Gland Design: Scraper, Landing Gear, Installation
|Publication Date:||1 April 2001|
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 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.
AS4088 is similar to this document, but 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. Since landing gears are more susceptible to dirt contamination, the additional clearance provides a larger path to allow excessive dirt accumulation to exit the gland.
This document is intended to present a groove which will accommodate an improved scraper/assembly design and is not intended to obsolete MS33675.
Present Practice: Currently the most common scraper gland is a MS33675, which accepts either a MS28776 bronze scraper or a comparable TFE scraper. It has been noted that this arrangement is deficient in two aspects: the metal scraper ring is split, allowing contaminants to pass through and around it; and, being installed in a very abbreviated gland, the scraper frequently dislodges under conditions of shock strut deflection. An improved metal, plastic, or elastomeric endless scraper ring would require additional gland volume with greater retaining surfaces than specified by MS33675.