Helical Gearbox Service Factor

I am struggling to get a handle on what 'Service Factor' means in terms of gearbox selection. I have managed to glean that there is a difference in meaning between American and ISO / DIN standards but am still unsure of what it relates to. Is it Power? Torque? Radial and Axial bending loads that the gearbox will withstand? or a combination of all?
How does it relate to gearbox selection?
Specifically I am interested in the application of large (150hp) mechanical surface aerators where service factors of 2 and above are typically quoted.
Can anyone please help?

 

[diamondjim]

Service factors are rough guidelines
that the individual industries apply
relying on past experience. The way
things fail actually follow the Weibul
Curve. If you can afford to replace
the gear box often and not expect the
full rated lifetime it is your option.
I do not think you have to worry about
the static side, since I assume it would
not be a dangerous situation if it failed.
Again are you looking for reliability or
at just the cost factor? Also consider
the cost of disassembly and installing the
new one. Anxious to hear from others.
What industry are you in?

 

Thanks for your input diamondjim.
I work in the Waste Water Treatment Industry.

I am more interested in achieving long trouble free life from the gearboxes rather than minimum capital cost - to a point.

Mechanical Surface Aerators have large overhung loads from their inherent design which usually comprises motor and gearbox sitting on a structure some 5 ft above the water where a large 10 ft diameter rotor rotates at the water surface at about 50rpm. The rotor (and all reactions thereof) are supported and reacted within the gearbox.

Is the high bending moment (developed through high radial forces at the water surface) have a direct impact on the size of the gearbox with respect to service factor, or is it dealt with separately by adding drop box bottom bearings (for instance) to increase the stiffness of the output shaft assembly? Hence the question in my previous submission.

 

[diamondjim]

You are lucky in that you probably see contant
loads with very little peaks and valleys and
also assume that you have constant rotation
in one direction. Wastewater treatment plants
have been around a long time and information
should be available. I know gear loads are
high and most take the max gear loads to possibly
60 percent times the tensile or near the plastic
load range and seem to be operating successfully.
I have always been a little surprised at the rating
of gear drives for the waste treatment bearings.
I hope you are taking advantage of the long addendum
or recess action gearing for this type of application.
I do not know that AGMA takes this into account, but
most have adopted this technology which also allows
for smaller pinions to be used as they are not
undercut and are stronger. Lubrication is a big
factor in life in these situations as you probably
are aware. Hopefully the gears are covered. Another
big plus in starting out with clean steel either vacuum
degassed or vacuum deladled to get a better grade of
steel. When I was in the bearing industry, we found
lifes increasing up to a 5 times increase only because
of the cleaner steel.

 

[GregPerry]

The service factor is related to the power. For a agitator or mixer application you need to check the stress in the low speed shaft, the gearbox manufacturer should be able to submit calculations. In some instances you may need to go to a larger gearbox to cater for very high bending moments on the low speed shaft. Most manufactures offer a standard mixer shaft, a reinforced shaft (thicker) and/or an extended bearing housing.

I recommend a service factor of 1.7 on absorbed (or design) power and 1.5 on installed power for 24 hour operation. Do check both conditions and use the larger. Check shaft stresses and dont forget to check the thermal rating of the gearbox to see if it does not require a fan.

 

[SAP]

TibbsB
There are several factors which go into the calculation of the rating of a gearbox. These things typically include bearing life, overhung loads, shaft torque capacity, gear tooth rating, housing strength, thermal loads, etc. Usually the gearbox manufacturer will specify the required service factors to apply to their gearbox to suit a particular application. Despite what the standards say, you will find that each gearbox manufacturer has slightly different methods for rating their own gearbox design.
My advice to you is to contact a gearbox manufacturer, give them your application details and let them suggest the appropriate gearbox. (Try

http://www.falkcorp.com)

Good luck
SAP