F. Rotor Head Maintenance

Transcription

1 There are several apparent advantages of the fiberglass construction over the metal. The major one is the life of the blade. From all indication the fiberglass blade will have no finite life, as does the metal blade. In addition, the fiberglass blade will be less susceptible to notch damage (Fig. 5-44), due to the threads of the roving. Corrosion of fiberglass is non-existent. Bonding or delamination problems can often be repaired by epoxy resin injections and may be performed in the field. For these reasons there will be undoubtedly more fiberglass or composites on the market with the new helicopters. NOTCH DAMAGE CHARACTERISTICS FIBERGLASS NOTCH UNDER LOAD AFTER CONTINUED LOADING UNNOTCHED FIBERS CONTINUE TO CARRY LOAD. LIMITED NOTCH GROWTH. UNNOTCHED FIBERS CONTINUE TO CARRY LOAD. STEEL NOTCH UNDER LOAD AFTER CONTINUED LOADING be accomplished in the field unless teardown capabilities are available. It is quite normal, however, for the greased head to sling grease, especially immediately after service. Both the grease and oil should be removed so that no accumulation occurs. This accumulation would not only make visual inspection impossible, but it would retain moisture and dirt that can be harmful to the head. Normal rotor head inspections are visual. Loose items, such as rod ends, will only add to a more rapid deterioration of the head. The feel of the movements and the possibility of cracks should not be omitted from any inspection criteria. On certain heads, periodic inspections may be required. These might include such items as DYE CHECK inspections, partial disassembly, and rotation of bearings. These could be required at different intervals of time or as the result of Airworthiness Directives. The rotor head is a highly stressed unit and usually has many time-change and mandatory retirement items. The constant checking of logbooks, replacement/retirement schedules, and historical records prior to each inspection is a time consuming task. RADIATING STRESS PATTERN DEFINES AREA OF WEAKNESS. NOTCH HAS GROWN AND CRACK HAD DEVELOPED. AREA OF WEAKNESS HAS GROWN. FAILURE IS IMMINENT. Fig Notch damage comparison of metal and fiberglass. F. Rotor Head Maintenance The servicing of the rotor head normally consists of lubrication. Proper lubrication of the rotor head cannot be stressed enough. The greasing of the head is done with a hand-type grease gun, using the manufacturer's specified grease. The intervals are also specified by the manufacturer. However, when operations are conducted in certain areas, more frequent greasing is advisable. For example: a sandy environment would require more frequent lubrication to remove the sand from the greased areas. Although the wet head should require less servicing and lubrication, leaks may occur. Most systems will have some leakage, but excessive leakage will cause bearing damage. Leakage of the head is quite obvious and the procedure for locating leaks is the same as in any other rotational item. The head must be clean before the origin of the leak can be discovered. Then the helicopter must be run for a short period and inspected. Quite often the repair of a leak cannot 115 Fig Bracket used to prevent damage to the tension-torsion straps during removal and installation.

2 Some manufacturers will allow the major inspections and overhaul to be performed in the field, while others have exchange programs or certain authorized repair stations that do work of this nature. Because of the complexity and specific requirements of this type of work, it will not be discussed in this section. General overhaul procedures will be discussed in a later section of this text. Special inspection of the rotor system will be required when certain circumstances exist, such as overspeeds, sudden stoppage and hard landings. The inspections will vary with the design of the head and the requirements of the manufacturer. Depending upon circumstances, the inspection required may be only a through visual inspection, while in other situations it may require a complete teardown or replacement of the rotor. Manufacturer's publications cover the special inspections in detail. Removal and installation varies from design to design. Some rotor heads may be removed with the blades installed, while others will require blade removal prior to removing the heads. This is usually dictated by the size of the rotor and the equipment available. STABILIZER BAR ASSEMBLY MIXING LEVER PITCH CHANGE LINK CONTROL TUBE DAMPENER LINK TUBE STABILIZER SUPPORT RETAINING NUT LOCK RETAINING NUT WASHER MAIN ROTOR MAST CONE SET STABILIZER DAMPENERS SCISSORS AND SLEEVE ASSEMBLY SWASHPLATE AND SUPPORT ASSEMBLY BOLT 17. NUT 16 Fig Typical rotor head removal for a semirigid rotor 116

3 Fig Special multiplier wrench used to remove a rotor head. The first step in the removal of any rotor system is to disconnect the flight controls attached to the rotor head. The controls are usually attached by bolts through rod ends and close tolerance bolts. HOISTING EYEBOLTS (4 PLACES) Pii A.", ) 111 HOISTING ADAPTER ' GROUND HANDLING PINS (4 PLACES) Often when pitch controls are disconnected, special holders are required to keep the blade pitch arms from moving to the point that damage may occur to the head. Specifically, damage can occur to tension torsion straps, equalizer links, or strap packs. Fig shows a typical pitch horn holder. After the controls are disconnected, some have other items that may be required to be removed, such as stabilizer bars, collective controls, dampener reservoirs, and driveshafts (Fig. 5-46). The mast nut used to hold the rotor head to the mast assembly are of a special nature and will require a special wrench for installation and removal. The torque used may require a hydraulic wrench or multiplier wrench (Sweeny wrench) (Fig. 5-47). Once the nut is removed, the head may need special hoisting slings or eyes for lifting the rotor system (Fig. 5-48).The rotor, removed with the blades, requires a suitable stand on which to sit the rotor assembly (Fig. 49). MAST STAND AFT JACKING PAD STA FWD JACKING FITTING BOTH SIDES STA Fig Hoisting eyes are often used to remove the rotor system. 117 Fig Typical stand used to place the rotor after removal.

