US2381325A - Damped drive-mechanism - Google Patents

Description

Aug. 7, 1945.

' A. M. WAHL DAMPED DRIVE MECHANI SM Fi-led Nov. 5, 1941 WITNESSES:

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ATTORNEY Patented Aug. 7, 1945 1 UNITED STATES PATENT OFFICE Arthur M. Wahl, Wilkinsburg,'Pa., assignor to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania Application November 5, 1941, Serial No. 417,886

2 Claims.

My invention relates toa driver-and-load combination in which the driver or .prime-mover, or the load, or both, are subject to torsional oscillations or hunting, superimposed upon the steady rotational movement of a shaft; and my invention particularly relates to a damping-means which is attached to the rotating part, preferably at a point approximating the point of maximum oscillatory movement, for damping such oscillations.

My invention, `while vol? broad application in other situations, was especially designed for an induction-motor fan-drive in which a large centrifugal fan, having an inertia several times larger than that of the motor, was driven by an, electric motor having a speed-torque curve of a type in which the torque increases with increasing speeds;

over a certain speed-range at which the motor is intended to operate for material periods of time, so that, if the speed should momentarily increase, for any cause, the torque would also increase, .tending to still further increase the speed, thus producing what is known as negative damping, resulting in oscillatory hunting or speed-variations above and below an average or steady-state speed. The particular motor which was used in the'installationfor which my invention was first and other vulnerable portions of the drivingmechanism, and these excessive torques were believed to be considerably in excess of the safe fatigue-1imits of the materials, so as to'seriously endanger the life of the apparatus as a result of the probable failure thereof, due to fatigue, if this torsional vibration were not very materially reduced.

' to provide a damper utilizing a filling of a lowdeveloped was an eillcient or low-slip 60-cycle motor having a low-resistance squirrel-cage winding, this motor being utilized to drive a heavy centrifugal fan through long shafting and speedreduction gearing, and requiring an unusually long period of time to bring the load up to speed during the starting-period, because of the high inertia of the load, -this starting-period lasting for a material period of time, which was from some 25 to 44 seconds, in different applications of the drive. Such induction-motors are well known to have a speed-torque characteristic in which the torque-speed curve is sharply inclined in the range from-the vicinity of half-speed up to the pull-out point, which is fairly close to the synchronous speed, after which the torque fallsl oil! 'to zero at full synchronism, and the motori Thisv viscosity liquid. which is moved back and forth, as a result of the torsional oscillation, and is caused to flow through an oriiice,in which damping is produced approximately in accordance with the square of the velocity of the liquid, and substantially independent of the viscosity, and hence the temperature, of the liquid.

With the foregoing and other objects in view,

. my invention consists in the parts, combinations,

apparatus, and methods hereinafter described and claimed,l and illustratedin the accompanying drawing, wherein:

Figure 1 is a somewhat diagrammatic view of a motor-drive combination in which my invention is utilized, y

Eig. 2 is a transverse cross-sectional view through my damper, and

Fig. 3 is a longitudinal sectional view thereof, on the broken plane indicated by the lines III-III in Fig. 2.

In Fig. 1,v I have illustrated my invention as being applied to a motor-drive combination in which a heavy centrifugal fan 4, having a very considerable rotative inertia, is driven from a coupling 5 and a long shaft '6 having considerable torsional resilience, from a reduction-gearing l which is illustrated as a gear-'wheel 8 which isl carried bythe shaft 6, and a pinion 9 which is carried by a drive-shaft Il. The drive-shaft Il. is driven, at any one ofa plurality of speeds, by either a large high-speed motor M, whichy is coun pled thereto at I2, or a small low-speed motor M, in the same installation, was rated at 40 horset power at 1170 R.. P. M., this large motor M being an eiiicient six-pole, GO-cycle, squirrel-cage, induction-motor having a low-resistance squirrelcage winding for causing the motor to operate at high emciency and a low slip. This motor had the, negative-damping torque-speed characteristic, as previously described, which was particularly evident at speeds in the vicinity of twothirds f the full speed.

