GB2181513A - Bifilar pendulum damper - Google Patents

Abstract

A bifilar pendulum automotive engine vibrational damper is provided to partially or completely cancel undesirable engine torsional vibrations of a particular order or orders, the damper containing rollers 56 movable along paths 58, 60 containing detuning ramps operable at higher swing amplitudes to maintain the swing amplitude within safe mechanical limits within the capacity of the pendulums, the detuning ramps having radii that progressively decrease with increases in swing angle to cause the pendulum 54 to be progressively overtuned so as to be progressively less responsive to the excitation frequency of the engine crankshaft 42. <IMAGE>

Description

SPECIFICATION Automotive crankshaft bifilar pendulum vibration damper This invention relates in general to an automotive crankshaft vibration damper, and more particularly to a bifilar pendulum type damper.

Bifularpendulum dampers are well known, especially in the aircraft industry, for suppressing engine vibrations by turning the pendulum mass to a natural frequency that equals the crankshaft excitation frequency. For example, U.S. 3,932,060, Vincent, U.S.

2,184,734, Chiiton, U.S. 2,306,959, Knibbe, U.S.

3,540,809, Paul metal, U.S. 2,272,189, Depew, U.S.

2,535,958, Sarazin, and British 401,962, Salomon, all showthe useof bifilarpendulum dampersto balance or cancel out undesirable order vibrations of an engine. In these cases, the crankshaft is connected to the pendulum mass by rollers that in different ones ofthe references move in circular, cycloidal, elliptical or epicycloidal paths in an attempt two maintain a constanttuning ortautochronic action to the pendulum so that all of the engine vibrations of one or more orders are exactly baianced. However, the majority of the prior art devices are for use in aircraft engines where large masses with corresponding small swing amplitudes can be tolerated to offset the vibrational forces ofthe impulses, such masses, however, not being practical in an automotive engine where space is limited.

Of interest is the patentto Madden, U.S.4,218,187.

This patent shows in Figure 4the use of oval tracks forthe rollers which produces cycloidal pendulum motions, and describes the effects of circular and non-circular holes on th e the transmissibilityof vibra- tion, as well as the effect ofovertuning and undertuning. The patent attempts to provide constant tuning even at high load levels by the use of a cycloidal path for the rollers. The cycloidal path provides a decrease in radius of curvature with increase of swing am pair tudeofthependulum mass to maintain close to a constant tuning. However, again, this device is for use in helicopter rotors and is not concerned with physical size.Also, there is no attempt two prevent the pendulum mass from swinging beyond its allotted space and thereby damaging the mechanism, other than by installation of large enough pendulum mass to generate sufficient vibrational force at low swing amplitude.

American Helicopter Society Paper No.354, dated May 14-16, 1969, describes theory and advantages of a bifilar pendulum damper as applied to a helicopter.

In this case, however, the rollers move in circular holes and provide increasing swing amplitudes with increases in excitation frequency.

This invention is directed to a bifilar pendulum damperthat is designed to prevent mechanical damage to the system by preventing swing amplitudes of the pendulum masses beyond their mechanical limits. That is, it maintains the swing amplitudes within the capacity of the pendulum mass. The pendulum masses ofthe invention function in a normal manner at low amplitude swing angles to completely cancel engine vibrations by tracking of the rollers along a path which generates approximatelytautochronic motion of the pendulums; the rollers, however, subsequently tracking orfollowing a path of progressively decreasing radius with increase in swing amplitude to progressively become less and less tuned to the excitation frequency of the crankshaft.This results in a progressive decrease in the response of the pendulum to the vibrational impulses, which limits the swing amplitude at higher torsional vibration excitation levels to maintain the pendulum mass within its swing angle capacity.

More particularly, the invention provides detuning ramps consisting of holes having a nearly constant radius during very low swing amplitudes, followed by progressively decreasing radii ofthe holes at larger swing angles progressively detuning the pendulum. At the higher excitation levels, the pendulum mass reaches a point near or essentially at its maximum swing amplitude where it becomes overtuned and thereafter has minimal responsetoafurtherin- crease in vibrational impulses of the crankshaft.

It is a primary object ofthe invention, therefore, to provide an automotive engine bifilar pendulum damper with roller holes which generate essentially constant tuning (tautochronic) at low swing amplitudes of the pendulum, contiguous to progressively decreasing radii of curvature roll paths as the swing amplitude increases with increased crankshaftvibrational amplitude to progressively decrease re- sponse of the pendulum to the higher excitation forces, thereby permitting the pendulumstocontinuefunctioning at engine vibrational levels higher than would be the normal absorption capacity level of the pendulums.

