US2788758A

US2788758A - Sewing machines

Info

Publication number: US2788758A
Application number: US339112A
Authority: US
Inventors: John G Attwood; Schoij Edgar
Original assignee: Union Special Machine Co
Current assignee: Union Special Machine Co
Priority date: 1953-02-26
Filing date: 1953-02-26
Publication date: 1957-04-16
Anticipated expiration: 1974-04-16
Also published as: DE1012809B

Description

April 16, 1957 J. G. ATTwooD ETAI- sEwING MACHINES Y 9 Sheets-Sheet l Filed Feb. 26, 1953 April 16, 1957 J. G. A'rTwooD ETAI- 2,788,758

' SEWING MACHINES Filed Feb. 26, 1953 9 Sheets-Sheet 2 Apil 16, 1957 .1. G. AT-rwoon Erm.

SEWING MACHINES 9 sheets-shim s Filed Feb. 26, 1953 April 16, 1957 J. G. A'r'rwoon E'rAL SEWING MACHINES Filed Feb. 26, 195s 9 Sheets-Sheet 4 April 16, 1957 FiledFeb. 26. 1953 FIG.9

J. G. ATTwooD EI'AL SEWING MACHINES 9 Sheets-Sheet 5 April 16, 1957 |.G.AT1-wo b Erm, 2,788,758

A SEWING MACHINES Filed Feb. 26, 1953 9 sheets-sheet e J. G. ATTWOOD ETAL Apfil 16, 1957 SEWING MACHINES 9 she'ets-sneet 7 Filed Feb. 26,. 1953 .E .mi

y April 16, 1957 J. G. Ar'lfwoon ErAL SEWING MACHINES 9 sheets-'sheet a Filed Feb. 2e, 195s Qw ma.

April 15, 1957 J. G. A'rTwooD ETAL SEWING MACHINES 9 sheets-sheet s Filed Feb. 26g 1953 nnb b. IMM" m a F v i... 1v N N N ,r/ 7,. an. Y, w. w m W w United States Patent SEWING MACHEES John G. Attwood, Oak Park, and Edgar Schoij, Chicago, lll., assignors to Union Special Machine Company, Chicago, lll., a corporation of Illinois Application February 26, 1953, Serial No. 339,1i2

12 Claims. (Cl. 112-220) The present invention relates to sewing machines and more particularly to provisions for the damping or absorbing f torsional vibrations in high speed sewing machines.

lt has been found that in the operation of sewing machines at high speed there is a tendency to set up torsional vibrations in the rotating shafts which serve to drive other machine elements back and forth at high speeds. When these shafts are rotated, they are subjected to twisting forces alternately in opposite directions and when they are rotated at or near certain speeds, generally termed critical speeds, they become dynamically unstable because of resonance eiects as a result of which relatively large torsional vibrations are likely to develop. The creation of such vibrations is objectionable not only because of the noise which results but also because of the excessive wear of the parts and the dillculties encountered in the handling of the work and in the proper formation of stitches.

There are two principal types of forces which give rise to torsional vibrations in the rotating shafts of the machine. One of these is the inertial type which is due to the mass and acceleration of parts having reciprocatory, oscillatory or intermittent movement. The other is the impact type which arises when moving members, because of clearance tolerances, strike their confining members. Impact forces arise also from the striking of certain parts, such as the feed dog and presser foot, in the performance of their function. ln a high speed sewing machine, torsional vibrations may arise at a number of points. For example, in moving a needle bar down and up, the rotating driving shaft starts the needle bar from a standstill at the top of its stroke, the force required for this purpose serving to twist the driving shaft in one direction. At the bottom of the stroke, the needle bar is slowed to a stop, thereby twisting the shaft in the other direction. Two additional twists are imparted to the driving shaft in moving the needle bar from the bottom to the top of its stroke, so that the driving shaft receives two alternate twisting forces for each revolution thereof. These inertial twisting forces imparted to the shaft give rise to torsional vibrations. Similar torsional vibrations may be set up in the shaft by impact forces which arise when the needle bar and the associated moving parts strike their confining members in taking up the tolerances between the parts.

If the twisting or vibratory frequency coincides with a natural resonant or critical frequency of any independently vibratable portion of the driving linkage or ofthe driving linkage as a whole, the amplitudes of the resulting torsional vibrations will be greatly amplified, tending to cause extreme deections and resulting in objectionable noise, wear, loosening of screws and even breakage. of parts, and in; poor sewing. The torsional vibrations may e considered as oscillating components. of motion superimposed on the normal rotary motion ofthe shaft or other machine element. NormallyV a shaft and its associated driving connections are suliicientlystiff that. the resonant or critical frequency thereof corresponds to a speed substantialiy higher than the normal operating speed of the machine. However, when a belt is used as a part of the driving connections, the exibility thereof is sulflciently great that the resonant or critical frequency of the system, comprising the shaft and the driving connections, is likely to fall within or adjacent the operating speed range of a high speed machine. Such coincidence or nearness of the operating speed range and the critical or resonant frequency sometimes occurs in sewing machines employing gears or other relatively rigid inter-connecting means between the shafts. Furthermore, since the vibrations set-up by the motion of a driven part, such as a needle bar, are made up of a fundamental frequency and a number of harmonic frequencies, there is a possibility that one or more of these harmonic frequencies may coineide with the resonant frequency of the driving connections.

In many sewing machines, the torsional vibrations set up in the top rotary shaft by reciprocation of the needle bar are much greater than similar vibrations set up in other parts of the machine. However, such vibrations are present elsewhere in a sewing machine, and while they may not produce undue noise, they do nevertheless tend to produce Wear or loosening of screws and other partsv and to interfere with proper sewing, so that it is frequently desirable to suppress these vibrations as well as the vibrations set up in the top rotary shaft. For example, undesirable torsional vibrations are frequently set up in rotating elements included in a sewing machine looper actuating mechanism or feed dog actuating mechanism.

A primary object of the present invention has been to provide means for suppressing torsional vibrations that tend to arise in various parts of high speed sewing machines. Toward this end, appropriate damping means are provided at oney or moreselected points.

The selection of a particular damping means for a given location in a sewing machine depends upon a number of factors such as the type and severity of vibration encountered, the frequency or frequency range of vibrations involved, the accessibility for adjustment, they space available and other like considerations. The number and location of damping meansY to be' employed in a particular sewing machine depends upon similar considerations. While satisfactory damping may be achieved in many cases with a single suitably placed damper or absorber, in other cases two or more may be desirable or necessary. rlhe locationv and construction of suitable damping means, as well as the reasons for the location and construction employed, will be apparent from the d" eussion hereinafter. The term damper, as used in the specification and claims herein, is synonymous with or includes the term absorber.

