Main Rotor Hub Assembly
Description
The main rotor hub assembly is mounted at the top of the mast of the transmission assembly and attaches the main rotor blades to the main rotor shaft of the helicopter. It provides a power path from the main rotor shaft to the blades. The parts of this assembly are made of aluminum alloy and steel. Moving parts are lubricated with grease.
The main rotor hub assembly is broken down into the following subassemblies:
Yoke
Trunnion
Blade grip assemblies
Yoke
The yoke assembly forms the main structural member of the main rotor hub assembly. It has two hollow preconed spindles that serves as the pitch change axis and form the surface on which the blade grip assemblies are mounted. The preconed is to relieve stresses. Each spindle has the hollow bore especially machined at the outboard end to fit a seal assembly, and the inboard end is machined to fit the tension-torsion (T-T) strap end fitting. Provisions are also included to mount two pillow blocks that, with the trunnion, form the flapping axis mechanism for the rotor head. On the underside of the yoke are provisions for two static stops that contact the main rotor shaft if excessive flapping occurs.
Tension-torsion straps
The tension-torsion straps, which consists of stainless steel wire wrapped around two steel ends, are flexible structural tension assemblies which attach the blade grip assemblies to the yoke assemblies. They transfer centrifugal loading from the blades to the yoke and also assist to counteract aerodynamic forces while allowing the blade grip assemblies to rotate about the axis of the spindle. During this “rotor feathering”, the flexibility of the strap allows the grip bearings to rotate on the yoke journals. They are mounted inside the hollow spindles of the yoke using a steel strap pin and strap fitting with an aluminum radius ring.
Static stop
On the underside of the yoke are two static stops that contact the main rotor shaft if excessive flapping occurs. The static stop also retains the strap fitting in position with a pinch fit using a shim.
Static stop shim
The static stop shim is a stainless steel laminated shim, easily peelable into fine .002 of an inch strips. The shim is used to precisely fill the space between the static stop and yoke, to achieve a .002" to .004" "pinch fit" on the strap fitting.
Rubber stop
The rubber stop protects the strap fitting from the static stop
Weight Retainers
The main rotor hub assembly utilizes weights attached to the yoke with a weight retainer to achieve chordwise balance. The weights reduce the number of sweep adjustments during the main rotor chordwise dynamic balance procedure.
Weights are added to the weight retainer when other than a full point of sweep is required for balancing.
The weight retainer will hold eight thick weights (800 grams). When more weight is required to chordwise balance the hub and blade assembly, remove the eight thick weights and sweep the blade one point, then add the weight required to chordwise balance.
Trunnion
The trunnion mates the main rotor to the mast, transmits drive to the yoke assembly, and serves as a pivot point for the rotor flapping axis. It is attached to the yoke assembly by two pillow blocks. The trunnion and the pillow blocks also serves to provide hub centering provisions. Each spindle of the trunnion mates with the inner race of the flapping bearing that is between the trunnion and the pillow blocks.
Pillow blocks
The pillow blocks and the trunnion provide the flapping hinge mechanism for the main rotor assembly and are located at the top of the main rotor shaft. The pillow blocks attach the outer race of the bearing (that is between the pillow blocks and the trunnion) to the yoke. The bearings are machined race needle roller bearings. The rollers are separated by a steel retainer (cage) to help achieve higher speeds and provide a lubricant reservoir. The bearings are constructed with radial lubrication hole and groove on the outer raceway for relubrication through the pillow block grease fitting. A precision ground inner ring, thermally fitted and pressed onto the spindle of the trunnion, provides a hardened raceway for the rollers. Seals are provided to help prevent contaminant entry and contain the lubricant.
Pillow block shim
The shim is a stainless steel laminated shim, easily peelable into fine .002 of an inch strips. The shim is used to precisely fill the space between the pillow block and yoke, to achieve the truest possible fit within 0.002 inch.
To Determine the required thickness for the shim at each of the four pillow block attachments. One of the two following methods is used:
While pressing the pillow block against the yoke with firm pressure, measure the gap between the washer and the bushing at several locations. Round down the measured gap to the nearest multiple of 0.002 inch (0.051 mm) to get the required thickness for the shim.
Once shimming and final installation of hardware is complete, a 1/8 inch (3 mm) wide paint mark across the heads of bolts to flange on the pillow block and from nuts to flange on yoke identifies any slippage.
Trunnion Center
The trunnion is centered using a 1/4 in. hex wrench to reposition trunnion, as necessary, to obtain equal dimensions from the center of the trunnion and the outsides of the machined journals on the spindle of the yoke within 0.002 in. or less.
Blade grip assemblies
The blade grips assemblies are the structural members which attach the main rotor blades and provide a mechanism to rotate the blade grips over the spindle of the yoke. They are cylindrical in shape and rotate about the hollow spindle of the yoke by means of two bearings with a spacer. The outboard end of each grip has a machined hole for the blade bolt, and the grip body has drilled tangs to fit the blade latch bolt that retains the outboard end of the tension-torsion straps. Attached to each blade grip assembly is a pitch horn which provides the means to attach the control rods that change the blade pitch. For proper lubrication, packing and seals keep the lubricating grease contained inside the blade grip assemblies.
Pitch horn assemblies
The pitch horn assemblies attach to the blade grip assemblies and provide an attachment point for control rods from the swash plate which move vertically to change the pitch of the blades. The parts of this assembly are made of aluminum alloy and steel. There are two pitch horn assemblies per main rotor assembly.
Blade Latches
The blade latches are adjustable mechanical stops located at the ends of the latch bolts that hold the blade grip assemblies to the tension-torsion straps. They also position and hold the blades so as to allow the precise adjustment of blade alignment (blade lag).
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