Other objects, features and advantages of the invention will become more apparent upon reference to the succeeding, detailed description thereof, and to the drawings illustrating the preferred embodimentthereof; wherein: Figures 1,2 2 and 3 graphically illustrate char- acteristics of bifilar pendulum systems ofthe prior art and ofthe invention; Figure 4 schematical Iy compares the theory and construction of the invention to that ofthe prior art; FigureS is a side view of a pendulum carrier moun- ted to a crankshaft and viewed from one side ofthe crankshaft.

Figures 6and 7 is a cross-sectional view taken on a plane indicated by and viewed in the direction ofthe arrows VI-VI and VII-VII, respectively, of Figure 5and 6.

The standard orconventional bifilar pendulum damper is provided with circular rollers that operate in circular holes in both the crankshaft and pendulum masses. This results in the swing amplitude or angle of oscillation of the pendulum mass increasing progressively in proportion to increases in the excitation forces or vibrations. It also results in a detuning ofthe pendulum mass progressively as the oscillation angle increases, as pointed out in Figures 6 and 7 of Madden referred to above and shown in Figures 1 and 2 herein.

Figure 1 herein graphically illustrates the trans- missibility of crankshafttorsional vibrations versus pendulum tuning. Transmissibility is plotted versus reasonancy. At resonance tuning, where the pendulums' natural frequency is equal to the crankshaft exciting frequency, transmissibility is at a minimum.

This means thatthe pendulums' oscillations create a torsional vibration force which is 180 opposite in phase to the exciting vibration and aimost equal in amplitude, effectively cancelling most if not all of the exciting vibrations. lithe pendulum mass becomes overtuned, the natural frequency of the pendulum is higher than that ofthe excitation force, and the pendulum response begins to decouple from the exciting vibration. Now the swing amplitude of pendulum oscillation will be less than itwould bewere it at re- sonance but still within its capacity.It now still reaches a maximum swing amplitudewithouta mechanical contact between the parts, but the pendulum torsional vibration force now cancels only a percentage or portion of the total exciting vibration astheexcitationfrequencyincreases, ratherthanall, as was the case at lower excitation levels which require correspondingly less swing amplitudes. The decreasing hole radius makes the pendulum mass less responsive to the excitation force, causing an increase oftransmissibility, as seen in Figure 1.

On the other hand, ifthe pendulum mass becomes undertuned,the pendulum mass is now at a natural or resonantfrequencywhich is lower than thefrequency of the excitation force and the phase of pendulum oscillation changes. The pendulum torsional vibration forces now no longer cancel the exciting vibration, but add to it, causing a transmissibility greater than one, as seen in Figure 1 herein.

Figure 2 curve A, illustrates howthe standard circular hole bifilar pendulum becomes progressively undertuned as the oscillation amplitude increases, putting it rapidly into an area oftransmissibility greaterthan one, as seen in Figure 1. To stay out ofthis undesirable area of high transmissibility, therefore, at low amplitudes, the standard bifilar pendulums normally are biased to be slightly overtuned atthat point, as illustrated by the modified curve A' in Figure 2, and care is taken to not operate the pendulum at high oscillation amplitudes. This is readily accomplished in aircraft engines because large pendulum masses resulting in low swing angles can be used because space generally is not a major consideration.

The Madden device referred to previously com pensatesforthe undesirable characteristics ofthe circular hole bifilar by providing cycloidally contoured holes to maintain a constantly tuned condition of the pendulum mass, up to high response swing amplitudes, as indicated by the nearly linear curve B in Figure 2. Atthe iarge swing amplitudes, it be- comes slightly overtuned, which keeps it above the crankshaft excitation frequency, as illustrated in Figures 6,and 7 of Madden. The amount of overtuning of Madden, however, is insignificant.

Knibbe, also referred to above, provides an operation similar to that of Madden, except in this case oval shaped holes with decreasing radii are provided to maintain a constantly tuned condition of the pendulum mass at all times independent ofthe swing amplitude, as indicated by the line Cthat overliesthe axis.

Figure 3 illustrates the disadvantages of using aircraft type devices, such as Knibbe and Madden, in automotive applications. The normal engine speed operating ranges of both aircraft and automotive en gines are as indicated, as well as vibrational levels with and without pendulum dampers. The torsional vibration level of an engine increases dramatically as the engine is lugged to a low speed at enginewide open throttle condition, as indicated by curve D. The pendulum's mechanical limit of swing amplitude is indicated at point 2. Therefore, at the lower lugging speeds for an automotive engine, the pendulum's capacity to absorb vibrations would be exceeded.An aircraft engine cannot be lugged to a lowspeed like an automotive engine because opening ofthe throttle in a helicopter aircraft, for example, results in an increase in propeller speed. If, however, an aircraft engine with the Knibbe or Madden tuned pen dulums,forexample,were lugged to a low engine speed like an automotive engine, the torsional vibrations would also create such a large swing amplitude oroscillationanglethatitwouldbegreateror beyond the mechanical limits of the pendulums. That is, the vibrations would exceed the level that the pendulums are capable of cancelling because the rollers or pendulums would strike metal to metal againstthe ends of the space available for their motion and the pendulums would be damaged.

