WHEN a belting man thinks of a motor belt drive, the normal load on it,
the starting load, the peak load, he is likely to think in terms of
effective tension in the belt in pounds. When an electrical man talks
of the load which the motor can start, the load which stalls the motor
or peak loads on the motor, he is likely to speak in terms of torque.
Since the belt man must frequently proportion his belt drive to suit
the performance of the driving motor, he must frequently obtain
information from the data furnished by the electrical manufacturer. It
is there-fore needful that he understand the meaning of the terms
Torque is simply another name for twisting force.
Torque is not a measure of work, power, or energy. Torque is a measure
of a force . . . a particular sort of force, to be sure, a force
tending to produce turning or rotation . . . a twisting force. Torque
is ordinarily measured by the product of the force which tends to
pro-duce rotation (measured in pounds) and the lever arm or leverage in
feet. The lever arm is simply the shortest distance from the line of
action of the force to the center of the shaft upon which it exerts
rotational effect. Many writers, including the most prominent of the
motor manufacturers, state the torque characteristics of their produce
in "pounds at one foot radius." In ordinary engineering practice this
is more often contracted to pound-feet and frequently to foot-pounds.
This latter designation though quite common is to be avoided. To avoid
confusion it is best to reserve the designation foot-pounds for the
unit of work. Torque is occasionally measured in inch pounds. This unit
is now little used except by structural engineers.
In Fig. 14 [i.e., Torque 2] we have three cases of torque produced by
cord wrapped around a pulley.
In (a) the force is 4 pounds, the lever arm or radial 20 distance to
the line of application of the force is 6 inches or / foot, the product
is 4x 1/2 pound-feet torque.
Similarly in (b) the torque is 2 pounds x 1 foot lever arm = 2
It is evident that these three cases involve identical twisting effort
exerted upon the shaft. It is therefore allowable in obtaining a mental
picture or physical con- s ception of the phenomenon to consider all
torques ex-pressed in pound feet as though they were exerted at a 2
radius or lever arm of 1 foot as in Fig. 14-b. That means, that in
order to get a clear picture in our minds we may well think of any
torque expressed in pound feet as though it were a force of just that
many pounds 4 exerted at a radius of one foot from the shaft center.
Matters of torque in foot pounds seldom enter into belting problems,
but such occasions do arise. The district agencies of many of the
principal motor manufacturers have data on the starting torque and the
maxi-mum torque (in pounds at one foot radius) of each type and size of
motor in their line. This data may be furnished in case of a specific
motor about which you or the customer enquire. In cases where the load
starts particularly hard, or the peaks expected are particularly
severe, the purchaser's engineer frequently asks for starting torque or
running torque data in his inquiry and such data in pounds at 1 foot
radius is frequently included in the quotation. It is in such
relatively infrequent cases that data in this form will be available to
the transmission engineer or salesman.
More often he has available only the manufacturer's general data
covering a certain type of motors. Thisdata is not expressed in pound
feet tork. In the motor agency price books of most manufacturers the
descriptive pages, preceding the pages of prices, contain general
statements such as these:
"Maximum running torque or pull out torque will be not less than 200%
of the full load running torque."
In planning mechanical transmission we must not lost sight of the fact
that actually most motors develop somewhat higher torques than the
guaranteed minima. In the exigencies of fitting a complete line of
motors to a limited number of frame sizes a large majority of ratings
have some margin of performance over minimum guarantees, frequently a
With any given size of pulley which may be under consideration, belt
pull is proportional to torque. When a motor exerts 200 per cent of
full load running torque it is equally true that the effective belt
pull will be twice as great as it is when the motor is carrying normal
nameplate horse power.
It will be noted that this, the form in which motor torque data is most
frequently presented, does not involve units of torque at all. It is
simply stated that whatever the torque corresponding to full load on
the motor may be, the torque at starting, the maximum torque, etc.,
bear certain relations to them.
The easiest way to figure problems from data in this form is to
determine the size of belt required for full load horse power, from the
belting manufacturer's tabular data, and then increase the belt width
from a consideration of the application, of manufacturer's torque
guarantees and a knowledge of the amount by which actual motors are
likely to exceed guarantees.
There is one matter in this connection which is worthy of consideration
in the design of drives where the belt speed is high. It is of no
importance for drives where the full load speed of the belt is less
than 4,000 feet per minute.
Above 3,000 to 4,000 feet per minute, the effective pull which a belt
can exert is not quite as great as at lower speeds because centrifugal
force reduces the pressure between the belting and the pulley. Most
motors develop their maximum torque at 2/3 to A full speed. Thus when
the belt must exert the most extreme effective tensions it has the
advantage of somewhat improved grip on the pulley due to this reduction
in speed. If the belt speed at normal operating load is 5,000 f.p.m.
this reduction of / to 1/3 in speed at maximum torque gives an
advantage of 10 per cent to 15 per cent while for a belt operating
normally at 6,000 feet per minute the advantage is about 20 per cent.
If we neglect this minor effect the error is on the safe side.
Another matter which introduces considerable uncertainty into estimates
of maximum torque and starting torque . . . the voltage of the motor.
Voltage is electrical pressure. If a motor is operated at precisely the
voltage for which it was designed, the voltage marked on its name
plate, it will perform about as the manufacturer states. However, if
the motor is operated where the pressure is 10 per cent higher than
normal, this motor will develop 20 per cent more torque at starting and
20 per cent more maximum torque than at normal voltage. Similarly, if
operated 10 per cent below rated voltage, starting and stalling torques
are reduced 20 per cent.
Now most plants carry a voltage at the power house or sub-station 5 per
cent to 10 per cent above the normal voltage for which their motors are
designed, in order that the voltage on the outlying motors may not be
too much below normal. It is generally safe to conclude that the
starting torque and maximum torques of motors in the immediate vicinity
of the power house, such as air compressor motors and the motors
driving the station auxiliaries and coal preparation plant . . . that
for these motors the starting and maximum torques will be 10 per cent
to 20 per cent higher than indicated by the manufacturer's statements,
based on normal voltage conditions.
It is equally likely that the torques of motors in out-lying locations
far removed from the transformers which feed them will have extremes of
10 per cent to 20 per cent below normal. This, however, is not certain,
for tapping up of transformers and such expedients may be resorted to
to avoid it. In general, outside the immediate vicinity of the power
house itself, and considering the heavy current which a motor takes
when starting or approaching maximum load, we may say that usually any
such error is likely to be on the safe side.
As we consider the matter then, we see that an estimate of the starting
torque and maximum torque of a motor, based upon best information which
a belt sales-man or transmission engineer is able to obtain without a
special investigation, involves three uncertainties, one 20 to 50 per
cent, which is not on the safe side, and two, neither of which is
always present, each of 10 per cent to 20 per cent, in a direction
tending to offset the first.
There is some difference between different manufacturers and some
difference between different motors in a manufacturer's line, but it
will be useful to tabulate the starting and stalling torques of normal
motors as now built in the United States. We have seen above that if we
have the manufacturer's figures we cannot approach precision in
estimating such performance. Ordinarily such a tabulation as follows in
the next report will enable us to estimate as closely as necessary for
the purpose of planning mechanical transmission, without obtaining any
special information from the motor manufacturer or his agents.