Patent Publication Number: US-3874829-A

Title: Sealing device for rotary mechanisms

Description:
[ Apr. 1, 1975 United States Patent 11 1 Pratt SEALING DEVICE FOR ROTARY MECHANISMS Inventor:  
 Primary Examiner-C1. Husar Assistant E.\&#39;aminerLeonard Smith n g Pratt North Hdledon Attorney, Agent, or FirmRaymond P. Wallace [73] Assignee: Curtiss-Wright Corporation,  
 Wood-Ridge, NJ.  
 Nov. 19, 1973 [22] Filed:  
 planetates about a shaft axis, a sealing device for each Appl. No.: 417,031  
 of the rotor apex portions, which device comprises a seal blade disposed for reciprocation toward and away from the trochoidal surface of the housing, and pressurized fluid cushioning means responsive to centrifu- [58] Field of Search 418/1 13, 115,122, 123,  
 gal forces acting in alternating directions with regard to the shaft axis, and coacting resiliently with the seal blade to allow it to recede rapidly from sealing en- [56] References Cited UNlTED STATES PATENTS gagement with the housing surface and to return slowly, at predetermined conditions of centrifugal acceleration.  
 3.229.673 1/1966 Ehrhardt............1............ 418/123 x 3,796,527 3/1974 Woodier ct 418/123 X 3 x 10 724 5/1974 Luukkoncn 123/849 x 6 Clams 4 Drawmg figures \I id s Z s 6 1 FMENTEB APR 1 SHEET 1 9 3 PATENIEBAPR um 3,874,829  
 SHEET a g5 55 SEALING DEVICE FOR ROTARY MECHANISMS BACKGROUND OF THE INVENTION This invention relates to sealing devices for rotary mechanisms, and more particularly to sealing devices disposed at the apex portions of the rotors of rotary mechanisms of the type disclosed in U.S. Pat. No. 2,988,065.  
  In rotary mechanisms of the trochoidal type having a rotor rotating about an axis which in turn planetates about the shaft axis, all portions of the rotor trace a trochoidal path, and hence the peripheral housing, which forms chambers of variable volume with the working faces of the rotor, has an inner surface of trochoidal profile along which the rotor apexes slide in sealing relation to isolate the chambers from one another. Each of the rotor apexes is provided with a longitudinal slot parallel to the rotor axis, in which is disposed a sealing bar or blade which sweeps the peripheral wall, the sealing bar being resiliently loaded in the radially outward direction to maintain good sealing contact.  
  As set forth in US Pat. No. 3,456,625, there is also a centrifugal force acting on the sealing bars, which is directed radially outwardly throughout the larger portion of the trochoidal path, but which changes its direction at each cusp of the trochoid and becomes radially inward, owing to the change in the direction of curvature of the path. This characteristic is fully explained in the patent referred to.  
  If the spring force urging the seal outwardly is sufficient to maintain the seal in contact with the trochoidal wall at the cusps where the curvature is convexly inward with respect to the axis, when centrifugal force is added to this at the outwardly curved portions the seal wear becomes excessive and power loss from friction is noted. On the other hand, if the spring force is not so great the seal will leave the trochoidal surface at the cusps and then rebound into contact where the direction of curvature changes again to outward. This hammering of the housing wall by the sealing bar results in the so-called chatter marks, slight ripples in the trochoidal surface in the region just downstream from the cusp, which impair sealing efficiency.  
  Various expedients have been proposed to overcome this difficulty. In the patent referred to above, pivotally mounted counterweights are attached to the sealing bars, so that as the weights are centrifugally urged outwardly they pull the seals inward. Another scheme is shown in U.S. Pat. No. 3,456,626, wherein after the seal leaves the trochoidal surface at the cusp it is seized by a centrifugally actuated pinching mechanism which holds it out of contact. A further device is disclosed in U.S. Pat. No. 3,482,551, in which a centrifugally actuated toggle cams the seal inwardly as the toggle weight is urged outwardly. Still another mechanism is found in U.S. Pat. No. 3,496,916, wherein the tendency ofa ring to walk around a cylindrical member on which it is centrifugally swung is used to pull the seals inwardly.  
