Backup rings may be installed singly, if pressure
acts only upon one side of the seal. In this case, the
backup ring is installed next to the O-ring, opposite
the pressure force. See view A of figure 2-51. When
dual backup rings are installed, the split scarfed ends
must be staggered, as shown in view B of figure 2-51.
View C of figure 2-51 shows an improper dual ring
installation.
Wipers
Wipers (scrapers) are used to clean and lubricate
the exposed portion of piston shafts. This prevents
foreign matter from entering the system and scoring
internal surfaces. Wipers may be of the metallic
(usually copper base alloys) or felt types. They are
used in practically all landing gear shock struts and
most actuating cylinders. At times, they are used
together, the felt wiper being installed behind the
metallic wiper. Normally, the felt wiper is lubricated
with system hydraulic fluid from a drilled bleed
passage or from an external fitting.
Wipers are manufactured for a specific hydraulic
component and must be ordered for that application.
Wipers are normally inspected and changed, if
Figure 2-51.—Teflon® backup ring installation (external).
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