PROPELLER HUB ASSEMBLIES, PN 3745; used on Continental 65,75,80,85,50 engine
The Continental propeller hub (tapershaft), assemblies have an interesting history. The taper shaft hub was designed as a relatively cheap unit designed to be damaged and replaced in a nose over or ground loop, so that the expensive crankshaft itself would be protected and preserved.
The original propeller hubs were made of a “J” series steel (now obsolete), that was quite useable and machinable on high speed turret lathes. None of these hubs have been produced by Continental motors since the mid 1950’s. Continental had hoped that the supply of hubs would be depleted at some point, after which the engines would be removed from service, thus removing that operational liability. Good products do not expire so easily.
There would be MANY of these hubs remaining in service except for the abuse in service, over-torqueing (correct torque is 225 ft/lbs), and of course the nose over problems.
In attempting to procure serviceable hubs from 1990-2005, we discovered that only 1 in 10 hubs were serviceable, the other 9 were bent, cracked, or had damaged threads and worn bolt holes. The hubs often crack at the keyway groove, or between the lightening holes in the flange. A cracked hub will not ‘ring’ when tapped, while a hub without cracks will have a good and lasting tone upon tapping it. Cracks are often very small, and may not be detected by the naked eye. Bent flanges (or straightened flanges), can cause cracking, or may simply affect propeller track and cause engine asymmetrical loading. Worn bolt holes, damaged threads, and other defects may or may not be immediately obvious to the uninitiated buyer/owner.
After a large number of these defective hubs passed through our hands and into the garbage (expensive), the Luscombe Foundation decided to have a drawing made of the product, and we specified a high-quality aircraft steel to replace the obsolete material. We had that drawing approved by the FAA, and we undertook several production batches of these hubs under FAA-PMA for our use in restorations, and for use by vintage aircraft owners in need. We utilized 4 axis CNC processing, and the duplication tolerances were very tight, yielding a product far superior to the original hubs.
Unfortunately, the FAA oversight of PMA is not equal, nor is it reasonable in its application- especially for small shops. Over the course of 4 (annual certification), inspections, the SAME hub was targeted for audit no less than 5 times, (all with successful documentary and dimensional proofs). Never-the-less, one inspector of military background,(with no budget constraints), demanded that all future PMA production must submit to CMM inspections- because that was what he understood, (and because he was unable to properly use calipers and common ball gauges for QA). While we agreed that CMM is a great tool, the dimensions had always checked within tolerance, and thus, we did not understand why an additional $400 charge for each hub inspection was warranted.
Thereafter we ceased PMA inspection/production, and instead chose to produce hubs to the same drawing and standard, but we only conform them pursuant to custom Owner requests on an ‘as needed’ basis, pursuant to the owner’s request and their specification requirements. Such part production or product ‘alteration’, is common for vintage aircraft, and often necessary when the original factory has ceased product support for airframe or engine items. This customer maintenance support is compliant to the re codified regulations CFR 14, 21.9 (a)(5), and (a)(6). This section is a re-numbered version of CFR14, part 21.303 (b)(2), as opined and clarified by the FAA legal counsel in their Opinion date August 5, 1993; which is entitled, “definition of owner produced part (21.303(b)(2)”.
To meet the regulatory requirements, we collect a production authorization form from the Owner which defines the product required, AND we make the owner the final inspector and arbiter of suitability, confirmed by his acceptance and use of the part on his aircraft.We conform the part to the FAA approved drawing and data, and we certify an inspection and release of the part to service on an FAA form 337 to the end user with inspections by both an A&P, and an FAA Inspection Authorization.