2. Field of invention

Design of Repair of MGB housing bores wear out of a helicopter by using Steel Liners is an invention in the field of Mechanical engineering, particularly in the field of gear boxes maintenance of rotary wing. It is used for repairing the Magnesium Gearbox Housings bores wear out which are becoming after usage of few 100 hours of flying.

3. Background of Invention

MGB is used in any helicopter for combining all the powers through engine and transmitting to Main Rotor Blade, tail rotor blades and accessories drive.

MGB housings are made up of magnesium material and manufactured through precision forging and machining process. Housings are very critical because of selection of Raw material, forging techniques for achieving the shape and precision machining to achieve the tolerances.

Because of dynamic loads of Main rotor and tail rotor bores wear out were being observed in MGB housing after completion of one or two overhaul life.

US 2011/0030503 A1 is about manufacture of complete housing assembly having improved bearing liner. It describes assembly of bearing liner in the housing of newly manufactured gearbox. This publication uses sintered bearing liner which are selected from the group consisting of ceramic material, ceramic coated material, ceramic and metallic hybrid material, lightweight metallic material, light weight metallic alloy material and lightweight metallic coated material made of ceramic and metal fused composition.

There are other several prior art related to bearing liner, however they are related to composition of the sintered bearing liner.
4. Summary of the present invention

The repair scheme is established by identification of satisfactory level of bore wear out, selection of steel liner of appropriate material and dimension and establishing the procedure of application of steel liner on the affected MGB housing. Procedure is achieved through Pre-machining of Housing Bores, Selenic acid touch-up at machined bores locally, Installation of Steel liners by applying anti corrosive paste in the Housing bores, Installation of Dowel pins by drilling of Housings/ Liner interfaces with applying Loctite, Post machining of Housing bores as per drawing,CD checks by FDP Method, Magic blue touch-up at Liner inner diameters & Final Inspection.

5. Brief description of drawings

Fig 1: Drawing of MGB Housing

Fig 2: Shows the section view BB of MBG housing with bearing Liner 1 Fig 3: Shows the section view CC of MBG housing with bearing Liner 2 Fig 4: Shows the installation of Dowell Pin Fig 5: Drawing of Bearing Liner (1 & 2)

Fig 6 : Drawing of Dowell Pin
6. Detailed description of invention:

MGB in a helicopter fulfills following functions-

a) Transmission of power and reduction of rotational speed between the engines and the rotors.

b) Provision for the fixed anchoring of the flight servo controls.

c) Driving various accessories such as hydraulic pumps, alternators, cooler fan.

MGB housing is made-up of magnesium metal and it having openings where bearings are press fitted and through arrangement of different gears ahd pinions transmission of power and reduction of rotational speed from engine to rotor is achieved.

MGB housing wear out is observed after gear boxes are flown for few 100 hours because inadvertent load transfer on the housing.

A repair scheme is invented for application of steel liner on MGB housing by maintaining close tolerances of original MGB housing drawing requirements. Procedure is as follows-

a) Initial Measurement of Bores:

Gearbox Bores to be measured locally for wear out and also with respect to the geometrical tolerances of other bores & Faces surfaces Housings and recording as reference purpose. Use CMM machines for geometrical tolerances purpose. Gearbox housing drawing is shown in Fig 1. Further bearing liner position where wear out is observed is shown in sectional view drawing Fig 2 and Fig 3.

b) Pre-machining of Bores: Open the existing bores of item no. 1 for installing the steel liners by loading on jig boring machines with rotary tables/ Fixture plates. Carryout through dialing of liner or radial surfaces of bores with respect to Housings main reference surfaces before machining.

c) CD Check & Selenic Acid touch-up: Carry-out CD check by FDP method locally on machined bores of item no. 1. Carry-out Selenic acid touch-up in the machined bores locally.

d) Preparation of Steel liners: Manufacture the Steel liners by considering tolerances and material specification as per drawing & keeping the stock inner bore for finishing in assembled stage. Carryout surface protection with Phosphate or
Cadmium plating method. Steel liner (1 & 2) item no. 2 and 3 drawing is shown in fig 5.

e) Installation of Steel liners in to gearbox housings:

1) Heat the gearbox housing 1 to a temperature of 60 to 75deg centigrade in oven.

2) Apply anticorrosive paste into the gearbox housing bores uniformly,

3) Cool the Liners up to -180 to -190 Deg C by using Liquid Nitrogen.

4) Install the Steel liners 2 & 3 into Gearbox Housings 1 by using suitable Tooling without damaging the Housing surfaces as per Fig 2 and Fig 3.

f) Installation of anti- rotation pins:

1) Drill holes at 3 places through Liner item no,2 & 3 and Housing interfaces 1 to required depth for installing the dowel pins for avoiding anti-rotation of Liner while using on Helicopter as per Fig 4

2) Install Dowel pins at item Housing/liner interfaces by applying Loctite as shown in Fig 4.

g) Post machining of Housings:

1) Load the Housing on Jig boring machine bed with rotary table and fixture plate.

2) Dial the axial and radial surfaces and check with respect to drawing requirement for geometry of the Housing.

3) Carry-out machining of Bores at one stage or in two stages as per criticality of the Housings geometry for intermediate check in coordinate measuring machine (CMM) in order to ensure all parameters while finish machining of bores,

4) Unload the machine and carry-out removal of burrs or sharp edges as per assembly requirement

h) Inspection of Housings: Carry-out detailed bores check at liner surfaces & geometrical tolerances on Coordinate measuring machine (CMM) for further operations.

i) CD Check & Magic blue touch-up: Carry-out CD check by FDP method locally on machined bores. Carry-out magic blue touch-up in the machined bores locally.