Bifilar vibration dampers
United States Patent Saunders et al. Apr. 1, 1975 [54] BIFILAR VIBRATION DAMPERS 3,372,758 3/l968 Jermcy 7. 416/500 X 3.4, 90 Pl l v.4I6I44X 175] Inventors: James Edwin Saunders, Bradford 5 0 809 7 au eta I Abbas near Sherborne; Alan Henry vmcem East Coker near Yeovll, Primary Examiner-Everette A. Powell, Jr. both England Attorney, Agent, or Firm-Larson, Taylor &amp; Hinds [73] Assignee: Westland Aircraft Limited, Yeovil,  
 Somerset, England [22] Filed: Oct. 1, 1973 57 ABSTRACT [21] Appl. No.: 402,140 I ln bifilar vibration dampers adapted for cancelling vibrations in helicopter rotor systems a mass is con- Appl&#39;camm pnorlty Data nected by two pins to a support so that the mass is ca- Oct. 19, I972 United Kingdom .4 48178/72 pable of pendulous movements relative the support. The pins are located in opposed arcuate slots in the [52] U-S- Cl 416/1 4/574 mass and the support, so that the washers for the pin [51] Int. Cl. B64c 27/32 have only to be of slightly larger diameter than the [58] ield Of Search 16/1 74/574 width of the slots in order to retain the pins, thereby minimising inertia effects to significantly increase the [56] References Cited operational efficiency of the damperv UNITED STATES PATENTS 2,353.68] 7/1944 Martin et al. c. 416/500 X 9 Claims, 3 Drawing Figures PATENTEB APR 1 I95 FIG.2  
 BIFILAR VIBRATION DAMPERS This invention relates to bifilar vibration dampers, and particularly to such dampers adapted for cancelling vibrations in a helicopter rotor system.  
  By a bifilar vibration damper we mean a damper having a mass which is connected to a support by two pins to that, during operation, the mass moves in a pendulous movement with respect to the support. When the pendulum natural frequency is equal to an external exciting frequency the pendulum exerts a force on the support which is equal and opposite to the force applied by the external excitation so that vibration is either reduced or eliminated.  
  One of the parts, usually the mass in existing designs, is of U-shaped cross-section and is arranged to receive the other part, i.e. the support, between its parallel side portions. The pendulous movements of the mass are permitted by rolling of the two pins along the surfaces of circular apertures in both of the arms of the U- shaped mass and in the support. Two large washers are clamped on each pin and located between the inner surfaces of the side portions of the U-shaped mass and the adjacent surfaces of the support to retain the pins in their operative positions.  
  When adapted for use in a helicopter rotor system, at least three bifilar vibration dampers are arranged symmetrically about the rotational axis of the main rotor system at the ends of supports in the form of arms extending radially from a rotor hub. The dampers rotate with the rotor system in a plane perpendicular to the axis of rotation, and can be located either above, below, or coincident with the plane of rotation of the main rotor blades. The dampers can be adapted and tuned to cancel either in-plane or rotor mement and shear forces active on the rotor head at any particular frequency.  
  According to the invention we provide a bifilar vibration damper having a mass connected by two pins to a support so that the mass is capable of pendulous movements relative the support, either the mass or the support being of U-shapcd cross-section and arranged to receive the other part between its parallel side portions, wherein the pins are located and retained in opposed arcuate slots in the mass and the support, so as to permit pendulous movements of the mass relative the support by rolling of the pins through the length of the arcuate slots.  
  The invention will now be described by way of example only and with reference to the accompanying drawings, in which:  
  FIG. I is a sectioned side view of an existing bitilar damper construction,  
  FIG. 2 is a plan view of an improved bifilar damper according to one embodiment of the present invention, and  
 FIG. 3 is a sectioned view on lines A-A of FIG. 2.  
  An existing bifilar damper construction is shown in FIG I. and comprises a mass in the form ofa U-shaped weight I I connected by two pins 12 at an end ofa supporting arm 13. One only of the pins 12 is hown in the drawing, and it is to be understood that the assembly of parts is identical at each pin location.  
  The pins I2 are located in circular apertures I4 in both of the parallel side portions of the weight II and in the support 13 to permit pendulous movement of the weight II relative the support 13 by rolling of the pins I2 over the surfaces of the apertures I4. The pins I2 are retained by cylindrical washers I5 located between the surfaces of the parallel side portions of the weight II and the surfaces of the support I3. the washers 15 being located and clamped by annular spacer members I6 retained by nuts I7.  
  It will be apparent in relation to the typical existing bifilar damper shown in FIG. I that the outside diameter of the washers I5 must always be greater than the diameter ofthc apertures 14 in order to retain the pins I2 in their correct operative position. The arrangement means, therefore, that the washers have a relatively large inertia which is detrimental to the operational efficiency of existing bifilar dampers as is more fully explained hereinafter.  
  The improved bifilar vibration damper shown in FIGS. 2 and 3 again includes a mass in the form of a U- shaped weight Il connected by two pins I2 at an end of a supporting arm I3. In this arrangement, the pins are located in opposed arcuate slots I8 and I9 provided respectively in both of the side portions of the weight I I and in the support l3 to permit the required pendulous movement of the weight II relative the support 13. The pins 12 are similarly retained by cylindrical washers 15 located by annular spacer members I6 (FIG. 3) forming the rolling surfaces and retained by nuts I7. however, in this arrangement it will be apparent that the outside diameter of the washers I5 is greatly reduced over that of existing designs (FIG. I This reduction is possible because in the assembly according to the present invention the pins I2 have only be retained in a space represented by the width of the slots I8 and I9 as opposed to a space represented by the diameter of the apertures I4 in existing designs (FIG. I). This means that in the improved design the diameter of the washer I5 has only to be slightly larger than the width of the slots 18 and I9, thereby significantly reducing the inertia of the washers I5 during rolling of the pins I2, resulting in improved operational efficiency.  