A feature of the invention residesI in the provision at one or more appropriate points of a uid restrained disc type damper, suitable for use in high speed sewing machines, in which axial movement of the inertia element is suppressed.

Another feature of the invention has been the provision of a bitilar pendulum type damper suitably arranged with respect. toY a. sewing machine needle bar actuating crank toy suppress torsional vibrations in the needle bar driving shaft and associated driving connections- A further feature of the invention has been the provision of damping means. for suppressing belt whipping or flexing in high speed sewing machine having a belt connection betweentwo rotary shafts.

Still another feature ofthe invention resides in the provision of a hysteresis type damper for suppressing torsional vibrations in a rotating or oscillating shaft of a high speed sewing machine.

Other objects, features and advantages of the invention Ywill appear from the following detailed description of Y certain illustrative embodiments of the same. .'In accordance with the invention a sewing machine having a plurality of cooperating stitch forming and work feeding elements, driving connections therefor and means in the driving connections serving to apply periodic reversals of torsional force to one or more portions of the driving connections, thereby tending to create torsional vibrations therein, is provided with damping means associated with at least a part of the driving connections for suppressing the torsional vibrations.

The invention will now be described in greater detail with reference to the appended drawings in which:

Fig. 1 is a longitudinal, vertical sectional view of a sewing machine constructed in accordance with the invention;

' Fig. 2 is an end elevational view of the sewing machine of Fig. 1 as viewed from the left;

Fig. 3 is a horizontal sectional view taken just below the work supporting surface of Fig. 1;

Fig. 4 isV an enlarged detail view, taken substantially along the line 4 4 of Fig. l, showing a bifilar pendulum damper according to the invention incorporated in the needle bar crank as a part of the counterweight therefor;

Fig. 5 is a sectional View taken along the line 5--5 of Fig. 4;

Fig.V 6 is an enlarged sectional View of a portion of the machine shown in Fig. 1, illustrating a disc type torsional vibration damper constructed in accordance with the invention and associated with the upper shaft belt sprocket;

Fig. 7 is a face View of one end of the disc type torsional vibration dampers of Fig. 6;

Fig. 8 is an enlarged sectional view of another portion of a sewing machine similar to the one shown in Fig. l but with a frictional and dynamic type damper associated with the hand and belt wheel on'the lower shaft, in place of the disc type damper shown in Fig. l;

Fig. 9 is a face view of the frictional damper of Fig. 8 with the cover plate partially broken away;

Fig. 10 illustrates a modified, partially frictional damper arranged to connect the hand and belt wheel to the lower shaft;

Fig. 1l is a detail view, partially in cross section, taken `along the line .l1-1l of Fig. 10; v n

Fig. l2 is a longitudinal, sectional view through the overhanging arm of a machine similar to that shown in Fig. l and illustrates a hysteresis type damper associated with the upper shaft of the sewing machine;

Fig. 13 is a View, partially in plan and partially in cross section. showing a flat bed sewing machine, different from the machine of Fig. l and illustrating a hysteresis type damper associated with an oscillating shaft mounted at the rear of the overhanging arm;

Fig. 14 is a sectional view taken along the line 14-14 of Fig. 13;

Fig. 15 is a fragmentary view, partially in elevation and partially in cross section7 illustrating one end of the hysteresis type damper of Fig. 13;

Fig. 16 is a vertical, transverse sectional View through the vertical standard of a sewing machine similar to the oneY shown in Fig. 1, and illustrating means embodying the principle of hysteresis damping, as applied to a power transmitting belt. for suppressing torsional vibrations;

Fig. 17 is a fragmentary view, partly in longitudinal section and partly in cross section, of a driving and damping belt arrangement according to the invention, and constituting a modification of the arrangement shown in Fig. 16; and

Fig. 18 is a vertical, transverse sectional view through the vertical standard of a sewing machine similar to the one shown in Fig. 1 and illustrating means for reducing belt deection and suppressing resonant torsional vibrations in accordance with the invention. Y

Referring now Vto the drawings and more particularly to Figs. l, 2 and 3, the invention has been illustrated as embodied in a flat bed sewing machine of known type. It should be understood, however, that the invention is adaptable to substantially any form of high speed sewing machine.

As shown in Figs. l, 2 and 3, the sewing machine comprises a hollow base having a bottom wall 20 and a top wall or cover 21 on which is supported a cloth plate 22. The hollow base contains an oil reservoir 23 and carries certain elements of the stitch forming and work feeding mechanisms which areV outside of the'enclosed reservoir. The frame of the sewing machine includes a hollow vertical standard 24 which communicates at its lower end with the hollow base, outside of the reservoir 23, and which has integrally formed therewith a hollow overhanging arm 25 which terminates in a needle head 26. Y

Au upper or needle bar driven shaft 27 of the machine extends through the overhanging arm 25 and is journaled in bearing

members

28 and 29 located, respectively, adjacent opposite ends of the overhanging arm 25. A belt pulley 30 is mounted on the end of the shaft 27 within the hollow standard 24, this pulley being rigidly affixed to the shaft 27 by means of a set screw 31 provided in a hub portion 32 of the pulley. A needle bar 33 is mounted for vertical reciprocation in

bushings

34 and 35 provided in the needle head 26. At its lower end the needle bar 33 is provided with a needle 36. To reciprocate the needle bar, a crank member 37 is mounted on the end of the shaft 27 within the needle head 26. A crank pin 3S carried by the crank 37 provides a pivotal connection with the upper end of a pitman 39 which, at its other end, is pivotally connected to the needle bar 33 by means of a pin 40 extending from a split collar 4l clamped to the needle bar. The arrangement is such that one complete reciprocation is imparted to the'needle bar 33 with each revolution of the shaft 27.

