A balance weight having one or more journals with a reduced mass portion disposed between a pair of journal portions that define circumferentially extending bearing surfaces. The reduced mass portion has a body, which defines a circumferentially extending bearing surface that is concentric with the bearing surfaces on the journal portions, and at least one stiffening structure that is configured to resist flexing of the journal portions relative to one another due to the transmission of bending loads through the balance shaft.

A balance weight having one or more journals with a reduced mass portion disposed between a pair of journal portions that define circumferentially extending bearing surfaces. The reduced mass portion has a body, which defines a circumferentially extending bearing surface that is concentric with the bearing surfaces on the journal portions, and at least one stiffening structure that is configured to resist flexing of the journal portions relative to one another due to the transmission of bending loads through the balance shaft.

A balance weight having one or more journals with a reduced mass portion disposed between a pair of journal portions that define circumferentially extending bearing surfaces. The reduced mass portion has a body, which defines a circumferentially extending bearing surface that is concentric with the bearing surfaces on the journal portions, and at least one stiffening structure that is configured to resist flexing of the journal portions relative to one another due to the transmission of bending loads through the balance shaft.

A counterweight forms a crankshaft of an engine, and includes: an arm connecting a crank journal and crank pin of the crankshaft; a neck extending from a first connection face of the arm in a direction opposite to the crank pin; and a substantially fan-shaped weight continuous with a portion of the neck opposite to the crank journal. The weight has left and right shoulders continuous with the neck, and each of the shoulders is tilted to be away from the neck at an angle with respect to a horizontal line orthogonal to a crankshaft center when viewed along the crankshaft center, the angle being 15 or more and 22.5 or less.

A counterweight forms a crankshaft of an engine, and includes: an arm connecting a crank journal and crank pin of the crankshaft; a neck extending from a first connection face of the arm in a direction opposite to the crank pin; and a substantially fan-shaped weight continuous with a portion of the neck opposite to the crank journal. The weight has left and right shoulders continuous with the neck, and each of the shoulders is tilted to be away from the neck at an angle with respect to a horizontal line orthogonal to a crankshaft center when viewed along the crankshaft center, the angle being 15 or more and 22.5 or less.

A balance weight having one or more journals with a reduced mass portion disposed between a pair of journal portions that define circumferentially extending bearing surfaces. The reduced mass portion has a body, which defines a circumferentially extending bearing surface that is concentric with the bearing surfaces on the journal portions, and at least one stiffening structure that is configured to resist flexing of the journal portions relative to one another due to the transmission of bending loads through the balance shaft.

An air compressor includes a rotating device pivotally connected to a piston body. The rotating device includes a gear body and a counterweight. The gear body is formed together with a counterweight, wherein the counterweight is provided with a crankshaft corresponding to the center of the gear body and a crankpin placed at a distance from the crankshaft. The counterweight is horizontally divided into two integrally formed halves, including a left half and a right half, with a common line connected between the center of the crankshaft and the center of the crankpin, wherein the left half has a weight different from the right half, so that the piston body gathers more rotational momentum from the counterweight at BDC to facilitate upstrokes, so that the piston body can conduct reciprocating motion more smoothly, and thus the service life of the air compressor can be increased.

A torque arm structure includes a first member which is fixed to the reduction gear or a member integrated with the reduction gear, and a second member which is fixed to an outer member to oppose the first member, and an elastic member configured to transmit a load by which the reduction gear is rotated around the output shaft from the first member to the second member. The elastic member is to be fitted into a gap between the first member and the second member and is detachably disposed in the gap.