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Physical Measurement of Corrosion and Wear Damage on Splined Surfaces


TECHNOLOGY AREA(S): Materials, Air Platform

OBJECTIVE: Develop an innovative measurement system capable of quantifying depth of corrosion and wear damage on rotor mast splines (located on the gearbox output driveshaft) to quantify the depth of damage present prior to repair effort to justify repair decision and quantify the depth of material removal during repair/final surface profile to support the justification of the mast’s continued use.

DESCRIPTION: There is currently a need for a measurement device to identify the extent of, and depth of, corrosion present in and around mast splines. Surface corrosion and pitting damage on the V-22 Proprotor Gearbox (PRGB) mast splines result in removal of the PRGB from the aircraft. The mast has three splined areas external to the PRGB. Current damage tolerances of 0.0005 inches allow for minimal material removal in the splined areas of the mast. Current measurement tooling is limited to C-micrometer for mast diameter dimension-over-pins measurement and a ball scribe/surface defect probe for determining depth of damage on mast splines. The geometry of the mast splines precludes use of other standard measurement processes (i.e., depth micrometers). Initial damage inspection does not currently measure accurately the depth of damage. After damage is removed dimension of pins is used to confirm that enough material remains on the mast. If measurement is below allowable limits, the PRGB is removed resulting in replacement and repair cost of approximately $8.5K including travel costs.Two types of measurements are necessary to evaluate and repair mast spline damage. First, existing corrosion/wear damage must be quantified to determine feasibility of repair within damage limits. Second, the material condition of mast (surface profile) after repair must be measured to ensure the repair did not exceed limits. The proposed system must allow for damage depth measurement with a tolerance of 0.0005 inches while mast is installed in the PRGB on the aircraft wing.The mast material, Cadmium Plated, per SAE AMS-QQ-P-416 [Ref 3], Type II, Class 2, is 4340 stainless steel. The spline geometry can be found in ANSI B92.1-1996 [Ref 1]. The mast has three splined surfaces, two of which have the same geometry. Spline A (upper mast spline) is an external involute, fillet root, side fit spline with 56 teeth, a pitch of 10/20 and pressure angle of 30 degrees. Splines B and C (center mast splines) are also external involute, fillet root, side fit splines with 104 spline teeth, a 16/32 pitch and pressure angle of 30 degrees.

PHASE I: Design, develop, and perform a concept demonstration of a technology that can determine damage depth on spline tooth-face representative of mast geometry for all three splined areas of mast. The Phase I effort will include prototype plans to be developed under Phase II.

PHASE II: Further develop and demonstrate a prototype of the technology under operational conditions. Verify and validate measurement of spline damage, both pre- and post-repair, on a mast-installed on-wing.

PHASE III: Perform final testing and integrate the developed technology into platform Planned Maintenance Interval (PMI) lines. Document the appropriate level of maintenance (organizational or intermediate) depending on the complexity of the system.This technology could be useful for all other vertical lift application with mast corrosion issues. Successful development could have application with commercial variants of the H-1 (Bell 204 and 205) as well as the Leonardo AW609. Additionally, the offshore helicopter industry has constant exposure to corrosive saltwater environments that drive the need for increased inspections.

KEYWORDS: Transmission Mast, Corrosion, Pitting, Splined Surface, Damage Depth, Surface Profile Measurement


1. “Involute Splines and Inspection (ANSI B92.1-1996).” American National Standards Institute, 1996. (cost is $97) 2. “Guide for Eddy Current Testing of Electrically Conducting Materials Using Conformable Sensor Arrays (ASTM E2884-13).” American Society for Testing and Materials (ASTM), International, Book of Standards, Vol. 03.03. 3. QQ-P-416F, FEDERAL SPECIFICATION: PLATING, CADMIUM (ELECTRODEPOSITED) (01 OCT 1991) [S/S BY SAE-AMS-QQ-P-416]

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