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CONTINENTAL TAPERSHAFT CRANK

Tapershaft mountings

Early engines by Lycoming and Continental (among others) used a standard taper shaft output with a keyway, instead if the familiar propellor flange seen on more modern engines. The tapered crankshaft with a separate propeller hub drives by the use of a square key, and the friction of the taper itself.

Pictures of the two types of crankshaft are available in the Continental A-series service manual.

Some of this installation might have been the ‘convention’ of the time where industrial taper hubs were utilized in a variety of engines, thus making those "aircraft engines" suitable for other applications with the change of a hub/drive unit. There were a variety of industrial tapers identified in the early 1900’s, some still survive, but most have been replaced with splines, transfer plates, and other devices to harness the energy of engines and machinery at the output.

The Propeller installation for airplanes was ideally suited for bolting to a plate mounted on a crankshaft. This plate became integral to a hub, and that hub was then bolted to the taper shaft crankshaft output end by means of a large nut. This mounting was likely because that standard taper was convention, but the taper shaft installation offers other advantages to be discussed later.

The later flanged cranks represent a typical design evolution enabled by new heat treat and alloy processes which allowed different portions of the same part to have different levels of temper.

Application

The taper shaft Continental crank was utilized in the A-65, A-75, A-80, and the 85HP engines (same crankshaft PN, and stroke, different RPM and a bore upsizing for the 85 HP engines)

The taper shaft crank is directly interchangeable with the later version flanged crankshaft made for the 65-85 HP Continental motors.

The taper shaft propeller hub uses the same propellers as the flanged crankshaft, but require the use of bushings in the rear of the propeller to fill the counter bored area.

On a Luscombe the original cowlings were not split, so the removal of the propeller with one nut facilitated removal of the single piece cowling. You can use the tapered hub to remove the propeller quickly and conveniently without many tools or safety devices.

Advantages

1. It is much easier to replace the crank seal on a taper shaft crank, because there is no flange or prying with which to deal.

2. The propeller hub is generally softer material than the hardened crank, so in the event of a prop strike the hub fails or deforms, not the entire crankshaft (as in the flanged cranks).

3. Few taper shaft cranks fail a run-out test after a propeller strike. Most flanged cranks will fail such a test under similar conditions and after similar propeller damage.

4. The hub is fastened to the crank with a large nut that pulls on a "c" ring and ledge which is integral to the hub. The nut can be turned with a large steel bar. The hub is acts as its own puller to remove the prop.

5. The Nut or Hub or retainer clip may be replaced individually in the propeller hub assembly, facilitating repairs and return to service.

Disadvantages

1. Propeller Hub availability

2. Antique styling

3. Over-torque or damage to the "c" ring can inhibit removal.

Removal of damaged "c" ring or over-torqued hub.

Subject to be explored at a later or revised posting. There are special techniques for this.

Service difficulties

Continental propeller hubs used on these tapered crankshaft installations are prone to service difficulties if they are over-torqued, operated loose, or if operated in a damaged condition (i.e. bent flange).

An over-torque of the center mounting nut will cause cracks radiating from the keyway into the rear flange, usually to the lightening holes. Sometimes cracks are located in between the propellor bearing surfaces on the hub assembly. The cyclical loads from operating loose (banging on the keyway) causes the same problem. Likewise, the thrust differential and P-factor differential caused by operating with a bent flange can cause failure from cyclical loading of the hub.

Service and inspection

Removal of the propeller from the hub on odd years is a good idea so that the flange and hub can be cleaned and inspected with a magnifying glass. First a cleaning and visual inspection for cracks or deformity of the hub is appropriate (on a leveling table, with a magnifying glass). Because the hub is relatively soft as compared to the crank, and designed to fail first in a strike situation. Few tapered cranks are ever found to be 'bent'.

Generally the cracks in the hub radiate from the keyway at the rear flange toward the lightening holes in the flange. I personally will not reinstall a hub that is not magnafluxed also, since sometimes that finds cracks in the thinner hub area between the two bearing surfaces that the propeller rides upon. Many units in service will not pass proper tests. (Any suspect hubs should be inspected with magnaflux). Inspection methods and limits for the Hub and Crank are available in the Continental overhaul manual.

Condition and availability

Hub adapter problems can be difficult. Used units are available, but in my inspection of those I have found that 4 in 5 fail to pass a magnaflux inspection due to hidden cracks and stresses. The NOS units alleged to be available from a west coast distributor are without paperwork or inspections and appear to be of a different manufacturing quality than the original. Some caution is urged as the vendor has suffered a recall of engine gears and other parts advertised as NOS, which were found to be NBG and NFG (New Bogus Stock). The inexpensive price is indicative of source and likely condition. Inspection alone costs nearly $150 per hub.

Reconditioned or like new hubs, with certified inspection papers are available should you desire such an item. Few sellers of "used" or NOS hubs are willing to provide inspection and condition certification with their product.

Contact Donna Losey at 480-650-0883. Condition and your satisfaction are guaranteed. Feel free to call or inquire if you have further questions.

Doug Combs
© Copyright of original content, 2004

 

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Last modified: 02/01/06