  All these devices require very precise proportioning of masses, careful computation of lever arm moments, and accurate positioning of pivot points and linkages; and in the main they effect a positive retraction of the seal at a given centrifugal force and hold it out of contact until that force diminishes. If the connection between the seal and the actuating mechanism is positive in both directions, when the curvature of the path changes the seal will be driven back against the housing surface, with the possibility of causing chatter marks. If the connection is only in the direction of retraction, at some speeds the seal is perhaps pulled farther away from the housing than is desirable, with consequent gas leakage, and may not return at all until the speed diminishes.  
  A different approach is taken in U.S. Pat. No. 3,229,673, wherein inwardly directed movements of the sealing bar are prevented by a hydraulic damping means. This prevents the seal member from leaving the trochoidal surface at the cusps, but since the rotor is filled with oil at all times and the damping means receives its pressure from this source, the seal is held outwardly during the large sweeps of the trochoid by what can be an excessive force, therefore resulting in considerable wear.  
  The present invention provides means of overcoming these difficulties of the prior art.  
 SUMMARY This invention provides oil cushioning means by which at a predetermined centrifugal force the apex seal bars are allowed to fly inward readily from the cusps of the trochoid, but return only slowly when the direction of force is reversed, so that there is no bouncing of the seal bar to cause chatter marks. At high speeds of operation when the centrifugal force is great the seals remain out of contact with the trochoidal peripheral wall so that they produce no friction therewith. The slight amount of fluid leakage which this mode of operation may produce can be tolerated, since fluid leakage is a function of time and at high rotational rates there is less time for fluid to escape from a working chamber at high pressure. At low operational speeds when close sealing is required the seal bars will not leave the peripheral wall and good contact is maintained.  
  This mode of operation of the seal bars is effected by having each of them attached to a double-acting piston movable in a chamber of pressurized oil, with suitable valving and venting means arranged so that the oil under the piston can readily flow to the top side of the piston under the influence of an inward thrust, but can be displaced in the other direction only slowly under an outward thrust.  
  It is therefore an object of this invention to provide a trochoidal rotary mechanism with a device which holds the apex seal out of contact with the peripheral sealing surface at a predetermined speed of rotation of the rotor.  
  Another object is to provide means to compensate for the variable effects of centrifugal forces on the apex seals of the rotor.  
  A further object is to provide means to prevent excessive frictional wear of apex seals and the peripheral wall.  
  Other objects and advantages will become apparent on reading the following specification in connection with the accompanying drawings.  
 BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an end view of a rotary trochoidal mechanism with one end wall removed;  
  FIG. 2 is an enlarged view in cross-section of a portion of the mechanism of FIG. 1, showing the oil cushioning device in its relation to the apex seal and other portions;  
  FIG. 3 is a much enlarged cross-sectional view of the cushioning device, in elevation taken transversely to the showing of FIG. 2; and  
  FIG. 4 is a cross-section taken substantially along line 22 of FIG. 1 on an enlarged scale and showing two cushioning devices.  
 DESCRIPTION OF A PREFERRED EMBODIMENT The invention will be described in terms of a twolobed epitrochoidal mechanism with a generally triangular rotor, but it is to be understood that it applies to trochoidal mechanisms of any number of lobes, the rotor having one more apex portion than the number of lobes in the epitrochoid. Also, although the mechanism is shown as an internal combustion engine, the invention may be applied to trochoidal mechanisms of other types, such as compressors, pumps, and expansion engines. Various elements such as rotor oil seals, bearings, cooling means, and other structures not necessary for an understanding of the invention have been omitted from the drawings for simplicity of illustration.  
  In FIG. 1 there is shown a rotary mechanism 11 comprising a peripheral housing 12 having an epitrochoidal inner surface 13 of two lobes, closed by a pair of end walls 14 of which only the rearmost is shown. A rotor 15 is mounted on an eccentric portion 16 of a shaft 17, rotation being imparted to the shaft by rotation of the rotor within the cavity. The rotor has three apex portions 18 with a working face 19 extending between each pair of apex portions, the working faces defining with the housing three working chambers 21 of variable volume.  
