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).

necessary, while component repair is in process. Metallic wipers are formed in split rings for ease in installation, and they are manufactured slightly undersize to ensure a tight fit. One side of the metallic wiper has a lip, which should face outward upon installation. Metallic wipers must be inspected for foreign matter and condition, and then installed by sliding them over the piston shaft in the proper order, as directed by the applicable MIM.

The felt wiper may be a continuous felt ring or a length of felt with sufficient material to overlap its ends. The felt wiper should be soft, clean, and well saturated in hydraulic fluid during installation.