Updated: 6/15/2024
The purpose of tail rods, as installed on steam engines, is to add extra support and stability to a horizontally oriented reciprocating component. They can be applied to parts such as piston rods and valve spindles. Piston and valve heads have side thrusts, predominantly but not exclusively vertical, imparted by the angular movements of connecting rods and valve gears on their respective drive rods. This causes the piston and valve heads in contact with liners in the cylinder to unevenly rub against them inducing uneven wear on all components in contact. Additionally the vertical mass of horizontally oriented components causes further wear. In steam engine practice the interposing of a crosshead between the crank (circular) driven connecting rod and the linear drive required for the piston rod provides a strong linear guide thus eliminating the majority of the vertical forces. Poor crosshead design on older locomotives worsened these side thrusts. Having tight cross head tolerances with plenty of support surface area and adding extension tail rods on the non-drive end decreases side thrusts and linear wear on cylinders and valves by providing a linear support along the whole direction of travel therefore decreasing wall, head, and ring wear. Tail rods can also normally be provided with a substantial wearing surface. The problem of cylinder side thrusts is significantly worse in internal combustion engines with arcing connecting rods because the piston head is directly connected to the arcing connecting rod and has no external guides to truncate and stabilize its motion.