Crosshead

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Piston drives with and without crosshead

The cross head is a machine element that is used in crank drives . It couples the translationally oscillating (reciprocating) piston rod with the translationally and rotationally oscillating (simultaneously swiveling out) connecting rod and is mainly found on large piston engines .

Working principle

Kinematics of a piston drive with and without crosshead

The axes of the piston rod and the connecting rod pin intersect on the same plane. The cross head has a separate slide bearing and is rigidly connected to the piston via the piston rod . The connecting rod swings around the cross head and is connected to the crankshaft .

The cross head absorbs the forces acting vertically and laterally on the crankshaft ( crank pin ). It keeps the piston free from side forces. As a result, the piston can be made very flat (disc piston) and the hot cylinder and piston running surfaces are less stressed. In the case of plunger piston machines, the piston skirt takes on the role of the cross head and transfers lateral forces to the cylinder. Machines with oscillating cylinders get by without a crosshead. The piston rod engages directly on the crank and the cylinder performs a pivoting movement.

The sequence of movements of a crank mechanism with a cross head is the same as that of a crank mechanism without a cross head with the same length connecting rod.

Applications

Crosshead of a steam engine
Crosshead of a steam locomotive

The cross head is mainly used in two-stroke large diesel engines and piston steam engines . This is related to several properties:

  • The stroke can be extended as required for a given bore. For two-stroke engines, 3 ... 4.2 times the stroke are common. In contrast, the stroke of the plunger is limited by the deflection of the connecting rod.
  • The method is suitable because of the great moving masses and the height only for speeds of slow-speed (up to 300 min -1 ). Large diesel engines, however, benefit from the fact that
    • the combustion process enables very good thermal efficiency (highest overall efficiency up to 55%, approx. 158 g / kWh)
    • for ships the crankshaft without transmission (and thus loss of efficiency) directly to the ship's propeller can act
  • The straight guidance of the piston rod allows the crank mechanism to be easily sealed off from the cylinder. This has several advantages:
    • Sealing of the crankshaft and the cross head against the hot exhaust gases and residues caused by blowby losses on the piston. Lubricants for the piston rings and for the crankshaft can be separated. Inexpensive fuels that burn with high ash content can be used . Piston rings require higher quality (especially with regard to temperature, ash tolerance and other additives) oil than the crankshaft.
  • The straight piston rod enables the construction of double-acting machines in which the piston in the same cylinder is alternately pressurized from both sides. This eliminates idle strokes, the power density increases and the torque curve becomes more even. Two-cylinder steam engines with a 90 ° crank offset can start from any position and can also be designed to be reversible (direction of rotation). Double action is the rule with steam engines. Double-acting combustion engines could not prevail because of the higher thermal load (cooling problems). However, it is quite common to separate the compression chamber below and the combustion chamber above.
  • The piston is free from side forces. The piston can be made flat and simple (short piston skirt, no joint, no piston pin) and has no piston tilting when it passes through top dead center (TDC).

In the case of smaller engines, the cross head is used in the opposed piston engine from Golle Motor AG in a new and patented way.

literature

  • Leo Hagedorn, Wolfgang Thonfeld, Adrian Rankers: Konstruktiv Getriebelehre , 12th edition, Springer, Heidelberg, 2009, ISBN 978-3-642-01613-4 .
  • Hans Jörg Leyhausen: The master craftsman's examination in the automotive trade part 1, 12th edition, Vogel Buchverlag, Würzburg, 1991, ISBN 3-8023-0857-3 .
  • Max Bohner, Richard Fischer, Rolf Gscheidle: Expertise motor vehicle technology, 27th edition, Verlag Europa-Lehrmittel, Haan-Gruiten, 2001, ISBN 3-8085-2067-1 .

Web links

Commons : Crosshead Steam Engines  - Collection of images, videos and audio files
Commons : Piston Drive with Cross Head  - Collection of images, videos and audio files

Individual evidence

  1. ↑ Leo Hagedorn, Wolfgang Thonfeld, Adrian rankers: Constructive Getriebelehre . 2009, p. 21
  2. ↑ See GP Merker, R. Teichmann (Ed.), " Fundamentals of Internal Combustion Engines ", 7th edition 2014, Section 3.8 " Large Diesel Engines ", Springer Fachmedien Wiesbaden, ISBN 978-3-658-03194-7
  3. ↑ a b Klaus Mollenhauer, Helmut Tschöke, " Handbuch Dieselmotoren ", 3rd edition 2007, section 18.4.1.3 " Features of modern two-stroke slow runners ", Springer-Verlag Berlin Heidelberg, ISBN 978-3-540-72164-2
  4. ↑ MAN G95ME-C9.6 operated with HFO ( Memento from May 26, 2018 in the Internet Archive )