  A fluid intake port 22 is provided for the intake of combustible gaseous fluid. After intake and compression the gaseous fluid is ignited by a suitable ignition means, as indicated by the zigzag arrow 23. The combustion gases then re-expand the working chamber producing rotation of the rotor, and are discharged through an exhaust port 24. To isolate the working chambers from one another, the rotor carries in an axially disposed slot at each apex a sealing bar 26 which sweeps the trochoidal surface in sealing relation thereto, and on each of its side faces a plurality of seal strips 27, each of which strips 27 is disposed near the periphery of the rotor and approximately parallel to the profile of the working face, to seal the space between the rotor side faces and the end walls of the housing. An intermediate seal pin 28 is disposed at each apex portion on each side of the rotor, and effects sealing be tween the apex seals 26 and the side seals 27.  
  It will be seen from a consideration of FIG. 1 that the inner surface 13 is concave in the outward direction over each of the two lobes of the epitrochoid. However, at the junction regions between lobes, the cusps 29 of the trochoid, the direction of curvature changes to convex. It will also be apparent that whenever an apex of the rotor is crossing a cusp the shaft eccentric 16 is on the opposite side from that cusp, since the shaft turns three times for each turn of the rotor. Hence, the centrifugal throw of the rotor mass and of the apex seals is in the direction away from the cusp.  
  The apex seals are resiliently loaded in the direction outwardly from the shaftaxis. This loading is slight, but sufficient to maintain sealing contact at the cusps at low speeds of rotation, when it is important to minimize gas leakage between working chambers. However, this loading is additive to the centrifugal throw in the other portions of the epitrochoid, and consequently at high speeds when the centrifugal throw is great it would produce excessive wear on the seals and the sealing surface. It has therefore been the prior art practice to make the resilient loading as light as possible, with the result that at high speeds the loading is overcome atthe convex cusp portions and the seals are thrown inwardly. Then when the curvature changes to concave the seals are thrown outwardly again with considerable force, resulting in chatter marks and still possibly two great a wear.  
  Since gas leakage between working chambers is a function of time, it is possible at high rotational speeds to allow the apex seals to be out of contact with the peripheral trochoidal surface 13 with only a minimal leakage such as can be tolerated. This invention therefore provides oil cushioning means coacting with each apex seal, whereby the seal bars are permitted at moderate 3 rotational speeds to fly inwardly from the sealing surface at the cusps, and return only slowly outwardly. When the rotational speed is greater and the centrifugal forces consequently large, the seals fly inward to greater depth and the time is not great enough for them to return to the surface at all during the sweep of a sinone apex of the rotor 15 as it crosses a cusp 29. The cushioning means will be better seen in the, enlarged 1 view of FIG. 3. i  
  The oil cushioning means 31 is disposed in a housing 32 which may be an integral portion of the&#39;rotor 15 as shown, or which may be a separate element attached by any suitable means. The rotor hub is bored for seating a bearing 33 surrounding the eccentric 16, and at least one housing member 32 extends radially outwardly from the rotor hub toward each rotor apex 18.  
 Each housing member contains a cylindrical cavity 34, in which is disposed a hollow barrel 36 for receiving oil, and containing other elements to be described. Cavity 34 is closed at its radially inward end by a plug 37 which may be retained by screw threads, press fitting, or other conventional means. The shaft 17 has. an  
 axial passage 38 for oil from an oil pump or other supply means (not shown), and the eccentric portion 516 has at least one radial passage 39 communicating with passage 38. The bearing 33 is provided with at least one aperture 41 communicating with passage 39, and an annular groove 42 in its exterior surface which is kept filled with oil supplied through the shaft passages. Plug 37 has a further communicating bore 43 which empties into barrel 36.  
  The barrel contains a double-acting piston 44 slid-. ably disposed therein and having attached thereto a pushrod 46 with its radially outer end connected to the.  
 apex sealing bar 26, as by riveting, welding, brazing, or i 3 other rigid means. It is important that the connection should allow no play between the pushrod and the seal member, so that centrifugal force transmitted from one element to the other will not be expended in lost mo.- tion.  
  The inward end of the piston 44 has a cylindrical platform 45 of a diameter closely fitting theinterior of the barrel and is provided with sealing means 47 such as an O-ring to prevent leakage of oil past it during reciprocating movement of the piston. A spring 48 is positioned between the piston and the plug 37 and provides the resilient loading of the apex seal in the radially outward direction.  
