Patent Application
20170334549
This invention applies to airplane propellers and horizontal-shaft wind turbine rotors that have variable pitch control of their individual blades, and particularly to the cases where such control is achieved by means of a passive balance of torques resulting from the application of springs and speed-sensitive and/or aerodynamic forces acting upon the pitch axis. (The terms “propeller” and “rotor” will be used interchangeably except as otherwise noted).
In the prior art, it has been customary to retain each individual blade relative to a central rotor or propeller hub by means of rolling-element thrust bearings to permit angular variation of the blade pitch. The blade-root hub often comprises a spindle or pitch shaft telescoped within and relatively movable angularly via two spaced radial bearings that define a pitch or feathering axis with respect to an outer housing or structure, where one of the spindle or housing is attached along the pitch axis to the individual blade and the other to the central rotational axis of the rotor. Within the configuration of the hub, between or outboard of the two radial bearings, means are provided for transfer of thrust loads between the spindle and the housing, Ball or roller thrust bearings may be heavily loaded by the centrifugal forces experienced by each blade due to its mass as it rotates rapidly about the central axis of the rotor.
Despite rolling-element bearings generally having a relatively low coefficient of friction, the high centrifugal thrust load may cause friction torque not only to be substantial compared to the torque available to overcome it, but it also has other undesirable characteristics. For instance, ball or roller bearings, particularly upon wear and lack of lubrication may develop a “lumpiness” or unevenness in their friction torque as they rotate through an angle of pitch, interrupting the smooth application of changes in control torque and resulting in uneven pitch angular response. This incremental change of torque is extreme, however, upon reversal of direction: friction torque of a rolling-element bearing abruptly reverses when the direction of feathering motion of the blades is reversed (as will be seen in the instant
This unevenness of response may be particularly noticeable when the net torque upon the pitch axis is the result of a balance between torques applied by a calibrated spring and those developed as a function of rotational speed (RPMs) about the central axis and/or aerodynamic forces acting upon the blades The intent for such systems is that the system will be driven to a desired equilibrium pitch angle at which the net torque is near zero.
Passive RPM control is thus typically achieved through the action of a spring that manifests a torque tending to reduce blade pitch in opposition to a torque implemented through suitable linkages from centrifugal forces upon flyweights that urge the pitch of the blades to be increased when speed increases, thereby causing aerodynamic drag to slow the propeller down. This is the basis for passive constant-speed control mechanisms for aircraft propellers. Aerodynamic force acting upon the blades to produce feathering-axis torque is also involved in pitch control with the so-called Aeromatic propeller.
In both cases, accuracy and smoothness of control can be affected by friction torque changes of rolling-element thrust bearings: roughness or abrupt steps in rolling-element friction torque adding in to the net torque on a blade may then result in an equilibrium pitch angle that is offset from that achievable in the absence of such bearing friction, to the detriment of accuracy of control.
In an attempt to increase accuracy of control in such cases, the magnitude of the calibrated spring, RPM and aerodynamic torques might be intentionally increased relative to the undesirable rolling-element bearing torques, leading to increased bulk and weight of a design.
Instead, this disclosure urges the adoption of laminated rubber (i.e., elastomeric) thrust bearings, well known in the prior art (indicated in the referenced patents, e.g., U.S. Pat. No. 2,900,182, etc.). Elastomeric thrust bearings have been developed to deal with this kind of situation, especially for helicopter blade retention: very high compressive loads and limited angles of motion, with small size and no lubrication. These characteristics are the result of an elastomer-metal composite construction, comprising a circular stack of thin metal laminations or layers normal to the load, separated and bonded together by thin layers of rubber (as will be seen in the instant
The torque of an elastomeric bearing is due to shear stress in the rubber layers, being essentially proportional to the rotation angle. The torque is little-affected by the thrust load; i.e., the elastomeric bearing is a torsional spring (as illustrated in the typical torque vs. deflection curve of
The torsional stiffness or spring rate is dependent upon the type of elastomer and the dimensions of the bearing: specifically, for a given elastomer, the stiffness is proportional to the 4th power of diameter and inversely proportional to the total axial rubber thickness, although these parameters may vary somewhat with age and temperature. This rubber-related spring rate would add to that of any otherwise desired total spring rate of an overall mechanism.
In some such cases, it might be found desirable to minimize the torsional stiffness of an elastomeric thrust bearing, which would generally lead to using the smallest diameters possible: in particular, when the overall spring rate is to be minimized, or the accuracy of calibration of the overall spring effect is of paramount importance and effects of rubber parameter variation relative to the whole should be minimized.
In cases where the configuration permits, one way of achieving minimality of rubber-influenced spring rate is to employ a relatively small diameter elastomeric bearing imposed within a loading structure at the end of, rather than encircling the spindle or pitch shaft of the rotor or propeller hub. This configuration removes the need for an aperture in the bearing for passage of the thrust-conveying shaft through it. This not only minimizes the diameter of the bearing for a given loading area and average internal hydrostatic pressure, but also tends to maximize its columnar capacity, both well-known concerns for elastomeric bearing design.
At the same time, this stack of laminations can sustain very high normal forces between its top and bottom surfaces, such as 10,000 psi or more (depicted by the black arrow acting downwards upon the stack and resisted by the surface upon which it rests). But compression is very slight, because the rubber layers are too thin to squeeze out from between the metal layers (as thin as 0.002″, but relatively few are shown, being thicker for clarity).
The laminations are flat in the illustrated cross-section, but their shape may instead may be truncated conical (as in a lampshade), hemispherical or wrapped into a cylindrical arc. So a laminated bearing can resist thrust, radial, or combined normal forces, depending upon the configuration of its laminate surfaces, while permitting limited lateral or angular movement between its opposed outer loading members.
The pitch angular freedom of motion is shown by arrows 10, and the thrust load transmitted between the spindle 5 and the housing 7 is indicated by the heavy arrows pulling apart along the pitch axis 11, while the well-understood means of application of control torques about the pitch axis are not shown (this applies to all the following drawings as well).
It will be understood that the embodiments described above are merely exemplary and that persons skilled in the art may make many variations and modifications without departing from the spirit and scope of the invention as defined in the appended claims.
