BACKGROUND OF THE INVENTION Marine propulsion units in the form of stern drive units are used to transmit rotational energy, provide from an engine crankshaft, to either forward or reverse rotational energy provided by a propeller, and to displace the axis of the rotational energy provided by the engine downwardly so that the propeller can be submerged a desired depth within the water. Referring to FIG. 1, marine stern drive units 10 are well known in the art and are conventionally mounted to a transom 12 of a boat 14. An internal combustion engine 16 is disposed within the boat and includes an output with a shaft 18 which extends through transom 12 and a mounting plate 20 secured to the stern drive unit 10 in a conventional manner.

The stern drive unit 10 generally includes a stern drive housing 22 forming an upper gear case 24, a lower gear case 26 suitably mounted to gear case 24. and a generally horizontally fore- to-aft extending torpedo housing 28 forming a portion of and disposed at the bottom of gear case 26. A propeller 30 is mounted for rotation generally aft of housing 22. For purposes of driving the propeller engine output shaft 18 is suitably connected to an input shaft 32 in the stern drive unit 10. Input shaft 32 is connected through a clutch assembly 34 to a downwardly extending main drive shaft 36 which in turn connects at its lower end though suitable gearing 38 and a propeller shaft 40 to drive the propeller 30.

Referring to FIG. 2. the stern drive unit 10 comprises an an input bevel gear 42 disposed within the upper gear case 24 and is attached to a end of the input shaft 32 that is opposite the engine output shaft 18. Gear teeth of the input bevel gear 42 are placed into communication with gear teeth of forward bevel gear 44 that is disposed within the upper gear case 24 and positioned below the input bevel gear 42. Gear teeth of the input bevel gear 42 are also placed into communication with gear teeth of a reverse bevel gear 46 that is also disposed within the upper gear case 24, and that is positioned above the input bevel gear 42. Arranged in this manner, the input bevel gear communicates with both the forward and reverse bevel gears simultaneously.

The forward and reverse bevel gears interact with an axial main shaft 48 that extends vertically through the stern drive housing 22. The clutch mechanism 34 is disposed around the axial main shaft 48 and is interposed between the forward and reverse bevel gears 44 and 46.

The clutch mechanism 34 is actuated by a shifting mechanism 50 that is generally located within the housing 24 at a position that is diametrically opposed from input 39.

Configured in this manner, rotational energy taken from an engine crankshaft is provided

to the stern drive unit via the input shaft 32. which in turn operates to rotate both forward and reverse bevel gears 44 and 46 by gear teeth interface. Depending on the position of the shifting mechanism 50 and related position of the clutch mechanism 34, the rotational energy transmitted to the forward and reverse bevel gears can be transmitted to provide forward or reverse directed rotation of the axial main shaft 48, and ultimately the propeller. Bearings are interposed between the forward and reverse bevel gears and the housing, and between the input shaft and the housing, to facilitate rotational movement of these members within the stern drive unit.

An inherent property of such conventional stern drive unit is that when excessive load or torque is applied to the input shaft, input bevel gear, and forward and reverse bevel gears, the contact planes between the input bevel gear and the reverse bevel gear deflect away from one another, thereby increasing the design running clearance or backlash of the gearset. The backlash increases due to both wear that is caused on both the input bevel gear bearing and the reverse bevel gear bearing, and that is caused on the gear teeth themselves. As this backlash increases, the mesh planes of the input bevel gear and the reverse bevel gear continue to deflect from one another and make contact with one another correctly. This deflection reduces the ability of the gearset to transmit an input shaft load and ultimately leads to premature component failure.

The above-described deflection between the input bevel gear and reverse bevel gear also causes the forward bevel gear to tilt or deflect away from its rotational axis. This deflection is permitted by the design clearance provided by the roller bearings disposed inwardly of the forward bevel gear. The tendency of the forward bevel gear to be tilted or deflected away from its axis of rotation increases when the gear is under load, thereby causing the input bevel gear and forward bevel gear to become deflected away from one another during use. Such unwanted gear set deflection causes premature the roller bearing wear and ultimately stern drive unit failure.

It is, therefore. desired that a stern drive unit be constructed having a drive gear assembly that is designed to minimize or eliminate gear set deflection between the input bevel gear and the forward and reverse bevel gears, thereby increasing stern drive unit service life. It is desired that such stern drive unit provide such improved drive gear assembly without compromising other operational functions of the stern drive operation, e. g., shifting. It is further desirable that such stern drive unit with improved drive gear assembly be compatible for use with other existing stern drive elements to facilitate retrofittability.

SUMMARY OF THE INVENTION A stern drive unit, constructed according to principles of this invention, comprises a housing that accommodates a gear assembly therein. The gear assembly includes an input gear that is coupled to an input shaft for transmitting rotational energy from an engine to the stern drive unit. A forward gear is part of the gear assembly and is in gear communication with the input gear. A reverse gear is also part of the gear assembly and is gear communication with the input gear simultaneously with the forward gear.

