Complete specification of

ANALYSIS ON CAUSE OF STALLING OF ACTUATOR ON ACTUATED FREEWHEEL ASSEMBLY OF HELICOPTER TRANSMISSION SYSTEM

FIELD:

The present invention relates to a mechanism to avoid stalling of actuator on actuated freewheel assembly of helicopter transmission system.

BACKGROUND:

The freewheel unit ensures unidirectional torque transmission from the engine(s) to the rotor and isolation of engines when the engine fails. On the ALH, which is a twin engine .helicopter there are two freewheel units. The one on the port side is fitted with an actuating mechanism which facilitates, systems checking on the ground without the rotors turning by use of an electro mechanical actuator. This actuated freewheel comprises of a main freewheel and two small accessory drive freewheels. The star board side freewheel consists of only main freewheel. It is referred to as non-actuated freewheel. Both the actuated and non-actuated freewheels are of roller type and have cage type construction. A torsional spring ensures return of the rollers to the no load position after a freewheeling operation.

During ground run operation there was a sudden jerk observed, as a consequence crack, was observed on the MGB housing. During modular disassembly it was revealed that there was dislodgement of the torsional spring. Also, stalling or drawing higher current by the electro mechanical actuator is observed due to excess of load at the freewheel module. Repeated stalling/ spring dislodgement on any particular unit could warrant for servicing of that actuated freewheel module. To investigate and isolate the cause for stalling/ spring dislodgement, meticulous test validations were carried out. This, in turn, has resulted in a conceptual change in the design of actuated freewheel parts like actuator lever, actuator bearing fork, actuating pin, bearing bushes and actuator mounting plate attachment lo free wheel housing

SUMMARY:

The present invention deals with freewheels to avoid dislodgement of the torsional spring and stalling or drawing higher current by the electro mechanical actuator observed due to excess of load at the freewheel module.

BRIEF DESCRIPTION ON THE MECHANISM:

Freewheels are provided one per engine (i.e port side & star board side). The freewheel units, fitted between each engine output shaft and MGB input shaft ensures unidirectional torque transmission from the engines to the rotor. Port side Engine free wheel can be selected for actuation by employing electro mechanical actuator and is called actuated free wheel; the star board side free wheel cannot be actuated hence called as Non-actuated free wheel. The free wheels are cam and roller type. The freewheels at both port side & star board side facilitates autorotation.

A. Function of Free Wheel

i. To connect port side or star board engine to rotor when speed of the respective engine
(free turbine NF) is higher than rotor. ii. In auto rotation when rotor rpm greater than free turbine speed, drive from rotor to
engine is disconnected (called freewheeling). The same situation occurs when only
one engine is operating. (Freewheeling occurs on. the free wheel of the Non-operating
engine). iii. Actuation of portside freewheel for system checking on ground without rotation of
main rotor.

B .Free wheel Mechanism

The schematic Fig.] & 2 shows a typical cam-roller- cage type freewheels. The cage is provided with limited angular rotation against a torsional spring load. These accessory freewheels are mounted on a common drive shaft while being driven, one from the input shaft of the main freewheel (Connected to engine) i.e. accessory F/W 1 and the other from the output shaft of the main freewheel (Connected to rotor drive) i.e. accessory F/W 2. Under normal operational mode, all the three freewheels will be in engagement thereby transmitting power from engine to rotor as well as to accessories.
Under actuated operation the main freewheel is intentionally disconnected and accessory freewheel 2 will be under freewheeling (over running) condition and the power from the engine to the accessories is provided by the accessory freewheel 1. During this period the rotors are disconnected.

Under auto rotation operation (power from the engine not being available and during only starboard side engine drive mode the accessory freewheel 1 and main freewheel will be under freewheeling condition while power from the rotor is ensured to the accessories through the accessory freewheel 2.

B. Working Principle of mechanism:

Fig. 2. shows arrangement of main and accessory freewheels arrangement of an actuated freewheel. There are 14 rollers in Main free-wheel arrangement and 8 rollers in accessory free wheel arrangement.
During system check on ground by actuation of actuator, the cage at the main freewheel can receive the angular rotation taking the compliment of rollers out of wedge into relieving pockets, thereby disconnecting the main drive to the output shaft. Drive to the LH side accessories is ensured by two accessory free wheels which are internal part of the actuated free wheel.
Fig. 3 shows a schematic arrangement of the actuating mechanism employed. A hinge pin, on the free wheel housing carries an external lever and an internal fork The fork engages the outer ring of the bearing while the inner ring slides on the splines of the input shaft. The cage has rotational hinge against a torsional spring load and has a taper on the end face. When the lever is operated, the pin on the inner ring presses against the taper on the cage, thereby inducing the rotation of the cage and taking the compliment of rollers out of wedge.

