A load due to combustion pressure is applied to each of the crank pins via a connecting rod in a direction from the axis of a piston pin to the axis of the crank pin. Each of the crank arms includes a recess in a surface adjacent to the journal, in a region inward of a peripheral region along the edge of the surface. The recess extends along the peripheral region, and is asymmetric with respect to a crank arm centerline connecting the axis of each of the crank pins to the axis of each of the journals. Each of the crank arms has a maximum flexural rigidity at a point of time when the load onto each of the crank pins due to the combustion pressure reaches a maximum. The crankshaft has a reduced weight and an increased torsional rigidity in combination with an increased flexural rigidity.

A load due to combustion pressure is applied to each of the crank pins via a connecting rod in a direction from the axis of a piston pin to the axis of the crank pin. Each of the crank arms includes a recess in a surface adjacent to the journal, in a region inward of a peripheral region along the edge of the surface. The recess extends along the peripheral region, and is asymmetric with respect to a crank arm centerline connecting the axis of each of the crank pins to the axis of each of the journals. Each of the crank arms has a maximum flexural rigidity at a point of time when the load onto each of the crank pins due to the combustion pressure reaches a maximum. The crankshaft has a reduced weight and an increased torsional rigidity in combination with an increased flexural rigidity.

A propeller shaft (1) that is a power transmission shaft has a shaft member (2) as a tubular body made of iron-based metal and a balance weight (3) made of iron-based metal and welded to an outer peripheral surface of this shaft member (2). At least a part of the balance weight (3) of the propeller shaft (1) is covered with a sacrificial metal coating (4) made of sacrificial corrosion prevention material that contains metal whose ionization tendency is higher than that of metal forming the shaft member (2). With this, it is possible to suppress local progression of corrosion at a periphery of the balance weight (3) and improve durability of the propeller shaft (1).

A crankshaft includes journals that define a central axis of rotation; crank pins that are eccentric with respect to the journals; crank arms connecting the journals to the crank pins; and counterweights integrated with the crank arms. Each of the crank arms has recesses in a surface adjacent to the crank pin. The recesses are disposed inward of peripheral regions in both sides along an edge of the surface, and are disposed along the peripheral regions. The crankshaft has a reduced weight, an increased torsional rigidity and an increased flexural rigidity.

A damper device is attached to a rotating shaft to suppress amplitude at resonance of the rotating shaft. The damper device includes: a damper housing formed annularly and concentrically with the rotating shaft; a plurality of mass bodies annularly arranged around the rotation shaft inside of the damper housing and configured to be movable in the diameter direction by centrifugal force; an annular elastic body, formed of a circular spring-shaped elastic body abutting on the outside of the plurality of the mass bodies, so as to bias the mass body inward; and a biasing member, being a leaf spring-shaped elastic body abutting on the outside of the annular elastic body, so as to bias the annular elastic body inward.

A damper device is attached to a rotating shaft to suppress amplitude at resonance of the rotating shaft. The damper device includes: a damper housing formed annularly and concentrically with the rotating shaft; a plurality of mass bodies annularly arranged around the rotation shaft inside of the damper housing and configured to be movable in the diameter direction by centrifugal force; an annular elastic body, formed of a circular spring-shaped elastic body abutting on the outside of the plurality of the mass bodies, so as to bias the mass body inward; and a biasing member, being a leaf spring-shaped elastic body abutting on the outside of the annular elastic body, so as to bias the annular elastic body inward.

A balance shaft for balancing forces of inertia and/or moments of inertia of a reciprocating-piston internal combustion engine, including: at least one elongate main body; at least one bearing seat, disposed on the elongate main body for the mounting of a radial bearing. In the center point of the bearing seat there is provided the rotational axis of the balance shaft. The elongate main body may be formed of an integral tubular element, and the center of mass of the elongate main body may lie outside the rotational axis of the balance shaft.

A balance shaft for balancing forces of inertia and/or moments of inertia of a reciprocating-piston internal combustion engine, including: at least one elongate main body; at least one bearing seat, disposed on the elongate main body for the mounting of a radial bearing. In the center point of the bearing seat there is provided the rotational axis of the balance shaft. The elongate main body may be formed of an integral tubular element, and the center of mass of the elongate main body may lie outside the rotational axis of the balance shaft.

A crankshaft includes journals serving as a central axis of rotation, crank pins decentered from the journals, and crank arms connecting the journals and the crank pins. Some or all of the crank arms have counterweights integrated therewith. At least one of the crank arms has a recess in a crank pin facing surface. The crank arm includes ribs formed respectively in two side portions of the recess along a periphery of the crank arm.

A crankshaft includes journals serving as a central axis of rotation, crank pins decentered from the journals, and crank arms connecting the journals and the crank pins. Some or all of the crank arms have counterweights integrated therewith. At least one of the crank arms has a recess in a crank pin facing surface. The crank arm includes ribs formed respectively in two side portions of the recess along a periphery of the crank arm.