When structural engineers talk about “efficient” structures, this may be their way of saying they didn’t leave much unstressed metal in the design.
Case in point: Have you been hearing heavy clunking sounds from the upper ends of your Yak 50 gear lately, during ground handling?
The kind of comforting ‘Clunk’ you’d always want to hear two of in the circuit, doesn’t sound right at all when you’re simply manhandling the plane by its tail on the ground.
TYC has replacement Main Gear Axles and Bronze Bushings.
Get the plane safely on jacks, remove one gear fairing and have somebody carefully apply side force to the wheel while you look for relative movement at the pivot joint.
You might see this....
A well-designed pivot joint distributes loads to the participants, (axle, bushings and housings) so they all grow old gracefully together. When failure finally occurs, the designer knew how to make it non-catastrophic - and like a painful knee, noise is there to alert us something is worn or abnormal.
The steel top of the Yak 50 strut ( the ‘trunnion’) has two outrigger towers through which the head and tail of the bolt pass. The outside of the trunnion head has a flat milled in it to prevent the axle’s hexagon head from turning - Shown in white at lower right in the picture below.
In the outriggers, steel bears on steel. By contrast, the inner supports (attached to the wing) are made of Aluminum with press-fit bronze bushings. These bushings take the landing loads into the wing structure. Here the Steel axle runs on Bronze and because of relative movement during gear retraction & extension, grease is delivered through a drilled passage down the axle from a Zerk fitting in its hex head.
The axle is loaded in shear - so its castellated nut with cotter pin just keeps things lined up - no need for it to be so tight that it’s creating axial tension.
-The first surface to go might be the bearing surface of the steel bolt itself. Or the outer diameter of the bronze bushings that have worn the wing housings slightly oval - or the steel bores going out of round in the trunnion outriggers. A replacement axle/ bushing set
needs to be sized to accommodate any of these wear/failure combinations.
Your mechanic will know how they want to go about fixing it, probably involving an adjustable reamer carefully piloted between the both sides of the joint to ensure axial alignment.
The dimensions of this repair axle and bushings set are all over-sized so that material from any interfacing surface can be judiciously removed. This is challenging work, best undertaken by those with prior experience. In the case of the Yak 50 trunnion, (being quite rare these days), needing to ‘put metal back’ (after too much was removed during the repair), would be very undesirable. Do a skills self-assessment before attempting this, and ask for help upfront if you know you’ll need it.
The only good clunks are airborne clunks, and only when commanded by the gear lever.
Please ( have your mechanic) call us if special dimensions are required.
John flies out of KSEE whenever he can scrape together a few bucks to fill the tanks