Marine propulsion – Screw propeller – Shafting
Reexamination Certificate
2000-06-23
2001-10-16
Sotelo, Jesus D. (Department: 3617)
Marine propulsion
Screw propeller
Shafting
C440S112000, C277S336000
Reexamination Certificate
active
06302750
ABSTRACT:
TECHNICAL FIELD
The present invention relates generally to a marine motor drive assembly, particularly useful as part of the drive shaft mechanism in a marsh motor on a shallow water boat.
BACKGROUND
A marine mud motor is a specialized marine motor used to propel a boat in shallow water applications. These motors are useful in marshes or other shallow water areas where the propeller frequently comes in contact with rocks, mud, logs, and weeds, etc., at the bottom of the body of water. The design of the motor and drive shaft propels boats in extreme conditions by allowing the propeller to ride gently over these obstacles with minimal damage to the drive.
A marine mud motor, illustrated in
FIG. 1
, includes a frame
8
that supports an engine
14
. The frame is mounted on a boat
10
by an engine mount
30
. A long drive tube
16
is coupled to the frame by supports
28
and the drive tube contains a drive shaft
26
. A propeller
22
is connected to the drive shaft and moves a boat forward. This design has been used for more than 30 years and is in use throughout the U.S. and Asia today to propel boats through adverse shallow water conditions.
The conventional design described above has a long drive tube
16
that encases a drive shaft
26
which is three to seven feet in length. This design includes a drive shaft, which is supported during rotation by bronze or composite bushings pressed into the drive tube, one on the bottom end and one on the top end of the drive tube. The bushings are contained in the drive tube assembly
16
. Marine mud motors using bronze or composite bushings pressed into the drive tube have been in production around the world for over 30 years.
As with any device operated under water, moisture is a serious problem. Seals are used to keep water out and lubricants inside the drive. Because bronze and composite bushings do not corrode easily, they have conventionally been used as a wear surface in the drive. Roller bearings have been tried, but have repeatedly failed due to corrosion and alignment difficulties associated with this conventional design. The use of roller bearings has failed due to the extreme conditions in which the motors are used, and the eventual intrusion of water, dirt and sand, which quickly rusts and seizes the bearing, leading to extremely early failure.
To avoid this early drive shaft failure, multiple seals conventionally are used and the drive tube is filled completely with grease. Even with multiple seals and filling the tube with grease, water, dirt, and particles enter the drive and cause early bearing failure. This early bearing failure usually takes less than two years, even with constant attention and lubrication. For example, users must frequently fill the drive tube with grease, which tends to leak out.
Another significant problem is due to the drives' inherent design. The rotating shaft causes a Venturi effect which draws water up inside the drive tube past the multiple seals. Because the drive is in frequent contact with the bottom of the body of water, silt and sand accompany the water, which is pulled into the drive tube. Of course, the silt, particles and sand significantly accelerate bushing and drive tube wear and induce early failure.
Even more water and silt is pulled into the drive tube when the drive is at rest in the water. This is caused by cooling parts that create a vacuum, which draws water inside the drive. This additional water, silt and sand further contribute to the problem of the Venturi effect.
Because of these problems, bronze bushings, composites and even ceramics have been used as a wear surface for the rotating drive shaft. Other bearings have been tried, but the sand, dirt and water wear them out in a very short time. Accordingly, the use of bronze or composite bushings in the drive tube has been the industry standard.
Conventional drive tubes are completely, or nearly completely, filled with grease which lubricates both the top and bottom drive bushings. This lubrication helps combat any moisture that enters the system and helps to hydraulically balance the long drive shaft, which tends to vibrate during operation due to its length. Seals mounted on each end of the drive tube keep the grease inside the tube. The lower end of the drive tube supports the propeller and commonly features multiple seals, usually three. One of the seals is used to keep the grease inside the drive tube and two of the three seals are reversed, thus keeping water from entering the drive tube. Multiple seals are used because the long drive tube causes a Venturi effect, which attempts to draw water inside the drive tube. The two reversed seals slow this “pumping” process down. Regardless, the Venturi effect is strong and eventually small amounts of water with silt enter the system and cause accelerated wear and contamination of the lubricant. For this reason, bearings have not been used successfully because the moisture and dirt causes early bearing failure. Thus, only bushings have been used for the past 30-plus years.
SUMMARY OF THE INVENTION
It has been recognized that it would be advantageous to develop a system which would allow bearings to be used in a marine motor drive shaft. In addition, it would be valuable to have a device which can overcome the Venturi effect and stop the flow of water, silt and sand into a drive shaft.
The present invention provides a drive assembly for a marine mud motor with an elongated drive tube containing a lubricant. The drive tube rotatably receives a drive shaft including a propeller on the drive shaft end. The drive assembly comprises a drive assembly housing having an upper end, lower end, and housing cavity. An outer seal is mounted in the lower end of the drive assembly housing and structured to stop lubricant flow from the housing cavity. An inner seal is provided, between the outer seal and the upper end of the drive assembly housing, and spaced apart from the outer seal to provide an area for pressurized lubricant.
In accordance with one aspect of the present invention, the system includes a drive assembly containing a roller bearing and a structured seal arrangement. The roller bearing drive assembly provides a drive rotation pressure which overcomes the system's inherent Venturi effect by first reversing the Venturi effect, and then creating a pressurized lubricant chamber. The drive assembly contains a bearing set and two or more seals. Two seals can develop the desired results, although more than one bearing or two seals may be used. Hydraulic pressure is generated on the lower end of the bearing due to its accelerated rotation and slope. This pressure is used to force lubrication or grease past the lip of the innermost seal, which is installed in a reverse direction. The grease then flows under pressure into a chamber between the inner and outer seals. The pressurized lubricant in the lubricant chamber cannot escape because the two seals are installed in opposite directions, allowing pressurized lubricant to enter from the bearing side but not escape the area between the two seals. The developed pressure reverses the normal tendency of the drive to create a Venturi effect that attempts to draw water inside the drive tube past seals into the drive tube which leads to increased wear, corrosion, and early failure. Alone, this pressure helps keep water out of the system, but when captured in a seal assembly, a secondary barrier is provided that maximizes its sealing capability, thus extending the life of the drive. This configuration allows the use of roller bearings in mud motors that could not use roller bearings as a wear surface due to water contamination. The use of an integral assembly having a removable housing, containing a bearing, seals, and seal cap benefits this application.
Additional features and advantages of the invention will be set forth in the detailed description which follows, taken in conjunction with the accompanying drawings, which together illustrate by way of example, the features of the invention.
REFERENCES:
patent: 4936593 (1990-06-01
Sotelo Jesus D.
Thorpe North & Western LLP
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