A key feature of the stern drive unit of this invention, that is unlike conventional stern drives known in the art, is that it includes an idler gear disposed within the housing and in gear communication with both the forward and reverse gears simultaneously with the input gear. The idler gear serves to control thrust loads imposed on the reverse and forward gears, and reduce the torque loads imposed on the reverse and forward gears, thereby increasing the service life and load carrying ability of the stern drive.

DESCRIPTION OF THE DRAWINGS These and other features and advantages of the present invention will become appreciated as the same becomes better understood with reference to the specification, claims and drawings wherein: FIG. 1 is a schematic cross-sectional side view of a prior art stern drive unit; FIG. 2 is a schematic cross-sectional side view of an enlarged section of the prior art stern drive unit of FIG. 1 ; FIG. 3 is a schematic cross-sectional side view of a stern drive unit of this invention ; and FIG. 4 is a schematic perspective cross-sectional end view of the stern drive unit of FIG.

3.

DETAILED DESCRIPTION OF THE INVENTION Stern drive units, constructed according to principles of this invention, have an improved drive gear assembly comprising a constantly meshing idler pinion gear that is positioned within the stern drive gear housing opposite the input shaft. and that is designed to engage both the reverse and forward bevel gears simultaneously with the input bevel gear, thereby helping to carry and distribute the torque and thrust loads transmitted to the reverse and forward bevel gears.

Constructed in this manner, the idler pinion gear serves to minimize or eliminate tilting of both the reverse and forward bevel gears, thereby reducing bearing wear. maintaining controlled gear set mesh plane orientation, and improving stern drive unit service life.

FIGS. 3 and 4 illustrate a stern drive unit 52, constructed according to principles of this invention, comprising a gear housing 54, and an input shaft 56 disposed within a shaft opening 58 within the housing. The input shaft 56 includes a first end 60 extending outwardly from the housing that is configured to accommodate attachment with a drive shaft or the like that is connected to an engine crankshaft. An input bevel gear 62 is attached to the input shaft 56 within the housing, and is carried by at least one set of roller bearings 40 to facilitate rotational movement within the housing. The number of roller bearings used to support the input bevel gear will depend on the particular stern drive unit application. For example, a stern drive designed for high horsepower applications can include two sets of roller bearings.

A reverse bevel gear 66 is disposed within the housing 54 and is positioned therein having an axis of rotation perpendicular to the input bevel gear 62. A roller bearing 68 is interposed

between an inside diameter of the reverse bevel gear 66 and a housing top cover 70 to facilitate rotational movement of the reverse bevel gear within the housing. An upper thrust bearing set 69 is interposed horizontally between the reverse bevel gear 66 and the top cover 70 to control axial thrust movement of the reverse bevel gear. The reverse bevel gear includes gear teeth that are positioned to engage gear teeth of the input bevel gear.

A forward bevel gear 72 is disposed within the housing 24 at a positioned below the reverse bevel gear 66. and having an axis of rotation perpendicular to that of the input bevel gear.

A roller bearing 74 is interposed between an inside diameter of the forward bevel gear 72 and an inside shoulder portion 76 of the housing to facilitate rotational movement of the forward bevel gear within the housing. An lower thrust bearing set 77 is interposed horizontally between the forward bevel gear 72 and the housing 54 to control axial thrust movement of the forward bevel gear. Like the reverse bevel gear. the forward bevel gear also includes gear teeth that are positioned to engage gear teeth of the input bevel gear.

Configured in this manner, the input bevel gear 62 simultaneously drives the counter- rotating reverse and forward bevel gears 66 and 72. This configuration is common to known stern drive units, such as that described in U. S. Patent Nos. 4. 244. 454 ; 4, 957, 506 ; and 4. 630. 718, which are each herein incorporated by reference. As is well known in such arrangements, the forward and reverse bevel gears are disposed around an axial main shaft 78 that is positioned within the housing along the rotational axis of the reverse and forward bevel gears. Roller bearings 80 are interposed between the shaft 78 and the housing to facilitate rotational movement of the main shaft therein.

A clutch assembly 82 is disposed within the housing to control engagement of the axial main shaft with either the reverse or forward bevel gear. The clutch assembly 82 comprises a sleeve member 84 that is disposed within the housing and that is slidably connected to and around the main shaft 78 at a positioned between the reverse and forward bevel gears 66 and 72.

The clutch sleeve member 84 includes conical clutch faces 86 and 88 that are designed to cooperate with adjacent conical clutch surfaces 90 and 92 disposed along an inside surface of respective reverse and forward bevel gears. The reverse and forward bevel gear conical clutch surfaces 90 and 92 are designed to engage respective sleeve member clutch faces 86 and 88 when the sleeve is axially actuated and moved into position against either the reverse or forward bevel gear.

As will be described in better detail below, the clutch mechanism 82 is actuated by a shifting mechanism that resides within the housing and that is operated by the user to move the clutch sleeve member into reverse, neutral, and forward positions.

