page_title: Coupling rod
section_title: Allowance for Vertical Motion
hierarchical_section_title: Coupling rod / Allowance for Vertical Motion
caption_attribution_description: English: Drivers on MILW 261 steam locomotive
context_page_description: A coupling rod or side rod connects the driving wheels of a locomotive. Steam locomotives in particular usually have them, but some diesel and electric locomotives, especially older ones and shunters, also have them. The coupling rods transfer the power of drive to all wheels.
context_section_description: In general, all railroad vehicles have spring suspension; without springs, irregularities in the track could lift wheels off the rail and cause impact damage to both rails and vehicles.  Driving wheels are typically mounted so that they have around 1 inch (2.5 cm) of vertical motion.  When there are only 2 coupled axles, this range of motion places only slight stress on the crank pins.  With more axles, however, provision must be made to allow each axle to move vertically independently of the others without bending the rods.  This may be done by hinging the side rod at each intermediate crank pin, either using the pin itself as a hinge pin, or adding a hinge joint adjacent to the pin, as shown in the illustration.
  An alternative is to use a side rod that spans multiple axles with a scotch yoke used at each intermediate axle.  This approach was quite common when side rods were used to link a jackshaft to 2 or more driving wheels on electric locomotives and some early internal combustion locomotives.  The Swiss Ce 6/8ᴵᴵ Crocodile locomotive is a prominent example, but there were others.
caption: Hinged side rods connecting the driving wheels of a 4-8-4 locomotive.
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