Structure of drive shaft of outboard motor

Abstract: A drive shaft (14) for transmitting rotary force of an engine (8) to a propeller shaft (17) of an outboard motor (1), wherein at least one waist portion (14b) of the drive shaft (14) is tapered off the engine side to the propeller shaft side.

Description: This invention relates to an outboard motor comprising a drive shaft for transmitting rotary force of an engine to a propeller shaft. Such a device is described in document US 3 368 420. In the outboard motor mounted on small boats, the engine is disposed in the top cowling located in the upper part of the unit, the propeller is attached to the lower case located in the lower part of the unit and the drive shaft for transmitting the rotation of the crankshaft of the engine to the propeller shaft extends from the engine in the top cowling through the upper case in the state of being isolated from the oil pan and the exhaust passage by means of the drive shaft chamber to the propeller shaft in the lower case. The vertically elongate drive shaft of the outboard motor is conventionally formed as follows; part of the drive shaft in the lower case near the propeller shaft is made with a large diameter so that the part may be journal-supported in a stabilized manner, part of the shaft above the large diameter part extending from the part connected to the crankshaft and located above the large diameter part to the water pump constituting part (to be the water pump rotary drive shaft) located in the vicinity of the lower part of the upper case is formed as the waist part with a smaller diameter than the diameter of the parts above and below the waist part. With such an arrangement, even in the case for example the propeller of a moving boat hits a stone in a shoal and is locked although the engine is running, the drive system including the gears is prevented from being damaged as the impact of the lock is absorbed by the torsional deformation of the small diameter waist part of the drive shaft. The impact force exerted on the drive shaft when it is locked on the propeller side while the engine is running is greater toward the engine side. On the other hand, the above-described waist part for absorbing the impact force of the conventional drive shaft is formed with a uniform diameter (in a straight shape). Therefore, the torsional angle is not constant along the length of the waist part. That is, the plastic deformation in the waist part is greater toward the engine side. Therefore, the drive shaft is likely to be damaged in the waist part. Accordingly, it is an objective of the present invention to provide an outboard motor with a drive shaft as indicated above facilitating the prevention of breakage of the shaft in case the drive shaft is locked on the propeller side by an obstacle while the boat is moving by improving the impact absorbing effect of the waist part of the drive shaft. According to the present invention, this objective is solved for an outboard motor with a drive shaft as indicated above in that at least one waist portion of the drive shaft is continuously tapered off from the engine side to the propeller shaft side. According to the present invention, the at least one waist portion may be twisted uniformly in case the drive shaft is locked on the propeller side. The invention for solving the above problem further relates to outboard motors with a drive shaft for transmitting the rotary force of the engine to the propeller shaft, characterized in that the small diameter bar-shaped waist part of the drive shaft is continuously tapered off from its engine side toward the propeller side. With the above arrangement, while the impact force on the drive shaft when it is locked on the propeller side with the boat moving and the engine rotating is greater toward the engine side, since the waist part is made thinner gradually along its length from the engine side, the torsion in the waist part caused by the impact is uniform along its length. As a result, locally great torsion does not occur, the drive shaft is prevented from breaking, the total torsion angle increases, and thus the impact absorbing effect is enhanced. Other preferred embodiments of the present invention are laid down in further dependent claims. Embodiments of the drive shaft structure of the invention for outboard motors will be hereinafter described in reference to the appended drawings in which: FIG. 1 is a schematic side view of an example of outboard motor to which the drive shaft structure of the invention is applied; FIG. 2 is an entire side view of the drive shaft for use in the outboard motor shown in FIG. 1; FIG. 3 is a graph showing a comparison of torsional angle of the waist part of the drive shaft shown in FIG. 2 between two different drive shafts; one with the tapered waist and the other with the straight waist; and FIG. 4 is a side view of the drive shaft shape of another embodiment of the invention. In the following, the present invention is explained in greater detail with respect to several embodiments thereof in conjunction with the accompanying drawings, wherein: FIG. 1 is a general view of an example of outboard motor to which the drive shaft structure of the invention is applied. An outboard motor 1 is removably mounted on a transom of a hull (not shown) through a clamp bracket 2, a tilt shaft 3, and a swivel bracket 4. The housing part of the machine consists of; a top cowling 5 made up of an upper cowl 5a and a lower cowl 5b, an upper case 6, and a lower case 7. In the top cowling 5 of the upper part of the machine is disposed a driving source, an engine 8. A propeller 9 for propulsion is attached behind the lower case 7, the