There are many factors to consider in the selection process of the proper gear speed reducer. Unfortunately, it is not as simple as calculating speed, torque, horsepower, and output RPM. With the introduction of computerized engineering manuals, most gear reducer manufacturers have made it easy to make basic gearbox selections.
The basic selection process is as follows:
Determine the service factor – each gearbox manufacturer publishes service factors for each speed reducer type. Service factors can be found in their engineering guides.
Determine the gearbox (unit) size – This refers to minimum horsepower and is found in the manufacturer’s engineering tables. Based upon the proper service factor, the motor horsepower and RPM are used to select the proper unit size.
Check thrust and overhung loads – Again, the manufacturer’s engineering catalog will have a chart for making this determination.
Check dimensions – After the unit size is selected (and the thrust and overhung loads are determined) the unit’s dimensions must be looked at to determine if the selected unit will fit into the desired application.
Select gearbox accessories – Configuration, mounting type, cooling fans, etc.
While these basic selection guidelines assist in proper speed reducer selection, there are many more considerations that go into the selection process. The manufacturer usually considers the following factors when designing and applying speed reducers:
Cooling, Noise, Radial and/or axial forces on the input and output shafts (for example extruders), External forces on the casing (for example mining), Heavy impacts, or torque peaks, Vibration, Extreme environmental influences, Sealing methods, Reversing, Reversing operation, Back driving and backstops, Clearance and stiffness, Precision, Lubrication, Minimum maintenance, Retrofitting, Arrangement and Accessibility of measuring points.
Bearing Types for Industrial Gearboxes
Rolling element bearings are used almost exclusively for shaft and gear wheel support in industrial gearboxes. The exceptions are in some specialized areas, such as high-speed, high-power turbo-compressor drives, where hydrodynamic plain bearings are used. There are many good reasons for this
dominance of rolling bearings:
1. Good shaft location with minimum radial and axial play enables optimum gear meshing to be achieved.
2. High specific load-carrying capacity with low friction.
3. Wide range of internationally standardized products produced in high volumes at reasonable prices, and good availability.
4. Can be calculated using reliable load-carrying capacity values.
5. Simple arrangement
6. Axially compact so that short and stiff shafts can be used.
7. Normal tolerances and surface finishes for shaft and housing seatings.
8. Less sensitive to misalignment than plain bearings.
9. Standard bearings can accommodate radial and axial loads.
10. Not influenced by the direction of load or rotation
11. Low starting and running torque
12. No starting problems in intermittent operation
13. Relatively easy to lubricate
14. Favorable behavior under emergency conditions
15. Economical maintenance
Lubrication
Gear sets and rolling bearings only perform reliably when they are adequately lubricated. The lubricant prevents inter-metallic contact between gears and rolling elements, raceways, and cages. It also protects gear and bearing surfaces against corrosion. The task of choosing the most suitable lubricant and method of lubrication is made more difficult due to different and varying demands on lubrication that exist within the same gearbox. Generally, the lubrication must not only be appropriate for the gears but also for the bearings.
Additionally, the operating conditions for the individual gears and bearings in a gearbox are often very different. One type of lubrication can be the optimum for high-speed, lightly loaded gears and bearings, but unsuitable for heavily loaded gears and bearings that rotate slowly. The operating temperature, which has a significant influence on the quality of lubrication, is often dependent on the load and speed and is affected by changes in ambient temperature. Generally, only one method of lubrication and one lubricant can be used throughout a gearbox, which results in seldom achieving optimum lubrication for all gears and bearings. To find the best compromise, all the demands regarding lubrication and lubricant properties must be carefully evaluated. Use the manufacturer’s recommendations for lubrication whenever possible.
Grease Lubrication
The most important advantages of grease lubrication are:
1. Good protection against corrosion, as grease, adheres well to the gear and bearing surfaces.
2. The efficiency of seals against external contaminants is reinforced. There is little risk of lubricant leakage.
3. Reliable lubricant supply (particularly when operation is intermittent), as the grease is retained at the gear and bearing position.
4. Freedom from maintenance for sealed, lubricated-for-life bearings.
From this, it is possible to define the main areas where grease lubrication can be employed in gearboxes. It is used mostly acceptable for small units and gear motors, as the gears are also grease-lubricated. Small gearboxes may often be used in varying positions (horizontal, vertical, or inclined at an angle). In such cases, lubricant supply is more reliable if grease is used rather than oil bath lubrication. Sealing arrangements can also be simpler if grease is used. The life requirements are often very moderate for small units, and if they are only used for short periods at a time, they require little to no maintenance. For oil bath lubricated vertical shaft gearboxes, grease lubrication of the upper bearings is common as the amount of oil splashed up is generally inadequate. The grease can be retained in position by baffle plates.
