Figure 13-18.  Axles machined out of the same material as the  landing gear lower cylinder. Figure 13-17.  Some landing gear shock struts use an internal  metering tube rather than a metering pin to control the flow of fluid  from the bottom cylinder to the top cylinder.  Figure 13-19.  Torque links align the landing gear and retain the  piston in the upper cylinder when the strut is extended.  Nose gear shock struts are provided with a locating cam  assembly to keep the gear aligned. A cam protrusion is  attached to the lower cylinder, and a mating lower cam  recess is attached to the upper cylinder. These cams line up  the wheel and axle assembly in the straight-ahead position  when the shock strut is fully extended. This allows the nose  wheel to enter the wheel well when the nose gear is retracted  and prevents structural damage to the aircraft. It also aligns  the wheels with the longitudinal axis of the aircraft prior to  landing when the strut is fully extended.  [Figure 13-20]  Many  nose gear shock struts also have attachments for the  installation of an external shimmy damper.  [Figure 13-21] Nose gear struts are often equipped with a locking or  disconnect pin to enable quick turning of the aircraft while  13-10 Inner cylinder Piston rod Piston Centering cam Metering tube Air valve Axle Lower cylinder Torque links