Abstract:
A damping apparatus for damping vibrations in a vibrating system including a piston which is reciprocally movable in a fluid filled chamber, piston movement in either direction of movement being resisted by fluid pressure in the chamber behind the piston, which resistance provides damping forces which act to oppose piston movement, the piston being connected to one component of the vibrating system and the chamber being connected to a second component of the vibrating system, characterized in that means are provided to relieve the fluid pressure behind the piston to allow substantially unopposed piston movement in the chamber without damping forces being provided, during piston movements which occur in response to vibrations in the vibrating system at frequencies other than a fundamental frequency which the damping apparatus primarily is intended to damp.

Description:
BACKGROUND TO THE INVENTION  
         [0001]    This invention relates to a vibration damping apparatus for damping vibrations in a vibrating system.  
         DESCRIPTION OF THE PRIOR ART  
         [0002]    Hydraulic dampers are known for use in helicopter rotor systems for example, for damping helicopter blade movements in a plane of rotation as the blades rotate (so called “lead lag dampers”), which vibrations can give rise to a phenomenon known as ground resonance, although similar hydraulic dampers are provided in other vibrating systems to damp vibrations.  
           [0003]    In one form of damping apparatus, the apparatus includes a piston movable in a chamber in response to vibrations, piston movement being resisted by fluid pressure in the chamber at either side of the piston. Restricted fluid flow from one side of the piston to the other is permitted so that the piston may move in the chamber whilst providing damping forces to counteract such piston movements and provide damping.  
           [0004]    Mechanical vibrating systems employing such hydraulic damping apparatus may experience modes of vibration other than at a fundamental frequency which primarily it is desired to damp. Such additional vibrations need not necessarily require damping, but the operation of a damping apparatus which responds to such additional vibrations may result in unnecessarily high damping forces which the surrounding structure will have to be strengthened to support.  
           [0005]    In an effort to alleviate this problem it is known to incorporate into the damping apparatus a load limiting device such as a pressure relief valve, which limits the maximum damping force which can be provided by relieving fluid pressure at either side of the piston when high fluid pressures are produced. However such devices tend to degrade the ability of the damping apparatus to produce useful damping forces to damp the fundamental frequency, particularly in the presence of higher additional frequency vibrations since during part of each oscillation forces will be produced which assist the motion which it is intended to damp.  
         SUMMARY OF THE INVENTION  
         [0006]    According to one aspect of the invention we provide a damping apparatus for damping vibrations in a vibrating system including a piston which is reciprocally movable in a fluid filled chamber, piston movement in either direction of movement being resisted by fluid pressure in the chamber behind the piston, which resistance provides damping forces which act to oppose piston movement, the piston being connected to one component of the vibrating system and the chamber being connected to a second component of the vibrating system, characterized in that means are provided to relieve the fluid pressure behind the piston to allow substantially unopposed piston movement in the chamber without damping forces being provided during, piston movements which occur in response to vibrations in the vibrating system at frequencies other than a fundamental frequency which the damping apparatus primarily is intended to damp.  
           [0007]    Thus in circumstances where the generation of unwanted damping forces in response to vibrations at frequencies other than a fundamental frequency it is desired to damp, can act in the same direction as the disturbing force giving rise to the fundamental vibrating frequency, damping forces are relieved. Thus the damping apparatus can be tuned to provide damping forces primarily in response to vibrations at the fundamental frequency.  
           [0008]    In one embodiment the fluid pressure relief means includes a fluid bypass means which has a first passage for fluid including a first one way valve means which permits substantially unopposed flow of fluid through the passage from a first side of the piston to an opposite second side of the piston, and a second passage for fluid including a second one way valve means which permits substantially unopposed flow of fluid through the passage from the second side of the piston to the first side, and a fluid control means which controls the flow of fluid through one or other of the first and second passages of the by-pass means depending upon the direction of the velocity of the fundamental vibrations frequency to be damped.  
           [0009]    An actuating means may be provided to operate the control means in response to the direction of the velocity of the fundamental frequency vibrations to be damped and to changes in the direction.  
           [0010]    Thus irrespective of the direction of the disturbing force giving rise to vibrations other than at the fundamental frequency, the fundamental vibrating frequency is damped at least when fluid flow through the by-pass means, is prevented.  