The lever can be actuated from cockpit through an electromechanical linear actuator. The actuator is mounted on the freewheel housing. A fork fitted on the actuator

shaft is connected to the vertical slot at the lever. The position of the fork on the actuator shaft can be adjusted and locked through a lockout to facilitate rigging.

The actuator receives 28 V DC power supply from the Helicopter and is operated through a toggle switch in cockpit. When the actuator stroke is completed, a green advisory light illuminates on the overhead panel.

The actuation being ground based preflight systems check; it is protected against inadvertent operation in flight through a mechanical lock for the toggle switch and also includes safety features in the electrical circuit to prevent inadvertent operation.
DETAILED DESIGN REVIEW OF ACTUATED FREEWHEEL ASSEMBLY:

A. cause of Stalling:

Based on design review of actuated freewheel assembly, two actuated freewheel units
(#01 & #02) where built initially and taken up for test validations.
The test specimen #01 passed through test validation satisfactorily. In addition, the trials
of the unit on MGB load test gear box were also satisfactory. The test specimen #02
during test validations it was observed that the actuator was drawing higher
current/stalling was observed at few cycles. However the stipulated 50cycles test

requirement was met. During such stalling, the stroke was withdrawn and reattempted. The reattempted stroke was normal. It was decided to look into the cause of stalling.

To investigate & isolate the cause for stalling, many iterations & test validations were repeated on replacement of various components in the unit systematically one after the other like actuator, actuating fork .

The observations of higher current drawn/stalling was repeating without any significance influence of these. During stalling 8 to 10 kgf additional pull for the stroke completion was required. But however any such stalling has no functional impairment either on actuator or on the freewheel hardware and this was demonstrated during investigation. The stroke can be withdrawn & reattempted as in vogue for engine restart.

Hence, the other main c omponent, cage was considered. With replacement of the main freewheel cage, the test validations for 50cycles carried out was satisfactory without any higher current drawn/stalling observations.

Visual/ Dimensional inspection was carried out on the test specimen cage at the cage-actuator bearing interface appreciable deviation was observed.

Inference: observations of actuator higher current drawn/stalling on the test specimen #02 was due to main freewheel cage under deviation. Test specimen & other freewheel units with normal cages (without deviation) have proven to be satisfactory. Actuated freewheel operations for system checks are ground based operations. Actuator stalling is easily detectable. The stroke could be withdrawn and reattempted. After such reattempt, freewheel units with unsuccessful stroke could be considered as defective unit needing replacement/servicing.

The current configured actuated freewheel with present actuator is satisfactory for the actuated freewheel operations on the helicopter.

B. Incorporation of modified spring with additional Features to avoid dislodgement.
During ground operation a sudden jerk was observed as a consequence crack was observed on MGB housing. During modular disassembly it was observed that there was dislodgement of torsional spring assembly between input shaft and freewheel cage. Hence, there was a need to review the design features of the freewheel spring/other interfacing components to prevent dislodgment of the spring by addition of fool proof safety feature without changing the physical configuration of the spring.

The correction/safety features includes increase in length of the spring to accommodate kinks/bends at the spring ends as shown in the figure 4A & 4B.
To validate the modified spring, meticulous test validations were carried out against behaviour for an advertent pressing, down of the spring at the slot end. Spring with its interface assembly in cage/shaft is checked for torsional pull.

a )In addition to above I hr functional run with drive from LH.
b) Similarly with drive from LH, to carry out speed increase to I00%;to dwell for a 1
minute at 100% speed &to bring back speed to Zero. This is repeated for 50 cycles.
c) repeat a & b with drive from RH side.

The test specimen of modified spring with additional features to avoid dislodgement unit passed through all these test rigorously and found satisfactory. In addition, the trials of the unit on MGB load test gear box were also satisfactory.

C. Design review at Actuating Lever, .Actuauting fork, actuating pin, Bearing bush, Actuator mounting plate
Actuating Lever, Actuauting fork: Hard anodizing was introduced at the interface surfaces.

Actuating pin: Teflon coating was introduced

Bearing bush, Actuator mounting plate: Dimensional tolerance @ various interfaces was advised to be controlled.