For industrial gearboxes, the service factor is the numeric value describing gear reducer service duty. It takes into consideration daily operating conditions, load variations and overloads connected with each gear reducer application.
An example from the Bonfiglioli VF-W series of workboxes is shown in the graph below. After selecting the proper ‘daily working hours’ column, the service factor is given by intersecting the number of starts per hour and one of the K1, K2 or K3 curves. K curves are linked with the service nature (approximately uniform, medium and heavy) through the acceleration factor of masses K, connected to the ratio between driven masses and motor inertia values.
Data To Assist Selection
When selecting a gearbox, some fundamental data are necessary to assist the correct selection, namely:
- Prime mover
- Duration of service
- Driven machine load classification
Each of these are usually referenced in the front section of gearbox selection manuals and are used to assist in gaining an understanding of the series and size of gear reducer required. A service factor of between 1.25 and 2.0 is typically chosen, which is then multiplied by the motor nameplate power to establish that required by the driven equipment.
Gear drives must also be sized so as not to exceed the torque limits of the individual components. This peak torque is either listed separately in gear selection catalogues or is a factor applied to the running torque.
In determining the gearbox required for an application by utilising the service factor, be mindful that published service factors can be described as “the minimum recommended”. In particular, applications involving unusual or severe loading, or those requiring a higher degree of dependability, should be reviewed with the gearbox manufacturer.
The American Gear Manufacturer’s (AGMA) practice for enclosed speed reducers contains a listing of applications with their proper service factors. Gear drives, supplied in combination with electric motors, may be designated with a “service class number” (such as I, II, or III rather than a numerical service factor). Class I, II, or III are equivalent to service factor values of 1.0, 1.41, or 2.0. The terms service class and service factor can be used interchangeably.