           [0011]    The actuating means may include velocity direction responsive means, such as an accelerometer, for sensing the direction of the velocity of the fundamental frequency it is desired to damp.  
           [0012]    Typically the damping apparatus includes a first restricted fluid flow path for fluid from a first side of the piston to a second side of the piston so that when fluid flow through the by-pass means is prevented, piston movement in the chamber in a first direction is permitted controlled by the rate of fluid flow through the first fluid flow path, and a second restricted fluid flow path is provided for fluid from the second side of the piston to the first side so that when fluid flow through the by-pass means is prevented, piston movement in the chamber in a second direction is permitted controlled by the rate of fluid flow through the second fluid flow path.  
           [0013]    According to a second aspect of the invention we provide a vibrating system including a damping apparatus of the first aspect of the invention.  
           [0014]    According to a third aspect of the invention we provide a helicopter including a rotor system having at least one damping apparatus of the first aspect of the invention.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]    The invention will now be described with reference to the accompanying drawings in which:  
         [0016]    [0016]FIG. 1 is a diagram showing the relative displacement in time of one component of a vibrating system relative to another component at a fundamental and another vibrating frequency arising in the system;  
         [0017]    [0017]FIG. 2 is a diagram of the corresponding velocity of the components due to vibrations at the fundamental frequency, and the total resultant relative velocity due to vibrations at the fundamental frequency and the other frequency indicated in FIG. 1;  
         [0018]    [0018]FIG. 3 is a diagram similar to FIG. 2 but superimposing a typical damping force response to the total resultant velocity of a prior art hydraulic damping apparatus;  
         [0019]    [0019]FIG. 4 is an illustrative side cross sectional view of a damping apparatus in accordance with the invention;  
         [0020]    [0020]FIG. 5 is a diagram similar to FIG. 3 but superimposing a damping force response to the total resultant velocity of using damping apparatus in accordance with the present invention.  
         [0021]    [0021]FIG. 6 is an illustration of a helicopter including a rotor system including a damping apparatus in accordance with the invention.  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0022]    Referring to FIG. 4 and FIG. 6, in a vibrating system such as a helicopter rotor system  33  (which is shown in FIG. 6 as the main sustaining rotor, but may alternatively be the tail rotor) a first component such as a rotor blade  34 , is attached in use to a piston  18  of a vibration damping apparatus  15 , and a second component such as a rotor hub  35  is attached to a chamber  16  of the damping apparatus  15 , the piston  18  being adapted thus to be attached by means of a first attachment formation  21  at one end of a piston rod  19  which is connected to the piston  18 . The chamber  16  is adapted to be attached to the second component by means of a further attachment formation  20 .  
         [0023]    The first and second components in use, will relatively vibrate due to disturbing forces to which they are subjected.  
         [0024]    Piston  18  movement in the chamber  16  is however resisted by fluid pressure in the chamber  16  behind the piston  18 . When the piston  18  is urged in a first direction i.e. to the left as seen in the drawing, hydraulic fluid in the chamber  16  behind the piston  18  may pass from a first side  31  of the piston  18  to a second side  32  of the piston  18  through a flow path  18   a  through the piston  18 . However the flow path  18   a  is restricted and as a result damping forces which oppose piston  18  movement will be provided. When the piston  18  is urged in an opposite second direction i.e. to the right in the drawing, fluid may flow from the second side  32  behind the piston  18  through a second flow path  18   b  through the piston  18 , again in a restricted manner to provide a damping force to oppose piston  18  movement. Moreover, each fluid path  18   a ,  18   b  through the piston  18  has a one way valve (not shown) in this example so that only fluid flow in response to one or other first or second direction of piston movement can occur.  
         [0025]    In this example, the piston rod  19  extends from the attachment means  21  beyond the piston  18  through an opening in the axial end of the chamber  16  wall so that the piston  18  is positively guided throughout its entire range of movement.  
         [0026]    In another example, a single flow path  18   a  or  18   b  may be provided for the fluid, without any one way valve means, or a restricted flow path external of the chamber  16  may be provided, but in each case, piston  18  movements in the chamber  16  in response to vibrations in the vibrating system are opposed by the hydraulic or other fluid in the chamber  16  behind the piston  18 .  
