Coupling rod

Not to be confused withConnecting rod.

Hinged side rods connecting thedriving wheelsofMilwaukee Road 261.

Acoupling rodorside rodconnects thedriving wheelsof alocomotive.Steam locomotivesin particular usually have them, but somedieselandelectriclocomotives, especially older ones andshunter locomotives,also have them. The coupling rods transfer the power of drive to all wheels.

Development

Locomotion No. 1was the first locomotive to employ coupling rods rather thanchains.In the 1930s reliableroller bearingcoupling rods were developed.

Allowance for vertical motion

Connecting rod and coupling rods attached to a small locomotive driving wheel

In general, all railroad vehicles havespring 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 thecrank 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 ahinge 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 ascotch yokeused at each intermediate axle. This approach was quite common when side rods were used to link ajackshaftto 2 or moredriving wheelsonelectric locomotivesand some early internal combustion locomotives. The SwissCe 6/8^IICrocodile locomotiveis a prominent example, but there were others.

Balancing

Counterweight on a smalloutside-framedual-modeelectro-diesel locomotive,aSwissTem IIshunter locomotive.

The coupling rod's off-center attachment to thecrank pinof the driving wheel inevitably creates an eccentric movement and vibration when in motion. To compensate for this, the driving wheels of an inside-frame locomotive always had built-incounterweightsto offset the angular momentum of the coupling rods, as shown in the figures above. Onoutside-framelocomotives, the counterweight could be on the driving wheel itself, or it could be on the crank outside the frame, as shown in the adjacent figure.

Where the motion of the side-rods is purely circular, as on locomotives driven byjackshaftsor geared transmission to one driver, counterweights can balance essentially all of the motion of the side rods. Where part of the motion is non-circular, for example, the horizontal motion of a piston rod, counterweights on the wheels or drive axles cannot be made to balance the entire assembly perfectly. On a driving wheel supporting both side-rods and theconnecting rodto apiston,the counterweight needed to balance the horizontal motion of the piston and connecting rod would be heavier than the counterweight needed to balance the vertical weight of the rods. As a result, a counterweight chosen to minimize the total vibration will not minimize the vertical component of the vibration.

The vertical component of the vibration that could not be eliminated because of the weight needed to balance the pistons is calledhammering.This is destructive to both the locomotive and the roadbed. In some locomotives, this hammering can be so intense that at speed, the drivers alternately jump from the rail head, then slam down hard on the rails as the wheels complete their rotation. Unfortunately, hammering is inherent to conventional two-cylinder piston-driven steam locomotives and that is one of the several reasons they have been retired from service.

Materials

Initially, coupling rods were made ofsteel.As technology progressed and better materials became available, the connecting rods were manufactured of lighter and strongeralloys,which in turn permitted smaller counterweights and also reduced hammering.

See also

• Steam locomotive components