ओलियो-न्यूमेटिक शॉक एब्जॉर्बर

अवलोकन

The oleo-pneumatic shock absorber — commonly called the oleo strut — is the primary energy absorption device in commercial aircraft landing gear, responsible for cushioning the impact of landing and smoothing out ground taxiway surface irregularities. On a firm landing at maximum landing weight, the gear must absorb a vertical velocity of up to 3.0 m/s (approximately 600 ft/min sink rate) with descent rates up to 6.1 m/s (1,200 ft/min) for certification limit load demonstrations. The energy involved is comparable to dropping the fully loaded aircraft from a height of 46 cm (18 inches), and this energy must be absorbed in approximately 150–300 milliseconds — far faster than any spring-based system could manage without transmitting destructive loads into the airframe.

The oleo strut uses a combination of compressed gas (nitrogen) acting as a spring and hydraulic oil flowing through calibrated orifices as a damper. This dual mechanism allows the strut to act as a progressive spring during compression while simultaneously dissipating energy as heat through fluid flow resistance, preventing the rebound oscillations that would result from a pure spring system.

यह ��ैसे काम करता है

The oleo strut consists of an outer cylinder (upper, attached to the airframe) and an inner cylinder (lower, carrying the wheel axle), telescoped together with sliding seals. The internal volume is divided by a metering pin or orifice plate into upper (gas) and lower (oil) chambers. In the extended (unloaded) position, compressed nitrogen occupies the upper chamber at a pre-charge pressure of approximately 1,000–1,500 psi (depending on aircraft weight), and hydraulic oil fills the lower chamber and a portion of the upper chamber.

On touchdown, as the strut compresses under aircraft weight, oil is forced from the lower chamber through calibrated orifices into the upper chamber. The oil flow resistance creates damping force proportional to the square of the velocity — fast compressions generate high damping forces, slow compressions generate low forces. Simultaneously, the oil entering the upper chamber compresses the nitrogen gas further, increasing the spring force resisting compression. This combined spring-damper action converts kinetic energy into heat in the oil and absorbs the impact progressively rather than abruptly.

A metering pin — a tapered rod attached to the inner cylinder that passes through the orifice plate — progressively reduces the orifice area as the strut compresses, providing variable damping that maintains near-constant deceleration force across the compression stroke. Rebound is damped by a one-way (rebound) valve that restricts oil flow back from the upper to lower chamber, preventing the gear from bouncing the aircraft back into the air after touchdown.

प्���मुख घटक

• Outer Cylinder: The structural tube attached to the airframe trunnion. Contains the upper nitrogen charge and oil reservoir, and provides the bearing surface for the inner cylinder.
• Inner Cylinder: The lower sliding element carrying the axle/bogie assembly. Moves telescopically within the outer cylinder against the spring and damper forces.
• Metering Pin: Tapered rod providing variable orifice area through the compression stroke to maintain progressive damping characteristics.
• Orifice Plate: Fixed or variable restriction through which oil flows between chambers. The primary energy dissipation element.
• Nitrogen Charging Valve: Ground-serviceable valve allowing strut extension check and nitrogen pre-charge pressure adjustment to maintain correct strut extension per maintenance manual.
• Torque Links (Scissors): Articulated link pair connecting outer and inner cylinders to prevent rotation while allowing telescoping. Also limit maximum extension to prevent strut separation when the aircraft is jacked.
• Sliding Seals: High-pressure dynamic seals at the cylinder interface preventing oil and nitrogen leakage. Seal condition is the primary maintenance focus; a weeping seal causes strut extension loss and reduced damping capacity.

विमान में अनुप्रय���ग

• Boeing 737-800 — twin-strut main gear (one per side); single nose gear oleo; strut extension visible as the height of exposed chrome inner cylinder
• Airbus A320-200 — identical oleo-pneumatic principle; strut extension specification maintained in weight-on-wheels condition per AMM
• Boeing 777-300ER — large-diameter oleos on six-wheel main bogies; additional trailing-arm pivot geometry buffers bogie pitch on touchdown
• Boeing 787-9 — standard oleo-pneumatic struts retained; composite gear doors fitted around conventional aluminium/steel struts

लाभ और सीमाएँ

The oleo-pneumatic design is remarkably efficient — energy absorption efficiency typically exceeds 80 percent compared to 50 percent for a simple coil spring — and provides a smooth, progressive load curve that protects both airframe and passenger comfort across a wide range of sink rates. The system is passive (no electronic components), inherently reliable, and requires only periodic nitrogen pressure checks and seal replacement as maintenance actions.

The primary limitation is the dependence on correctly maintained nitrogen pre-charge pressure. An over-extended or under-extended strut alters the aircraft's ground clearance, nose attitude (and therefore wing angle of attack during rotation), and nose gear steering geometry. Maintenance personnel use published strut extension tables (correlating visible chrome length against ambient temperature and aircraft weight on wheels) to verify correct servicing. Seal leakage is the most common failure mode, leading to gradual strut collapse and requiring immediate servicing to prevent ground clearance and handling issues.