lower part, of the machine. An oil pan 11 is provided in the upper case 6 located between the top cowling 5 and the lower case 7 to collect lubricant oil for use in the engine 8 through an exhaust guide 10 connected to the bottom end of the engine 8. An exhaust passage 13 for introducing exhaust gas, discharged from an exhaust pipe 12 extending downward from the engine 8 through the oil pan 11, to the interior of the lower case 7 is disposed under the oil pan 11. A drive shaft 14, in the state of being isolated from the oil pan 11 and the exhaust passage 13 by means of a drive shaft chamber 15, for transmitting the rotation of the crankshaft of the engine 8 passes vertically through the upper case 6. A water pump 16 for supplying cooling water to the engine 8 is disposed on the drive shaft 14 in the vicinity of the lower case 7 so as to be driven with the rotary force of the drive shaft 14. The lower end of the drive shaft 14 extending into the lower case 7 is connected through gears to a shift changing gear section 18 of the rotary shaft, or the propeller shaft 17 of the propeller 9. The outboard motor 1 is attached to the hull as follows: The swivel bracket 4 for supporting for horizontal rotation the steering shaft 19 of the outboard motor 1 and a clamp bracket 2 removably secured to the transom of the hull are rotatably interconnected at their top ends through a tilt shaft 3 extending horizontally. That is to say, the outboard motor 1 is removably attached to the hull through the clamp bracket 2, the tilt shaft 3, and the swivel bracket 4; and is rotatable horizontally relative to the swivel bracket 4, and vertically about the tilt shaft 3. FIG. 2 shows the drive shaft 14 for use in the outboard motor 1 described above, and has a portion 14a connected to the crankshaft located above it, a portion 14c constituting the (rotary drive shaft of) the water pump located near the lower part of the upper case, and a long continuous bar-shaped waist portion 14b of a diameter smaller than that of the portions 14a and 14c. A part 14d continued downward from the water pump constituting part 14c, namely the part near the part connected to the propeller shaft located in the lower case 7, is formed with a diameter greater than that of the crankshaft connecting part 14a and the water pump constituting part 14c. The large diameter part 14d is provided at its upper end with a thrust bearing part 14e and its lower end with a gear securing part 14f, made of a material (such as SCM of JIS specification, wherein SCM and JIS means chromium-molybdenum steel according to Japanese Industrial Standard) different from the material (such as SUS of JIS specification, wherein SUS and JIS means stainless steel according to Japanese Industrial standard) of the upper parts 14a to 14c, and welded together with the upper parts into the single drive shaft 14. In the drive shaft 14 for the outboard motor 1 of this embodiment, the small diameter long bar-shaped waist part 14b between the crankshaft connecting part 14a and the water pump constituting part 14c is tapered off from the upper part (engine side) toward the lower part (propeller side). That is, the waist part 14b of an axial length L is tapered off from upper to lower parts so that its diameter D1 at its upper part (engine side) is reduced gradually to a diameter D2 toward its lower part (propeller side). Incidentally, the taper of the waist part 14b is not clearly visible on the drawing because the difference between the diameters D1 at the upper part and D2 at the lower part is in the order of 0. 5 to 1 mm over the length L of about 300 mm. The graph of FIG. 3 shows a comparison of torsional angle tested with two different drive shafts; one of this embodiment with the waist part 14b tapered off, and the other with the waist part in a straight shape (with the lower part diameter D2 being the same as the upper part diameter Dl). Each drive shaft is fixed at its lower end, its upper end (crankshaft connecting part 14a) is twisted about its axis by the same torsional angle, and the torsional angles of the waist part at several points along its axial direction are plotted on the graph for comparison of data between the two drive shafts. For the graph showing a dash-and-dotted line and a dash-and-dotted line in Fig. 3, the values in abscissa axis represent "torsional angle per unit length" resulting from the occurrence of elastic deformation. Each data shown in Fig. 3 was measured and obtained by application of impact torsional torque of a given amount in the direction of rotation about the drive shaft, on the upper part of the drive shaft with the lower part being secured. The amount of the impact torsional torque is predetermined on the basis of the measurement of impact force actually applied on the drive shaft when the screw side is in locked condition, with the engine being operated. As seen from the comparison shown in the graph of FIG. 3, the torsional angle per unit length at each point along the length of the waist part is almost the same for the drive shaft of this embodiment as shown with the dash-and-dotted line while that of the drive shaft with the waist part formed in the straight shape increases sharply toward the upper part on which rotating force is applied, as shown with the dash-and-double-dotted line. As described above, with the drive shaft structure for the outboard motor of this embodiment with the waist part tapered off, although the impact force exerted on the drive shaft when the propeller side is locked with an obstacle or the like while the boat is moving and the engine is rotating is stronger toward the side near the engine, since the waist part of the drive shaft is