Cog belt shred-o

brs

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I've stripped teeth from two belts on the Shredo. The first, I'm fairly sure was too loose. Two weeks ago I trashed my second belt (pic) and this time I believe it was too tight (I cut the belt to remove it - didn't break). So please tell me how you RAF and SH drivers tension your belts as I can't find any simple howto's for this.

Also, looking at this belt makes me think that if it were two belts (split) one might have survived. Any experience out there with two smaller belts?
 

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birdy

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Pully alinement is the most probable cause.
Wen i still had the brick [ ej22] on the wasa it almost shredded [ wen i found the problem, 60% of every tooth was detatched] the first belt in less than 100 hours.
Checked the pullys for paralel and found they were only a poofteenth out. [ cant remember zactly how much, but it was only a few thou]
Shimmed the prop shaft to correct it and the second belt was still good at 300 hours.
 

RayPierce

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Try a different manufacturer. What is your engine idle rpm? Too low and jerky will put a lot of strain on teeth. Engine idle rpm should be smooth.
 

Blue Chips

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brs,
Belts have a shelf life, check to be sure you are getting fresh belts.
They can look perfect but will fail under stress.
 

brs

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brs,
Belts have a shelf life, check to be sure you are getting fresh belts.
They can look perfect but will fail under stress.
Thought this one smelled a bit moldy.
The belt pictured is a Gates. The belt I'm fix'n to install is a different brand but I'll have to go look. The idle is set at 1200 rpm which is smooth once warmed up.
 

Vance

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The Gates engineers said it couldn’t be too tight.

The Gates engineers said it couldn’t be too tight.

I worked with the Gates engineers when I was working at Indian.

The rear belt was a challenging application because in first gear it was high torque at low rpm and in high gear it was low torque and higher rpm.

It also got loose as the swing arm went through it’s travel.

We looked at using it for the primary drive and as the aluminum inner primary got warm the distance between the crankshaft and the clutch grew a lot, I don’t remember the number. They felt it should be tight like a bow string and that there was no such thing as too tight.

They had a video of it climbing the teeth if it wasn’t tight enough.

Your redrive grows with heat and makes the belt tighter. The belt does not grow as much as the aluminum with heat so it just gets tight.

I feel that you will ruin the bearings before you will hurt the belt from being too tight.

We found even a slightly worn sprocket dramatically reduced the life of the belt and the torque it could manage.

The engineers felt the belt should only need to be adjusted once, more was an indication that something was not right.

Gates feels their belt should not be used to drive a propeller.

Good luck, Vance
 

bryancobb

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Vance's Comments Pertinent to ME

Vance's Comments Pertinent to ME

Thanks Vance,

When you speak of your motorcycle exploits...I LISTEN! You know what you are talking about because you worked very closely with the experts in these fields and proved out what they said with actual experience.

The Mini-500 I am building uses this same type Cog-Belt as the one above. I had one fail on my Mini-500 in 1997 because at the time, deliberate shaft mis-alignment was the method used to make the belt ride in the center of the pulleys. Tension was set by adjusting the idler once, and using a spring scale to measure deflection and tightening it there. It didn't throw off teeth like the one in the picture. It actually shredded perpendicular to the teeth like it had been cut with a slitter. When I landed normally after I heard the machine-gun sounds back there, There was less than 1/2" of the belt width driving the rotor. The Revolution helicopter factory had me elongate one of the idler pulley axle holes and experiment with tilting that pulley to track the belt. I had no problems in the next 50 hours until I sold it.

Now I have the newer "Bravo" style idler setup which involves the engine crank and MRGB input pinion being in perfect alignment. Then the new style idler is spring-loaded with a dampener to keep belt tension constant at manufacturer spec., under all operating conditions and as parts wear or warm up.

Tracking of the belt on the cogs is accomplished by a micro-precision axle adjuster that slightly tilts the idler pulley.

Seems good to me, but I have never flown the new setup. I don't remember anyone having any problems with it after the re-design.

Now my NEW 15 year old kit came with a "WOOD'S" brand belt (PRINTED ON THE BELT) that was made somewhere in Europe. (Can't remember where) My 1997 Mini-500 came from the factory with a Gates brand one. I know that rubber degrades over time so even though my new belt had been in a vacuum pack for 15 years, I wanted a new one.

