lower bearings neighbouring the double lip-seals are lubricated by grease, rendering contamination of the mixture impossible.

The additional housing that is introduced through the EBD concept not only benefits the drive by adding strength and durability, but also grants that extra room to incorporate a drywell. The EBD design concept has resulted in a chain reaction of improvements: larger bearings, larger shafts and improved lubrication systems. The gearbox is now specifically tailored to mixer and agitator applications in the manufacturing industries.