The belt pulley 30 is connected to a belt pulley 42 by means of a toothed or ribbed belt 43. Pulley 42 is mounted on a lower shaft 44 and is afiixed thereto by means of a set screw 45 provided in a hub portion 46 of the pulley. The shaft 44 is supported within the base of the machine by suitable

bearings including bearings

47 and 48. One end of the shaft 44 extends beyond an end wall 49 of the base and has axed thereon, by Ameans of a set screw Si), a combined hand and belt wheel 51 by which the machine may be operated either manually or by power. An oil slinging disc 52 is mounted `on the shaft 44 so that the periphery thereof dips into the oil within the reservoii 23. Through rapid rotation of the disc 52 during operation of the sewing machine, oil is dispersed in the form of a mist within the reservoir portion 23 of the hollow base and is also thrown upward through a hollow tube or passage 53 within the standard 24, thereby supplying lubricating oil to bearing 29 and,

through a tube 54, to the bearing 28 and the moving Y elements within the head 26.

A looper 60 is disposed within an accessible portion of the hollow base, outside of the enclosed reservoir, and is arranged to cooperate with the needle 36 in the formation of stitches in well known manner. The loop taking and needle avoiding movements of the looper 6d are provided through a mechanism actuated from lower shaft 44. This mechanism includes an eccentric (not shown) on the shaft 44 and a pitman 61- having a strap at its upper end surrounding the eccentric and connected at its lower end, through a ball and socket connection 62, to a horizontal arm 63 carried by a rock shaft 64, within the reservoir 2.3.V This shaft passes through the front wall of the base and has secured to its outer end an upright arm 65 which, through an extensible pitman rod 66, is connected by a strap 66 to a ball stud 67 mounted on a carrier 67 for the looper 60. The carrier 67 is arranged to swing on a stud 68 lextending laterally from a` sleeve- 69 which is ath-nedto a rock shaftv 7) (Figs. 1 and 2)- extending parallel to the lower shaft 44. An eccentricv (not shown) on the shaft 44 serves to rocl; the shaft 7@ in properly timed relation. The swinging of the carrier 67 about its pivot 68 imparts loop taking and shedding movement to the looper 69. Through rocking of the shaft 79, looper 66 is given its needle avoid movements.

The work feeding devices may suit-ably be of the character disclosed in the patent to Peterson et al. No. 2,577,430, granted December 4, 1951. lt may comprise a feed dog 71 (Fig. l) carried by a feed bar 72 (Fig. 2.) pivotally mounted on a feed rocker 73'. The latter may be rocked by connections, including an arm 74 and a pitman 75, from an. adjustable eccentric 76 which may be adjusted to any desired position along a diameter of u iscv 76' secured tothe outer end of shaft 44. Another eccentric (not shown) on the shaft 44 is arranged to raise and lower the feed bar to carry the feed dog into and out of'engagement with the work. Apresser foot 77 carried by a spring urged presser bar 73y of any suitable construction cooperates with the feed. dog in advancing the work.

As was pointed out hereinbefore, torsional vibrations may be set up in rotating or oscillating shafts of a sewing machine through inertial forces or through impact forces incident to the driving of certain: parts. ln a sewing machine of the type illustrated, the most severe torsional vibrations. are set up in the upper shaft 27 and are caused primarily by the inertia forces arising from starting and stopping. they needle bar 33 at each end of its reciprocatory movement. These vibrations may be suppressed, in accordance with the invention, by means of a` bifilar pendulum damper arranged to operate in conjunction with the upper shaft 27 and constructed andV arranged to suppress vibrations at the critical frequency of the shaft 27 and its driving connections. The effective length of the pendulum is so selected as to provide a natural period for the pendulum which coincides with the frequency of the major, alternating twisting forces applied to the shaft to which the bililar pendulum is applied. Also it is so arranged that the restraining forces whichV it applies to the shaft are out of phase with and effectively oppose the forces which tend to set up torsional vibrations in the shaft.

lt should be observed that the critical frequency of shaft 27' alone will generally fall well outside the operating speed range of the machine so that. there is little likelihood of serious vibrations being set up at a frequency in the neighborhood of this critical frequency. However, when the shaft 27 is included in a power transmission system having appreciable flexibility, the critical frequency of the system may be lowered to a value at which serious amplications of the torsional vibrations imparted to the shaft may occur due to resonance effects resulting from the coincidence of the vibratory force fre quency and the critical frequency. This flexibility may be introduced by means of a belt coupling between the driven shaft and the driving shaft, as in the machine illustrated in the drawings, or may be introduced through other relatively flexible connections. Similarly, flexibility rnay arise because of a belt or other flexible connection between the hand and belt wheel and an electrical transmitter or other primary power source.

lt has been found that, where it is desired to suppress torsional vibrations at a particular frequency such as those introduced through reciprocation of the needle bar, the detuning action of an appropriate bitilar pendulum damper will produce very satisfactory suppression of the undesired torsional vibrations. Moreover, this damping or detuning action is effective at all speeds of operation of the machine since the centrifugal force acting on the pendulum increases in proportion to the increase in speed of the machine. As the speed of the machine increases the frequency of the twisting forces resulting from the Si driving ofthe needle bar increasesV but the ynatural period of the pendlumcorrespondingly increases.

Referring now particularly to Figs. 4 and 5, there is illustrated'. a suitable bililar pendulum damper mounted on the reduced diameter end portion 27' of the upper shaft 27 and associated with the needle bar crank 37. The crank 37 is provided with a slot adapted to receive a plate which serves as a support member for the

bilar pendulum masses

81, 82 and S3 and which is provided with a hole adapted to accommodate the reduced diameter portion of the shaft 27. The pendulum masses have generally U-shaped cross sections (Fig. 5) and the radial center line of each mass is spaced 90 from the radial center line of the adjacent mass. The mass 8l is supported by

rollers

84 and 85; the mass 82 is supported by rollers 85 andy 87; and the mass 83 is supported by

rollers

33 and 89. As is best shown in Fig. 5, with respect to the roller S6, the rollers are each provided with a radially extending flange on either end thereof, flange S4 through 87 being shown in Fig. 4 and flanges 86' and 86 bei-ng shown in Fig. 5. Rollers S4 througl S9 are arranged to act in circular holes 90 through 95, respectively, providedV in the pendulum masses and in circular holes 96 through 10i, respectively, provided in the pla-te S0. As is best shownin Pig. 5, with respect to hole 92, each of the holes provided in the pendulum masses are counterbored to accommodate the flanges provided on the ends of the rollers. The counterborings 99 through are shown in Fig. 4', while the

counterboring

92 and 92" are shown in Fig. 5. The rollers S4 through 89 act in holes 90 through 95'-, respectively, in the. penduluinmasses and in holes 96 through 101, respectively, in the plate S0..