Protection Against Corrosion
Usually gearboxes are well protected against the penetration of water. Nevertheless, the presence of water or moisture cannot be completely prevented, as differences in temperature allow condensation to form. Since any water-induced corrosion in the rolling contacts of a bearing or gears quickly destroys bearing surfaces, only greases with good rust-inhibiting properties should be used.
Compatibility
If, for some reason, it is necessary to change to another grease, ensure the base oil and thickener of the old and new greases are compatible. When a combination of oil and grease lubrication is used (for example grease lubricated bearings and oil-lubricated gears) the lubricants should also be compatible with each other. This is particularly important when synthetic gear oils and mineral oil-based bearing greases are used.
Oil Lubrication
Gearbox bearings are generally oil lubricated when it is simpler to use a single lubricant and the gears are to be oil lubricated. The use of oil lubrication for gears and bearings has the following advantages:
1. Oil can remove heat when gears and bearings operate at high speeds and high temperatures.
2. At very slow speeds and under heavy loads, oil penetrates to the gear’s bearing surfaces more easily than grease.
3. Less maintenance is required in respect of supplying oil to the gear and bearing positions than for grease lubrication, enhancing operational reliability.
4. The intervals between oil changes are longer than grease re-lubrication intervals, particularly for medium and large-sized gears and bearings.
5. Changing oil is simpler than changing grease.
Behavior in the Presence of Air
At moderate, to high speeds, there is a danger of air becoming mixed with the oil. As a result, foaming may occur. Therefore, it is important for oils to have good anti-foaming properties.
Aging
Lubricating oils oxidize as a result of external influences, such as high temperatures and exposure to air. This oxidation is accelerated in the presence of some metals, such as copper or iron (wear particles). Anti-oxidant additives slow down the process. Synthetic lubricating oils are more resistant to oxidation than mineral oils but are not always as good with respect to lubricant film formation. Synthetic oils are used for worm gears because of lower friction and for gears used in a wide range of temperatures (i.e. wind turbines).
Maintenance
Speed reducer gear and bearing maintenance basically consists of monitoring the operating conditions in the gearbox and monitoring the condition of the gears and bearings. This preventive maintenance should enable early identification of any malfunction so that remedial action can be taken. Such action should either prevent premature ending of the bearing service life or, at least enable bearing replacement to be planned.
Monitoring Lubrication
Lubricant supply and lubricant quality should be checked. To check the lubricant supply, simple means are available, such as a sight glass for oil bath lubrication. For circulating oil lubrication on the other hand, complex systems are required to check oil pressure, flow rate, and temperature at each lubrication position. It must also include an alarm system. When choosing the monitoring arrangements lubricant supply reliability should be weighed against the costs that could occur in the event of a blockage. Measuring the temperature in the oil bath, the return duct, and the bearings can monitor oil quality. This allows the operating viscosity to be evaluated. Additionally, regular oil analysis is recommended.
A common recurring problem in gearboxes is particle production. Because many gearboxes use the same oil for both gear and bearing lubrication, wear particles from the gear sets migrate into the bearing assembly, causing premature bearing failure. There are solutions for this potential problem. In a wind turbine for example, more advanced oil systems and/or filters are installed to keep gear wear particles from getting into the bearings.
Monitoring Load
The power consumption of the drive is sometimes used as a measure of load. However, this is not suitable for monitoring bearing loads, as peak loads are very much smoothed in the recording. Better information is obtained by measuring torque, and if possible, stress at the root of the gear teeth. A reliable bearing load measurement can only be obtained by using a special force-measuring bearing equipped with strain gauges. As this method is very expensive, it is generally only used for new developments or during damage analysis.
Monitoring Temperature
An indication of incipient bearing damage occurs quite late by monitoring the temperature. At low speeds, there may be no indication at all. Therefore, measuring temperature is only appropriate for condition monitoring of gears and bearings at high speeds as an indication of trends. Temperature measurements of bearings, gearboxes, and oil are very suitable for monitoring the operating viscosity of the oil. This allows important deductions to be made with respect to the operating conditions.
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Keywords: Gearbox Selection Criteria, Types of Bearings, Lubrication and Performance Monitoring