Prop Shaft Part II and Prop Assembly


Work continues on the prop shaft, and the means I use to attach a Graupner prop. Mike's plans called for a taper to be turned into the propellor shaft. That method will work fine, but understand that the taper would be fixed longitudinally on the prop shaft. By making the taper in the form of a bronze collet, the physical location of the taper, during final assembly, is moveable; by assembling this collet firmly rearward on the prop shaft, the collet captures the inner race of the front, main prop shaft bearing!

From front (right) to back; not to scale -
  • Spinner, or prop nut - secures the prop assembly
  • Gall ring; centers the propellor pressure disk relative to the prop shaft and the propellor
  • A pressure disk. This part, combined with 4 cap screws through the prop hub, and the prop driver, forms a prop sandwich assembly.
  • The prop itself
  • The prop driver. This part mates with the collet, and is the surface to which the prop is secured
  • The prop collet. Bored internally to a constant 10mm ID, the exterior is tapered and this taper mates with the interior boring of the prop driver.
  • Front main bearing

Hopefully, these parts, and their relationship, will become clear with these photos. This entire process is a big deviation from Mike's plans, but the concept is quite normal for powerful engines, including full-sized examples.

  The pressure disk: Turned from 303 stainless. Not shown, it is drilled through for the 4 ea. 4-40 bolts which secure the prop to the driver. The disk is bored to a greater dimension than the 10mm prop shaft for the bronze gall ring. If the boring here were 10mm, it would be a close fit to the prop shaft, but the contact would be steel-on-steel, pressure disk to prop shaft, and vibrations would ultimately mar and wreck the prop shaft. Thus, it is bored significantly greater than 10mm to accept the following part....
The Gall Ring: The gall ring is turned of bearing bronze, SAE660. Without this simple turning, the pressure disk would really chew up the prop shaft at the front of the propeller hub. You will see how it works momentarily. The gall ring is bored to a tight sliding 10mm fit to the prop shaft, and the shoulder shown here is inserted into the pressure disk. Bear with me, it'll be obvious soon.
The parts are on hand, and we begin to assemble the prop shaft, the front main bearing, and the rest of the propellor parts. Shown here, the front bearing is installed on the prop shaft, with the inner race butting againt the larger dia. shoulder on the shaft. The seal faces out.

Next, the prop collet is installed onto the prop shaft, and slid down until it makes contact with the forward rim of the bearing inner race. Note the external taper, which will ultimately do two things - first, as the entire assembly is tightened, the collet will be forced backwards against the inner bearing race, and it will simultaneously tighten onto the 10mm prop shaft. Second, the external taper will center and provide a slow taper for the prop driver. Much like a Morse Taper in a lathe tailstock... due to its shallow taper, when correctly installed, the two parts will mate and prevent rotation.

The prop driver is installed over the collet. Note the four 4-40 tapped holes for the prop hub, which is secured to the driver using the pressure disk and 4 long 4-40 cap screws. At this stage, the entire assemply is pressed downward only by hand. The final force will be the prop nut/spinner.
On goes the Graupner prop! The bore of the prop is close to 10mm but a bit looser fit than the collet.

Postscrip - The graupner prop was suitable for tests, but ultimately is not the best choice for the engine... it has been replaced with a much bigger, lighter, and stiffer, Mejslik carbon-fibre prop.

The pressure disk is installed. Note from the first photo on this page, the addition of 4 ea spotfaced through-holes for the 4-40 prop bolts. Note too the large boring in the pressure disk, to accept the gall ring.
On goes the gall ring. Again, the gall ring is a sliding 10mm ID fit to the prop shaft, and a shoulder (not visible) is a nice fit to the bored center of the pressure disk. The gall ring aligns everything nice and square. The 4-40 prop bolts are tightened firmly.

At this point, everything except the 4 prop bolts is still only hand-tight. The entire propellor, along with the pressure disk, gall ring, and driver, can be removed from the prop shaft.

It's time to tighten and secure everything. For simplicity, I will call this odd prop nut a spinner. As mentioned elsewhere, it is turned to mimic a variable-pitch piston. The interior is threaded female 3/8" x 24. The four holes in front accept a custom pin-spanner. When vigorously tightened, the entire prop assembly is firmly fixed and simultaneously trued. The front main ball bearing inner race is completely secured, being pinched by the existing prop-shaft shoulder at the rear, and the prop collet.

The spinner is longer than the prop-shaft male threading. This allows the final part to be secured, an interior lock nut...

...shown here. I took a stub 3/8" x 24 set screw and cut and turned it to this short length, installed on a hex wrench for the photo. Once the spinner is completely tightened, the set screw is installed in the bore of the piston and given some torque to lock everything together. Initial tests show a perfectly running prop with no runout of the prop tips.

You may or may not like the appearance. I do personally, and can always replace the piston with a profiled spinner at a later date.

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