With respect to each pendulum mass, the holes therein and the associated holesl in the plate 80 have substantially equal diameters. However; the diameter of each of the rollers is smaller than the diameter of the associated holes, so that each point on a pendulum mass swings in parallel relation to all other points on the mass along an arcuate path having a radius equal to the difference between the diameter of the roller and the diameter of the associated holes. ln this swinging movement the rollers rol-l back andforth along the surfaces which dene the holes in the plate 80. The effect of each pendu-lum. mass depends on the distance between the axis of rotation of the shaft 27 and the center of mass of the pendulum mass, as well as on the length of the pendulum link. The bifilar pendulum is preferably designed toy damp out vibrations; in the upper shaft 27 occuring at or nea-rthe critical frequency of this shaft and its associated driving connections including the belt 43. ln general, the design of the bifilar pendulum damper may be: determined approximately from the equation:

In this equation D represents the effective length of the pendulum represented by each mass and is equal to the difference between the hole diameter and the roller diameter, R represents the distance from the center of rotation to the center of oscillation of the pendulum mass and "n is the required pendulum frequency in swings per revolution. The biflar pendulum damper is particularly suited for use in association with a needle bar cranl: because of its efficiency in damping out a single relatively high critical frequency. lt has been found that bitilar pendulum damper constructed as shown in Pigs. 4 and 5 and employing three 90 spaced pendulum masses will damp out approximately 90%y of the critical frequency vibrations. Use of the three pendulum masses, as described, increases the damping efliciency over that which would be secured by using a single mass and also permits the assembly of relatively small pendulum masses to mt as an efficient balancing counterweight for the needle bar crank.

In a sewing machine which is driven by a belt or has aY belt for, transmitting power between two shafts in the machine, the flexibility of the belt lowers the critical frequency of the system comprising the belt and the shaft driven by it to a point within the frequency of the vibratory forces applied to the shaft. This renders the system highly susceptible to torsional vibrations of large magnitude. It is therefore desirable in many high speed sewing machines to provide damping means in association with the system comprising the belt yand the shaft driven by it. Where the vibratory forces occur at a particular discrete frequency related to the speed of operation of the parts, it may be desirable to employ, at some point in the system, a damper such as the biiilar pendulum of Figs. 4 and 5. One the other hand, where vibratory forces occur over a range of frequencies, such as those produced by impact effects, a damper responsive to a wide frequency range of vibrations is preferred. Dampers of this latter type are shown in Fig. l in association with the upper shaft belt pulley 30 and the lower shaft combined hand and belt wheel 51. These dampers are both of the same type, i. e., uid restrained disc type dampers. It should be understood that it may be desirable to use a damper of this type on the belt pulley 3i). either in lieu of or in addition to damping means provided directly on the needle bar actuating crank. When both are used, one may be arranged to take care of certain vibrations at a definite frequency while the other may take care of other vibrations occurring at a different frequency or at different frequencies within a wide range.

Referring now particularly to Fig. 6, there is shown in enlarged form the disc type damper associated with the belt pulley Sti. The disc type damper comprises a hollow casing 120 rigidly affixed to the belt sprocket 3) by means of machine screws 121. Casing 120 is provided with a central hub portion 122 and a rear cover plate 123 affixed to the rim of the casing 120, by machine screws 124, to form an annular chamber within the casing. The casing 126 is provided with an outwardly eX- tending, centrally located flange portion 125 having a diameter approximately equal to the diameter of the shaft 27 and fitted in the central bore of the belt pulley. Within the casing 126 there is provided an annular inertia member 125 having an outer diameter somewhat smaller than the inner diameter of the casing 120 and an inner diameter slightly larger than the diameter of the hub 122. The spacing between the hub 122 and inertia member 126 is preferably selected to provide a running fit between these parts. The space between the inertia member 126 and the casing 12() is filled with a viscous fluid, such as a relatively heavy oil or silicone uid, which, when casing 129 is rotating with the shaft 27, causes inertia member 126 to rotate at substantially the same rate as the shaft 27. Any changes of direction or speed of the casing 121), such asfoccur when torsional vibrations are set up in the shaft 27, are opposed by inertia member 126. Inertia member 126 thus provides a damping force in opposition to torsional vibrations in the shaft 27, the vibratory components of motion of the shaft 27 being transmitted to the inertia member 126 through the belt sprocket 39 and the fluid within the casing and the damping action of the inertia member being transmitted to the shaft 27 through the same path. The effect of the disc type damper is to suppress'critical frequency vibrations by resisting vibratory motion. Since it is untuned, it will act to suppress vibratory motion over a wide range of frequencies.

Sufficient clearance should be provided between the sides of inertia member 126 and the adjacent surfaces of the casing 120 and cover plate 123 to provide adequate fiuid capacity. However, since a substantial spacing would tend to permit lateral movement of inertia member 126 on the hub 122, which movement would cause undesirable longitudinal thrust on the shaft 27, projecting

annular bosses

127 and 128 are provided on inertia member 126 adjacent the hub 122. The

bosses

127 and 128 provide a relatively close iit with the surfaces of the casing and cover plate 123 adjacent the hub 122, thus keeping inertia member 126 centered axially in the casing 120.

It will be observed that lubricating action will be required between the hub 122 and inertia member 126 and between

bosses

127 and 128 and the adjacent surfaces of casing 120 and cover plate 123. Silicone fluid is not a good lubricant for steel on steel, although it is as between steel and some metals such as chromium. Therefore, if silicone fluid is used, one of each pair of mating surfaces is preferably chromium plated. Oil may not be satisfactory as a uid in some installations because its viscosity usually changes materially with temperature so that the damping action providedwould tend to vary substantially with temperature. However, oil may be used as the fluid for the damper in some cool running sewing machines or in cases in which the viscosity of the oilr at the usual operating temperature of the machine is adequate to provide a good damping action. When oil is used as the fluid the chromium plating indicated would not be required.

The disc type damper generally designated at and associated with the hand and belt wheel 51 of Fig. l is constructed and operates in the same manner as the damper associated with the belt sprocket 30. The damper 140 comprises a hollow casing member 141 affixed to the wheel 51 by means of screws 142 and having an annular chamber, a cover plate 143 affixed to the casing 141 by means of screws 144, and an annular inertia member 145. inertia member 145 is provided with

bosses

145 and 145 corresponding to

bosses

127 and 128 of member 126. Damper 14) suppresses torsional vibrations in the shaft 44. Since the combined hand and belt wheel 51 is connected to the electricaltransmitter or other power source by means of a belt (not shown), the location of the damper 140 in association with the wheel 51 render this damper suitable for suppressing vibrations introduced into the sewing machine driving connections because of the fiexibility of the connection between the sewing machine and the power source.

