Sikorsky S-55 Gearbox and Rotor Assembly

S-55 Transmission

The transmission system conveys power between the engine and the main and tail rotors and at the same time reduces engine rpm to proper rotor rpm. This system is made up of the centrifugal clutch assembly; the main drive shaft from the engine; the main gear box; the tail drive shaft; the intermediate gear box; the drive shaft to the tail rotor; and the tail gear box.

Centrifugal clutch

The automatic centrifugal type clutch installed on the engine crankshaft consists primarily of a lower or driving disc and upper or driven disc. The driving disc is mounted on the splined engine crankshaft and has a single row of 24 clutch shoes. The driven disc is supported by two single row bearings and the engine shaft nut on the upper part of the crankshaft. The four purposes of the clutch assembly are: to provide cooling air to the engine at all times; to provide ram air for the carburetor; to provide a means of “soft” engagement between the engine and the transmission system; and to act as a flywheel for the engine.

Main gear box

The main gear box is located on the top of the cabin just aft of the cockpit. It has three primary functions: it changes the angle of the drive from the engine to the main rotor assembly; it reduces the engine rpm to rotor rpm; and it proves a means of driving the tail rotor and the gear box accessories.
The gear box is made up of an upper housing assembly, a lower housing assembly, and a ring gear, bolted between them. Gear reduction is accomplished by the input bevel gears and two-stage planetary gear system. The gear reduction ratios are as follows:
Input Bevel Gear 1.4193 to 1.000
First Stage Planetary 2.8235 to 1.000
Second Stage Planetary 2.8235 to 1.000
Overall 11.3148 to 1.000
Input to Tail Drive 1.00 to 1.11

The gear box lower housing incorporates an accessory drive section at the rear. Accessory drives are provided for the generator, the generator blower, the transmission oil pump, the rotor tachometer-generator, the hoist hydraulic pump, and the servo mechanism hydraulic pump.

Main rotor assembly

The main rotor assembly is mounted above the center section of the helicopter and is splined to the drive shaft rising out of the main gear box. The main rotor assembly serves as an attachment for the three main rotor blades, rotates them by engine power, and controls their pitch. The main rotor assembly consists of a hub assembly including three vertical hinge assemblies, three sleeves, spindle, and blade horn assemblies, an upper plate, a lower plate, and a bracket, three damper assemblies; three droop restrainer assemblies; three anti-flapping restrainer assemblies; and a star assembly.

The S-55 helicopter in the courtyard employs this transmission. It is powered by a Pratt & Whitney