formed with smaller diameter away from the engine side so as to be more likely to be twisted, the torsion of the waist part by the impact force is uniform over its entire length. As a result, locally great torsion does not occur on the waist part and the drive shaft is prevented from breaking. And the increase in the total torsional angle as a whole of the waist part increases the impact absorbing effect of the waist part. The use of such a drive shaft of a high impact absorbing effect in the outboard motor makes it possible to use a high gear ratio for the gear connection between the drive shaft lower end and the shift change gear section of the propeller shaft although a high gear ratio is disadvantageous for the bending strength of gear teeth, the high impact absorbing effects of the drive shaft offsets the disadvantage. It is also possible to transmit a greater engine torque than is conventionally possible. As a result, it is possible to improve the propeller thrust, and reduce weight and size of the drive system. While the drive shaft structure of outboard motors according to the invention is described above by way of an embodiment, the invention is not limited to the above embodiment but may be embodied in various ways: For example, while the waist part 14b of the above embodiment is formed with a single taper over the entire length between the crankshaft connecting part 14a and the water pump constituting part 14c, it is also possible to form only part of the waist part 14a or 14c may be formed as the tapered waist part. However, the longer the waist part 14b of the drive shaft 14 is, the more it is possible to secure the total torsional angle over the entire length of the waist part while securing the torsional rigidity in each waist part, and to improve the impact absorbing effect. In view of the above, as shown in FIG. 4, when a bush support part 14h is formed between the crankshaft connecting part 14a and the water pump constituting part 14c of the drive shaft 14 to journal-support the middle part of the drive shaft 14 passing through the upper case with the surrounding wall of the drive shaft chamber 15 through a bush 20, the tapered waist part 14b may be formed in two parts, above and below the bush support part 14h, so that the total length of both of the waist parts 14b, 14b is long enough, although it is impossible to make a single long waist part 14b because of the presence of the bush support part 14h in the middle of the drive shaft 14. In this way, the impact absorbing effect as a whole is secured while securing the torsional rigidity in each waist part. The outboard motor with drive shaft structure of the invention described above makes it possible, in the case the propeller side is locked with an obstacle or the like while the boat is moving, to prevent the drive shaft from being damaged with a locally great torsion by making the waist part of the drive shaft uniformly twisted, and to enhance the impact absorbing effect by increasing the total torsional angle as a whole.

Claims: An outboard motor (1) comprising an engine (8), a propeller shaft (17) and a drive shaft (14) transmitting rotary force of said engine (8) to said propeller shaft (17), wherein said drive shaft (14) comprises at least one waist portion (14b) and is connected at an upper side to said engine (8) and at a lower side to said propeller shaft (17), characterized in that said at least one waist portion (14b) of said drive shaft (14) is continuously tapered off from said engine side to said propeller shaft side. An outboard motor (1) according to claim 1, characterized in that said at least one waist portion (14b) of said drive shaft (14) is of a bar shape with small diameters. An outboard motor (1) according to claim 1 or 2, characterized in that said two or more waist portions (14b) of said drive shaft (14) are provided. An outboard motor (1) according to claim 3, characterized in that said waist portions (14b) of said drive shaft (14) are separated. by a bush support part (14h). An outboard motor (1) according to at least one of the claims 1 to 4, characterized in that a part (14d) of the propeller shaft side of the drive shaft (14) is continued downward from a water pump constituting part (14c) and is formed with a diameter larger than a crankshaft connecting part (14a) and the water pump constituting part (14c). An outboard motor (1) according to daim 5, characterized in that said large diameter part (14d) is provided at its upper end with a thrust bearing part (14e) and at its lower end with a gear securing part (14f). An outboard motor (1) according to daim 6, characterized in that said gear securing part (14f) is made of a material different from the material of the upper parts (14a, 14b, 14c). An outboard motor (1) according to at least one of the claims 1 to 7, characterized in that a difference between the upper and lower diameter (D1, D2) is about 1/6% to 1/3% of a length (L) of said waist portion (14b) of said drive shaft (14). An outboard motor (1) according to at least one of the claims 1 to 8, characterized in that the propeller side is connected through gears to a shift changing section (18) of said rotary drive shaft (14) or said propeller shaft (17) of a propeller (9). An outboard motor (1) according to claim 9, characterized in that the gear securing part (14f) is made from SCU of JIS specification and that the other parts (14a, 14b, 14c) are made from SUS of JIS specification.

Inventors: OGUMA TAKAHIRO, OGUMA, TAKAHIRO, Oguma, Takahiro, c/o Sanshin Kogyo K.K.
Applicants: YAMAHA MARINE KK, YAMAHA MARINE KABUSHIKI KAISHA
Application year: 2000
Main classification: B63H20
IPC codes: F02B61, B63H20, B63H23, F16C3