Document 43: Torque. What is It?

Copyrighted and published in 1933, this is a article on torque is Report Number 5, one of Fifty Reports on Mechanical Power Transmission from Motor Drive to Industry.
The transmission of mechanical power from electric motor to driven machine ... by belting and other means. The problem being considered fro the standpoint of design, maintenance, trouble elimination, comparative production and comparative costs, under a wide range of conditions and applications.

Robert W Drake, E.E., M.E.

New York: American Leather Belting Association

Torque. What is it? 

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 ordinarily employed.

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.

torque 2

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 pound-feet torque.

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:

torque 1

Maximum Torque

"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 large margin. 

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.