It doesn't matter whether the reduction drive is a gear, belt, or chain if it is properly designed. There are a lot of sucessful belt drives and gear drives. We don't have much experience with Hi-Vo chain drives on Subaru engines but they have a very good track record on big block Ford and Chevy engines in ag planes.
We have built about 350 cog belt drives and I only know of one crankshaft failure and that was a DOHC EJ25 which has a runout problem on the crankshaft flange. We use an outer support bearing on the outer end of the drive sprocket so that bending loads on the crank are mininimized. RAF uses the same type of system except that they use two bearings back to back.
Some of their outer support bearings are non precision bearings and do not hold up but lately they have selected a precision bearing, or at least that is what I found on one reduction drive.
Gear drives require a lot more attention to torsional vibration problems than cog belt drives and can not be as easily inspected as belt drives. The HTD cog belt drive system has been well proven in industrial applications and they will run for thousands of hours with minimal maintenance in harsh environments.
The HTD belt absorbs some of the torsional vibration and the use of a five or six pound flywheel will eliminate any problems in that area. Look at all of the belt drives that have been put on the market over the past 15 years. Belted Air Power builds and is still building belt drives for GM V6 and V8 engines. Jess Myer ran his old Globe Swift airplane for 14 years with his original reduction drive and never changed the belt. He put 4,000 hours on it.
Rotary Air Force has around 600 RAF 2000 gyroplanes out there and there have been very few complaints about the reduction drive other than the Chinese bearings for the outer support assembly.
Reductions, Inc. in Manitoba, Canada built several hundred cog belt drives. Dave Johnson started about the same time that we did. He had a few problems which I think was attributed to mechanical resonance of his light weight "box" style upper support assembly for the driven sprocket.
Stratus, Inc. has built a lot of cog belt drives for the EA81 and the EJ22 and have a very good track record. They do use a large support bearing on the EA81 crankshaft flange.
Cog belt reduction drives are typically less expensive and easier to maintain and are less expensive and less complex than gear drives. The down side for gyroplanes is that the typical offset of around 8 inches is a deterrant to obtaining center line thrust unless the drive is "turned over" like the AAI SparrowHawk. Cog belt drives are well suited for fixed wing airplanes with cowled engines with the stock intake manifold.
It's like Fords and Chevys--a matter of opinion as to which is best. I am not trying to plug my reduction drives either--I am out of the business. Tracy Crook in North Florida is developing a planetary reduction drive for the EJ series engines which is capable of handling up to 250 Hp. The drive utilizes a rubber donut torsional dampner and is well proven on Mazda engines. If it will hold up on a Mazda it will hold up on a Subaru. Although not commonly known, Mazda engines have a lot of torsional vibration. His Mazda drive sells for $2500 which is much less expensive than most gear drives and the unit for the Subaru engines will probably be about the same price.
If you are in the market for a planetary reduction drive check out his website:
www.rotaryaviation.com.
and that was more than 10 years ago.