In general, it may be stated that the most effective point of application of a torsional vibration damper to a system subject to torsional vibrations is at a point remote from that at which the system receives its power. Thus a torsional vibration damper is preferably associated with a driven shaft or member rather than a driving shaft or member. lVhile the damper 140 could be placed in direct association with a transmitter or motor shaft7 greater damping emciency is secured by attaching the same to the combined hand and belt wheel 51.

As has been indicated hereinbefore, periodic reversals of torsional forces are applied to the elements of the driving connections in a sewing machine. These forces produce torsional vibrations which, when the frequency thereof is equal to or near the critical frequency of the driving connections, are amplified by resonance effects. The amplitudes of the torsional vibrations may be materially reduced by shifting the critical speed of the driving connections, or one or more elements thereof, out of the range of frequencies of the vibratory forces. This may be effected by varying the exibility of the driving connections. For example, fiexibility may be addedl to the driving connections to lower the critical frequency thereof. Alternatively, the driving connections rnay be stiffened to raise the critical frequency thereof.

Excellent suppression of resonant torsional vibrations can be achieved by introducing a tuned mass and spring, which imparts additional flexibility into a particular vibratory system to shift the critical frequency thereof downward, and by providing friction to absorb .or suppress vibrations. In the embodiment of the invention illustrated in Figs. 8 and 9, there is shown means for adding flexibility and friction between' the electrical transmitter, or other power source, and the lower main shaft of a sewing machine. twill be observed that the sewing machine of Fig. 8 corresponds to the machine illustrated in Fig. l except that the disc type damper 140 has been replaced by the frictional damper which will now be described.

The combined hand and belt wheel 51', which corresponds to the wheel 51 of Fig. l, is mounted on the shaft 44 so as to be freely rotatable thereon. The wheel 51 comprises a pulley portion 156 which is adapted to receive the belt (not shown) which transmits power from the motor or other prime mover to the sewing machine. An annular housing portion 151 is formed integrally with the pulley o and is provided with a cover plate 152 secured thereto by means of screws 153. An annular disc member 154 is located within the housing 151 and is rigidly affixed to the pulley 150 by means of screws 155. A collar 156 is mounted on the end of the shaft 44 within the housing 151, and is rigidly aflixed to the shaft by means of set screws 157. The collar 156 is secured to the disc 152i` by means of three resilient springlike arms 158. The arms 15S are preferably spaced 120 relative to each other, as is best shown in Fig. 9. The disc 155.', the collar 156 and the arms 158 may conveniently be formed as an integral unit. Rotational motion imparted to the pulley 159 by means of the driving belt is imparted to the main shaft 44 through the disc 154, the spring-like arms 158, and the collar 156. The exibility of the spring-like arms 153 should be selected to provide the desired flexibility in the driving connections so that the desired change in the critical frequency of the related system will be effected and a tuned mass and resilient connection is provided having approximately the same critical frequency as the driving connections to oppose the resonant vibration of the latter. In most instances, it will be found desirable to combine such shifting of the critical frequency with the positive damping effect produced by a frictional element. This friction, which serves to suppress vibratory motion, is introduced in the embodiment illustrated in Fig. 8 by a disc 159 which is rigidly afiixed to the shaft 44 by means of set screws 160 provided in hub portion 161. The radial surface of the disc 159 abuts against the opposing surface of the pulley portion 15G. Relative rotation between these surfaces may take place but is opposed by frictional forces therebetween, thus providing the desired damping effect. The frictional force between the surfaces of the disc 159 and the pulley 150 is, of course dependent on the spring tension in the arms 158. Some additional frictional damping will be produced by relative rotation between the pulley 1543 and the shaft 44.

In the arrangement shown in Fig. 8, there is a positive connection betwen the disc 154 and the pulley 150, this connection being provided by the screws 155. 1f desired, however, the screws 155 may be omitted and the friction between the disc 154 and the adjacent surface of the pulley 15@ may be used to transmit driving power from the pulley 151% to the shaft 44. 1n such an arrangement, friction between the disc 159 and the pulley 156 will assist in transmitting the power. Thus, a double frictional drive between the pulley 15G and the shaft 44 will be provided. 1n sewing machines having a high rate of acceleration, such a frictional coupling isV not as satisfactory as the rigid coupling illustrated because, if sutiicient friction is provided to eliminate undue slippage, relatively little damping effect will be obtained, whereas, if the friction is made smaller to insure the desired damping, then slippage from acceleration may be excessive.

An important advantage of the friction type damper lies in the fact that damping is obtained without an appreciable addition of. mass to the sewing machine mecha- 10 nism. The low mass assists in securing the high speeds which are desirable in many modern sewing machines.

The construction shown in Figs. l0 and 11 is similar to that of Figs. 8 and 9, except that relatively little friction is provided, the structure being intended primarily to introduce flexibility into the @driving connections. in this embodiment, the combined hand and belt wheel 51, which corresponds to the

wheels

51 and 51 of Figs. l and 8, respectively, is mounted on the shaft 44 and is freely rotatable thereon.V An annular disc is arranged to abut against a portion of the surface of the wheel 51 facing the sewing machine and may, if desired, be rigidly connected thereto by welding, by means of screws, or in some other suitable manner. A collar 166, which is rigidly affixed to the shaft 44 by means of set screws 167, is connected to the disc 165 by resilient, spring-like arms 168. The collar 166, the arms 168 and the disc 165 may conveniently be formed as an integral unit. Power imparted to the wheel 51 is transmitted to the shaft 44 through the spring arms 168, the latter serving to introduce liexibility in the same manner as the arms 158 of Figs. 8 and 9. Some friction is provided between the wheel 51" and the shaft 44, although the frictional damping produced will be much less than that secured in the structurershown in Figs. 8 and 9. If desired, a greater frictiona'l drive between the shaft 44 and the wheel 51" may be secured by not providing a rigid connection between the disc 165 and the Wheel 51". Friction may be relied upon for the transmission of power between these elements. As was pointed out above, such an arrangement is suitable primarily for sewing machines which do not have a high rate of acceleration. The outside surface of the wheel 51" is provided with a vertical or radially disposed portion adapted to abut against a washer 169. The washer 169, which is held in place by a nut 170, limits longitudinal movement of the wheel 51 and serves to retain the parts in assembled relation. If only a frictional connection between the disc 165 and pulley 51" is provided, the friction between these parts may be varied by suitable adjustment of the nut 170.

