1 Title of the Invention

Repair procedure for damaged stud of MGB housing of a helicopter

2 Field of the Invention

The invention relates to general mechanical engineering and more specifically to methodology of repair of magnesium main gear box housing of a helicopter

3 Background of the Invention

Main Gear Box is heart of any helicopter system and fulfills following major applications-

a) Transmission of power from engines to rotors and reduction of rotational speed from engines to rotors

b) Fixed anchoring of main rotor actuators which transmit the pilot commands to Main rotor blades

c) It also drives various accessories like alternator, hydraulic pumps, and cooler fan

Main gear box housing is one the most critical system of any helicopter. In case of ALH it houses all the gears and pinions of the main gear box. It also houses Upper control surface interconnections of main gear box. There are three booster rods which are connected one end to main rotor actuators and another end to Housing of MGB. The actuators are mounted on housing through studs and through which Fixed anchoring of main rotor actuators is achieved. Also Housing top surface is having studs, which will be held for fastening of mast bearing housing/mast bearing.The studs installed at these places are of special ROSAN type metric insert having internal ring lock feature.

Due to the mishandling/ overloading at local surfaces of housing or inappropriate torque the studs of housing were observed having damages,
thread wear out or loosening. As these studs are of special ROSAN type and cannot be removed without breaking the stud ring and enlarging the diameter of the mounting area (MGB Housing), it used to lead to complete rejection of gear boxes.

3.1 Prior Art

Previously the attachment area used to be inspected regularly for any damage and if damages were noticed, it used to lead to rejection of housing. There was no standard repair procedure available to carryout repair.

4 Brief Summary of the Invention

This repair procedure was developed to salvage the Main Gear Box housing when stud damage or wear out of stud is observed. The repair procedure starts with removal of lock ring from MGB housing. Further MGB housing was machined to open out stud maintaining a proper edge distance. It further involved fabrication of bush and pins, assembly of bush and pin, peening at rework surface and lastly installation of same size of stud with bush.

5 Detail Description of the Drawings

Figure 1 the damaged stud (1) of the Main Gear Box housing which is at the booster rod and main rotor actuator interface.

Figure 2 shows another damaged stud at another location of MGB housing. It is shown as (2).

Figure 3 shows the two views of opening out of the stud hole to higher approved size maintaining the required tolerance.

Figure 4 shows the two views of assembly of newly fabricated bush (3) on housing stud hole.
Figure 5 the installed view of the bush and diagonal drilling of it for installation of pin (4). Figure 6 shows the installation of Pin (4) on MGB housing.

Figure 7 shows the peening operation at reworked surface.

Figure 8 shows the two views of Bush (3).

Figure 9 shows the drawing of pin (4).

Figure 10 shows standard installation of stud.

6 Detail Description of the Invention

Whenever main Gear Box comes for overhaul/ repair, MGB housing is thoroughly checked for any damage as it holds critical components of MGB and upper control surface.

Studs are installed on MGB housing at booster rod area interface where main rotor actuators are interfaced with booster rod which in turn changes the main rotor blade angle as desired by pilot. As part of checks studs at booster rod area interface and also at top surface are checked for any damage or looseness. After flying of 1000 hours in few of the MGB housing defects like (1) in Figure 1 or (2) in Figure 2 were observed. For them this process is evolved and is as follows-

a. MGB housing is clamped on an angle and with the help of standard tool lock ring of damaged stud (1) or (2) is removed. Damaged stud is removed by drilling the hole.

b. MGB housing is moved to jig boring machine and for damaged stud (1), boring is performed to enlarge the hole from M12 x 1.5 Diameter to Diameter 15 (H7)) mm up to depth 29 (+0.2) mm with chamfer 0.5 x 45 degree & open out groove as shown in Figure 3.
For damaged stud (2), boring is performed to enlarge the hole from M12 x 1.5 Diameter to Diameter 18 (H7)) mm up to depth 29 (+0.5) mm with chamfer 0.5 x 45 degree.

c. Check is performed so that proper edge distance of 4 mm is available after carrying out opening of the hole.

d. Crack detection is performed afterwards for checking any cracks after machining operation.

e. Afterwards application of Selcenic acid is performed on reworked area

f. Assembly of the bush (3) into the MGB housing hole after applying the locktite-609 on Outer Dia of Bush (3) as per Figure 4. Figure 4 also shows face X of the assembly. If the protrusion of the Bush (3) is observed from Face X it is flush machined to remove any protrusion. Bush (3) is fabricated as per Figure 7 by using specific Aluminum material. Alodine treatment is performed as a final operation for fabrication of bush.

g. After this diagonal drilling of 1.5 mm of MGB housing is performed at 45 degree up to a depth of 7 mm as per Figure 5. Pin (4) is driven in this hole with application of loctite-609 ensuring that it does not protrude out of the surface of the housing Figure 5. Flush machining is performed to remove any kind of protrusion. Pin (4) is fabricated using specific stainless material as per Figure 8. Phosphating is performed on Pin as last operation of fabrication.

h. Peening operation is performed at 4 places as shown in Z of Figure 6.

i. Studs of same size needs to be installed on this reworked surface. Counter bore and thread tapping as per standard details is performed on assembled bush.

j. Lastly the stud is installed as per Figure 9 and Lock ring is swaged on the MGB housing.
k. After installation of stud, nut is installed and torque tightened to twice the standard requirement. Any movement in stud or bush is verified. If no movement is noticed, this verifies the repair procedure.