         [0027]    In FIG. 1 there is illustrated a waveform  10  of a fundamental high amplitude (displacement) low frequency vibration such as might be experienced by a helicopter rotor blade as a result of transient disturbances of the rotor in the plane of rotation, and a waveform  11  of a relatively low amplitude high frequency vibration which result from normal forced oscillations of the rotor in flight. In the example shown, it is desired primarily to damp the high amplitude low frequency vibration  10  to prevent a phenomenon known as ground resonance. In this example the frequency of the low amplitude higher frequency vibrations is shown as being four times the frequency of the fundamental frequency vibrations it is desired to damp, but this ratio may differ in other operating conditions and/or other vibrating systems.  
         [0028]    The product of the amplitude and frequency of the respective vibrations provides a resultant velocity waveform. The velocity of the fundamental frequency  10  of FIG. 1 is shown at  12  in FIG. 2, and the sum of the velocities of the high and low frequency waveforms  10  and  11  of FIG. 1 i.e. the total resultant velocity, is shown at  13  in FIG. 2. It will be appreciated that the low amplitude high frequency vibrations  11  of FIG. 1 make a greater contribution to the total resultant velocity at  13  in FIG. 2 than the low frequency vibrations  10 .  
         [0029]    In a damping apparatus such as that shown at  15  in FIG. 4, relative movement between the piston  18  and chamber  16  occurs in response to relative movement of the vibrating components, but in a conventional damping apparatus, the damping forces which are produced to damp such vibrations are a function of the total resultant velocity  13  of the vibrating components. Thus in the vibrating system described with reference to FIGS. 1 and 2, conventionally damping forces will be provided as a function of the total resultant velocity  13 .  
         [0030]    A typical damping force response  14  of a conventional hydraulic damping apparatus to total resultant velocity of the vibrating components is illustrated in FIG. 3. The velocity waveform of the fundamental frequency which is required to be damped, is shown at  12   a , and the total resultant velocity waveform of the vibrating system is shown at  13   a.    
         [0031]    It can be seen that the damping apparatus responds to the resultant velocity  13   a  by providing rapid reversals in the direction of applied damping force  14 . Thus for some phases of operation, which are the areas which are cross hatched, the damping forces produced act in the same direction as the disturbing force augmenting the low frequency vibrations  12   a  it is desired to damp, and so rather than damping the fundamental frequency vibrations  12   a , the damping force produced tends to assist the disturbing force. Particularly, at the position indicated at A in FIG. 3, the resultant velocity  13   a  changes direction, and the damping force provided changes direction to compensate and to attempt to damp the resultant velocity  13   a . However the damping force thus provided, indicated at  14   a , is in the same direction as the velocity of the fundamental frequency vibrations  12   a , and thus the performance of the damping apparatus to damp the vibrations at the fundamental frequency is substantially degraded in this phase of operation.  
         [0032]    Referring to FIG. 4 it can be seen that the damping apparatus  15  in accordance with the invention includes a fluid by-pass means  22  which in certain phases of operation of the damping apparatus  15 , relieves fluid pressure behind the piston  18 .  
         [0033]    The by-pass means  22  includes a first fluid passage  24  and a second fluid passage  25 , each passage  24 ,  25  including a one way valve means  26  and  27  respectively. A switchable controller  28  is also provided which in this embodiment is a cylindrical rotatable valve member which, depending upon the rotational position, either permits fluid flow through the by-pass means  22  via the first fluid passage  24 , which is the rotational position shown in the figure, or permits fluid flow through the second fluid passage  25 . The controller  28  is in fluid communication with the chamber  16  at the first side  31  of the piston  18  via a line  29 , and each of the passage  24 ,  25  are in fluid communication with the chamber  16  at the second side  32  of the piston  18  via a line  30 .  
         [0034]    Thus when the controller  28  is rotated to a first open position, as shown, fluid may flow from the first side  31  of the piston  18  through the first passage  24 , past the one way valve  27  therein, to the second side  32  of the piston  18  in response to piston  18  movement to the left i.e. in the first direction, and conversely, when the controller  28  is rotated to a second open position opposite to that shown in the drawings, fluid may flow from the second side  32  of the piston  18  through the second passage  25 , past the one way valve  26  therein, to the first side  31  of the piston  18  in response to piston  18  movement in the second direction i.e. to the right.  