Suitable ldampening of the torsional vibrations may be achieved in many instances by providing a hysteresis effect. This may be accomplished in a variety of different ways. It may be brought about by dividing an existing flexible member into two parts, and providing friction therebetween. Alternatively, one or both of two exible elements may be added to the machine and friction created therebetween to provide the desired damping. In providing the hysteresis effect the system may be rendered somewhat less flexible. A suitable construction is illustrated in Fig. l2 wherein a hysteresis type damper is shown associated with the upper main shaft of a sewing machine similar to the one illustrated in Fig. l.

Referring now to Fig. l2, a cylindrical sleeve member is arranged to encompass aV portion of the shaft 27 (corresponding to shaft 27 of Fig. l). The sleeve 180 is held in place about the shaft 27 by means of a radially inwardly :extending end portion 181 which is rigidly ajxed to the shaft 27' adjacent the needle head 26. The end 181 may be welded to the shaft 27 or it may be secured thereto by other suitable means. To the other end of the sleeve 180 there is aflixed an annular friction disc 182. A cylindrical sleeve member 153, which is similar to but preferably shorter than the sleeve 180, is mountedy on the shaft 27 by means of a radially inwardly extending end portion 184 suitably aixed as by welding or the like, tothe shaft 27 at an intermediate point thereon. The open end of sleeve 183 is similarly provided with an annular' friction disc 185 adapted frictionally to engage the disc 182. it will be understood that the two

sleeve sections

180 and 183 are so mounted on the shaft 27 as to provide a substantial pressure between' the

friction elements

182 and 185. If desired, one' or both of these may be resiliently mounted 1 1 Von their respective sleeve sections to maintain the same in contact under a suitable, lspring force regardlessY of 'wear ,of the friction-faces. The arrangement is such that the iiexibility of the shaft 27 between bearing rmembers 28 and 29 is modified to some extent, and the relative turning of two longitudinally spaced portions of theV shaft, due to torsional vibration producing forces, is resisted by the friction between the

elements

182 and 185.

e Vibratory motion in the shaft 27 vis thus suppressed.

The detuning action'secured by the ,arrangement of Fig. 12 is attained without the addition of appreciable mass, which is an important advantage in sewing machines operating at relativelyV high speeds and having relatively small diameter shafts. The shaft ,27' together with the

sleeves

180 and 183 will constitute a much stiffer clement than the/shaft 27 alone, so thatthc critical frequency of the shaft 27 is raised. By proper proportioning of the parts, the critical frequency of the shaft 27', considered in relation to its driving belt, may be raised well above the rangek in which critical torsional vibrations are likely to occur. Y

A hysteresis type damper may conveniently be applied to an oscillating or rock shaft such as that used to operate a rutlling blade or a cover thread laying finger. The hysteresis type damper shown in Figs. 13, 14 and 15 is illustrated in connection with such an oscillating shaft. The shaft is mounted at therear of the overhangiug arm of a at bed sewing machine, and is similar to the corresponding shaft shown in U. S. Patent No. 2,577,430, issued December 4, 1951, to A. C. Peterson and C. C. Smith; It is a relatively long shaft which receives its oscillatory driving forces at the right end thereof (Fig. 13) and transmits the motion at its yleft end to .the parts lto be oscillated. The damper comprises a sleeve 190 surrounding the oscillating or rock shaft 191 an'd rigidly connected to the shaftradjacent one end thereof by a strap 192. The other end Vof the sleeve 190 is arranged to be in frictional engagement with the shaft 191 by means of another strap 193. As is best shown in Fig. 14, the strap 192 is split and is provided with a projecting annular ange 194, likewise split, which is of smaller outside diameter than the main body of the strap and is adapted to t into a recessed portion of the sleeve 199 provided in the vbell-shaped end thereof. The strap 192 may be secured to the sleeve 190 by any suitable means, such as welding, the en'd of the sleeve being split, as indicated at 190a, to correspond with the splitting of the strap. Strap 192 is provided with

flange portions

195 and 196 having threaded holes therein adapted to receive the

screws

197 and 198, respectively. It Vwill be understood that only the portions of the

flanges

195 and 196 which are below the line of split are provided with threads in the holes therein while the upper portions of the flanges have non-threaded holes through which the Shanks of the screws pass freely. VThe screws 197 and 198 serve to force the split portions of the strap 193 together whereby the strap firmly engages the shaft 191, thus providing a rigid connection between the sleeve 19t) and the shaft 191 at the righthand end of the sleeve in Fig. 13.

The construction of the strap 193, which is located adjacent the other end of the'sleeve 190, is similar to that of the strap 192. More particularly, the strap 193 is split, as 'shown in Fig. 15, and is provided with a split annular ange 198 adapted to be fitted into the bellshaped end of the sleeve 190. The strap 193 and the sleeve 190 may be rigidly fastened together by welding or other suitable means. Screws 199 and 200, cooper; ating with threaded holes provided in the lower parts of ange portions 201 and 262, respectively, of the strap 193, and passing freely through non-threaded holes in the upper parts of these flanges, serve to tighten the strap on the shaft 191 to provide the desired frictonal engagement between the strap 193 and the shaft 191. When the shaft 191 starts to twist, due to torsional vibrations set 12 up therein, relative angular movement of the parts of the shaft engaged by the straps 192 and 1937is suppressed lby the friction provided by the strap 193. This friction can be varied by tightening or loosening the strap 193. ln addition to the frictional damping action provided, it will be evident that the sleeve 196 and the

straps

192 and 193 tend to stiften the shaft 191, thus raising the critical frequency thereof and providing corresponding detuning thereof out of the normal range of the reversals of torque applied to the shaft.

