Planet gear for air turbine starter system

A planet gear for use in an air turbine starter is formed of a first part having a set of gear teeth at a first axial location. A shaft extends axially away from the first set of gear teeth. A second part is interference fit on the first part, with the second part having a second set of gear teeth. The second part is mounted on the shaft of the first part. An outer diameter of the shaft is selected to be significantly larger than an inner diameter of a cylindrical portion of the second part which is interference fit on the shaft. A ratio of the outer diameter to the inner diameter is between 1.0005 and 1.0100. A planetary gear system, an air turbine starter and a method of installing a planet gear are also disclosed.

BACKGROUND

This application relates to a planet gear for incorporation into an air turbine starter.

Planetary gear systems are well known, and have been utilized to provide a speed change between an input and an output. A sun gear rotates about a central axis, and a ring gear rotates outwardly of the sun gear. A plurality of planet gears are positioned to transmit rotation between the sun and ring gears.

In one known type of planetary gear systems, the planet gears are mounted on stationary shafts positioned inwardly of the gears. The planet shaft provides an inner ring for bearings which support the planet gears.

One application of a planetary gear system is in an air turbine starter system. In an air turbine starter system, air is delivered across a turbine rotor to drive the rotor. The rotor drives a sun gear, which drives a ring gear through planet gears. The ring gear in turn drives a starter shaft for a gas turbine engine.

In one known air turbine starter system, the planet gears are mounted on a stationary shaft, and there are needle bearings positioned between the shaft and the interior of the planet gear. It is known to provide a surface treatment to the inner periphery of the planet gear to harden the surface such that the needle bearings are able to successfully run relative to the surface. One known hardening surface provides a carburized surface on the inner diameter of the gear.

However, in at least one known planet gear, the input teeth and the output teeth are provided by two separate gear portions which are secured together. In this known prior art, the two parts have been welded. The weld joint occurs at the outer periphery of a gear shaft, and relatively radially close to the inner periphery which is carburized. There has sometimes been a loss of hardness at the inner peripheral surface.

SUMMARY

A planet gear for use in an air turbine starter is formed of a first part having a set of gear teeth at a first axial location. A shaft extends axially away from the first set of gear teeth. A second part is interference fit on the first part, with the second part having a second set of gear teeth. The second part is mounted on the shaft of the first part. An outer diameter of the shaft is selected to be significantly larger than an inner diameter of a cylindrical portion of the second part which is interference fit on the shaft. A ratio of the outer diameter to the inner diameter is between 1.0005 and 1.0100. A planetary gear system, an air turbine starter and a method of installing a planet gear are also disclosed.

These and other features of this application will be better understood from the following specification and drawings, the following of which is a brief description:

DETAILED DESCRIPTION

An air turbine starter assembly20may be associated with an aircraft, or other systems including a gas turbine engine. A source of compressed air22, which may be from an auxiliary power unit, as typically utilized while an aircraft is on the ground, delivers hot, high pressure air into an inlet24. The high pressure air flows across a turbine rotor26, causing the turbine rotor26to rotate. As the turbine rotor26rotates, it rotates an output shaft28through a planetary gear system. The output shaft28may be utilized as a starter to start operation of a main gas turbine engine30. The planetary gear system18includes a sun gear34that is driven by a rotor shaft32that rotates with the turbine rotor26. The sun gear34in turn drives a plurality of planet gears40(only one of which is shown) through teeth50. The planet gears40include output gear teeth41, which drive a ring gear42. The ring gear42drives the output shaft28through a mechanical connection.

The planet gears40rotate on a stationary planet shaft44, mounted in a housing38. Intermediate needle bearings110are positioned between an inner diameter of the gears40, and an outer diameter of the planet shafts44.

FIG. 2shows a detail of the prior art planet gear40. As shown, there are two parts,51and54, which together make up the planet gear40. The part51has a shaft53which defines an inner bore52. A second part54carries the input gear teeth50. The output gear teeth41are formed on the part51. As shown, an interface56between the parts51and54is directly radially outwardly of an inner diameter surface52of the part51. It is this inner diameter52that is provided with a surface hardening treatment, or carburized surface.

As shown inFIG. 3, a weld location60is formed at the junction56. This weld location60is radially close to the surface52, and can result in loss of hardness.

A first embodiment of the present invention is illustrated inFIG. 4. In the planet gear70, a first part72is secured to a second part76. Gear teeth50are formed on part76and gear teeth41are formed on part72. As can be seen, the part76has a cylindrical boss78having a chamfer80, and an innermost side112. As can be seen, the innermost side112abuts an end74of the gear tooth41on the part72.

The chamfer80on the part76is at an angle B. In one embodiment angle B is 45°. In embodiments, angle B may range between 40° and 50°. As can also be seen, a relief82is formed at a location adjacent to the end74on the part72. The relief82is formed in part by a chamfer114, which is at an angle A.

Since the end112abuts the end74, the exact positioning of the part76on the part72is assured.

As can be appreciated fromFIG. 5A, the outer diameter of the part72is at a diameter D1. The inner diameter of the part76is defined as D2. In one embodiment, D1was 1.1592″ (2.9443 cm). In that same embodiment D2was 1.1566″ (2.9377 cm).

In embodiments, the force fit between the parts76and72is provided by D1being much larger than D2. The force fit is selected to be sufficient that the two parts will not rotate relative to each other, even without a weld joint. In this manner, the weld joint is eliminated, and the challenge to the reduction in hardness is also eliminated.

In embodiments, a ratio of D1to D2is between 1.0005 and 1.0100. More narrowly, the ratio may be between 1.0005 and 1.0025.

FIG. 6shows another embodiment90. In embodiment90, the parts92and98are again secured with a large interference fit. The right-most end100of the part98abuts a side surface96of the gear tooth41. A chamfer94is formed in the part92, and leads into a relief adjacent to the end96. Another chamfer102is formed at a radially inner surface of the end100of part98. The chamfer on the part92is at an angle C. In one embodiment an angle C was 45°. In embodiments, the angle C can be between 40° and 50°.

In one embodiment the chamfer102was at an angle E. In embodiments, the angle E is 20°, and may be between 15° and 25°.

The outer periphery of the part92in this embodiment is at a diameter D3. In this embodiment, D3is 1.1322″ (2.8757 cm). In the same embodiment, the inner diameter of the part98is at a diameter of D4. In an embodiment D4is 1.1300″ (2.8701 cm).

In embodiments, a ratio of D3to D4is selected such that a strong interference fit will occur, and such the parts98and92will not rotate relative to each other, even with the elimination of the weld joint. As with the above embodiment, the elimination of the weld joint eliminates the loss of hardness as mentioned above.

In embodiments, a ratio of D3to D4is between 1.0005 and 1.0100. More narrowly, the ratio may be between 1.0005 and 1.0025.

In a method, a planet gear, as shown in either embodiment, may be inserted onto mounting bearings on an outer periphery of the support portion of a planet shaft in an air turbine starter. The planet gear is configured as set forth above, and consistent with one of the two embodiments.