Type 1-2 Prop-Hub

 
Rev - Nov 24, 2017

Steve Bennett
 

Force One Prop Bearing & Hub

Introduction

The original design of the Force One Prop Hub dates back to 1985. I was a member of EAA Chapter 153 in Schaumburg, Illinois at the time. Several members in the EAA Chapter, had 2180cc Aero-Vee conversions in their Sonerai's and had broken their 82mm crankshafts as used with their shrink fit prop hubs. These crankshafts were determined to be forged 1045 with a high amount of sulphur in the material. At the same time, two people I knew broke 82mm crankshafts with a short tapered hub. This crankshaft was determined to be a Forged E4340 Crank.

Upon examination of the crankshafts, all broke exactly in the same area and manner. Since we had never had a stock VW 1045 forged steel 69mm VW German crankshaft brake, we decided to cut one apart to use as a sample in the following accumulation of data. We were very fortunate at that time to have an EAA member in our chapter that was also a metallurgist. We obtained samples from the different cranks that had broken, analyzed them and formed some conclusions that still guide us today:

  1. A stock VW crankshaft is made from 1045 forged steel.
  2. A SCAT crankshaft is made from E4340 steel.
  3. Cast cranks are made from poured steel.
  4. Generic 1045 cranks had extra sulphur which speeds up the machining process = lower cost.
Crank Material

Yield

Break

1045 Forged VW

80k

120k

E4340 Forged

122k

144k

Cast

18k

18k (some as low as 11k)

1045 Forged/sulphur

33k

75k

Properties of tested cranks.

  • TThe 1045 forged VW crank has a high yield and a good spread between eyelid and break, and it is flexible!
  • The E4340 crank is very strong, but does not have a big spread between the yield and break, or it is very strong, but not as flexible.
  • The cast crank is not strong to start with and has no spread. It is kind of like a piece of glass.
  • The 1045 forged/suplhur crank, even though it is advertised as forged and it is, has a very low yield and break point even though it has a good spread between the two.

Every crankshaft failure started at the back of the woodruff key, left side, and went counterclockwise around the crankshaft, ending up in the crankshaft timing gear woodruff key hole where the crank would finally fail.

We learned many lessons. The actual starting point of the cracks, that lead to breaks, was at the top of the threads of the 20mm x 1.5 bolt that holds the hub in place. There is only about .060" between the top of the thread and the bottom of the woodruff key hole. Every crankshaft that finally broke - quit, at the back of the threads and went in a slightly different helix around the crank to the large woodruff key hole. Clearly the bolt hole in the crankshaft, the threads in the crankshaft and the woodruff keys all play a major role in prop hub retention and non-crankshaft failure. So by trial and error from 1985 to 1987, we played with different designs of the Force One Prop Hub and Main Bearing. The late Al Campbell of San Antonio, Texas, did most of the initial testing of the hub in his turbo powered 2180cc KR-2.

We concluded that torsional vibration and torque, are the primary culprits. The small end of the aftermarket 82mm crankshaft was simply not large enough to absorb and dissipate the propeller loading and flywheel loading or twist, not to be confused with torque. A 2180cc engine as used in an aircraft has a 52 percent increase in engine torque over a 1835cc engine. When using a shrink fit hub or standard taper hub on the small end of the crankshaft where the pulley usually rides in a car, this increase in torque, combined with torsional vibration, sometimes causes the crankshaft to fail.

Notice once again we are not referring to the stock forged VW 69mm crankshaft. The small number 4 front main bearing may or may not play a role in the stroker cranks breaking. We were never able to reach a firm conclusion. I suspect yes, because it doesn't support the crankshaft from side loads as does a longer bearing.

So the two purposes of the Force One Hub is to transmit the prop loading to a larger surface area of the crankshaft so that prop loading can be absorbed, transmitted and dissipated over a larger area of the crankshaft and absorb and transmit to the prop, the torque of the engine.

You may note that in our catalog we do not offer a 2180 or 2276cc engine driven off the pulley end (Front Drive) of the crankshaft without the Force One Hub. It's not a sales ploy, it's a safety issue! We have seen enough failures, over our 29 plus years in business to dictate the use of the Force One Hub set up on stroker engines. Our 82mm crankshaft is unlike any other crankshaft made. The thread is 1/2" fine thread and goes about 3" into the crankshaft, of which the first 1" there is no thread. The woodruff keys are flat and only .090" deep and 180 degrees apart from each other. There is no snap ring groove in the crankshaft. This is our current design of which we are not aware of any failures. Our Top Bug Type 1 Crankshaft introduced in 1994, has never suffered a known failure!

The stock forged VW 1045 steel crankshaft gets by using the standard taper hub and the shrink fit hub simply because of the generous spread between yield and breaking - and it is strong. It works very well and has no real service issues, unless there is a prop strike. We recommend that in the event of a prop strike, that hub and crank become boat anchors (and not re-used). Why?

Remember the crack in the back of the woodruff key? One will usually start with a prop strike. Many times the prop hub will shear the woodruff key. If this happens, the prop hub can actually unscrew on the crankshaft. While I can't remember all the names, dates and faces; just in the last year alone, we have replaced 2 crankshafts in engines that had prop strikes that didn't replace the hub and crank after the incident. Will every aftermarket crankshaft, forged or cast break if using a shrink fit or short tapered hub? Of Course Not!

You can read Email-postings that confirm this. I do think, however, that looking at it from a historical perspective (a war vs. a battle) one can clearly see that most of the failures are with short tapered hub or shrink fit hubs on aftermarket forged, forged/sulphur, or cast crankshafts.

Propeller weight, prop material make up, rotating mass force, transmission, absorption, dissipation etc... all play real parts in keeping the hub attached to the crankshaft. Remember, even Continentals and Lycomings occasionally break crankshafts.

Some do's and don'ts

  1. When driving off the pulley end of the crankshaft, use wood based props only. Specifically when using an aftermarket forged crankshaft.
  2. When driving off the pulley end of the crankshaft, try to keep the wood props weight to 7 lbs. or less. (That rotating force thing.)
  3. Re-torque your lightweight wood prop with every change of the season. Normal torque is about 11 to 14 ft. lbs.
  4. Never ever use an aluminum hub prop and carbon fiber blades when driving off the pulley end of the crankshaft. This applies to both stock 69mm forged VW cranks, as well as, aftermarket forged crankshafts.

In addition to selling and making parts for use on sport aircraft and their engines, Great Plains Aircraft is a clearing house of information. We talk to hundred's of people a week and get to hear their accomplishments and heartaches. It is easier for us to spot trends simply because we are exposed to a wider group of flyers and builders and more of them. The Force One Prop Hub kit is a time proven safe way to attach a propeller to a forged crankshaft, stock VW or aftermarket and have it all stay together.

There is no doubt that the number 4 oil galley plug has played a role in some crankshafts with shrink fit hubs breaking. If the oil supply to the bearing is blocked and the bearing wears out, it is only X number of oscillations until the crankshaft will eventually break. Many of you may remember a very popular Arizona based VW conversion company that was a true leader in this field for a number of years. Do you recall what in part lead to their sale? It had to do with the replacement of 25 short tapered prop hubs on stroker forged crankshaft in a new Magnum series of engines. The crankshafts were breaking. History is a great teacher if you take time to study it.