Torque

“ A force tending to produce torsion around an axis. An example is the
pulling or turning moment of an armature of an electric motor upon its
shaft. It is often expressed as pounds of pull excited at the end of a
lever arm one foot long.
The expression is due to Prof. James Thompson, then of the University of
Glasgow.
"Just as the Newtonian definition of force is that which produces or
tends to produce motion (along a line) , so torque may be defined as that
which produces or tends to produce torsion (around an axis) . ”

“ Thirty-six strands of rubber, weight about 56 grammes, at 150 turns give a torque of 4 oz. on a 5-in. arm, but an end thrust, or end pull, of about 31⁄2 lb. (Ball bearings, or some such device, can be used to obviate this end thrust when desirable.) A series of experiments undertaken by the writer on the torque produced by twisted rubber strands, varying in number, length, etc., and afterwards carefully plotted out in graph form, have led to some very interesting and instructive results. ”

“ A torque may be completely represented by an arrow drawn parallel to the axis of the torque, pointing in the direction in which a right-handed screw would travel if turned by the torque, and having a length which represents the magnitude or value Fl of the torque to scale. Thus the arrow T in Fig. 4 represents the torque due to the two forces FF.
The effect of an unbalanced torque upon a body is to produce a spin-velocity about the same axis as the torque, the amount produced
Fig. ”

“ For maximum torque obviously both resistance and reactance should be equal and as small as possible. Where a small primary current in starting is of considerable importance, this extra resistance is frequently introduced at starting and cut out afterwards, particularly in cases where large torque is necessary. If great starting torque is not necessary, the primary electromotive force is often diminished by inductive resistances, or a change in the connexions of the transformer from which the motor is fed. ”

“ Considering therefore two shafts, T1ω1=T2ω2; that is ω1/ω2=T2/T1; i.e. the angular velocity ratio is inversely as the torque ratio. Hence the higher the speed at which a shaft runs, the smaller the torque for the transmission of a given horse power, and the smaller the tension on the belts or ropes for the transmission of a given horse power. ”

“ Torque, neglecting field distortion, is at a maximum when the current is the greatest possible at the given applied voltage—that is, when the motor is at rest. With a small armature resistance this current is generally far too great for convenience; hence the motors are usually started with a rheostat in series with the winding if the current is not limited by the generator itself. The torque then depends on the sum of the resistances in circuit, and can be made just sufficient to start the motor under the required load. ”

“ The assumption tacitly made hitherto that the torque T remains constant is rarely true in practice; it usually varies from instant to instant, often in a periodic manner, and an appropriate value of f must be taken to suit any particular case. Again it rarely happens that a shaft sustains a torque only. There is usually a bending moment associated with it. For a discussion of the proper values of f, to suit cases where the stress is variable, and the way a bending moment of known amount may be combined with a known torque, see Strength of Materials. ”

“ If, therefore, the motor is mounted on a cradle free to turn about knife-edges, the reacting torque is the only torque tending to turn the cradle when it is in a vertical position, and may therefore be measured by adjusting weights to hold the cradle in a vertical position. ”

“ In the first class of motors the torque is due to the magnetism established in different portions of the motor by currents from the same or from independent sources, and exhibiting time differences in phase. In the second class the torque results from the energizing effects of a current upon different parts of the motor which differ in magnetic susceptibility—in other words, parts which respond in the same relative degree to the action of a current, not simultaneously, but after different intervals of time. ”

“ The torque in these does not change sign with reversal of the current in both field and armature, and consequently alternating current can still produce in them unidirectional torque. Practically the first step toward an alternating current series motor is lamination of the field to reduce parasitic currents; the second is to keep down the reactance. A laminated field motor performs fairly well at a frequency of 10 periods or thereabouts, but to render it useful at ordinary frequencies requires modification in design. ”

“ Yet it was explained by the author [3] on many occasions, that according to relativity theory, this torque causes no change of motion because the material parts of the condenser (like all elastically stressed bodies) require a torque for uniform translatory motion; at this place, only the procedure shall be considered, by which that torque is calculated in both theories. ”

“ In a motor, the direction of the actuating current is the reverse of that of the armature current of a dynamo, consequently, the armature 353 reaction which constitutes a drag, acting against rotation of the armature of a dynamo, becomes a pull in the direction of rotation of the armature of a motor and constitutes its real turning effect or torque which is used at the pulley to do mechanical work. The greater the load applied to the motor, the greater will be the amount of current taken from the supply mains, and consequently, the greater the torque. ”

“ Speed and Torque of Motors.—The speed of an induction motor depends chiefly on the frequency of the circuit and runs within 5 per cent. of its rated speed; it will produce full torque if the line voltage do not vary more than 5 to 10 per cent.
At low voltage the speed will not be greatly reduced as in a direct current motor, but as the torque is low the motor is easily stopped when a light load is thrown on. ”

“ In these motors the motor electromotive force is directly proportional to the output, the torque being constant. They will not start with more than a certain definite load, but once started the speed will increase until added work (internal or external) balances the torque. The type is intrinsically bad in speed regulation, and must be treated by the same methods as are adopted to secure constant current in arc machines. ”

“ An even torque, or great uniformity of rotation, is transmitted to the air-screw by these engines, while the design also permits of such good balance of the engine itself that vibration is practically eliminated. The angle between the two rows of cylinders is varied according to the number of cylinders, in order to give working impulses at equal angles of rotation and thus provide even torque ”