rfwo other applications of the principles of hysteresis damping are shown in Figs. 16 and 17. Referring now to Fig. V16, which is a vertical, transverse sectional View taken through the vertical standard of a sewing machine similar to the one illustrated in Fig. l, there is shown an upper shaft 214) and a lower shaft 211, corresponding to the

shaft

27 and 44, respectively, of Fig. 1. The shafts 219 and 211 are provided with two

toothed belt sprockets

212 and 213, respectively. A notched or toothed driving belt 214, corresponding to belt 43 of Fig. 1, engages the

pulleys

212 and 213 to transmit power from one Shaft to the other. A second or damping belt 215 is arranged to encompass the belt 214. The belt 215 tends to travel with the belt 214, although the arrangement is such that some slippage therebetween may occur. This slippage introduces friction which provides a damping action. The belt 215 also tends to stiften the connection between the

shafts

210 and 211 and to reduce flexing of the belt 214, the net effect of which is to raise the critical frequency of the belt 214 and the associated driven and driving parts, such as the

shafts

210 and 211. Control over the flexibility of the damping belt 215 and the driving belt 214 may be achieved by providing an idler roller 216 at an intermediate point along one leg of the belt 215, The idler roller 216 is mounted -on a shaft 217 suitably journaled in the walls of the vertical standard. The shaft 217 may be made laterally or vertically adjustable to vary the effect of the roller 216 on the

belts

214 and 215. For example, the roller 216 may be made to engage the belt 215 only when the deection thereof exceeds a predetermined amount. While the roller 216 is preferably positioned adjacent the center `of one leg of the belt, it may, if desired, be located nearer one pulley than the other.

ln Fig. 17 there is illustrated a modied form of construction also employing a driving belt and a damping belt. While Fig. 17 shows only a fragmentary portion of a sewing machine, the machine as a whole may be similar to the one illustrated in Fig. 1, the upper shaft 220 being provided with a notched or toothed belt sprocket 221 adapted to cooperate with a notched or toothed driving belt 222. The belt 222 also passes around a similar belt sprocket 223 which is rigidly aixed to a lower shaft 224 by means `of a set screw 225 acting in a hub portion 226 of the belt sprocket 223. Power is transmitted from a combined hand and belt wheel 227, secured to the lower shaft, to the upper shaft 220 through the belt 222 in the same manner as described in connection with belt 43 of Fig. 1. A smooth belt pulley 228 is ailxed to the shaft 220 by means of a set screw 229 carried by a hub portion 230 of pulley 22S.

Pulleys

221 and 228 may, if desired, be integral or otherwise connected for .rotation together. A similar smooth belt pulley 231 is aiiixed to the shaft 224. If desired, the pulley 231 and the sprocket 223 may be formed integrally or may be rigidly fastened together by suitable means. A smooth belt 232 is engaged by the

pulleys

228 and 231 and rotates therewith when power is applied to the hand and belt wheel 227. The belt 232 is arranged to Vbe free to slip on its pulleys in order to provide desired frictional damping action. Belt 232 also tends tostiffen the connection between the

shafts

220 and 224, as in the arrangement described in connection Vwith Fig. 16. Flexing of 'the belt 232 and hence the magnitude of the damping and stiffening actions provided by the belt 232 can'V be controlled by providing an idler roller 234 arranged to adsense engage the belt 232 atV an intermediate point in its travel in a manner similar to the' roller 21'6' of Fig. 1.6. The roller 23dv is mounted on a shaft 235" which is suitably journalled in the vertical' standard. Shaft 235v may be made laterally or vertically adjustable to vary the position of the roller 234 relative to the belt 232. The belt 215 of Fig. l6y and the belt. 232 of Fig. l-7` each act' to limit relative motion between the Iupper and lower main shafts of a sewing machine,- thus suppressing torsional vibrations in the sewing machine;

In sewing machines having belt connected shafts, a substantial reduction in' torsional vibrations may be achieved by controlling the iiexing of the driving belt. When belt-driven sewing machine is running, each leg of the belt tends to assume the shape ofV a sine curve with three nodes and tw'o a-ntinodes. A node is produced at each` end of thev belt, and at the center of each. leg. The two antinodes are produced in each leg midway between the center node and arespective endV node. In accordance with the' invention, the exing of the belt may be reduced by limiting or preventing belt deflection at the antinode points as a further means of suppressing vibrations.- A suitable arrangement for accomplishing this reduction in eX-ing is illustrated in Fig. 18, which i's a vertical, transverser sectional. view taken throughl the hollow standard of a sewing machine' similar tothe one shown in Fig. l.

Referring now to Fig. 1.8, an upper shaft 24% and a lower shaft 241 are provided with toothed or notched

belt sprockets

242 and 243,-V respectively..

Sprockets

242 and 243 are connectedv for rotationI in unison by a toothed driving belt 244-, corresponding` to the belt 4'3 of Fig. l'. In operation, the belt. 244 will tendto assume a configuration such that nodes will` be formed along the points of contact of the belt 244 with itssprockets and at points MSV-andv 2de midway between the sprockets. Antinodes will` be formed at

intermediate points

247, 248, 249 and located, respectively midway between adjacent nodes. The antinode positions represent` points of maximum outward deflection ofthe belt 244. Dedection or flexing of the belt may be limited by preventing or restraining the formation of these antinodes. This is achieved in Fig. I8 by providingr

idler rollers

251, 252, 253 and 254 adjacent the outer'surface of belt 244 at the p oints 247, 24S, 249 and 250, respectively. Rollers 251 through 254 are mounted on

shafts

255, 256, 257 and 25S, respectively, the shafts being suitably journaied in the walls of thevertical standard. The rollers, each of which engages belt 244 at a respective antinode point thereon, prevent or limit outward movement of the belt 244 at the antinode points, thereby materially reducing eXing of the belt di?. If desired, the shafts 255 through 258 may be made laterally adjustable to compensate for stretching of belt 244 with wear. Provision of the idler rollers in effect stiffens the belt 244, thus raising the critical frequency of the belt and associated driving connections.

The type of damping means employed and the number of such means provided in a given sewing machine will, of course, vary with the type of sewing machine and with the character of vibrations encountered. In some sewing machines serious vibrations will occur only in connection with a single rotating or oscillating shaft, such as the upper main shaft of a at bed sewing machine. In other machines, the vibrations will be encountered in widely separated parts so that a plurality of dampers may be desirable. If the forces creating torsional vibrations are eective at a frequency dependent entirely upon the speed of operation of the machine a damper of the bilar pendulum type shown in Figs. 4 and 5 may be best suited for suppressing vibratory motion. On the other hand, where forces which create objectionable vibrations at a plurality of frequencies at a given speed of operation of the machine are encountered, an untuned damper, such as the disc type damper, ofv Fig; 6, mayE be best. suited.- The combina,b tion of dampers employednray be varied toy specific requirementsV as encountered. That shown in Fig l, which comprises threev dampers in different parts of a filat bed sewing machine, has been found: well suited to the suppressing of resonant torsional vibrations occurring in such a machine when. it is operated at veryy high speeds, i. e., upwards of 5,000 stitches per minute.- For particular applications, substitutions may suitably be made. For example, inFig. 1 the disctype damper 141) could advantageously be' replaced with the damper! ilius'- trated in Fig. 8i. A disc type damper, because of the wide frequency range in which it is effective, may desirably be used in a machine wherelittle or nor adjustment is convenient.

