Whether for solvent extraction, leaching or flotation, the mixing requirements of mineral processing industries are among the most arduous.
In many cases heavy-duty dense slurry is mixed in vessels that can reach up to many millions 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 that need torques between six to 65,000Nm, the SEW-Eurodrive range includes the MC Compact series of helical and helical-bevel gear units. This range has recently expanded with the introduction of the extended bearing distance (EBD) feature, built specifically for mixing applications in the mineral processing industry.
The company says as it encompasses improvements to the bearings, shaft and housing, the EBD makes the MC Compact series a suitable gearbox for mixer and agitator applications of five to 250kW.
Similar to household hand-held blenders, mixers 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 (see Figure 1 on page 70).
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 one, the EBD increases by about 50% the distance between the upper and lower bearings, adding rigidity to the mixer shaft.
SEW-Eurodrive national product manager industrial gear units Ian Tribe says the company has a strong philosophy of modularity and standardised parts.
“Multiple drive solutions have always been possible using a limited number of different components,” Tribe says.
“The same design values have been applied here. We’ve taken the popular MC Compact series, added a component, and subsequently increased its capacity to deal with external high loads on the low speed shaft (LSS),” he says.
According to Tribe, two types of additional bearing housings are available depending on the requirements of the specific application: the “Z” housing with additional support arms and the “N” housing without support arms (see Figure 2 on page 70).
The options are further increased by three shaft variants for most of the eight MC Compact unit sizes: standard, heavy duty and a hollow type. These, in turn, all impact on the choice of bearing size.
The various combinations of these are selected to suit the individual mixing application, and are 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,” Tribe says.
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,” Tribe says.
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.
Usually, drive solution providers opt for an over-sized gear unit in order to match the mixer shaft dimensions, but this leads to an over-priced, over-weight and over-engineered mixer drive. With the EBD concept, SEW-Eurodrive can increase the gearbox shaft diameters across the board for all MC Compact unit sizes.
“The EBD design allows us to select the smallest gear unit that can mechanically meet the load requirements, and we still match the shaft size,” Tribe says.
Bearing sizes have also increased, resulting in a significantly longer bearing life.
Crucial for meeting the mechanical load requirements, the EBD MC Compact 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 lock-nut. This way they act as a fixed bearing and absorb axial and radial forces experienced by the shaft.
Using the “Z”-type housing, the EBD Compact series are flange-mountable. The axial and radial forces absorbed by the paired tapered roller bearing arrangements are then transferred to the flange, which protects 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,” says Tribe.
“They lead to a high-load capacity and low axial displacement, which makes for a more efficient and longer-lasting mixer drive,” he says.
Minding the mixture
That these mixer drives are longer-lasting means lubrication of the bearings and gears is very important.
“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 mineral processing industry, is to ensure that the lubricant never comes in contact with the mixture,” Tribe says.
In designing the EBD MC Compact series, SEW-Eurodrive has paid special attention to the lubrication and sealing systems.
Two options are available for lubricating the moving parts of the gearbox, while protecting the mixture from contamination. For the smaller gear units (up to 12,000Nm), 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 to the operator’s panel. This warns the operator that oil leakage is imminent,” Tribe says.
For larger gear units, (16 to 65,000Nm), SEW-Eurodrive has introduced a drywell arrangement to keep the bearing housing free of oil (Figure 5).
The company says the drywell arrangement stops oil leaking from the shaft or bearing housing.
It has an oil reservoir in the main gear housing above the extended bearing housing with 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, which makes 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 us that extra room to incorporate a drywell,” Tribe says.
“From the EBD design concept has resulted a chain reaction of improvements: larger bearings, larger shafts and improved lubrication systems.
“We’ve now have a gearbox specifically tailored to mixer and agitator applications in the mineral processing industry,” he says.