Accessory Gear Drive for a Two-Cycle Engine

1. Technical Field

This invention relates to two-cycle internal combustion engine and particularly to gearing for driving accessories.

2. Background Art

Prior two-cycle engines such as those disclosed in U.S. Pat. No. 3,765,393 to Pipes, issued on Oct. 16, 1973, and U.S. Pat. No. 4,372,258 to Iwai, issued on Feb. 8, 1983 have disclosed worm gears on the engine crankshaft to drive accessories. In the prior systems the worm has been formed as part of the crankshaft and lubrication of the worm and the worm gear has been critical to prevent early failure of the gearing.

The invention provides a two-cycle crankcase compression engine having a cylinder block including a cylinder. A crankcase member defines with the cylinder block a crankcase and a crankshaft is mounted for rotation in the crankcase. A piston is mounted for reciprocation in the cylinder and is connected to be driven by the crankshaft. An intake means is provided to admit lubricating oil and a combustible mixture to the crankcase. A worm member formed of a low-friction plastic material and having a helical tooth is attached to the crankshaft and drives a worm gear mounted on the engine in engagement with the helical tooth.

The plastic of which the worm member is formed can be a plastic resin filled with a lubricating material. Nylon impregnated with molybdenum disulfide has proven particularly satisfactory.

Preferably the worm member has the form of a split annulus and further includes an attachment means for attaching the ends of the worm member together around the crankshaft. The engine crankshaft can include a circumferential flange and the worm member can include a circumferential groove mating with the flange to restrain the worm member from axial movement.

The invention is particularly suitable for driving an oil pump for supplying lubricating oil to the crankcase of the two-cycle engine.

FIG. 1 is an elevational view of an outboard motor engine broken away to show details of the crankshaft.

FIG. 2 is view taken along the line 2--2 of FIG. 1.

FIG. 3 is a sectional view in elevation taking along line 3--3 of FIG. 2.

Referring to FIG. 1 a two-cycle crankcase compression engine 10 suitable for use on an outboard motor is illustrated. The particular engine illustrated has six cylinders 11 arranged in a V, though other cylinder arrangements could be utilized with the invention as well.

The engine 10 includes a crankcase defined between the forward end of the cylinder block 12 and a crankcase member 13. A crankshaft 14 is mounted for rotation about a normally vertical axis in the crankcase and has six crank throws 15, each connected to a piston 16 by a connecting rod 17 in a conventional manner. The crank throws 15 are supported by circular crankshaft cheeks 18 which also serve to divide the crankcase into six crankcase compartments 19, one for each corresponding cylinder 11. Each crankcase compartment is connected to transfer ports 20 in the corresponding cylinder by transfer passages, not illustrated.

An oil pump 21 is provided to supply lubricating oil from a reservoir 22 to mix with the fuel prior to its introduction into the carburetors 23 in a manner like that shown in U.S. Pat. No. 4,388,896 to Sheridan et al, issued on June 21, 1983. The conventional oil pump 21 is driven by a worm gear 24 which in turn is driven by a worm 25 mounted on one of the crankshaft cheeks 18.

The oil from the pump 21 is mixed with the fuel and supplied to the crankcase compartments 19 through an induction system which includes carburetors and reed valves not illustrated for admitting an air fuel mixture to the crankcase compartments 19. In the crankcase compartments 19 the oil seves as a lubricant for the crankshaft 14, connecting rods 17, and pistons 16.

The worm 25 is formed as a split ring to allow it to be mounted on the crankshaft 14. Referring to FIG. 2 the worm member 25 is split along a diagonal cut 26 to allow it to be placed on the crankshaft 14. The worm 25 must be sufficiently flexible to allow it to be spread apart and mounted on the crankshaft 14. The worm member 25 includes a radial projection 27 extending inward to engage a corresponding indentation 28 in the crankshaft check 18. The radial projection 27 assures that the worm 25 will rotate with the crankshaft 14. A circumferential flange 29 formed on the crankshaft 14 extends around the crankshaft cheek 18 as shown in FIG. 3 and engages a corresponding circumferential groove 30 in the worm member 25 to restrain the worm member 25 from axial movement. An axial hole 31 through the worm member 25 adjacent to the cut 26 houses a lock barrel 32 which includes a threaded bore 33. A retaining screw 34 is provided to close the worm member 25 around the crankshaft 14. The retaining screw 34 extends through the worm member 25 across the cut 26, and engages the screw threads in the lock barrel 32. Four helical teeth 35 are provided on the external circumference of the worm member 25 to drive the worm gear 24.

A bore 36 through the crankcase member 13 and into the cylinder block 12 houses the worm gear 24. A cylindrical projection 37 at the end of the worm gear 24 is mounted in a bushing 38 in the bottom of the bore 36 in the cylinder block 12 to support the worm gear 24 for rotation. A shaft 39 formed integrally with the worm gear 24 extends outwardly towards the oil pump 21, and has a flat side 40 at the end to mate with a correspondingly shaped coupler bushing 41 to support the outer end of the worm gear 24. The bushing 41 is coupled directly to drive the oil pump 21.

The worm member 25 is molded of a low-friction plastic material, preferably nylon II impregnated with molybdenum disulfide, while the worm gear 24 is machined steel. Because of the lubricated nature of the material forming the worm member, no special provisions are necessary for lubricating the worm member 25 and worm gear 24. Sufficient lubrication for the meshing gears is provided by the simple expedient of mounting the worm member 25 on one of the lower cheeks 18 in a crankcase compartment so that the normal accumulation of oil in the compartment 19 provides lubrication.