In the installation just described, the centrifugal fan 4 had a moment of inertia many times greater than that of the large motor M, so that any torsional oscillations which were set up as a result of the negative damping in the motor became apparent at the shaft I4 of the large motor M, rather than at the shaft I5 of the fan 4. Inl one such installation, the frequency of the hunting which developed during the starting period was of the order of 3.7 to 3.9 cycles per second, while in another installation of somewhat different size, the hunting-frequency was of the order of 5.7 to 5.9 cycles per second.

My present invention consists in the application, to the motor shaft I4 of a motor-driven combination such as that just described, of a small damper I6 which is coupled to the driveconnection which extends from the motor M to the fan or load 4,- being connected to said drivemechanism at a point close to the motor, or, in general, at a point close to the member 'having the smaller inertia. In the illustrated case, this means that the damper i6 is coupled to the motor-shaft I4.

As shown in detail in Figs. 2 and 3, my damper fle comprises an annular housing I1., lcomprising an annular cup-shaped housing-member having an outer cylindrical wall I8, an inner cyrespect to each other that the damper-member has a naturalfrequency of the oscillatory weightmovement which corresponds approximately t0 the frequency of the` most objectionable oscillatory shaft-movement components which are to be damped. In this manner, the amplitude of the arc through which the weight-member 24 oscillates is several times larger than vthe arc or amplitude of the oscillatory movement which is superimposed upon the steady rotational movement of the motor-shaft I4; and means are provided, in accordance with my invention, for interposing some sort of resistance to this relative amplified movement of the weight-member 24 with respect to the housing-member I1 of the damper I6. This movement-opposing resistance can be just plain friction, which would be fairly constant, or viscous oil-damping, which would vary approximately as the velocity of the relative movement of the weight-member with respect to the housing-member, b'ut I believe that very much better results are obtainable with a movement-damping means which develops a movement-opposing force which is proportional to, or nearly vproportional to, the square of the relative velocity of the weight-member 24 within the housing-member I1; and I find that a very d esirable means for obtaining such a squared-velocity damping is by means of a filling of a light oil, or other low-viscosity liquid, which is shown within the spaces 26 within the annular housing-member I1. The lpartition-members 21 or the weight-member 24 or both, are provided with orifices 32, as shown in connection with the partition-members 21 in the illustrated embodiment of my damper.

If the viscosity of the damping liquid is low,

l such as is obtainable in. a very light oil, the relindrical wall I9 which surrounds the motorshaft i4, and which is keyed thereto at 2l (Fig. 2), and a radially extending annular disk-member 22 which joins the inner and outer cylindrical members I9 and I8 together in a rigid, unitary structure. The open side of the cupshaped annular member is adapted to be closed by a removable, annular disk-member '23 which is bolted thereto. Disposed within the annular housing I1 is a rotatably, mountedweight-member 24 which'is illustrated as having two cutaway portions 25 therein for providing a space 26 within the outer cylindrical wall I8 of the housing I1. In the center of each of the spaces 26, there is disposed a partition-member 21 which is screwed to the outer cylindrical wall Il at 28 (Fig. 2).. and which serves as an abutment for two springs 30 which resiliently center the weight-member 24 with respect to the partitions .21, while permitting the weight-member to have a certain yieldable oscillatory movement in response to the inertia-forces resulting from the oscillatory component of the rotating movement which is imparted thereto by the motor-shaft I4.