While theY invention has been described in particular embodiments thereof and in particular uses, it should be understood that various modifications thereof will occur toV those skilledin the art without departing from` the spirit and scope of the invention as set forth. in the appended' claims.

What is claimed is:

l. A sewing machine comprising a frame having mounted thereon a plurality of cooperating stitch' form ing and work feeding elements' and driving connections therefor including a driven shaft, a driving shaft,l a exible driving belt intercoupli-ngsaid shafts. in non-slip relation thereto, and means for actuating said driving shaft thereby to operate saideleme'nts, said driving connections constituting a vibratable system having a'- given critical frequency the magnitude of which is relatively low' due to the presence of said belt therein, means in. said. driving connectionsv serving to apply periodic reversals of torsional force to said: driven shaft' induced by the driving of certain of said elements, thereby tending to create torsional vibrations in said sha-ft at substantially said critical frequency and tending to cause the legs of said belt to whip and assume a sine curve,` and resonant vibration opposing means connected to a part of said driving connections to which poweris supplied through said belt and arranged to' react therewith at substantially 180 out of phase with said periodic reversals of. force to suppress a substantial part of said resonant vibrations;

2. A sewing machine' in accordance with claim l. in which said resonant vibration opposingv meansl comprises bifilar pendulum means tuned to the critical frequency of said independently vibratable' system, said bifilar pendulum means being mounted on and arranged to' r0- tate with said driven shaft.

3. A sewing machine in accordance with claim l in which said vibration opposing means comprises a uid tight hollow housing member rigidly connected to said driven shaft, said housing member having an axial hub rotatable about the axis of rotation of said shaft, an annular inertia member mounted for free rotation on said hub within said housing, the inner diameter of said inertia member relative to the outer diameter of said hub providing a running fit therebetween and the clearance between the peripheral surfaces of said annular inertia member and the adjacent surfaces of said housing being substantially greater than the clearance between said hub and said inertia member, said housing being lled with a viscous iuid for producing rotation of said inertia member with said housing.

4. A sewing machine in accordance with claim l in which said vibration opposing means comprises a flexible damping belt mounted on said driving belt and disposed in sliding frictional relationship therewith, whereby deection of said damping belt is suppressed and torsional vibrations of at least one of said shafts are reduced.

5. A sewing machine in accordance with claim l in which said Vibration opposing means comprises a exible damping belt intercoupling said driving and driven shafts and disposed in frictional sliding relation there- Yin which said bitilar pendulum means comprise three pendulum masses connected with said crank, said masses having their radial centre lines disposed, respectively, at 90, 180 and 270 from the radial centre line of said needle bar crank, the elfective length of each pendulum mass and the spacing between the centre of rotation and thecentre of oscillation of the mass being such as to provide a frequency of oscillation of said masses substantially'equal to the frequency of said periodic reversals of torsional force.

'7. A sewing machine comprising a frame having a work supporting base, a vertical standard and an overhanging arm, said frame having mounted thereon a plurality of cooperating stitch forming and work feeding elements and driving connections therefor including a driven shaft extending longitudinally of said overhanging arm, :a driving shaft in said base, a flexible driving belt intercoupling said shafts in non-slip relation thereto, and means for actuating said driving shaft thereby to operate said elements, said driving connections constituting a vibratable system having a given critical frequency the magnitude of which is relatively low due to the presence of said belt therein, means in said driving connections serving to apply periodic reversals of torsional force to said driven shaft induced by the driving of certain of Ysaid elements, thereby tending to create torsional vi,-

brations in said shaft at substantially said critical frequency and tending to cause the legs of said belt to whip and assume a sine curve, and resonant vibration opposing means connected to a part of said driving connections to which power is supplied through said belt and arranged to react therewith at substantially 180 out of phase with said periodic reversals of force to suppress a substantial part f said resonant vibrations.

8. A sewing machine in accordance with claim 7 in which said resonant vibration opposing means comprises bifilar pendulum means tuned to the critical frequency of said independently vibratable system, said biiilar pendulum means being mounted on and arranged to rotate with said driven shaft.

9. A sewing machine in accordance with claim 8 in and a crank on s aid driven shaft arranged to reciprocate said needle bar, reciprocation of said needle bar serving toapply said periodic reversals of torsional force, and in which said biilar pendulum means comprises three pendulum masses connected with said crank, said masses having their radial centre lines disposed, respectively, 'at 180 and 270 from the radial centre line of said needle barY crank, the eifective length of each pendulum mass and the spacing between the centre of rotation and the centre of oscillation of the mass being such as to provide a frequency of oscillation of said masses'substantially equal to the frequency of said periodic reversals of torsional force. v

10. A sewing machine in accordance with claim 7 in which said vibration opposing means comprises a fluid tight hollow housing member rigidly connected to said driven shaft, said housing member having an axial hub rotatable about the axis of rotation of said shaft, an annular inertia member mounted for free rotation on said hub within said housing, the inner diameter of said inertia member relative to the outer diameter of said hub providing a running tit therebetween and the clearance between the peripheral surfaces of said Vannular inertia member and the adjacent surfaces of said housing being substantially greater than the clearance between said hub .and said inertia member, said housing being filled with a viscous uid for producing rotation of said inertia member with said housing.

11. A sewing machine in accordance with claim 7 in which said vibration opposing means comprises a flexible damping belt mounted on said driving belt and disposed Vin sliding frictional relationship therewith, whereby deflection of said damping belt is suppressed and torsional vibrations of at least one of said shafts are reduced.

12. A sewing'machine in accordance with claim 7 in which said vibration opposing means comprises a flexible damping belt intercoupling said driving and driven shafts and disposed inV frictional sliding relation therewith, whereby slippage of said damping belt opposes torsional vibration of at least one of said shafts.

References Cited in the le of this patent UNITED STATES PATENTS 1,548,458 Grieb Aug. 4, 1925 2,112,984 Chilton Apr. 5, 1938 2,267,581 Zonis Dec. 23, 1941 2,280,364 Atteslander Apr. 21, 1942