Filler plug Bleed hole Piston A B B A Replenishing piston Abutment and valve assembly Spring Replenishing check valve Rotating vane Dowel pin Spring retainer Mounting flange Wing shaft Packing spring Filler cap Hollow dowel pin Fluid port Fluid port Replenishing chamber Key Abutment flange Indicator rod connected to replenishing piston Wing shaft packing Hydraulic seal Wing shaft Rotating vane Valve orifice 13-33 Figure 13-55.  A typical vane-type shimmy damper. Figure 13-54.  A shimmy damper on the nose strut of a small aircraft.  The diagram shows the basic internal arrangement of most shimmy  dampers. The damper in the photo is essentially the same except the  piston shaft extends through both ends of the damper cylinder body. filled with fluid. The chamber size can only change as fast  as the fluid can be forced through the orifice. Thus, the gear  oscillation is dissipated by the rate of fluid flow. An internal  spring-loaded replenishing reservoir keeps pressurized fluid  in the working chambers and thermal compensation of the  orifice size is included. As with the piston type shimmy  damper, the vane-type damper should be inspected for leaks  A vane-type shimmy damper is sometime used. [Figure 13-55]  It uses fluid chambers created by the vanes separated by a  valve orifice in a center shaft. As the nose gear tries to  oscillate, vanes rotate to change the size of internal chambers  A piston-type shimmy damper may contain a fill port to  add fluid or it may be a sealed unit. Regardless, the unit  should be checked for leaks regularly. To ensure proper  operation, a piston-type hydraulic shimmy damper should  be filled to capacity. Aircraft not equipped with hydraulic nose wheel steering  utilize an additional external shimmy damper unit. The case  is attached firmly to the upper shock strut cylinder. The shaft  is attached to the lower shock strut cylinder and to a piston  inside the shimmy damper. As the lower strut cylinder tries to  shimmy, hydraulic fluid is forced through a bleed hole in the  piston. The restricted flow through the bleed hole dampens  the oscillation.  [Figure 13-54] As mentioned above, large aircraft with hydraulic steering  hold pressure in the steering cylinders to provide the required  damping. This is known as steering damping. Some older  transport category aircraft have steering dampers that are  vane-type. Nevertheless, they function to steer the nose  wheel, as well as to dampen vibration. Steering Damper Piston-Type Vane-Type