“Well begun is half done”, a quote that most reference
materials attribute to Aristotle, certainly applies when
selecting mechanical power transmission products. A
selection process that is well thought out at the start can
ensure that the product selected will be properly sized and
appropriate to the application at hand.
Different manufacturers have different names for
them: service factor, safety factor, overload factor, load
factor, etc. They all do basically the same thing: they
factor or adjust for the hours of usage and the nature of
irregularity, shock or vibration in the application. For our
purposes, we will use the term “service factor” as that
seems to be most common across the widest range of
power transmission product types.
Service factors are used as a multiplier applied to
the product’s Horsepower or Torque rating. For example,
a 1HP motor used in an application with a 1.5 service
factor will be seen as having a Design HP or 1.5 HP for
Use of the proper service factor for an application
ensures that the product selected will provide the desired
life in the application as well as tolerate the levels of
shock or vibration that is anticipated in the application.
A few examples will illustrate this point.
Table 1: Service Factors for Gearing
The service factors for open gears such as spur
gears, helical gears or worm gears will typically be
provided by the manufacturer as shown in Table 1.
Looking at Table 1, we can see that the service
factors for gear sizing addresses two key variables: the
number of hours per day the product will be used, and
the variability of shock and vibration there is in the
application. For example, a liquid mixing tank motor
that has a slow acceleration to full speed and very low
variation in load would be considered a uniform load
application. However, a stamping press or die cutter drive
could easily be considered a heavy shock load application.
These factors must be considered when selecting gears
since each situation will apply different levels of stress
and wear on the required gearing.
Some products, such as overrunning clutches, will
require consideration of not just the nature of the load but
the type of prime mover as well. As can be seen in Table
2, there can be an additive nature of an irregular load
combined with the prime mover that will generate spikes
as well. A basic AC motor will provide a much smoother
output to an application than a diesel engine might and
these differences can have a significant impact on the
Table 2: Service Factors for Overrunning & Backstopping Clutches
Products such as V-belt drives will require a
combination of all three factors: variability of the
application load, the hours of service and the output
nature of the prime movers. Other belt considerations
include factors for the use of such devices as idler pulleys
and how they are mounted.
When considering service factors it is important to
think about not just how a product might be used in the
immediate future but further ahead as well. Note again
in Table 1 that there is one factor for a uniform load at
10 hours or less per day and another that is 25% higher
for more than 10 hours per day. Market demand may
require that the system run only one shift now, but in the
future that might change to require a two or even three
shift operation. The gearing unit that provides perfectly
acceptable life in one instance may become a maintenance
headache in a higher usage situation. Worse, upsizing may
require more than just a change in one component.
For example, a worm gearbox selected for 1 HP
20:1 ratio for single shift operation may be a 1.8 in.
center distance but to provide proper life for a two shift
operation that same 1 HP application would require a
2.1 in. center distance gearbox. The height to the output
shaft would change from 2.06 in. to 2.28 in. with the
requirement that bearings, shaft alignment and other
drivetrain components change as well. The difference
between the cost of the initial smaller gearbox and the
cost of the larger unit may be a approximately $150, but
selecting the properly sized gearbox for how the system
is going to be used in the future can avoid very significant
machine redesign later.
Table 3: Coefficient of Sliding Friction for Non-Vertical Loading Applications
So far we have discussed how service factors can
result in upsizing products. When we consider linear
products, such as ball screws or linear actuators, load
factoring can decrease the impact of the load. Vertical
load is always comprised of the weight of the load
plus any force applied by the process. But when linear
movement is horizontal the load is likely supported by
rails or by linear bearings of some type. Depending upon
the coefficient of friction of the support, the load being
moved can decrease by a very sizable amount, as shown
in Table 3.
Industrial component manufacturers have become more and
more sophisticated in providing downloadable or online selection tool
software. These include an automatic calculator that will incorporate
appropriate service factors for your application parameters, as shown in
the online belt drive selection tool screen.
Online Belted Drive Selection Tool Screen
Whether they are called service factors, safety factors or some other
term, it is important to follow manufacturers’ guidance when selecting
a power transmission product. Use of proper service factors will ensure
selection of correctly-sized product that will provide optimal unit life
and performance. Consideration of the entire application for both today’s
need and next year’s can ensure performance now and then. Lastly,
manufacturers’ tools can help simplify the selection process. Nearly all
manufacturers gladly provide application assistance resources to help
you make smart, cost-effective choices.