Arrangement for Bearing of a Crankshaft

This invention relates to an arrangement in a crankshaft of a combustion engine, preferably a two-stroke engine with crankcase compression. It has for an object to provide a separate crankshaft bearing band which encloses the roller bearings without joints.

In the development of light combustion engines for portable tools, such as power saws, new materials within the engine technique have been used, preferably fibre-glass reinforced plastics material. Constructions having a crankcase of plastics material have been tried but difficulties have arisen as concerns the sealing of the joint surfaces of the two parts of the crankshaft bearing seats. Due to the different properties of the materials of the various parts with regard to linear expansion, heat conduction and stiffness, plastics material has so far not been directly applicable to combustion engines. Even if plastics material of refined properties, which would satisfy the requirements, are available, these are not advantageous with regard to costs compared to constructions already available. One solution is however possible if high-quality material is used only in the connection between the cylinder including the crankcase top part on one hand and the bottom part of the crankshaft on the other hand.

The present invention offers a solution for joining a crankcase of plastics material to a cylinder of a metal alloy via a crankshaft bearing sleeve which when mounted encloses the crankshaft bearings without joints and separates the cylinder and the crankcase in the parting plane through the crankshaft. The material of the crankshaft bearing sleeve is selected for example among high-temperature resistant polyester plastics, which both endure the temperature of the cylinder part and limit the heat transfer to the bottom part of the crankcase which is made by a cheaper plastics material. Thus the idea of reducing the manufacturing cost and the weight of the product is realized.

An embodiment of an arrangement according to the invention will now be described with reference to the accompanying drawing in which

FIG. 1 shows an engine with the said arrangement in a vertical cross section,

FIG. 2 is another cross section perpendicular to the first-mentioned,

FIG. 3 is an end view of a crankshaft bearing sleeve and

FIG. 4 shows the crankshaft bearing sleeve in a horizontal projection.

As seen in FIGS. 1 and 2, the engine of this embodiment is a two-stroke engine with a cylinder 10, a piston 11 and a crankshaft 12. The cylinder is coupled to the crankcase by four screws 13 passing from below through holes in the crankcase half and are screwed in the cylinder part. The crankcase 12 is produced by pressing and/or injection moulding of thermoplastics and has semi-cylindrical cavities 14 in its upper surface. These cavities correspond to cavities 15 in the cylinder 10. The crankshaft 16 is a through-shaft with a crankshaft bearing in the form of roller bearings 17, 18 in each cavity. Each roller bearing is surrounded by a sleeve 19, 20 which is placed exteriorly on the outer ring of the bearing before the crankshaft is mounted in the crankcase. The sleeves are shown in FIGS. 3 and 4, the four holes 21 indicating the location of the through-passing screws 13. The sleeves are included in a frame with short sides 22 and long sides 23. At the short sides there are connecting means 24 keeping the frame together. Each frame half, however, can be treated separately on mounting on the crankshaft bearings. The frame sides also serve as sealing between the cylinder and the crankcase. The sealing property can be improved by a low ridge 25 on each side surface of the frame. When the crankcase is assembled and the screws 13 are tightened the frame surfaces seal against the relevant surfaces of the parts 10 and 12. The elasticity of the sleeve contributes to filling up of possible uneveness of the cavities when the screws are tightened so that an effective sealing around the bearings is achieved. Therefore, the cavities can be left unfinished such as they appear after moulding because the tolerances already obtained are sufficient for fitting to the sleeve. Thanks to the sleeve also a uniform pressure around the outer bearing ring is obtained and the latter is thus not deformed when the screws are tightened.

The embodiment described should be considered as an example of realization of the invention. It would be evident that parts can be moved, completed, replaced or be differently shaped without deviating from the basic concept of the invention.