since 1940

Crankshaft gear separated from Crankshaft on Lycoming aircraft engine

Lycoming crankshaft showing counterbore where crankshaft gear installs.

The accessory drive geartrain is driven by an accessory gear mounted on the aft end of the crankshaft.  The gear is located on the crankshaft by an integral flange that registers in a counterbored recess in the crankshaft end and is retained by a 5/16 inch diameter bolt screwed into a threaded hole in the crankshaft. Bolt retention is by a lock washer with an outer tab locating in a slot in the gearwheel and an inner tab bent against the bolt head after the bolt has been torque-tightened. A shouldered dowel fitted into the crankshaft has a 1/4 inch diameter section that protrudes from the seating face and locates in a hole in the gear flange. A similar accessory drive gear design is used on all Lycoming reciprocating engines.

The engine manufacturer has stated that the design intention is for the rotational loads on the accessory gear to be reacted by torsional friction loads between the gear and the crankshaft generated by the clamping effect of the retaining bolt. The dowel is provided to ensure the correct rotational orientation of the gear on the crankshaft, and is not intended to transmit torque. The bolt has a right-hand thread and the crankshaft rotates clockwise as viewed from the rear and thus, in the event of a loss of torsional restraint between the gear and the crankshaft, geartrain loads will tend to unscrew the bolt. The manufacturer considered that loosening of the gear on the crankshaft had nearly always been caused by a torsional overload on the gear or by inadequate clamping loads applied by the bolt.

Procedures for installing the crankshaft gear were recommended in Avco Lycoming Service Instruction (SI) No 1179D, issued on 29­6­73. It noted that instances had been found of damage in the area of the crankshaft gear and attributed these to improper assembly technique or reuse of worn or damaged parts during overhaul. It noted that the eventual failure of any of these parts would result in complete engine stoppage. The procedures included checks of the crankshaft recess, the crankshaft bolt hole threads, the dowel and the gear, and reassembly with a new bolt and locking washer. They also specified modification of the gear to allow inspection after installation by cutting three 0.25 inch radius scallops in the rim of the flange. Compliance was recommended at overhaul.

The SI was superseded by Textron Lycoming Service Bulletin (SB) No 475 'Crankshaft Gear Modification and Assembly Procedures', originally issued on 31­10­86. The SB expanded the procedures and required machining of the gear to enlarge the rim scallops to 0.75 inch radius and to counterbore the central part of the mounting face. Compliance was required during overhaul, after a propeller strike, or whenever crankshaft gear removal was required. It was categorized by the engine manufacturer as mandatory, but was not mandated by the FAA or the CAA. A supplement (No 1) requiring assembly with the bolt threads clean and dry was issued on 24­5­88. The SB was re-issued as No 475A on 16­7­90, with the addition of a caution against the reworking of crankshaft threads except by Lycoming.

An FAA Airworthiness Directive, AD 91­14­22, was issued on 19­8­91 making SB 475A mandatory at each overhaul, after a propeller strike or sudden stoppage or whenever geartrain repair was required. Compliance with the FAA AD by UK operators was mandated by CAA Airworthiness Notice No 36. The SB was reissued, as Textron Lycoming Mandatory Service Bulletin No 475B, on 23­4­93, with additional cautions and expanded text related to ensuring correct assembly. The contents of this SB were incorporated in a revision of the Overhaul Manual issued in June 1993.

Copyright 2003 by Sacramento Sky Ranch Inc. All rights reserved.  Prices subject to change without notice. Not responsible for typographical or misprint.
Disclaimer: sacskyranch.com contains abundant information relating to aircraft maintenance. The information provided  is not intended to supercede or supplement the F.A.A. approved  maintenance and/or operator’s manuals. Those F.A.A. approved manuals must be utilized when performing maintenance and/or operating aircraft.