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Tips from Tye: P-51 Landing Gear Pivot Shafts

Tips from Tye: P-51 Landing Gear Pivot Shafts

Popping across town to the hangar often feels like a field trip: there’s always something cool to see and someone wearing wings happy to answer questions. For many, the intrigue in our surroundings is all about form, while for others the fascination is in function - the AirCorps team is a seasoned mix of both.

Tye is one who focuses on function. It’s evident in the orderly arrangement of workspaces, and the organization of the hangar in general, as soon as one walks in the door. Among the items carefully displayed on the hangar shelves are several pivot shafts that have undergone inspection. Pivot shaft is an abbreviated description from the actual component properly named SHAFT - LANDING GEAR PIVOT. This is a critical component that 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 landing gear to wing and absorbs the impact of the main wheel tires making contact with the ground upon landing.

Digital drawing of NAA pivot shaft on AirCorps Library

COMPONENT BACKGROUND

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.

TYPES OF PIVOT SHAFTS

Aircraft Maintenance Manager Tye Halvas 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.

Top view: Light pivot shaft drawing 73-33112 - Notice the common cracking point on the left and how inner diameter 3.359” leaves a wall thickness of only 9/32”.
Light pivot shaft - Notice lack of additional material in the cheeks area of the shaft. Only 3 of the 16 light pivot shafts inspected and tested by AirCorps have passed.
Heavy pivot shaft - Notice additional material through cheeks of shaft to strengthen area prone to cracking. Also the chamfered / symmetrical sloping edge is visible within the highlighted area where the upright portion makes the transition to the shaft.

INSPECTION

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 recommend visual inspection by pilot on each preflight of external surfaces as outlined in this discussion for all pivot shafts light or heavy.

PIVOT SHAFT FAILURES


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

Cracking location #2 - Radius of thrust face can be up to ¼” from face edge of internal radius.

Cracking in Pivot Shaft under magnification - Photo AirCorps Aviation
Example of a light pivot shaft that failed on landing - damaging the airframe, prop, and engine. We appreciate photo contribution from Mike Vadeboncouer

DEFORMATION

To ensure the landing gear outer cylinder piston tube (Bendix Part numbers 67428 / 67427) will fit the pivot shaft landing gear leg receptacle hole, measure for elongation of the internal dimensions.

Twist of spline shafts is an often overlooked element of inspection that can affect the landing gear not properly engaging and uplocking when retracted. Splines will twist under 73-33116 bushing which is not visible without careful inspection.

Heavy pivot shaft - Notice additional material through cheeks of shaft to strengthen area prone to cracking. Also the chamfered / symmetrical sloping edge is visible within the highlighted area where the upright portion makes the transition to the shaft.

INTERCHANGEABILITY

Heavy shafts are interchangeable with lights, but the bearings in the gear casting in the wing need to be chamfered to accept heavy shafts. Heavy pivot shafts (161271) require the use of bushing 122-33109 in the landing gear casting in wing. Damage to casting and shaft will occur if a 161271 shaft is installed in a casting with 73-33109 bushing.

WHEN PURCHASING

If you’re looking for replacement shafts, prior to purchasing ensure airworthiness by inspecting for cracks, straightness, and that the bore for the gear leg is not out of round. While replacement pivot shafts are quite difficult to source right now, AirCorps is available to perform inspections, answer questions, and perform modifications and services related to pivot shafts and landing gear.

Tye Halvas achieved his Airframe & Powerplant and Inspection Authorization certifications and has pilot ratings in tail wheel, float, and private. Tye leads the Maintenance department which works not only on prepping restored aircraft for flight readiness, but also on inspections, light and heavy maintenance, and component repair for a variety of aircraft. During a recent visit to the hangar, it was possible to view two P-51D models, a Stearman flown by George H.W. Bush, AT-10, and AT-11, all strategically spaced, as components of a B-25 and a P-51K were being attended to nearby.

Contact us for more info on ways we can assist with your project!

Special thanks to Eric Trueblood and Sara Zimmerman for co-authoring this post.

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