Belleville Washers in Gearbox of Rotax 912/914 Engine

Rotax 912 Gearbox. Note: There is no fuel pump since we have an all-electric fuel delivery system.
The Belleville washers in the Rotax gearbox, which exert pressure on the so-called dogs, must be replaced when they become worn beyond the required spec.

 This should not happen if the Belleville washers are replaced according to the schedule specified by Rotax. For our 912UL engine, this is to be done after 1,000 hrs. 

 Now we do all repairs and overhauls on condition and not on a fixed hours-in-service schedule. 

 If you also follow that rule, proved by the US Air Force in WWII to drastically reduce failures, then you want to look out for the symptoms described below.

 First indication of worn Belleville washers is the following: 

 You are past 1,000 hrs, and engine idle is a bit rough. Not too bad, but no matter what you do, such as balancing of carburetors, the roughness does not go away.

 Suppose you ignore this seemingly minor symptom. 

 The second indication of worn Belleville washers is an intermittent harmonic humming sound when the engine turns 2,200 to 3,000 rpm. The humming stops once you go past 3,000 rpm. 

 As time goes on, the humming becomes more pronounced. You might think that something is vibrating under the cowl, but the worn Belleville washers in the gearbox are the culprit. 

 At the first sign of humming, the Belleville washes of the gearbox must be replaced.

 Suppose you ignore the humming and simply go on flying. The humming will become more and more pronounced, and finally another symptom shows up: 

 During idle, the engine makes a clattering sound, and when you shut down, the engine stops with a sound as if a rod had failed. Do NOT ignore this message. This is your final warning that you must replace the Belleville washers. 

 If you choose to ignore this final warning message, you will destroy the gears of the gearbox.

Have any questions or feedback about the replacement of the Belleville washers in the Rotax gearbox? Please share your thoughts in the comments. 

Comments

  1. AnonymousFebruary 16, 2023 at 12:02 AM

    Hello Klaus, another interesting read here. Did you see any wear on the Belleville washers or did you just assume that they are out of specs? I am looking at my washers (80 hp 1200 hours) right now and have no way to check them to specs. I follow " replace as required" as you do. Thanks Peter

    ReplyDelete
    Replies
    1. Klaus TruemperFebruary 16, 2023 at 8:43 AM

      Lockwood in Florida replaced the Belleville washers of the gearbox. I do not know their condition when they were removed. Must have been bad since the engine idle was rough with funny sound. All this went away with the washer replacement. Agree with your "replace as required" philosophy, of course. In this case, I would replace the washers for two reasons. First, you may not have noticed that the engine idle gradually became rougher and noisier, and will be surprised how smooth it can be. Second, it is such a hassle to remove the gearbox later again and do the replacement. On top, this may well be the last replacement you will ever do. Am not sure about the latter thought since my engine is now past 2,300 hrs with no sign of wear.

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