         [0035]    When the controller  28  is rotated to either the first or second open position, the flow passages  24  and  25  and the lines  29  and  30  are dimensioned such that fluid flow from either respective side  31 ,  32  of the piston  18  to the other side  32 ,  31  respectively, no or substantially no damping forces to oppose piston  18  movement in the chamber  16  are provided.  
         [0036]    The controller  28  is operated by an actuating means  33  shown purely diagrammatically which responds to changes in the direction of the velocity  12   a  of the low frequency high amplitude vibrations it is desired to damp. The actuating means  33  may be a sensor such as an accelerometer, pendulum or similar device, which is adapted to sense changes in direction of the velocity  12   a  and rotates the controller  28  to an appropriate first or second open position so that damping forces are not provided which otherwise would assist the disturbing force giving rise to the fundamental frequency vibrations.  
         [0037]    Referring to FIG. 5, when the velocity  12   a  of the fundamental frequency vibrations is positive, the actuating means  33  may move the controller  28  to the position shown in FIG. 4. The controller  28  remains in this position even when the direction of the total resultant velocity  13   a  of the vibration changes, i.e. at the position indicated at A because it is tuned to respond only to the low frequency vibrations  12   a . By virtue of the by-pass means  22  though, at position A fluid may freely flow through passage  24  and the one way valve  27  therein in response to piston  18  movement in the first direction, so that the damping force is prevented. When the total resultant velocity  13   a  of the vibrations changes direction again, i.e. at position B, fluid flow through the by-pass means  22  will be prevented by the one-way valve  27  and thus damping forces will again be applied to oppose piston movement in the second direction.  
         [0038]    As illustrated in FIG. 5, before the resultant velocity  13   a  again changes direction, at position D, the velocity of the fundamental frequency vibrations  12   a  will change direction i.e. to negative at position C. As a result, the actuating means  33  will cause the controller  28  to rotate to the second, opposite open position in which first side  31  of the piston  18  is in communication with the passageway  25  of the by-pass means  22 . Thus piston  18  movement in the second direction (from left to right as seen in the drawing), i.e. due to the resultant velocity  13   a  being positive, will result in no damping forces being applied as fluid may freely flow through passageway  25  and the one way valve  26  therein from the second side  32  of the piston  18  to the first side  31 .  
         [0039]    When the resultant velocity  13   a  again goes negative, i.e. at position D, fluid flow through the by-pass means  22  will be prevented by the one way valve  26  and consequently damping forces in the first direction to oppose vibrations at the fundamental frequency  12   a  will be provided. Whilst the resultant velocity  13   a  and the velocity  12   a  of the fundamental frequency vibrations both remain negative, i.e. until position E indicated in FIG. 5, damping forces will continue to be applied. At position E, where the resultant velocity  13   a  again goes positive but the velocity  12   a  of the fundamental frequency vibrations remains negative, to prevent the damping forces contributing to the disturbing forces giving rise to the vibrations at the fundamental frequency, the damping forces are relieved by fluid flow through passageway  25  and one way valve  26  of the by-pass means  22 .  
         [0040]    At the position indicated at F in FIG. 5, the velocity of the fundamental frequency vibration  12   a  again changes direction, and as a result the actuating means  33  moves the controller  28  back to the position shown in FIG. 4 and damping forces will again be imposed. At position G shown in FIG. 5, damping forces are again prevented as the resultant velocity  13   a  again changes direction, and fluid may flow through passageway  24  and one way valve  27  of the by-pass means  22 , until the resultant velocity  13   a  again changes direction at position H when damping forces will again be applied.  
         [0041]    Thus in all positions where the provision of a damping force would assist the disturbing force giving rise to the fundamental frequency vibrations it is desired to damp, such damping forces are prevented by the by pass means  22  of this invention.  
         [0042]    Various modifications may be made without departing from the scope of the invention. For example although an actuating means  33  being an accelerometer has been described to actuate the controller  28  in response to changes in the direction of the velocity of the fundamental frequency vibrations, any other suitable velocity direction responsive means may be provided.