Eatin' up my prerotator belt - Merged

[KenSandyEggo]

I have the Sparrowhawk reduction plate on my gyro.....the reversed one for the prop, drive and starter. The prerotator belt comes within a hair of a bolthead that holds on the prerotator pulley-arm and it rubs and is wearing (photos attached). Can I just round off the sharp corners of the bolt-head, seeing as it's not structural per se? Or maybe some kind of aluminum shield under the bolt-head to cover it more smoothly? Any GOOD ideas?

[Ron Marlett]

Ken
Could you possibly run a shorter belt? It has been several months since I saw one of those drive assemblies up close but as I recall the belt tension is set by rotating the prerotator clutch assembly inboard or outboard about the axis of the bolt that passes thru the bottom of the aluminum assembly. I don't recall the mechanism that holds it in tension but your clutch/driven pully assembly appears to be nearly straight up and down. It looks to me that if you could install a slightly smaller belt that this would require that the clutch assembly be tilted slightly inboard to fit and tension the smaller belt. This would raise that upper belt a little higher in relation to that bolt head that it is interfering with.

Ron

[twistair]

I caught the same, Ken. The most possible damage is since the belt vibrates hard having amplitudes up to an inch at high engine rpm. I added two dampener wheels installed right at existing bolts (see photo) so that the lower wheel slightly tightens the belt (this allows you to tension the belt slightly less far and thus to improve it's clearance to that upper bolt) and the upper wheel only touches the belt when it's not rotating - it's only doesn't allow the belt to jump up'n'down. Wheels are standard plastic dampeners (two high speed bearings - a must!) used for cars CAM belts, weigh nothing. Works excellently and without any trace of wearing on the belt - tested for dozens hours to the moment.

[KenSandyEggo]

Thanks for that info Alex, Ron and Jim. I'll look into all 3 suggestions and determine as best I can, the simplest and easiest. I can't tell without looking at the belt and bolt in front of me. As to counter-boring, could that be done in place? If that all has to be disassembled, that would be a major undertaking. My prerotator arm looks different than Alex', so I'll have to wait until I play with it to see if a shorter belt can be used and if that would raise the belt enough.

Alex, next time you're that close with your camera, how about a couple shots of the lower prerotator unit so I can see if my hydraulic actuator would work?

[ScottTinnesand]

Ken-
It can be done in place. It's been done a few times in BXK to include Randy Coplen's converted RAF. It helped the issue but the solution Alex came up with looks likw a winner to me. We also did put a high speed to the hex head to shorten it a bit.
Scott

[KenSandyEggo]

Thanks, Scott. I sure have several options now.

[Cobra Doc]

Alex's idea looks like a great fix. Ithink I would go for a GM belt tensioner; steel pully and high speed bearings. The solution may be a redesign with a couple of spacers. JMHO.

[KenSandyEggo]

Wouldn't recessing the bolt-head be a lot simpler than rigging up 2 pulleys and all their attendant hardware? Maybe one pulley to keep the top section from flapping after recessing the bolt-head?

[Aussie_Paul]

I agree Ken!!!! Geez, I must be having a soft moment!!!!

I think that one pulley to stop the slack side of the belt flapping around would be sufficient. One of my modified Raf owners has done this single pulley mod but I have not seen or operated it yet.

Aussie Paul. :)

[KenSandyEggo]

I can't find the thread where we were talking about the belt slapping against the bolt head on the SH and conversions.

Sparky, a retired machinist who has a hangar in the next row had some sort of boring tool that attached to a drill. Hhas numerous diameters. We made the hole a little larger as to get a thin-walled socket around the head.

After several hours of flying, I notice no more belt-dust accumulation on the plate and other aluminum parts. Here are a couple pics. I just have a thin scuff-mark where the lower belt hits the plate a little. There's no bolt-head there.

[scottessex]

Looks good. Was the tool called a spot face? I have used those alot to counter bore bolt holes, works like a champ.

[KenSandyEggo]

He didn't call it that, but I'll be darned if I can remember what. It had a drill bit to go into the hole first. My hole was already there, so the bit was just a guide.

[ToddP]

Here is the original thread

http://www.rotaryforum.com/forum/showthread.php?t=2886&

[KenSandyEggo]

Thanks, Todd.

[KenSandyEggo]

http://www.rotaryforum.com/forum/showthread.php?p=41030&posted=1#post41030

This discussion continues in this new thread. I couldn't find this one at first.

[ToddP]

The two threads have been merged.

[r.coplen]

Ken,

I noticed excessive belt flutter on my SparrowHawk when I did ground high speed engine ground tests. The belt looked great at about 4400 rpm and than above 4500 rpm the belt started fluttering as much as 1 inch on both sides and was very unstable.. We solved it by going to one size smaller belt and making it as tight as we could. At 5150 rpm the belt was as true as could be. The belt is now part of every preflight inspection.