In the damper-member I6, the mass of the weight-member 24, and the resilience or springconstant of thel springs I0 are so chosen with sistance to the movement of the liquid through the orices 32 will be substantially proportional to the square of the velocity of such movement,

and substantially unaffected by the viscosity, so that any changes in viscosity which are produced by temperature-changes in the oil will have practically no effect upon the damping, which is a very desirable feature. The arrangement is such that the relative oscillatory movement of the weight-member 24 within, and with respect to, the annular housing member I1, produces a back-and-forth movement of the oil causing the latter to flow back and forth through the orifices 32, thus damping such movement. At very low amplitudes of oscillation of the weight-member in the damper, the frlctional damping, due to friction between the weightmember and the housing, and also that due to the viscosity of the oil, predominates over the velocity-squared damping du'e to the orifice. Of course, friction can be to a large extent reduced by using anti-friction bearings between the movable weight-member and the housing.

In the design of my damper, it is desirable that the orice 32 be of sufficient size to permit thev weight-member to have'a considerable amplitude of oscillating movement, so that its movement will be considerably larger than theoscillating component of the rotary shaft-movement.

This is so, because there can be no damping unless there is weight-movement, and it is desirable to make the damper so that a small oscillatory movement of the shaft will produce the amount of, weight-movement necessary to produce positive damping to overcome the negative damping in the motor M.

In actual operation, I have found my damperaasnsss means I6 to be so effective that a small damper,

of 12 inches diameter, is suflicient to reduce the jectionally large torsional oscillations of a particular frequencyrange, superimposed on a rotating-movement, over a particular speed-range at which the rotating-shaft means is intended to operate for material periods of time, and dampermeans coupled to the shaft, said damper-means comprising an annular` housing coupled vto the shaft, oscillatably movable weight-means within said annular housing, spring-means for yieldably resisting relative movement between the weight-- means and the housing, a lling of a low-viscosity Vliquid in said housing, and partition-means carried by the housing withinl said liquid, the .weightmeans being adapted to cause a movement of the liquid within said damper-housing as a result of the oscillatory movement of the weightmeansA relative to the housing, the partitionmeans being adapted to resist such movement of the liquid, and the system of weight-means and partition-means providing, in elect, orifice'- means of such adequate dimensions as to permit orifice-restricted liquid-flow in sulcient amounts, in both directions, to permit a relatively large amount of oscillatory movement of the weightmeans near the frequency-range of said torsional oscillations, the viscosity being so low and said o rice-means having. such orifice-opening as to damp said oscillatory movement with a force which varies with the velocity of movement at a rate greater than the first power of said velocity under intended operating-conditions, of the dampermeans, said weight-means and spring-means having a natural frequency of resonance approximating'the principal frequency of the torsional oscillations to which the shaft is subjected,

whereby the amount of angular movement of the weight-means relative to the damper-housing-is greater than the amount of the superimposed angular. oscillation which is transmitted from the shaft to vthe damper-housing. v

2. Damper-means adapted to be mechanically coupled to a shaft subject to torsional oscillationssuperimposed on rotating-movement, said damper-means comprising an annular housing adaptedto be coupled to the shaft, oscillatably movable weight-means within said annular housing, spring-means for yieldably resisting relative movement between the weight-means and the housing, a filling of a lowviscosity liquid in said housing, and 'partition-means carried by the vhousing within said liquid, the weight-means being adapted to caus'e a movement of ithe liquid within said damper-housing as a result of the oscillatory movement of the weight-means relative tothe housing, the partition-means being adapted to resist such movement of the liquid, and the system of weight-means and partitionmeans providing, in eect, orifice-means of such adequate dimensions as to permit orifice-restricted liquid-now in sufficient amounts, in both directions, to permit a relatively large amount of oscillatory movement of the weight-means under predetermined operating-conditions, the viscosity being so low'and said orice-means having su'ch orifice-opening as to damp said oscillatory movement with a force which varies with the velocity of movement at a rate greater than the first power of said velocity under intended operating-conditions of the damper-means, said weight-means and spring-means having a natural'freq'uency ofresonance approximating the principal frequency of thel torsional oscillations to which the shalt..,is subjected, whereby the amount of angular, movement ofthe weightmeans relative to the damper-housing is greater than the .amount of the superimposed angular oscillation which is transmitted from the shaft to the damper-housing.

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