TM 55-1520-228-10
Change 11 2-33
SECTION VII. POWER TRAIN SYSTEM
2-35. TRANSMISSION.
The transmission is mounted on the cabin roof deck,
forward of the power plant. The transmission transfers
engine power to the main rotor through the mast assem-
bly. The tail rotor is driven through the freewheeling unit,
tail rotor driveshafts and tail rotor gearbox. Transmis-
sion lubrication is provided by a pump, relief valves, two
each filters, spray jets, temperature bulb, and an oil
cooler. The pump is a constant volume type driven by
the accessory gear. An oil level sight gage is located on
the right side of the transmission case. A breather type
filler cap and two electrical magnetic chip detector drain
plugs are incorporated. The transmission also furnishes
lubrication for the freewheeling unit mounted in the en-
gine accessory gearbox. Transmission oil is cooled by
the oil cooler blower.
2-36. PYLON SUPPORT STRUCTURE.
a. The OH-58A/C has a design feature called a
focused pylon mounting system for its pylon. This sup-
port concept differs from conventional four or five point
spring mounting arrangements as follows:
(1) The pylon bi-pod supports aim or focus py-
lon input loads at the fuselage vertical center of gravity.
The purpose is to reduce cabin vibration by eliminating
pylon induced rolling moment about the center of gravi-
ty.
(2) Rigid mounting in the vertical plane mini-
mizes vertical vibration.
(3) Vibration should not increase with mount
aging as is possible with conventional spring mounted
pylon supports.
b. The elastomeric mount provides pylon static
centering.
2-37. PYLON SUSPENSION SYSTEM.
a. The main transmission is supported and at-
tached at its lower rear section by an isolation mount.
Bolted to the lower aft part of the transmission case is
the drag pin which bolts to the elastomeric isolation
mount.
b. The drag pin has a metal pin which is commonly
referred to as the spike. The drag pin or spike physically
is mounted so that it “rides” inside the square metal
opening of the transmission mount support.
c. The isolation mount is used to dampen pylon to
fuselage vibration and limit pylon rock. Movement of the
transmission and isolation mount is limited by means of
the drag pin (spike) which extends down into the trans-
mission mount support mounted on the deck.
d. Contact between the drag pin and the transmis-
sion mount produces a noise known as spike knock.
2-38. FREEWHEELING ASSEMBLY.
The freewheeling unit is mounted in the accessory gear-
box. Engine power is transmitted through the freewheel-
ing unit to the main driveshaft and tail rotor driveshafts
to provide power to the main and tail rotors respectively.
During autorotation the freewheeling unit provides en-
gine disconnect and transmission drives the tail rotor.
2-39. TAIL ROTOR GEARBOX.
The gearbox is located near the vertical fin and provides
a 90 degree change of direction of the tail rotor drive-
shaft. A breather type filler cap, electrical chip detector
drain plug and an oil level sight gage are provided.
2-40. DRIVESHAFTS.
a. Main Driveshaft. The main driveshaft connects
the engine output to the transmission.
b. Tail Rotor Driveshaft. The tail rotor driveshaft is
made up of four sections; the forward short shaft, the oil
cooler fan shaft, the aft short shaft, and the long shaft.
2-41. INDICATORS.
a. Transmission Chip Detectors. There are three
chip detectors located within the transmission lubricat-
ing system. Two electrical chip detectors are located on
the base of the transmission and are wired to the XMSN
CHIP DET caution light and the MASTER CAUTION
light. One magnetic chip detector is located on the free-
wheeling unit and is not connected to the caution lighting
system. All three are provided to give evidence of fer-
rous metal particles in the transmission lubrication sys-
tem.
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