“ It can start a load which requires much more than full load torque at starting, because the motor being nearly up to full speed, has available not only its maximum overload capacity, but also the momentum of the armature to overcome the inertia of the driven apparatus. ”

“ We can compare its field with an angle-lever, its material parts with the shell. Neither the electromagnetic momentum of one part, nor the mechanical momentum of the other part, is parallel to the velocity, therefore both need a torque for their translatory motion. However, since momentum as a whole is parallel to velocity, both momentum components (perpendicular to the velocity) are oppositely equal to one another, and the same holds thus for the torque. ”

“ The arrow S in the vector diagram represents the spin-momentum of the axle and drivers at a given instant, S' represents the spin-momentum at a later instant, ΔS represents the increment of spin-momentum, and T represents the torque which must act upon the axle because of its precession. To exert this torque the outer rail must push up with an excessive force against the outer driver, or, in other words, the outer driver must be forced downwards against the outer rail with more force than that which is due to the locomotive alone as it is rounding a curve. ”

“ The effect of external fields is eliminated by the astatic arrangement of the fields and the moving parts. A field which tends to increase the torque on one side of the armature diminishes it to a corresponding degree on the other side. ”

“ Washout.—Due to torque of the motor, the airplane tends to rotate in the opposite direction to the propeller. This tendency may be neutralized by giving one wing tip a smaller angle of incidence, called “washout,” so that the machine normally tends to neutralize the torque-effect.
[Pg 67]
PRINCIPLES OF AIRPLANE EQUILIBRIUM Fig. 28.—Balances of forces in an airplane.
Weight forward of lift, thrust below resistance. Thrust equals resistance, weight equals lift. ”

“ The torque of the motor renders it advisable for a novice to turn his machine to the right, if a right-hand propeller be used, and vice versa. If two propellers, turning in opposite directions, are employed, as in the Wright biplane, there is no inequality from the torque of the motor. ”

“ The passage of the alternating current through the armature winding produces a magnetic field that rotates about the armature core, and induces in the pole pieces eddy currents, which, reacting on the armature, exert a sufficient torque to start the converter from rest and cause it to speed up to synchronism. ”

“ The effect is rather startling since the motor may be giving a very great torque when it is merely turning over at a few revolutions per minute; and although the process is complicated, it leads to excellent results, and is widely used where delicate speed regulation is required. ”

“ There is considerable fluctuation of the torque in each revolution of the engine with five cylinders, but this fluctuation of torque is more easily smoothed out by the use of very light fly wheels than by increasing the number of cylinders, and thus adding to the complication of the engine. ”

“ They have rectangular holes punched near their outward periphery, through which the armature bars pass, and to which they are soldered. The internal resistance armature, fig. 1,778, is provided with a phase winding, starting (internal) resistance, and switch located on the shaft. The starting resistance is designed to give approximately full load torque with full load current at starting. A greater torque than full load torque can be obtained for starting, if required, by cutting out resistance. ”

“ Under these conditions, when the field of a motor is at its maximum strength, the motor torque will be at a maximum for any given strength of current, and the reverse electromotive force will also be at a maximum for any given speed, therefore, when the field strength is increased the speed will decrease and vice versa. ”

“ In an electric motor the forces acting on the conductors produce the mechanical output of the machine. In a generator these mechanical forces come into existence as soon as current is taken from the machine. In this case they produce a torque which is opposite in direction to the mechanical torque which is applied to the shaft of the generator in order to drive it. ”

“ The twist or surface-shear being proportional to the torque, the horse-power can be calculated if the modulus of rigidity of the steel employed is known or if the amount of twist corresponding to a given power has previously been ascertained by direct experiment on the shaft before it has been put in place. ”

“ In a four-cylinder engine there is, as with one or two cylinders, a reversal of the torque; that is, just before the end of each stroke the flywheel not only supplies all the power delivered by the engine, but also some of the power necessary for keeping the crankshaft and pistons in motion. Six cylinders are the smallest number delivering continuous torque at the crankshaft; but while continuous the torque is still far from being uniform ”

“ It is suitable for constant speed service where full load torque at starting does not exceed 140 per cent., and in general is adapted to drive all geared and belted machinery requiring constant speed with light or moderate starting torque. ”

“ Thus a series or separately excited motor can develop proportionately more torque in overcoming an increased load than it was developing before the load was enlarged. This effect may be increased by arranging the field magnets with a large amount of iron in their cores, and working under ordinary circumstances with a small magnetic density. ”

“ A torque about 5 inches in diameter, described as of copper, made of a simple wire, with the ends turned back so as to form hooks, and on each a lenticular button of metal, was found near Winslow, Bucks, [1467] and may also be Late Celtic.
Another form of torque is made from a stout wire expanding into small flat discs at the end, a type which is also common among bracelets both in bronze and gold. A torque of this kind, together with a bracelet, is shown in Fig. 471, kindly lent by the Council of the Society of Antiquaries. ”

“ These are presented to the explosion chambers, receive the impact of the explosion and then, running through an expanding groove, allow the charge to continue expanding and applying power till the groove ends in an open slot which instantly cleanses the cylinders or pockets of the burnt gases. By this arrangement there is only a twentieth part of the engine wheel where no power is being imparted, thus giving practically a continuous torque.”
“How’s ‘torque’?” laughed Stewart as he inserted a fresh sheet of paper in the typewriter. ”