  The barrel member 36 has one or more grooves 49 in its outer circumferential surface and extending in the radial direction. Each groove communicates with the interior of the barrel below the piston platform 45 by means of an aperture 51 through the barrel wall below the piston, and also with the space above the piston platform 45 by an aperture 52 through the outer end wall of the barrel. A spring 53 rests on the radially outer surface of platform 45 and surrounds the stern of piston 44, holding an annular valve plate 54 against the outer closure surface of the barrel to close apertures 52. One or more apertures 56 extend through platform 45 to provide communication between the oil chambers above and below the platform, but the total crosssectional area of apertures 56 is considerably less than the total cross-sectional area of the passage system comprising the passage 49, 51, and 52. In other words, the communication provided by passage 56 between the oil space above the piston platform and that below it comprises a restriction as compared to the communication provided by passage system 49, 51, and 52 between the oil space below the platform and that above It.  
  In operation of the rotary mechanism above a predetermined threshold of rotational speed, when the inwardly directed centrifugal force is great enough as the apex seal 26 crosses a trochoidal cusp 29, the seal will be impelled inwardly and the piston 44 will be pressed inwardly in its barrel 36, overcoming the outward bias of spring 48. There is no restriction to such inward movement of the piston, since the oil under platform 45 is transferred through the passage system of aperture 51, groove 49, and aperture 52, as shown by the arrows, displacing valve plate 54 against the slight pressure of spring 53 and allowing the oil to enter the expanding chamber above the platform.  
  When the seal has traveled inwardly as far as the magnitude of the centrifugal force will thrust it, there being no further movement of oil the spring 53 will then return the valve plate 54 to its closed position. The seal 26 is thus held out of contact with the peripheral wall by its connection with the pushrod 46 of the piston until reversal of centrifugal force starts to urge it in the outward direction. However, since valve plate 54 has closed oil cannot leave the outer chamber by the same route it entered. Instead, it bleeds slowly into the inner chamber through the small orifice 56, allowing the seal to return gradually to contact with the peripheral wall.  
  If the rotational speed has been only slightly over the predetermined threshold for operation of the cushioning system, the inward force will have been moderate and the seal will have moved only a slightly amount inwardly from the trochoidal wall, and will return thereto before it has completed its sweep of the epitrochoidal lobe. Nevertheless, its return is gradual so that it does not sharply impact the wall and thus the formation of chatter marks is avoided. If rotational speed is high and inward centrifugal force great, the seal may disengage to the full extent allowed by the dimensions of the equipment. It will then return only partially toward the peripheral wall and at the next cusp will again be impelled inwardly, so that itcan be maintained out of contact until speed is reduced.  
  The amount of radially inward travel of the sealing bar 26 can be limited either by the radial dimension of the seal itself and the depth of the seal slot in the rotor, or by the amount of travel permitted to piston 44 when its biasing spring is fully&#39;compressed&#39;, or by installing stop members at any convenient location. Such stop members may be adjustable in positioning. It will be understood that the clearances shown in the drawings have been greatly exaggerated for clarity of illustration, and that the actual amount of radial seal movement permitted is minute, of the order of a few thousandths of an inch, in order to limit the amount of leakage which may occur at high speeds.  
  The mode of operation described thus far has been that in which it is desired to permit the sealing edge of the apex seal bar to depart from an epitrochoidal path as a result of inwardly directed centrifugal force at the region of the trochoidal cusp, and to return in the direction of re-engagement with the trochoidal surface at a controlled retarded rate during periods of outwardly directed centrifugal force.  
  There exist, however, other modes of operation of the seal cushioning means of this invention which serve to reduce seal wear and wear of the trochoidal surface. The trochoidal housing surface may in certain portions deviate slightly from its designed profile because of minor machining inaccuracies causing slight dimensional irregularities, or because of thermal distortions causing such irregularities during operation of the rotary mechanism. In addition, the apex sealing bars may depart minutely from their theoretical epitrochoidal path as a result of mechanical clearances of the shaft and eccentric bearings, or of backlash in the phasing gears.  
