The invention relates to a method of manufacturing a rotor for an air motor. A shaft having a section with spiral grooves located adjacent to a cylindrical section is placed in a fixture. The shaft is placed in tension causing the cylindrical section to contract radially while expanding axially. A ring having spiral grooves is placed on the contracted cylindrical section. On removal of the tension, the cylindrical section expands to form a locking connection between the ring and shaft which holds the spiral grooves in a herringbone pattern.
The use of gears having teeth which meet to form a herringbone pattern is known by industry. A herringbone gear is a combination of two helical gear sets having equal but opposite helix angles. Two methods for manufacturing such herringbone gears are: (1) machine the opposing helix angles on a common gear blank (generally a void is necessary for cutting tool runout between the two gear patterns); or (2) machine each helix gear independently and then assemble them to a common shaft to form the herringbone gear. The most common method is to join two independently machined helical gears to a shaft to form a herringbone rotor. Thereafter, gears can be bound by welding, brazing, threading, diffusion bonding, shrink fit in conjunction with heating the ring member and/or cooling the shaft member, pinning, etc. Rotors made by some of these methods of joining the parts together were supplied with hot gas to simulate the operation of an air motor in an aircraft. The hot gas is compressor discharge air and can reach a temperature of 1050.degree. F. and pressure of 600 psig. The air acts upon a pair of rotors and drives the rotors at speeds above 20,000 RPM at acceleration and deceleration rates up to 16,000 revolutions per second.sup.2. Closed clearances must be maintained between the rotors and between each rotor and the motor housing to achieve reasonable engine efficiency and motor output torque.
Rotor tooth form accuracy limits approach those of conventional gearing though the rotor is a three-tooth gear, with a 45.degree. helix angle on each half of the herringbone gear form. The dimensional accuracies must accommodate thermal changes in growth between rotor centerlines and motor housing dimensions through transients of temperature change of several hundred degrees in less than a second of time. The herringbone pattern must withstand reversing loads which are radial and axial.
To achieve the herringbone rotor accuracy, the helical gears must be aligned rotationally within approximately 0.001 of an inch, and the gear end faces must mate without a gap and remain gap-free during operation in the motor. The gear profile must be maintained within about a 0.002 inch tolerance band.
The conventional methods of joining the gears described were evaluated for these conditions and were found undesirable.
When the gears were welded, it was difficult to inspect the weld joint sufficiently to ensure suitable reliability for use as a rotor in an air motor for an aircraft.
When the gears were brazed, it was difficult to inspect the joint adequately, and the close tolerance gear profile was jeopardized by the high temperature required to achieve the brazing.
The gears could not be threaded since the space limitations in the rotor precluded the design of sufficient threads to assure structural integrity.
When the gears were bonded by the diffusion process, the gear profile was distorted due to the high temperature of the process, and the joint could not be inspected adequately to ensure a suitab1e bond was achieved.
When the gears were joined by shrink fit by heating one gear and cooling the other gear, the close tolerance required to align the helix could not be achieved repeatedly, since only a few seconds were available to achieve the final assembly before the temperatures equalized sufficiently to lock the joint.
When the gears were joined to the shaft by pinning or staking the interface joint was not sufficiently tight to provide the structural rigidity needed of the rotor.
The method of manufacturing a rotor according to the invention disclosed herein is repeatable, controllable and does not result in distortions of the helix or spiral gears. In addition this rotor can now be manufactured by commonly used gear forming equipment. Whereas similar type rotors essentially limited to manufacture on a Sykes Gear Machine.
In the invention, a shaft is machined to have: (1) bearing sections located adjacent the ends thereof; (2) a series of spiral grooves that extend to the midpoint of the shaft; and (3) a locking section located between the end face of the spiral grooves and the other end of the shaft. At the same time, a ring is machined to have a series of spiral grooves on its periphery. Thereafter, the ring is placed on the bearing surface adjacent the locking surface and the shaft placed in a fixture. A tension load was placed on the shaft through the ends which causes the shaft to elongate axially and shrink radially. The inner diameter of the ring and the initial diameter of the locking surface are designed to be within a few thousandth of an inch (0.0004 inches) and as a result the required shrinkage is within the elastic range of the shaft material. Once the desired shrinkage is achieved, the tension load on the shaft can be maintained as long as necessary to slip the gear ring in place and index it rotationally to achieve the precise gear alignment, and to clamp the helical gears together to achieve intimate contact between the gear end faces.
When the tension load is released from the shaft ends, the shaft length shortens and the shaft diameter expands. Since the gear ring and shaft were sized or matched to achieve an interference fit in their free state, the resulting joint is a self-locking interference fit on the shaft journal and gear ring hub.
The mating gear end faces also are placed in compression by the shortening that occurs in the shaft length. This compression preload between the gear end faces provides the radial stiffness required of the rotor to withstand radial loads due to air pressure and rotor to rotor contact forces during motor operation.
An advantage of this method of manufacturing a rotor occurs through the individual machining of two helical gears which are later joined together to form a unitary structure.
An object of this invention is to provide a method of manufacturing a two piece herringbone rotor whereby a ring section is locked to the shaft and a first section of the rotor through an interference fit.