  In the particular embodiment of the present invention shown in FIGS. 2 and 3. the arcuate slots I8 and I9 extend through an arc of approximately 180. and are formed in plate members 20 rivetted to the surfaces of the side portions of the weight II and to the surfaces of the support 13 over circular apertures I4 provided in the weight II and the support 13. The plates 20 are supported centrally by annular spacer members 2| rivetted between the plates. This construction has the advantage that it facilitates the fitment of annular liners 22 in the apertures I4 to provide the rolling surfaces. and means for instance that a rolling surface of high grade steel can be provided in a weight I I and support I3 of a lower grade material. To ensure that the annular spacer members I6 roll along the surface of the liners 22 and not on the edges of the arcuate slots I8 and 19 in the plates 20, the outer radius of the tracks 18 and I9 is of slightly greater dimension than the internal radius of the liners, as shown in FIG. 3.  
  However, it is to be understood that this particular construction is exemplary only, and that many modifications are possible. For instance, the arcuate slots I8 and I9 could be machined in the weight II and the support 13, so that the surfaces of the tracks provide the rolling surfaces, thereby dispensing with the liners 22, the spacers 21, and the plates 20.  
  in operation. a hifilar damper is tuned to cancel out of balance forces of a particular frequency.  
  In the case ofexisting bifilar dmpers as shown in FIG. I. rotation of the pins 12 during pendulous movements of the weight II also causes rotation of the relatively large washers 15. thereby creating a high inertia in the washers 15. resulting in a reduction in operational frequency leading to detuning of the damper. Such detuning will have an adverse effect on the operational capabilities. and may well be ofsufficient magnitude to ren der existing constructions ineffective.  
  The present invention provides a bifilar vibration damper in which the outside diameter. and therefore the inertia. of the washers 15 is greatly reduced over existing arrangements. thereby minimising inertia effects of the washers 15 caused by rolling movements of the pins 12 to significantly increase the operational efficieney of the damperv The reduction in outside diameter of the washers I is accomplished in a bifilar damper according to the present invention because of the arrangement in which the pins 12 are restrained to name in arcuate slots 18 and 19 in the weight ll and the support 13. This means that the washers have a diameter only slightly larger than the rolling surfaces of the annular spacer members on the pins 12. as opposed to a diameter which must be larger than the diameter of the apertures 14 in existing designs.  
  In one application. at least three of our improved bifilar vibration dampers are arranged symmetrically about the rotational axis of a helicopter main rotor sys tem. each damper being supported at the end of an arm extending radially front a main rotor hub. The dampers rotate in a plane perpendicular to the axis of rotation. or parallel to the plane of rotation of the rotors, and are oriented and tuned to cancel iii-plane vibration forces acting on the rotor head. In other installations the dampers could be rotated through 90 and tuned to cancel rotor moment and/or shear forces effective on the particular helicopter rotor system in which they are installed.  
  Although one embodiment of the improved bifilar vibration damper has been described and illustrated, it is to be understood that many modifications are possible without departing from the scope of the appended claims, some of which have been previously detailed in this specification. The parts of the assembly could be reversed, so that the support comprises a Ushaped member and the mass comprises a weight located between the two arms of the support.  
 We claim as our invention:  
  l. A bitilar vibration damper indlucing in combination: a support arm adapted to extend radially from and to be rotated in a plane about an axis of rotation and having two arcuate slots extending therethrough, the slots being concentric about parallel axes which are parellcl to and equidistant from the axis of rotation and located with their concave surfaces closest the axis of rotation, at pendulous member of U-shaped cross section having substantially parallel side plate members positioned on opposite sides of said support member and spaced therefrom so as to define a spacing distance from said support member when said pendulous member is centered laterally with respect to the support member. each side plate member having two arcuate slots extending therethrough. the slots being concentric about parallel axes which are substantially parallel to the axis of rotation and being ofsubstantially the same dimensions as the support member slots. the slots in each side plate member being sligned respectively with the slots of the other side plate member and located with their convex surfaces closest the axis of rotation, a pin member extending through each of the slots of the support member and the aligned slots of the side plate members to connect the pendulous member to the support member. the pin member being of substantially circular cross section and concentric about a straight line axis and having a diameter less than the width of the slots, an annular washer positioned between the support member and each of the side plate members and connected at its inner diameter to the pin member and having an outer diameter greater than the width of the slots in the support and side plate members.  
  2. A damper as claimed in claim I, the said washers having an outer diameter slightly greater than the width of the arcuate slots.  
  3. A damper as claimed in claim 2. wherein the washers are fixed longitudinally on the pins by annular spacer memheers clamped by a nut. the outer surface of the spacer members forming the rolling surfaces of the pins in the slots.  
  4. A damper as claimed in claim I. wherein a liner is located on the concave surface of each slot.  
  5. A damper as claimed in claim 4. wherein the liners are of a different material than the support arm and pendulous member.  
  6. A damper as claimed in claim 1. wherein the support arm and pendulous member are supported cen trally of the slots by spacer members secured within the apertures.  
  7. A damper as claimed in claim I. wherein the opposed arcuate slots are machined through the side portions of the support arm and pendulous member.  
  8. A damper as claimed in claim I. wherein the arcuate slots extend through an arc of approximately Hit)&#34;.  
  9. At least three bifilar vibration dampers as claimed in claim I, the said axis of rotation being the axis of rotation of a helicopter main rotor system. each bifilar being secured at the end of an arm extending radially from a rotor hub for rotation with the rotor system in a plane perpendicular to the axis of rotation.