  For example, if any portion of the trochoidal surface is positioned radially inward from its theoretical location because of slight dimensional irregularity, the seal traversing it will be forced inwardly. If the irregularity is radially outward, the resilient loading of the sealing bar, or centrifugal force, or both such causes, will thrust the seal in the outward direction, but only at the retarded rate permitted by the bleed aperture 56. Bearing clearances may permit a rotor apex to travel either slightly inside or outside its theoretical epitrochoidal tracing path at some portion of the housing surface,  
 again with the result that the seal will be forced inwardly by the pressure of contact with the housing surface or allowed to move slowly outwardly as a result of failing to make contact with the housing surface. Such circumstances will produce a slight radially reciprocative motion of the sealing bars which is not due to centrifugal effects. Nevertheless, the oil cushioning means will moderate this reciprocative motion and lighten the average contact pressure between the seals and the housing surface, reducing the wear of the members.  
  The sizing of the crosssectional area of the passage system 49, 51, and 52 and its proportion to the crosssectional area of bleed passage 56 will be selected in accordance with the mass of the seal and the movable portions of the cushioning means, and in accordance with the magnitude of the centrifugal forces encountered at the selected threshold rate of speed for operation of the system. The resilience of springs 48 anad 53 will be selected in accordance with the same factor. Such dimensioning and resilience will vary according to engine size and the selected rates of speed producing the centrifugal forces to operate the cushioning system.  
  FIG. 4 shows an embodiment wherein a plurality of cushioning means 31 are employed with each apex seal member 26. Although a single cushioning means is considered to be ordinarily adequate, with a rotary mechanism having a rotor of great axial width it may be desired to use two or more such cushioning means with each apex seal, the construction of each such means being as previously described, each connected to its associated apex seal. An additional spring loading means 57 may be employed between the seal and the bottom of its seal slot, whether a single cushioning means is used or a plurality.  
 What is claimed is:  
  1. In a rotary mechanism having a housing with a peripheral wall and a pair of parallel end walls defining a cavity therein with an axis transverse to the end walls, the peripheral wall having an inner surface of trochoidal profile wherein the curvature is alternately concave and convex with respect to the axis, and a rotor supported for eccentric rotation within the cavity and defining with the housing walls a plurality of working chambers of variable volume, the rotor being of generally polygonal profile and having a plurality of apex portions, each apex portion of the rotor having an axially extending slot with a sealing bar disposed therein for radially reciprocative movement within the slot and sweeping the trochoidal surface in sealing relation thereto, the sealing bar being subject to radially outward centrifugal force while traversing the concave portion of the peripheral surface and radially inward centrifugal force while traversing the convex portion of the peripheral surface, and biasing means urging the sealing bar in the outward direction, wherein the improvement comprises:  
 a. oil cushioning means disposed within the rotor in each apex region thereof, the oil cushioning means comprising at least one barrel member having double-acting piston means therein slidable within the barrel in the radial direction with respect to the rotor axis and connected to the sealing bar, the barrel containing oil radially inward andradially outward of the piston means; l b. the oil cushioning means having first passage means for relatively free flow of oil from the position radially inward from the piston to the position radially outward from the piston, and having second passage means restricted for retarded flow of oil from the position radially outward from the piston to the position radially inward from the piston;&#39; c. inward centrifugal force on the sealing bar impelling the double-acting piston means inwardly at a relatively rapid rate to permit disengagement of the sealing bar from the peripheral surface, and outward centrifugal force on the sealing bar impelling the double-acting piston means outwardly at a relatively slower rate to retard re-engagement of the sealing bar with the peripheral surface.  
 2. The combination recited in claim 1, wherein the peripheral surface has dimensional irregularities in radial directions, and radially inward pressure of a dimensional irregularity on the sealing bar impels the doubleacting piston means radially inwardly at a relatively rapid rate, and radially outward relief of pressure on the sealing bar allows radially outward travel of the double-acting piston means at a relatively retarded rate.  
 3. The combination recited in claim 1, wherein there are two oil cushioning means coacting with each apex a sealing bar.  
  4. The combination recited in claim 1, wherein radially inward motion of the piston means pumps oil through the first passage means, the first passage means having normally closed valve means which opens in response to oil transfer from the position radially inward 6. The combination recited in claim 5, whereinthe,  
 barrel member contains resilient means urging the piston in the radially outward direction.