Main gearbox

Application description

The purpose of the main gearbox is to interconnect the engines (typically two) in order to drive and support the main rotor. The main transmission consists of different modules: typically two auxiliary modules, two input modules, and a main module. In most cases the left-hand input and auxiliary modules are identical to the right-hand input and auxiliary modules and are generally interchangeable. However, there are some exceptions presenting non interchangeable designs. A freewheeling unit, at each engine input to the main gearbox, permits the rotary wing to auto-rotate without engine drag. The action occurs in case of engine (or engines) failure or when engine r/min decreases below the equivalent of rotor r/min. The freewheeling unit also provides a means of disengaging the rotary wing head while providing power to operate accessories.

Finally, the freewheeling unit allows engine disengagement during autorotation. The main module provides mounting for the two input and auxiliary units. It has sensors that monitor oil temperature, low oil pressure, high temperature warning, and chip detector systems. A rotor brake mounted on the tail takeoff provides the capability to stop the rotor system. The rotor brake disc also provides the means to space-saving feature needed when transporting military helicopters. The module is driven by an output gear in the input module. This is linked to a bevel pinion by a quill shaft. The main bevel gear, which is driven by the main bevel pinions, and a sun gear, are part of a single unit. The sun gear drives the satellite elements, which are engaged onto the fixed external outer ring. The sun gears are connected to a sun carrier, which in turn transfers motion to the main rotor shaft. In other words, as the bevel gears turn, the sun gears rotate, which also turns the planetary gears linked to the main rotor shaft. The overall transmission ratio from the engine to the main rotor is around 50:1 as an order of magnitude.

A tail takeoff pinion rotates with the main bevel gear. The pinion provides drive to the auxiliary gearbox, and subsequent intermediate and tail gearboxes.

The run-dry requirement (Oil Off Condition)

Most current certification requirements specify that the aircraft must be able to continue safe flight for at least 30 minutes after the crew has detected lubrication system failure or loss of lubrication. This is sometimes referred to as the “30 minute run-dry requirement”, or “oil-off condition”.

There are five main factors determining the capability of an main gearbox to operate for a prolonged period after “loss of oil”. The design of the main gearbox must balance all of these factors in order to achieve and maintain for the longest possible period, a thermal equilibrium condition whereby the critical temperatures, although high, are constant or increasing at a low and controlled rate. These factors are:
  1. Low friction at gear and bearing contacts. This is achieved by keeping a low friction coefficient by means of the residual oil lubrication and by low surface roughness by super-finishing the gears and using special low-friction coatings. Ceramic materials are also excellent for friction coefficient reduction, nevertheless, ceramic rolling elements are disadvantaged due to detectability limitations with chip (metal particle) detectors in the gearboxes.
  2. Low sliding at gear and bearing contacts. This is influenced by design choices. Coarse pitch gears are stronger but have more sliding velocity. Rolling bearings, despite their name, experience sliding as well as rolling motion. In order of increasing friction, we can list: cylindrical, ball, spherical, and tapered roller bearings. In particular, tapered roller bearings exhibit significant sliding. For this reason they are not used at high or moderate speeds but only at very low speeds.
  3. High “hot-hardness” of gears and bearings such that they maintain their original size, shape and roughness up to the highest expected operating temperatures.
  4. Clearances must be maintained at all gear and bearing contacts throughout the highest expected temperatures. Loss of gear teeth backlash and bearing clearance will generate unwanted tightness, causing an uncontrolled and exponential increase of the contact forces.
  5. Heat removal from the hot spots by conduction and convection, in order to dissipate, at the highest possible rate, the heat produced by friction at the sliding contacts. It is important to avoid thermal gradients and allow uniform and progressive thermal expansion of all the parts.

SKF products and Typical solutions

In this section the usual positions of main gearbox subset are discussed with reference to a typical helicopter.

Input Pinion

The input shaft is directly connected to the engine and is supported by two cylindrical roller bearings taking the radial load and bending moment, and by a three-point angular contact ball bearing to support a reaction in both axial directions.

Typical bearing materials are AISI M50 VIM VAR steel for rolling elements and rings and AISI 4340 steel for cage with silver plating. Lubrication holes are normally supplied on the inner rings, and often, in helicopters gearbox applications, and especially in the main gearbox, the bearing inner ring is integrated in the customer shaft. A few other bearings can also have their outer rings integrated in the customer housing. These are convenient weight-saving solutions for the customer.

Satellite Bearing

The satellite bearing is usually a single row or double row spherical roller bearing.

In spherical roller bearings, the centre point of the sphere in the outer ring raceway coincides with the bearing axis. Therefore, the bearings are self-aligning and insensitive to misalignment of the shaft relative to the housing, which can be caused, for example, by shaft deflection. Spherical roller bearings are designed to accommodate heavy radial loads, as well as heavy axial loads in both

Outer rings are typically manufactured incorporating the external gear mesh; the raceway is then finished and the bearing assembled. All gears are case carburized with the material selection ranging from, but not limited to, AISI 9310, Vasco X2 or Pyro wear 53.

Mast Bearing

The mast assembly is commonly supported by a pair of bearings taking basically the rotor aerodynamic load, which has the same order of magnitude of the helicopter weight.

Three-point angular contact ball bearings (3PCBB) and radial single row angular contact ball bearings with raceways designed to support axial loads in both directions, make them ideal in this application. The operating load is mostly in one direction due to the rotor thrust; however it can vary from the primary thrust (flight requirement) to the reverse condition, which is represented by the weight of the mast and rotor system.

A flange is frequently incorporated into the bearing outer ring and is used as both an anti-rotation device and as an interface to the housing. Rings are typically produced in AISI M50 NIL VIM VAR case carburized steel, rollers in AISI M50 VIM VAR steel and cage in AISI 4340 steel with silver plating.

The same materials can be used for cylindrical roller bearings when paired with the 3PCBB.

Elastomeric main gearbox mounts

In many helicopter applications, the main gearbox is isolated from the fuselage structure of the aircraft using elastomeric mounts. Various configurations of mounts are used to isolate the vibration from the fuselage. Some common configurations of isolators are canisters (also referred to as cylindrical or radial isolators), rod ends and flat plate isolators. In all configurations, one metal component is attached to the gear box and another metal component is attached to the fuselage, with a layer of elastomeric material between them to isolate the vibration. Main gearbox isolators can utilize a single layer of elastomer or an elastomer and metallic shim package, depending on the requirements from the customer. The metallic shims provide greater stiffness in a particular load path direction. There are various configurations of elastomeric gear box mounts manufactured by SKF
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