Marine propulsion – Screw propeller – Having transmission
Reexamination Certificate
2000-04-25
2001-10-09
Sotelo, Jesus D. (Department: 3617)
Marine propulsion
Screw propeller
Having transmission
C440S037000, C416S129000
Reexamination Certificate
active
06299495
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to airboat propulsion mechanisms and, more particularly, to gear-based transmissions for airboats.
2. Description of Related Art
Airboats are often driven over land and water at high speeds. Airboats typically have employed aircraft engines operating at approximately 2500-3000 revolutions per minute (rpm) connected to solid direct-drive shafts, which rotate a single propeller. Aircraft engines are extremely expensive, and it is a general practice therefore to mount a used aircraft engine to an airboat to save on costs.
The steering apparatus of an airboat usually comprises a pair of rudders, with trim tabs added to correct for the torque that results from the rotation of the propeller, this torque tending to keep the boat from maintaining a level attitude.
Extreme gyroscopic forces can occur when airboats are turned rapidly, and these forces are borne, among other structures, by the driven shaft.
Previously known airboat systems utilize belt-driven transmissions, which are inefficient owing to power losses caused by belt friction, especially at higher rotational velocities. Belt breakage in these systems is a source of failure. Another disadvantage of belt-driven systems is their inability to permit reduction ofpropeller speed, since the driven shaft used to effect such a reduction would have to be too small to be practicable. Thus it would be advantageous to utilize a different transmission method in an airboat to enable engine speed reduction without loss of efficiency.
Propeller breakage is also a major source of failure, since at 3000 rpm extremely high forces are experienced at the propeller hub. It would therefore be desirable to reduce the load on the propeller.
It has been taught by Becker et al. (U.S. Pat. No. 4,932,280, dated Jun. 12, 1990) to use coaxial drive shaft systems for driving multiple outputs from a single input in an aircraft. Gearing means are disclosed for driving two outputs at different speeds.
The use of a gear-based transmission for airboats has been taught by Kaye (U.S. Pat. No. 5,807,149), including a transmission for driving a pair of counter-rotating coaxial shafts, to each of which is affixed a propeller. Such an arrangement can be used with an automobile engine, which is far more economical than an aircraft engine. This transmission has been shown to reduce noise and torque, to permit varying gear ratios, to increase fuel efficiency and engine life, and to be less expensive to operate.
An improved gear-based transmission for airboats has also been disclosed by Jordan (U.S. Pat. No. 5,724,867, the entire disclosure of which is incorporated herein by reference).
SUMMARY OF THE INVENTION
It is an object of the present invention to provide an airboat transmission that has improved strength, efficiency, noise, and stability characteristics for driving a pair of counter-rotating propellers.
It is a further object to provide such an airboat transmission having a minimum number of gears for driving two coaxial counter-rotating shafts.
It is another object to provide such an airboat transmission having a compact configuration to optimize space utilization.
It is an additional object to provide such an airboat transmission having reduced weight.
These and other objects are achieved by the airboat transmission of the present invention, which is for driving a pair of coaxial, counter-rotating propellers.
When the transmission is in use on an airboat, a drive shaft is mated at one end to a motor crank extending from and rotated by an engine. The opposite end of the drive shaft extends into the transmission from the fore side. As mentioned above, previously known airboats utilized aircraft-type engines; however, with the transmission of the present invention, it has been found that an automobile engine can be used. This has a benefit of reducing cost, as well as other benefits to be discussed below.
An inner shaft also extends into the interior space of the housing, typically from the aft side. The inner shaft is for rotating an outer propeller, that is, the propeller farther from the airboat body.
A hollow outer shaft likewise extends into the interior space of the housing and is further positioned in surrounding, generally coaxial arrangement to the inner shaft. The outer shaft is shorter than the first, and both ends protrude beyond the ends of the inner shaft. This outer shaft is for rotating an inner propeller, that is, the propeller closer to the airboat body.
The transmission of the present invention comprises a fore output gear rotatably affixable within a housing and affixable to the inner shaft for rotating the outer propeller in a first direction. An aft output gear is rotatably affixable within the housing and is affixable to the outer shaft coaxial with the inner shaft for rotating the inner propeller in a second direction opposite the first direction. The aft output gear is generally coaxial with the fore output gear.
An intermediate gear shaft has a fore end and an aft end, and both ends are opposedly affixable for rotation within the housing. This bracing on both ends confers additional stability to the transmission, conferring longer life and decreased vibration. An intermediate gear is mounted on the intermediate gear shaft, and is positioned so that the intermediate gear is in driving relation to only one of the fore or the aft output gear.
A drive gear is rotatably affixable within the housing and is affixed for corotation with the drive shaft. The drive gear has a first portion that is positioned in driving relation to the intermediate gear and a second portion that is positioned in driving relation to the other of the fore or the aft output gear, whichever is not being driven by the intermediate gear.
In a particular embodiment, the drive gear further has a central portion that has a diameter smaller than a diameter of the fore and the aft portions, and the central portion is positioned axially between the fore and the aft portions and further is radially coplanar with and in spaced relation to the output gear that is being driven by the intermediate gear.
Also in this particular embodiment, the intermediate gear is positioned axially in spaced relation from the output gear that is not being driven thereby and the drive gear by which it is not being driven.
The rotation of the drive shaft in one direction achieves, owing to the interposition of the intermediate gear between the drive gear and the output gear it is driving, a counter-rotation of the two output shafts and thus imparts counter-rotation to propellers attached thereto.
Using a gear-driven transmission permits driving an automobile engine at the point of maximum horsepower, which generally implies a motor crank rotational speed approximately in the range of 5000-5200 rpm, and then gearing down the rotational speed to roughly 1000-1800 rpm, a significantly quieter speed at which to run the propellers. In addition, the use of a gear-based transmission permits driving counter-rotating propellers a different speeds if desired.
The invention is not, of course, limited to the use of an automobile engine; in fact, the use of gears enables the user to optimize for efficiency and noise characteristics by altering gear ratios as desired.
The features that characterize the invention, both as to organization and method of operation, together with further objects and advantages thereof, will be better understood from the following description used in conjunction with the accompanying drawing. It is to beexpressly understood that the drawing is for the purpose of illustration and description and is not intended as a definition of the limits of the invention. These and other objects attained, and advantages offered, by the present invention will become more fully apparent as the description that now follows is read in conjunction with the accompanying drawing.
REFERENCES:
patent: 1329387 (1920-02-01), Fay
patent: 1734268 (1929-11-01), Moorhouse
patent: 1842055 (1932-01-01), Stout
patent: 2228638 (1941-01
Allen Dyer Doppelt Milbrath & Gilchrist, P.A.
Sotelo Jesus D.
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