(R) Gland Design: Scraper, Landing Gear, Installation
|Publication Date:||1 November 2008|
This SAE Aerospace Standard (AS) covers an alternate gland design for the installation of scraper/ wiper rings in the lower end of landing gear shock struts for the purpose of contaminant exclusion.
The defined scraper gland covered by this document, as shown in Table 1, is a variant of AS4716, the accepted gland standard for MS28775, O-ring packing seals. Piston rod diameters, gland internal diameters, groove sidewall angles and the surface finish are all defined by AS4716, but the gland outer retaining wall diameter is changed. The traditional scraper design installed into the glands detailed in Table 1 typically utilize components made from urethane or nitrile materials. These scraper designs, while still acceptable, must be reviewed in consideration to deicing, cleaners and disinfectant fluids applied to or in contact with the landing gear, as the materials of construction for the installed scrapers may not be compatible to these fluids. Exposure of the scraper to incompatible fluids is likely to reduce the performance of the scraper.
In addition, an alternative scraper gland is also covered by this document and shown in Table 2. It is also a variant of AS4716, however this gland has a reduced atmospheric gland lip and profiled lead in geometry to allow for a PTFE jacket metal spring energized scraper to be installed. The advantages of the PTFE jacket metal spring energized scraper design is that the materials of construction are chemically inert, greatly reducing the possibility of negative performance due to incompatibility with deicers, cleaners and disinfectant fluids.
AS4088 is similar to the hardware design in Table 1 of this document, which was developed by SAE A-6 for flight control and general-purpose cylinders. It differs from this document primarily by the clearance between the rod (piston) and outer gland wall.
This document is intended to present a groove which will accommodate an improved scraper/wiper ring assembly design and is not intended to obsolete the MS33675 gland standard.