Lower Sprocket Modification
I recently received lower sprockets from two relatively new RAF 2000 builders. Neither gyroplane has flown and only one has had any running time (approximately 1 hour). The sprockets were brought to me to see if I could replace the bearings and install the stainless steel sleeve which was developed by Jim Butler.
After checking the bearings by turning the inner race with my finger I discovered that the sprocket with about an hour of running time had very rough bearings. The unit that had not been run had no roughness at all. After further investigation I discovered that the bearings are Chinese made units that are used in Azusa wheels. These bearings have a 0.75 inch I.D. and a 1.625 inch O.D. I called my bearing supplier and asked if he had an American made replacement for Chinese bearings of that size and if they would hold up at 5,000 Rpm. He very ademately told me that no Chinese made bearings will hold up at 5,000 Rpm because the Chinese don't produce any precision bearings that are reliable. I told him that I would prefer a Fafnir brand bearing and he gave me a Fafnir part number and assured me that it would hold up at 5,000 Rpm. The Fafnir P/N is S8PP. Cost is approximately $28.00 each. The reason for such a high price is that SAE bearing sizes are non-standard and are not produced in very large quantities. Almost all bearings are metric sized.If the bearings have not galled the bore of the aluminum sprocket I don't see any reason to add a steel sleeve. The bearings are steel and there should be no diference between the surface of a steel sleeve or the bearing outer race. I have been using a single Fafnir bearing of slightly larger diameter and approximately 3/4 inch wide in my reduction drives for twelve years and have only had one failure out of several hundred reduction drives and it was because the engine was overheated which cooked the grease out of the bearing.
If the inner surface of the sprocket has been galled then it should be bored and possibly a sleeve added or the sprocket could be bored for a metric bearing. A new bearing boss would also have to be fabricated in that case. It is a simple process to replace the bearings. Just place the sprocket in an oven set at around 300 degrees for about fifteen minutes. The aluminum sprocket expands at a higher rate than the steel bearings so the bearings will just fall out when the sprocket is turned outer end down. Drop in new bearings and when the sprocket cools they will be pressed in place.
I recently talked with another RAF owner who said that he had Japanese made bearings in his sprocket. The Japanese made bearings may be okay if they are a precision bearing but to be on the safe side replace them with American brand bearings. Non precision bearings are very inexpensive---the Azusa wheel bearings retail for $3.00 each.
Don ParhamW.L., this is an area that needs to be upgraded. A lot of us have experienced this. You can remove the lower sprocket by going through the holes in it with a longer allen wrench if you haven't discovered this yet. I forget the size, but shouldn't take long to find one that fits. A precise, stainless steel insert then needs to be machined to hold the bearings after the pulley wheel is reamed out a hair. Don't know about replacing the stub. Maybe someone else can help there. I'm sure that a machinist could duplicate it.
Anyway, I have the engineered drawings for the lower sprocket upgrade with all the precise dimensions. Any competent machinist should be able to fabricate the part for you. Explicit instructions are given on how to insert the insert into the pulley (heating and freezing). Jim Butler is the one who designed the part and made the drawings and said it was O.K. to share them. He used to do the work, but doesn't anymore.
(EDITORS NOTE: The drawings are on this site at: lower_sleeve.jpg and lower_spindle.jpg.) It's an important upgrade. Some have tried to compensate by running the belt too loose, hoping that less pressure wouldn't make the bearings start to wobble. This flies against the belt makers intructions and specs and is not a fix. The bearing seat needs to be more than the soft aluminum of the sprocket. Of course it goes without saying to purchase only top-grade bearings, probably something you didn't get with your kit. If you don't do the upgrade though, you'll run into the same problem down the road, good bearings or not. The aluminum is too soft for any extended usage...like flying the thing.
Ken J. - SDCAUSA
Got my pulley back from Jim Butler with the steel insert and new spindle configuration. The bearings are now held onto the spindle and plate with a nut and bolt and we're talking tight and solid. After reinstalling the pulley, it was a bear getting the bearings into the insert. Tight, tight, tight. The last little bit is done by cranking down the four bolts on the plate. That gives them their final seating. My previous setup had the spindle coming out of the bearings like a hot knife pulling out of a chunk of butter. There was also a wobble.
Upon completion, I took her outside and ran her for awhile. I know it's subjective, but the engine seemed to be running so much smoother. I spun the blades to check the new prerotator belt and shut her down after about 15 minutes. I then noticed that the prop belt was centered on the small pulley, with a little bit of pulley showing on each side and neither side of the belt up against one of the pulley flanges as was always the case before the fix. This appears to be a significant upgrade to the lower pulley/bearings/spindle. I understand it wouldn't take too much wobble to start flirting with a broken crankshaft. I'll fly her tomorrow...finally.
Ken J. - Santa-E-A-Ho,ho,ho