The detuning ramp of the invention described herein detunesthe pendulum as the pendulum oscillation amplitude approaches its mechanical limit, the ramp creating an overtuned condition which in creases transmissibility (see Figure 1)with an increase inthe pendulum oscillation amplitude. Aswill be understood, the detuning ramps perm it the pen dulums to operate at close to theirfull capacity at vibration levels that are higherthan the pendulums capacity, by absorbing less and less percentage of the total vibrations with increased torsional vibration. In contrast, the prior art devices attempt to maximize pendulum efficiency by providing constant tuning right up to the mechanical limits of its travel.

The standard bifilar pendulum damper with circular holes provides tuning which decreases in- creasing amplitude. Madden and Knibbe, previously referred to, provide cycloidal (Madden), orelliptical (Knibbe) motion for the rollersto maintain resonant frequency independentofthe pendulum amplitude (tautochronic motion). These produce a decrease in radius of curvature of the path to compensate forthe non-linear amplitude restoring forces. However, they do nothing to limittheswingamplitudewithex- cessive excitation.

The invention, as illustrated schematically in Figure 4, provides non-circular overlapping holes 10, 12, shown in full lines, in both a radial extension of the crankshaft and the pendulum mass that are inter- connected by a constant diameter roller 18 inserted through both. The standard circular holes 20,22 are shown in dotted lines superimposed upon the holes 10, 12to illustratethechange in radii of curvature.

Forthefirst 15 or so, as an example in this part- icular case, of the contour of holes 10, 12 on either side of an initial at rest position 24, the contours of holes 10 and 12 are designed to produce tautoch ronic motion. Beyond the 15 , however, the radius of holes 10,12 is progressively reduced beyond the tautochronic shape. The effect is to tune the pen- dulum masstothe natural frequency ofthe crank shaftforcesforthefirstt 15 swing amplitudeto maximize cancellation ofthe engine vibrations.

Thereafter, there is a progressive overturning of the pendulums' oscillations. Ifthe pendulum should be excited to swing beyond + 15 , it now encounters an effective radius of swing which decreases rapidly with increased swing angle. This increases the resonans or natural frequency ofthe pen d u I u m. It now detunesfrom the frequency of the exciting torque, and thus large increases of excitation forces cause only a minimal increase in the amplitude of the pendulum swing. In effect, it limits the amplitude naturally without the use of external physical means.This is illustrated clearly by curve E in Figure 2 where resonanttuning is maintained upto larger swing amplitudes approaching the capacity of the pen dulums.Atthatpoint,overtuning ofthependulums causes a rapid flattening of the curve to prevent further increases in swing amplitude. The pendulums have reached their mechanical capacityto absorb vibration. The amount they absorb remains the same as the crankshaft vibrations continue to rise; only the percentage of absorption of the total excitation forces decreases. This results in the pendulum mass operating at near full capacity at crank- shaft excitation levels higherthan it normally could tolerate without the detuning ramps.This is indicated in Figure 3 by the lower curve F which showsthe detuned pendulum mass reaching the lower end of the automotive engine operating speed range at a point 4 on curve D still below the absorption capacity ofthe pendulum.

Itwill be noted, ofcourse,that angles largeror smallerthan 15'can be used forthe initial tautochronic motion without departing from the scope of the invention.

Figure 5shows a side view, and Figures 6 and7 show a cross-sectional view of a practical application ofthe principles illustrated in Figure4to an automotive engine crankshaft. 40 is a two-piece, U-shaped, clam shell like pendulum carrier member adapted to straddle the engine crankshaft 42. This portion ofthe crankshaft is provided with a flat 46 on top of which is mounted a nut 48 welded to the carrier. A cap screw 50 draws the nut up tight to the crankshaft, while a second set of cap screws 52 locate the carrier laterally to the crankshaft.

The carrier assembly in this case is designed to bal ance second order torsional vibrations of the engine by the use ofthree identical bifilar pendulums 54.

