Gear drives deliver power to industrial equipment such as bulk material conveyors, mixers, pumps and paper mills. The reliability that translates into greater uptime and profitability begins by specifying and selecting the proper gear drives for these critical applications. The choice of a gear drive depends on the application, its environment and the physical constraints of the system. The gearbox geometry is defined by four parameters which are determined by the characteristics of the driving and driven machinery:
- Horsepower transmitted
- Speed of the driving gear
- Ratio required (reduction or increasing)
- Arrangement of shafting
Gears can either be obtained as standard components from a manufacturer’s catalog or alternatively specially designed and manufactured. Gear catalogs tend to display only geometric and materials data of stock gears rather than specific operational information. This is because functional behavior will vary with an application and it is not feasible to give comprehensive data covering all operational conditions within a catalog. The operational factors for deciding the type of the gear are:
- Shaft orientation
- Operating environment
- Speed ratio
- Nature of load
- Service factor
- Gear drive rating
- Overhung load
- Gear lubrication
- Gear materials and heat treatment
- Efficiency
- Noise considerations
- Maximum speed
- Power transmission capacity
- Costs
All must be carefully evaluated to make the right decision.
Shaft Orientation
Various shaft arrangements are possible.
- Spur & Helical Gears, when the shafts are parallel
- Bevel Gears, when the shafts intersect at right angles, and
- Worm & Worm Gears, when the axes of the shaft are perpendicular and not intersecting.
- As a special case, when the axes of the two shafts are neither intersecting nor perpendicular, crossed helical gears are employed.
Operating Environment
Check your application and the operating environment.
Contact seals should be used on input and output shafts when the unit operates in dusty environments or where water is splashed around the unit. In atmospheres laden with abrasive dust or in areas hosed down with water under pressure, two contact seals may be required on each shaft. Typically, an enclosure around the gears with oil lubrication is the preferred design, but grease-lubricated open gears can be used in relatively clean environments.
Moisture or high humidity is another concern. A key instance of this is a food processing environment requiring washdowns. In such cases, consider reducers with special epoxy coatings, external shaft seals, and stainless steel shaft extensions and hardware.
Speed Ratio
You arrive at the specific gear ratio by dividing the motor full-load speed by the revolutions per minute (RPM) of the driven equipment. Each gear motor has a reduction ratio that can be selected. Theoretically, there is no limit to the speed ratio that can be designed into a single reduction gearbox, but there is an approximate ratio for each type of gear above where the materials are not being used economically. These ratios are:
| Type Normal Ratio Range | Type Normal Ratio Range |
| Spur 1:1 to 6:1 | Spur 1:1 to 6:1 |
| Straight Bevel 3:2 to 5:1 | Straight Bevel 3:2 to 5:1 |
| Spiral Bevel 3:2 to 4:1 | Spiral Bevel 3:2 to 4:1 |
| Worm 5:1 to 75:1 | Worm 5:1 to 75:1 |
| Hypoid 10:1 to 200:1 | Hypoid 10:1 to 200:1 |
| Helical 3:2 to 10:1 | Helical 3:2 to 10:1 |
| Cycloid 10:1 to 100:1 | Cycloid 10:1 to 100:1 |
For high-speed reduction, two-stage or three-stage construction should be used. For applications with variable frequency drives, exact gear ratios become less important. In such cases, it is best to select the manufacturer’s standard ratios, which are less expensive. Variable or multi-speed applications, however, require special considerations to provide adequate splash lubrication at the slowest speed, without excessive heating or churning at a higher speed.
Nature of load
- A gear drive is one part of a power system that has certain load characteristics peculiar to the specific application. The operating characteristics fall into two load categories: constant torque or constant horsepower.
- Constant torque occurs when load demand varies proportionally with a change in speed. Examples are conveyors, stokers, and reciprocating compressors. Gear drives are basically constant torque machines requiring no selection modifications.
- Constant horsepower implies load demand is constant regardless of speed. Examples are lathes, boring mills, radial drill presses, etc. The gear drive must be selected for the slowest speed at which the motor will deliver its rated horsepower capacity.
- The type of load on the gear drive also depends on the operational characteristics of the prime mover. Electric motors and turbines produce relatively smooth operation, whereas an internal combustion engine does not afford so smooth a load.
Service factor (SF)
Service factors are used to take into consideration intangible operating conditions such as misalignments, vibrations, transient loads, and shocks.
The actual horsepower is multiplied by the service factor to obtain an equivalent horsepower, and the gear unit selected must have a rating equal to or greater than the equivalent horsepower. Typically, this service rating is determined by multiplying the required horsepower by the appropriate service factor based on the equipment, duty cycle, and type of prime mover. An SF value between 1.25 and 2.0 is typically chosen and then multiplied by the motor nameplate power to establish that required by the driven equipment.
Unless otherwise designated, assume manufacturer’s ratings are based on an AGMA defined service factor of 1.0, meaning continuous operation for 10 hours per day or less with no recurring shock loads. If conditions differ from this, input horsepower and torque ratings must be adjusted for specific applications. A higher SF, or larger gear drive size, should be selected when peak running loads are substantially greater than normal.