[KenSandyEggo]

Randy, I don't see how a slightly smaller belt would make a difference, seeing as we can adjust the tension.....or is the S-H setup different from the RAF? Maybe the shorter belt has different harmonics. I'll chock the nose-wheel tomorrow, run her up to 4500, step out and see if the belt is fluttering. :eek:

[r.coplen]

Ken,

We were at the limit of the adjustment with the larger belt. It felt tight so we went with it to begin with. The smaller one allowed a little more tension

[Cobra Doc]

Hey guys, IMHO higher belt tension isn't the solution. You run the risk of bearing delamination, bearing housing warping, oil seal damage, crank shaft and drive shaft failure and heat build up in the belt and everything it touches. Our friend in Russia has the solution. If it was my problem, I would install a spring loaded idler. The idler spring arm helps absorb vibrations and shock loads. Shock loading isn't normally a problem in aircraft, unless you lose a spark plug. In that case you have a shock load every time a good plug fires after the missing one. Just some points to ponder.

[r.coplen]

Cody,

I have to tell you I don't think it is that tight. I will keep an eye on it and check for any heat build up.

[Cobra Doc]

Anytime I have non-motorcycle questions about drive belts, I contact Jess Meyers at Belted Air Power in Las Vegas. His phone number is 702-384-8006. I think he also has a website. Jess would be the real expert on this situation. I'm sure just about everyone has seen seen his Chevy powered RV-6A.

[KenSandyEggo]

Cody, this belt is driven off of a pretty large crank pulley that is next to and part of the prop pulley, so there should be no problems there. The other end is a pulley attached to the prerotator housing, and I understand what you're saying. When I got my kit, RAF re-designed the system so a longer belt is used. They forgot to send me the flanged pulley that went along with the new design. My belt kept coming off and as one possible solution, I tightened the belt so much that I bent the small axle in the lower prerotator housing. I just snug it up until it's fairly tight now, but I don't "gorilla" it down anymore.

The S-H must be a little different, as I'm nowhere near the max adjustment.

[Vance]

Gentlemen, Don't forget that when the plate that this is all atached to gets hot, it expands quite a bit. Belts with arimid fibers don't expand with heat, so Randy, next time you fly, check it with a spring scale before and after. You may find that it is getting tight enough to damage the bearings on the pre-rotor. Parts that are spit out of any part of that system could damage the prop and shoot off like a bullet. Thank you, Vance

[KenSandyEggo]

I dropped off my clutch plate at a machine shop to be trued today. I placed a straight-edge across the face of it and there were gaps. The machinist said that it looks like his buddy who has a precision grinding shop could do a better job, so he's going to pass it along to him for the work.

Paul Bruty suggested this a long time ago, but as usual, I don't like to do anything he says unless absolutely necessary. :p

Vance, the plate is uncoupled after prerotation, so there is no continuous heat-build-up on the plate, unless heat build-up on the pulleys would be a factor. Do car belts get tighter during usage from the pulleys? :confused:

[Vance]

Ken, I am talking about the redrive plates, they grow with the heat from the engine. I was realy speaking to Randy, if he starts with it tight it gets realy tight when the plate gets hot. I don't know how much it grows, so it may not be a problem. That is why I sugested he check it with a spring scale and a scale, to see if it is getting too tight. Thank you, Vance

[Cobra Doc]

Ken;
That's a good question. I never messed with hot belts on the car. Since both the pick-up and the car are Chevrolets with serpentine belts and spring loaded idlers, I s'pose I'll never know. But it was just a suggestion and a little "what if" from what was being described. I guess I need to go out Buckeye next time Jim and Terry invite me and take a hard look at both the AAI and the modded RAF. Who knows, I might see something the experts missed. :cool:

[AirgyroAviation]

I was going through belts about every 12 hours of flying. Now after a simple adjustment the latest belt I have has lasted me over 100 hours of flying on my Sparrowhawk. I was getting the wear right on that bolt head and also a little along the bottom side where the belt comes close to the angle support brackets.

All I did was put washers on the back side of the PSRU plate in between the Pre-rotation assembly brackets. Jaap at AAI talked me through it as we did some measurements and tests and figured out that the Pre-rotator pulley side was off by a little bit. (AAI is now working on thicker brackets for the prerotator as the current ones actually flex a little bit and cause belt wear on pre-rotation, ask Jaap about getting you some thicker brackets for your aircraft!)There are only 3 spots you have to put the washers and it moves the prerotator in line with the main drive of the engine! EASY fix and has worked great. Let me know if you have any questions or talk to Jaap at AAI and he will tell you how we did it.

[AirgyroAviation]

by the way I still have the origional brackets and have not upgraded to the thick ones yet, so the washer-fix works with the current brackets.