Abstract:
A friction damper is mounted in series with a viscous damper to accommodate the normal magnitude vibration and the higher magnitude vibration occasioned from a larger imbalance of a jet engine rotor manifested by a lost blade or the like.

Description:
CROSS REFERENCE 
     This patent application relates to the damper of the type disclosed in U.S. Patent Applications filed by D. C. Moringiello and S. H. Dallman and D. C. Moringiello, D. H. Hibner and M. D. Dulberger on even date and entitled FRICTION DAMPER and VISCOUS/FRICTION DAMPER respectively, and assigned to the same assignee as this patent application. 
     DESCRIPTION 
     1. Technical Field 
     This invention relates to rotors for gas turbine engines and particularly to dampers therefor. 
     2. Background Art 
     U.S. Pat. No. 3,756,672 filed by R. J. Comeau and myself on Sept. 4, 1973 and assigned to the same assignee as this patent application describes a viscous damper and for further details, reference should be made thereto. 
     The viscous damper serves to impede the vibratory motion that is manifested by the rotor imbalance. Oil is squeezed between the whirling cylindrical surface and a stationary cylindrical support pedestal. The gap therebetween is established by the required load and damping characteristics of the system. It is desirable to maintain the gap as small as possible to limit rotor excursions so that the tips of the attached blades do not touch the adjacent seals or casing. These design considerations therefore are limited to handle the normal vibratory responses occasioned by typical rotor imbalance to which the engine is subjected. It therefore, is not capable of handling the heavier loads that are manifested by blade loss or the like. 
     I have found that I can obviate this problem by providing a friction damper discretely located and mounted in series with the viscous damper, so that any load above a certain threshold is handled by the friction damper and until that load is exceeded, the friction damper is rendered inoperative, 
     DISCLOSURE OF INVENTION 
     An object of this invention is to provide for a rotor an improved damper system. 
     A feature of this invention is judiciously locating a friction damper in series relationship to a viscous damper to dampen loads exceeding the loads normally handled by the viscous damper. The friction damper is mounted concentric to the viscous damper. 
     Other features and advantages will be apparent from the specification and claims and from the accompanying drawing which illustrates an embodiment of the invention. 
    
    
     BRIEF DESCRIPTION OF DRAWING 
     The sole FIGURE is a partial sectional view of a shaft of a rotor rotatably supported by a bearing damped by the viscous/friction damper of this invention. 
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     As shown in the sole FIGURE, shaft 10 is rotatably supported by the roller bearing generally indicated by reference numeral 14. Bearing 14 comprises an inner race 16, outer race 18 sandwiching a plurality of circumferentially spaced rollers 17 (one being shown). 
     While this bearing support system is for a jet engine as is exemplified in the model TF-30 jet engine manufactured by Pratt &amp; Whitney Aircraft Group of United Technologies Corporation, the assignee of this patent application and which is incorporated by reference, it is to be understood that the invention has application for other rotor systems. The outer diameter of the outer race 18 is spaced from the inner diameter of the annular pedestal 20 for defining an annular cavity for continuously receiving oil in the customary manner. This defines the viscous damper. The side edges are sealed by piston rings 22. 
     Pedestal 20 is supported in position by hairspring 24 which has one leg 26 grounded. Hairspring 24 carries a pair of concentric cylindrical surfaces 30 and 32 for accommodating the friction damper generally illustrated by reference numeral 34. The friction damper comprises axially aligned friction plates or discs 36 and 36A that surround the shaft 10. The outer diameter of friction plate 36 is snugly fitted onto the surface 30 and the inner diameter of alternate friction plates is snugly fitted on the cylindrical surface 32. The plates are axially loaded against the shoulder 38 projecting inwardly from surface 30 by load nut 40. The load nut is torqued so that the friction damper is rendered inoperative until a certain load beyond that handled by the viscous damper is excluded. 
     It is apparent from the foregoing that the friction damper is in series relationship to the viscous damper and operates to handle different ranges of loads caused by certain imbalance characteristics of the engine&#39;s rotor supported thereby. 
     Slots or apertures 50 are located in the friction plates at alternating locations to allow for the passage of oil 52 so as to prevent overheating of the friction plates. 
     As is apparent from the foregoing, the friction plates 36 and 36A will rub together when the viscous damper bottoms out upon exceeding certain loads and causes the support to deflect. Resistance to this load is provided by the friction force between plates which results from the force normal to the rubbing surfaces. This normal force, which is necessary to produce the frictional force, is provided by the load nut. A spring may be disposed between the nut and friction plates to provide a known normal load. 
     The hairspring 24 serves to restore the friction plates 24 back to the center position as shown in the sole FIGURE. Hence, when this force manifested by an imbalance of the rotor exceeds a predetermined value, the centering spring deforms and simultaneously causes the friction plates to displace and dissipate that imbalanced load. When the load subsides the displaced spring returns back to its original position and concurrently restores the friction plates to the center position. Obviously, it is not significant that the plates be centered or that they be returned to the original position. It may be desirable in some installations to design the loads so that the plates merely move without any regards to the point where they return. 
     It should be understood that the invention is not limited to the particular embodiments shown and described herein, but that various changes and modifications may be made without departing from the spirit and scope of this novel concept as defined by the following claims.