4 14.0 ts.f. THREAD DETAIL B BLADE -- BLADE MUST BE RAISED TO RELIEVE PRESSURE ON THE BOLT SEE DETAIL A 11.0 FORCE GRIP I.' / OM" v Fig Blade removal requires lifting the blade. Different rotor heads and blade systems have different means of securing the blades to the rotor head. One method is the use of a blade retaining bolt in the grip assembly. Before the blade is removed, it must be properly supported to prevent binding between the grip and the blade. If an attempt is made to remove the bolt without proper support, galling will SEE DETAIL B AI- 6 NOTE ALL DIMENSIONS SHOWN ARE IN INCHES. -PP -AP , DETAIL A PULLER ROD ASSEMBLY R BETTER) LONG 3 HEY NUTS NE 1141 THREAD BEARING (THRUST) INNER RACE PLATE OR WASHER, STEEL OR ALUMINUM, , 1, THICK 5. PLATE OR WASHER, STEEL OR ALUMINUM am LD THICK 6 TUBE, STEEL OR ALUMINUM. WALL THICKNESS Fig Special tool used to remove blade retaining pins occur between the bolt and grip. This transfer will elongate the hole and destroy the airworthiness of the grip. Damage to the blade retaining bolt, the blade hole, bushing, or spreading of the fork of the grip may occur by heavy pounding. The bolt removal should only require moderate pressure (Fig. 5-50). Some blades required a special puller to remove the bolt (Fig. 5-51). Other blades are retained by taper pins (Fig. 5-52), requiring special pullers. Another method is where a circle of bolts is used to retain the blade (Fig. 5-53). Regardless of the method, the blade must be properly supported when it is removed, in order to prevent damage. When blades are to be removed and the rotor system is installed on the helicopter, the support of the blade is even more critical because of the height and the damage that may occur during removal. For this reason the blades are usually removed from the aft of the helicopter. This may also require support to the remaining blades. After the blades are removed, it is important that they be stored on blade racks. This will prevent blade damage. The rotor heads should be placed on a suitable stand to prevent damaging the head. As previously mentioned, the disposition of the head that has been removed varies with the different manufacturers. If the head is the type requiring a major inspection and return to service, adherence to the overhaul instructions is very critical. The steps are as follows: Determine which parts have time lives and if that time has been reached. Parts that have reached maximum life will be discarded without an inspection. The head will have to be disassembled for inspection. This procedure requires several special tools. No rebuilding should be attempted without the proper equipment. After disassembly, clean the parts and check part dimensions. Quite often these dimensions Fig Taper pin method of retaining the blades. 118

5 are in 10ths of thousandths, rather than thousandths. This will require micrometers capable of these readings. Typical dimensional checks are shown in Fig After these checks are taken, the parts of ferrous metal are magnafluxed and the nonferrous parts are zygloed to locate cracks. Usually the amperages used for magnafluxing and areas of concern are given in the overhaul manual. At this point, some part may require rework or updates. This could include adding bushings, changing radii, and removing nicks or scratches. At this time the finish of the part is examined and refinished. Manufacturers do not recommend replacement of plating in the field due to hydrogen embrittlement. If any plating is to be done, it must be only in accordance with the manufacturer's recommendation. This will usually require stress relieving of the part. After the parts have been inspected, they are ready for reassembly. The disposition of certain items, such as bearings and hardware, is done at the discretion of the operator. Some operators will never use certain items twice, while others will. Rebuilding of components has a direct relationship to the required maintenance during service life. The reassembly will require such procedures as shimming, bearing pinches, and end play measurements that will be discussed in another section of the text. When the head is reassembled, a color code of the component is usually placed on the parts for identification purposes because no left and right is present (Fig. 5-55). After the head is reassembled, the blades may be reinstalled, taking the same precautions used in disassembly. At this point, a series of maintenance procedures may begin. 1. Blade alignment Blade alignment is necessary on semirigid rotors. This procedure is sometimes referred to as chordwise balance, but this is really a misnomer. The procedure involves moving the blades about the lead-lag axis held stationary during the operation by the drag brace or latch pins, depending upon the design of the head. This movement is for the specific purpose of placing the blades in correct relationship with the hub of the rotor. This relationship places the center of BLADE NO. 3 (CODED YELLOW) BLADE NO. 2 (CODED BLUE) BLADE ROTATION asio din \ ROTATION BLADE NO. 1 (CODED RED) BLADE NO. 4 (CODED WHITE) BLADE NO. 5 (CODED BLACK) SEE DETAIL A DETAIL A Fig Blade removal and storage method used on S-76 helicopter. 119