Rotary Wing - Week 3

Primary purposes of the Main Rotor Transmission

• Deliver engine torque to the main rotor drive shaft (mast).

• Reduce engine RPM input to main rotor mast output.

• Transfer lift from the main rotor blades to the airframe.

Meaning of "Transmission Limited”

It means the engine can deliver more power/torque than the transmission is structurally rated to handle. The reserve engine power is primarily used up in hot/high operating environments

Why the main rotor cannot operate at engine speed

Engines produce maximum power at high RPM. The main rotor cannot run at these high speeds because blade tip speeds must remain subsonic to prevent supersonic shockwaves, and high rotational speeds contribute to retreating blade stall.

Explanation of a 16:1 gear reduction ratio

For every 16 rotations of the input drive shaft from the engine, the output shaft (main rotor mast) completes 1 full rotation

Element providing change of rotational axis and speed

Gears (such as bevel gears, spur gears, and planetary gear systems)

Element supporting axis and speed change devices

Bearings (and the transmission housing/case)

Planetary gear system components

sun gear, planetary gears, ring gear, planetary gear cage

Slowest rotating component in a planetary gear system

planetary gear cage

Three purposes of bearings

• Reduce friction between two moving parts.

• Act as a sacrificial wear surface between components.

• Constrain or limit relative motion of a moving body/shaft within a housing (in radial or axial directions)

Bearings support which kinds of movement

Radial movement, Axial movement (thrust), or a combination of both.

Purpose of upper/lower (inner/outer) bearing races or rings

To secure the bearing to machine components (such as the housing and shaft) and provide hardened, ground, and honed raceway surfaces for the rolling elements.

Identification and names of rolling surfaces

Identified by their geometric shape; their names are balls, rollers, cones, spheres, or needles

Purpose of the Cage assembly

To keep the rolling elements evenly spaced along the raceway between the inner and outer rings

Four principal types of bearing assemblies

• Ball Bearings

• Roller Bearings

• Plain Bearings

• Spherical Rod End Bearings

Type of bearing supporting radial and single-direction axial load

Thrust Type Ball Bearing (or Angular Contact Ball Bearing)

Commonly used wheel bearings

Tapered Roller Bearings

Bearings for limited space / pure radial load applications

Needle Roller Bearings

Purpose and application of spherical roller bearings

• Purpose: Provides a self-aligning bearing capability that accommodates shaft/housing misalignments while absorbing high radial and moderate axial loads.

• Application: Used in planetary gear assemblies to increase roller contact area and maintain alignment between the sun gear, planet gears, spider, and ring gear.

Bell 206: Component supporting total helicopter weight

Pylon Mount Links (A-frame pylon assemblies)

Bell 206: Attachment of pylon assemblies

They attach to a spindle on the transmission case via spherical bearings at the apex of the link, and to the top roof structure/cabin deck of the helicopter via deck mounts

Bell 206: Component containing elastomeric construction

Isolation Mount

Bell 206: Cause of scratches/gouging on isolation mount top

Contact by the ears of the forward drive shaft during excessive mast flapping

Bell 206: Components indicating excessive flapping action

The steel Drag Pin (Spike) and the soft aluminum Stop Mount Plate (which receives dents upon contact).

Bell 206: First gear driven by main rotor drive shaft

Input Pinion Gear

Bell 206: Two purposes of input pinion and spiral bevel gear assembly:

• Reduce rotational speed (by approximately 3.26:1).

• Change rotational axis direction by 90 degrees.

Bell 206: What drives the sun gear assembly

The Spiral Bevel Gear Carrier / Shaft (via splines)

Bell 206: What the Planetary Gear Cage/Spider drives

Main Rotor Mast

Bell 206: Fixed gear and its effect

• Gear: Ring Gear.

• Effect: Because it is fixed, it forces the planetary gears to "walk" along its inside circumference, causing the planetary carrier/spider to rotate and provide final speed reduction

Bell 206: Input pinion bearing support & installation check

• Bearing Type: Triple Thrust Ball Bearings (supported with alignment and thrust bearings).

• Verification: Scribed "V" marks on the outer races of the bearings must line up to form a completed "V" pointing in the specified inward direction

Bell 206: Three critical components driven by the bevel gear

• Transmission Oil Pump

• Hydraulic Power Pack (Hydraulic Pump)

• Main Rotor Speed ($N_r$) Tachometer Generator