Unlike other stern drive units known in the art, the stern drive unit 52 of this invention comprises a constantly meshing idler bevel gear 94 that is disposed within the housing on a side of the main shaft 78 that is opposite the input bevel gear 62. The idler bevel gear 94 has the same axis of rotation as the input bevel gear. that is perpendicular to the axis of rotation for both the

reverse and forward bevel gears. The idler bevel gear 94 is carried by a pair of roller bearings 96 interposed between the idler bevel gear and a bevel gear cavity 98 within the housing. The idler bevel gear 94 is positioned within the housing so that its gear teeth engage the gear teeth of both the reverse and forward bevel gears so that the idler bevel gear is driven by the counter rotating reverse and forward bevel gears.

The idler bevel gear 94 has a general configuration that is similar to that of the input bevel gear 62 to provide a symmetrically-opposed matching gear that cooperates with the reverse and forward bevel gears. The design and use of the idler gear is advantageous because it serves to minimize the reverse bevel gear 66 from tilting or deflecting away from its axis of rotation when placed under load by the input bevel gear. The placement of the idler bevel gear within the housing directly opposite from the input bevel gear provides two, rather than one, diametrically opposed contact points with the reverse gear that are each positioned along an identical contact plane, i. e.. the contact points between the reverse bevel gear and the input bevel gear and idler gear are provided at the same distance in the housing from the top cover.

Additionally, the design and use of the idler bevel gear 94 serves to minimize or eliminate unwanted tilting or deflecting of the forward bevel gear 72 away its axis of rotation and away from the input bevel gear. As discussed above for the reverse bevel gear, the idler gear provides a second contact point with the forward bevel gear that is positioned along a contact plane identical to that of the input bevel gear, thereby serving to stabilize and balance load effects on the forward bevel gear imposed by the input shaft.

Constructed in this manner, a drive gear assembly comprising the idler bevel gear 94 also functions to increase the load carrying ability of the drive gear assembly. Since the idler bevel gear is in mesh with the reverse bevel gear, the reverse bevel gear necessarily carries a portion of the load being transmitted through the drive gear assembly, i. e., between the input bevel gear and the forward bevel gear. Since the input bevel gear 38 is now transmitting load in two places, the load that is imposed on each individual gear tooth or cog of both the input bevel gear and forward bevel gear is divided in half. thereby greatly increasing the load-carrying ability of the drive gear assembly.

Stern drive units comprising drive gear assemblies of this invention can either be retrofit into an existing stern drive housing, or can be installed into a modified stern drive housing. In an example embodiment, stern drive units of this invention are constructed using a new housing that has been adapted to accommodate placement of both the idler bevel gear and the shifting mechanism therein. Specifically, that housing 54 includes a steering cap 100 that has been designed having a cavity 98 disposed therein for accommodating rotational placement of the idler bevel gear 94 therein. In this regard, the cavity 98 includes means for accommodating the bearing sets 96 and includes oil ports 102 for supplying oil to the bearing sets 96.

FIG. 4 illustrates a shifting mechanism 104, as used with the stern drive unit 52 of this invention, comprising a shift fork 106 that having an end 108 disposed within a channel of the

clutch sleeve 84. A shift pin 112 is attached to an end of the shift fork 106 opposite the clutch sleeve 84 and connects the shift fork 106 to a shift eccentric 114. The shift eccentric is disposed within a shift mechanism cavity 116 in a side portion of the housing 54. The shift fork 106, shift pin 112, and shift eccentric 114 extend radially outwardly from the clutch sleeve 84 to the housing at an approximately 90 degree angle to the input bevel gear 32.

A shift cavity cover plate 118 is positioned over the housing shift mechanism cavity 116 and includes a centering cavity 120 for accommodating a pilot shaft 122 of the shift eccentric 114. The cover plate 118 serves both to center the shift eccentric to facilitate its eccentric movement, and serves to prevent water from entering the shift cavity.

A shift lever 124 extends vertically along an outside surface of the housing 54 adjacent the shift cavity 116 and is connected at one end to the shift eccentric 116, and at an opposite end to a shift shuttle pin 126. The shift lever 124 is interposed between the housing 54 and the cover plate 118. The shift shuttle pin 126 extends from the shift lever inwardly into the housing through a pin cavity and is connected to shift shuttle 128. The shift shuttle 128 is a guided piston disposed within a piston cavity in the housing that transfers motion of a shift cable to the shuttle pin 128 and shift lever 124.

The shifting mechanism 104 is designed to actuate the clutch sleeve member 84, i. e., move the clutch sleeve member upwardly and downwardly along the main shaft 78, to shift the drive assembly between reverse drive, neutral, and forward drive operational positions.

Although limited embodiments of stern drive units having improved drive gear assemblies of this invention have been described herein, many modifications and variations will be apparent to those skilled in the art. Accordingly, it is to be understood that, within the scope of the appended claims, stern drive units having improved drive gear of this invention may be prepared other than as specifically described herein.