Mark on Maintenance – P-51 Landing Gear Pivot Shafts

Pivot shaft is an abbreviated description from the actual component properly named Landing Gear Pivot Shaft and Ring Assembly. This critical component can be described as the point at which the gear leg attaches and rotates for retraction of landing gear into the wing. It also bears the stress of attachment of a landing gear to wing and absorbs the impact of the main wheel tires making contact with the ground upon landing.

The pivot shaft was designed and engineered by North American as P/N 73-33112 on July 5, 1940 and checked / approved about a month later. The NAA drawing identifies two were used per ship on on all P-51 variants, F-51 & A-36 Series aircraft A thru K. On June 6, 1943 the drawing was made inactive and production was handed over to Bendix who produced the part as part number 66584 – a “light shaft” and as 161271 – a “heavy shaft” subsequently discussed.

Mustang expert Mark Tisler explains the three generations of pivot shafts commonly referred to as light, light cut, and heavy.

A light landing gear pivot shaft (P/N – 73-33112 & 66584) has two different generations of production, both of which share a light wall on the interior of the shaft. The first generation “light” pivot shaft has a sharp radius on the thrust face anda 90 degree corner around the radius of the chrome bearing. The second generation “light cut” pivot shaft has had the 90 angle undercut to give it a radius and reduce stress concentration. Tech Order 01-60JE-47 outlines replacement of light shafts with heavy shafts.

A heavy landing gear pivot shaft (P/N 161271) is machined differently on the inside bore of the pivot tube and has increased wall thickness. The pivot shaft thrust face has a large radius cut away to make a symmetrical sloping edge. There are also diagonal bands called “cheeks” on each side of the pivot shaft and an additional bead around the base to reduce the propensity to crack.

In overhauling 10 sets of landing gear over the past two years, AirCorps has established a standard process of handling these increasingly rare, high-value components. The importance of recording serial numbers, inspecting for airworthiness by checking for cracks, straightness, twist, and re-plating, if necessary, cannot be understated.

To ensure proper inspection, the pivot shaft must be removed from the aircraft to see cracks in the radius. Dye penetrant has been found to be insufficient in locating cracks meaning the pivot shaft must undergo magnetic particle inspection. A thorough dimensional analysis should be done to check the straightness and location of splines relative to the angle of the gear leg receptacle hole. Also measure for elongation of the top and bottom of the gear leg receptacle hole and the outer diameter (O.D.) of chromed surfaces, along with checking for flaking of chrome which is an indication of rust underneath.

Therefore, AirCorps recommends, Following guidance given Military T.O. 01-60JE-47 to replace “light weight” shafts 73-33112 & 66584 as soon as possible or at next inspection. If “light weight” shafts are operated, we recommend immediate magnaflux inspection if not previously inspected by removal and magnaflux. We also recomm

CRACKING

Pivot shafts are prone to cracks in a number of areas and require consistent inspection regardless of type. Experience in inspecting pivot shafts has identified the propensity for cracking in a few select areas:

Cracking location # 1 – Exterior Corner at intersection of pivot and landing gear receptacle on both light and heavy pivot shafts.

Location of external Pivot Shaft Cracking – photo – AirCorps Aviation

Heavy landing gear pivot shaft cracking circled in red. Thank you to Gary Norville for photo contribution