The carrier and pendulums are interconnected radially by pairs of rollers 56 movable on mating curved tracks 58,60. The tracks are arcuate portions of over- lapping holes constructed in the manner as illustrated in Figure4 bytracks 10 and 12. That is, the hole contour starts out with the proper shape to generate tautochronic pendulum motion forthe first 15'or so of swing amplitude of the roller; the hole contour or track ofthe rollerthereafter having a progressively decreasing radius ofcurvature as the swing ampli- tude increases beyond a15" swing angle, to pro gressively detune the pendulums to respond less and less to the excitation frequency of the crank-shaft and carrier 40, in the manner previously described.

Insofar as the mechanical construction is concerned, each pendulum 54 isfloatingly mounted in the cavity of the carrier between the nut 48 and crank shaft 42 on the one hand and a pairofdovetail shaped rollertrack inserts 62 on the other hand,with the rollers 56 inbetween. The inserts permit an easy assembly to the carrier upon mating together of the two sheet metal sections ofthe carrier as best seen in Figure 7 Rollers 56 have a thin flange 64 attheirends to prevent contact of the pendulums againstthe inner walls ofthe carrier assembly.Also, a pair of elastomeric bumpers 66 is provided in the base of each pendulum to prevent damage by metal to metal banging contact between the parts during engine cranking and starting operations when gravity on the pendulums is greaterthan the centrifugal force. They also reduce noise during th is time.

From the foregoing it will be seen that the invention provides a bifilar, pendulum vibration assembly that gradually detunesthe pendulums asthey reach higher swing amplitude angles, thus preventing further pendulum excitation that otherwise could damage them and render them inoperative.

While the invention has been shown and described in its preferred embodiment, it will be clear to those skilled in the arts to which it pertains that many changes and modifications may be madethereto without departing from the scope ofthe invention.

Claims (6)

1. A bifilar pendu I um type vibration absorberfor an automotive type internal combustion engine having a crankshaft and a mass projecting radiallytherefrom and rotatabletherewith,the mass having a pair of circumferentially spaced contoured holes therein, a pair of rollers freely rotatably received in the holes, and a pendulum mass radially aligned with and spaced from the crankshaft mass and supported thereon by means of a pair of holes in the pendulum mass of a contour and circumferential spacing mating that of the crankshaft mass also receiving the rollers therein,the pendulum mass being swingable arcuately in response to predetermined torsional vibrational impulsesofthecrankshafttoeffecta rolling of the pins upon the hole contour, the radius ofthe holes tuning the natural frequency ofthe pendulum to coincide with the excitation frequency of the crankshaft at low amplitudes of swinging movement ofthe pendulum to absorb the amplitude torsional vibrational impulses, and means operable at progressively larger swing amplitudes of the pendulum mass to progressively overtune the pendulum oscillations to thereby reduce the reactional response of the pendulum mass to the excitation forces and thereby progressively limit the maximum swing amplitude of the pendulum.

2. Abifilarpendulumtypevibrationabsorberfor an automotive type internal combustion engine having a crankshaft and a mass projecting radiallytherefrom and rotatable therewith, the mass having a pair of circumferentially spaced contoured holes therein, a pair of rollers freely rotatably received in the holes, and a pendulum mass radially aligned with and spaced from the crankshaft mass and supported thereon by means of a pair of holes in the pendulum mass of a contour and circumferential spacing mating that of the crankshaft mass also receiving the rollers therein, the pendulum mass being swingablearcuately in response to predetermined torsional vibrational impulses of the crankshaft to effect a rolling of the pins upon the holecontour,the radius ofthe holes at low amplitudes of swinging movement of the pendulum tuning the natural frequency ofthe pendulum to coincide with the excitation frequency ofthe crankshaft to absorb the lowfrequencytor- sional vibrational impulses, the radius ofthe holes decreasing progressively at progressively larger swing amplitudes ofthe pendulum mass to progressively overtune the pendulum oscillations to thereby reduce the reactional response of the pendulum mass to the excitation forces and thereby progressively limitthe maximum swing amplitude ofthe pendulum.

3. Avibration absorber as in Ciaim 1 or 2, wherein the radii ofthe holes at small amplitudes of oscilla- tion of the pendulum mass are designed so thatthe axis of a roller and thereby the center of mass ofthe pendulum follow approximately epicycloidal paths.

4. Avibration absorber as in Claim 1 or 2,wherein the small amplitudes are in the range upto approximately 15from an initial zero amplitude position.

5. Avibration absorber as in Claim 1 or 2, wherein the decreasing radii constitute detuning ramps four causing the pendulum mass to be progressively less responsive to excitation torsional vibration atthe higher amplitudes or excitation.

6. Avibration absorber as in Claim 1 or 2, wherein the radii ofthe holes at the larger swing amplitudes ofthe pendulum mass become progressively shorter and the curve generated by the axis of a roller during the smalier amplitudes is that of a tautochrone.