[Courtesy of Michael Kuper, Michael.Kuper "at" vw.com, July 1999 - originally posted on the IMAC mailing list to clarify concerns about a recent DA150 failure.]
The facts about the DA150 Crankshaft:
The broken DA10 crank seems to be a hot topic on this list. I have seen many broken cranks, from .15 FAI combat motors up to 454 chevy V8's. There is usually a number of contributing factors involved. I was very curious about the DA150 crank. Like Blaine Austin, the originator of this thread, I do not own a DA150 but made some inquiries anyway.
There are approximately 80 -90 DA150's flying right now. Two have broken the crankshaft. When Desert Aircraft heard about the first one (Mike Saul) they went into action analyzing what could have caused it. They did not concentrate on Mike's flying style or the way he sets up his airplane, they looked at the design strength and actual production of their design. I have to applaud this approach. They have run many engines through many tests and have not broken a crank, yet they still looked at it as their problem, not the customers. A few days into their analysis they were informed of a second crank breaking.
DA150 cranks are produced by a crankshaft manufacturer. Desert Aircraft does not have crank pins, wheels, etc. machined by a local machine and then press them together like most manufacturers. To analyze the failure, they cut a production crankshaft lengthwise to see the cross section, and inspected the area where the shaft and front throw meet behind the bearing. This is a high stress area. They found three factors that could contribute to failure here.
Stress does not travel around inside corners of a structure very well, it becomes concentrated there. To prevent this, a smooth radius is necessary anywhere an inside corner is formed. The larger the radius, the less concentrated the stress, as the stress is "fanned" out through the structure. Any sharp edge or malformation in this radius will cause a stress riser. To ensure that stress can be relieved to a greater degree, they changed the specification for the radius at the crank throw to 1mm. This radius is much larger than the calculated stress calls for and provides a large margin for any unanticipated stress riser such as machining mark left during production. This large of a radius also creates a manufacturing headache as the bearing inner race must be relieved to match this radius. Desert Aircraft felt this extra margin of strength was worth the extra manufacturing effort.
The material the crankshaft is made from has been upgraded to MC440 (a proprietary trade designation). From what I understand, this is what Desert Aircraft originally intended to use, however some cranks made from MC400 were supplied.
The crankshaft has a center hole machined in it. This remove approximately 20 grams of material (~3/4 oz). It was intended to make alignment during manufacturing easier and reduce weight. The new specification calls for the elimination of this hole. The center of the shaft doesn't provide nearly as much strength as the outer diameter, but again Desert Aircraft decided to allow an extra margin of strength at the expense of manufacturing ease and a small weight penalty. They also thought that any sharp edges that may have been created when this hole is machined may create another stress riser. If a crankshaft was made with a stress riser inside the shaft and a stress riser in the outer radius, the likelihood of failure goes up. Elimination of the hole means no inner stress riser can be induced during manufacturing.
The original crank design is probably more than adequate unless these stress risers are induced during manufacturing. This evident in the number of test hours flown without failure. Unless you are unlucky enough to have a crank that has an error or machining mark (microscopic groove) in the radius at the crank throw and a corresponding stress riser on the inside, I doubt if you will have any trouble. Desert Aircraft is not banking on this, that's why they made the specification changes noted. They will contact DA150 owners and inform them of the situation and remedy.
I could go on about many serial defects that are present in other motors where the manufacturer deems them acceptable, but I won't. I'll just say that Desert Aircraft is taking the most responsible approach I have seen and I intend to have one of their motors someday. Sorry for the long winded explanation, but these are the facts.