A crank mechanism for the use in an in-line boxer engine has at least two diametrically opposed cylinders, that has a crankshaft and the respective pistons as well as connecting rods for each cylinder of the in-line boxer engine, with the connecting rods cooperatively connecting the pistons with the crankshaft. Each of the connecting rods encompasses a respective piston connecting portion, at one end having bushings accepting a gudgeon pin. At the other end, the central connecting rod has a one-piece crankshaft bearing portion for the crankpin whereas the forked connecting rod has a crankshaft bearing portion with two spaced limbs resultant in bifurcated crankshaft bearing portions for the crankpin. The crankshaft possesses a cylindrical central middle crankpin, that is eccentric towards the crankshaft, onto which a cylindrical outer crank pin is immediately attached at each side without crank webs.

A system, in certain embodiments, includes a composite piston. The composite piston comprises a piston body made of a composite material, wherein the composite material comprises a reinforcing material distributed in a matrix material.

A system, in certain embodiments, includes a composite piston. The composite piston comprises a piston body made of a composite material, wherein the composite material comprises a reinforcing material distributed in a matrix material.

A crosshead for a piston rod is provided, which comprises a main body having a first and a second seat. The first seat is configured to hold a connecting rod, and the second seat is configured to hold a piston rod. The main body is made as a single piece.

A crosshead for a piston rod is provided, which comprises a main body having a first and a second seat. The first seat is configured to hold a connecting rod, and the second seat is configured to hold a piston rod. The main body is made as a single piece.

A piston rod assembly for a reciprocating compressor. The piston rod assembly may include a piston rod having a longitudinal axis and forming a piston rod neck and a piston rod shoulder. The piston rod may include a first end integral or coupled with a piston head and a second end axially opposing the first end and configured to couple with a crosshead including a flange. An inner surface of the flange may have a Brinell hardness. The piston rod assembly may also include a cylindrical sleeve disposed circumferentially about the piston rod neck. The cylindrical sleeve may have a lower Brinell hardness than the Brinell hardness of the inner surface of the flange.

A piston rod assembly for a reciprocating compressor. The piston rod assembly may include a piston rod having a longitudinal axis and forming a piston rod neck and a piston rod shoulder. The piston rod may include a first end integral or coupled with a piston head and a second end axially opposing the first end and configured to couple with a crosshead including a flange. An inner surface of the flange may have a Brinell hardness. The piston rod assembly may also include a cylindrical sleeve disposed circumferentially about the piston rod neck. The cylindrical sleeve may have a lower Brinell hardness than the Brinell hardness of the inner surface of the flange.

A crosshead-piston rod assembly for a reciprocating compressor comprises a piston rod; a crosshead having a bore; a connection element attached to the piston rod and at least partially inserted into the bore for connecting the crosshead to a piston.

A crosshead-piston rod assembly for a reciprocating compressor comprises a piston rod; a crosshead having a bore; a connection element attached to the piston rod and at least partially inserted into the bore for connecting the crosshead to a piston.

A self-aligning mud pump apparatus is provided. In one embodiment, the mud pump includes a rotatable crankshaft, a crosshead, a crosshead guide, and a hub disposed in a housing. The hub is disposed on the crankshaft and is operable to convert rotating motion of the crankshaft to reciprocating motion of the crosshead within the crosshead guide. The hub is coupled to the crosshead via a connecting rod, which is connected the crosshead such that the connecting rod has five degrees of freedom with respect to the crosshead guide. The mud pump may also or instead include a piston coupled to the crosshead with five degrees of freedom between the piston and the crosshead. Additional systems, devices, and methods are also disclosed.