My first contact was with an engineer at Gates. I sent drawings of the drive system to him and he was very helpful for the first 30 minutes ...UNTIL HE REALIZED IT WAS AN AIRCRAFT APPLICATION. Then "POWWWW!" He slammed the door in my face.

My second contact was with an engineer at "WOOD'S." I was very careful to NOT provide any info to her that would give a hint of aviation use.
She ran the calculations and roughly determined that the drive system could go to 2000 hours of constant 6500 RPM operation while transferring a constant 36 horsepower, with a safety factor of two.

Ouch! We all know the Mini uses 70 to 80 HP to hover and about 50 HP to cruise.

The engineer provided me the correct part number for the most current replacement belt that matched my tooth profile and identified my local distributor.

I ordered it through Applied Industrial Technologies, DIRECTLY FROM WOOD'S. $83.00 later, I had the new. modern belt in my hands...
GUESS WHAT? IT WAS A GATES BRAND POWER-GRIP GT BELT! The engineer at Wood's said "Oh...we don't MAKE them. The Gates Power-Grip GT is sold under our product name _ _ _ _ _ _ _ _." Can't remember the name.
 
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Stan V

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The Gates engineers said it couldn’t be too tight.
Gates feels their belt should not be used to drive a propeller
But of coarse, we use them all the time. The RAF has had a number of crank shaft failures from running the belt to tight.
 

Vance

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More ramblings about Gates belts.

More ramblings about Gates belts.

Hello Brian,

It is a challenge to do life calculations on a belt based on power or torque.

Generally they are imagining an electric motor and have some unstated safety margin and some unspecified duty cycle.

They may have become more sophisticated since I worked with them around ten years ago.

The person you were talking to was probably a “sales engineer”; who may not be an engineer at all.

Steady power is nothing like what we put them through with a two cylinder two stroke engine with a light flywheel and what amounts to a big flywheel on the other end. The firing of the engine quarrels with the rest of the systems desire to be smooth. The sprag may help some but it is still a discordant drive.

I would go with what has worked in the past and keep an eye on it rather than trying to imagine if it is suitable from an engineering standpoint.

Without a lot of sophisticated testing equipment we just don’t know what numbers to plug into the engineering calculations so we relied on destructive testing.

Tight and square are the two most important things for longevity.

Even the slightest out of parallel dramatically reduces the life of the belt in rough service.

In our testing we found the belts more sensitive to heat than Gates claimed.

We found that as Gates stated that the belt should need to only be adjusted once soon after it has been put in service.

We also found that aluminum sprockets didn’t hold up as well as steel no matter the alloy or heat treat and the condition of the sprocket has a lot to do with how long the belt will last. The small sprocket if there is much size difference is the one that shows the most wear.

If it has worked on other Mini 500 helicopters just keep them tight, square and replace the belts regularly and monitor the condition of the sprockets.

In a dirty environment we found that the sprocket and the belt would wear out about the same time. In a clean environment we found that we needed to replace the sprockets every other belt change. This is assuming a hard surface on the sprocket face.

Thank you, Vance
 

Vance

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It doesn't make it a good idea.

It doesn't make it a good idea.

But of coarse, we use them all the time.

The RAF has had a number of crank shaft failures from running the belt to tight.
I know that Stan.

It doesn’t mean it is a good idea.

I have reasonably direct knowledge of several belt redrive failures.

An important part of my preflight on modified RAFs and SparrowHawks was inspecting the propeller drive belt.

I suspect that Gates got tired of being sued because badly designed belt systems failed and the survivors wanted someone to pay for their loss.

I suspect what Gates was saying about not being a way to make the belt too tight is that you couldn’t hurt the belt or sprockets by being too tight and you could hurt the belt and sprockets by being too loose.

In my opinion the cranks broke because they did not properly support the crank outboard of the sprocket.

There is always a danger in putting a load on something in a direction it was not designed to manage.

I feel there is insufficient data to blame a too tight belt on a crank failure.

Thank you, Vance
 

Stan V

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I feel there is insufficient data to blame a too tight belt on a crank failure.

Thank you, Vance
I beleive you'd be in the minority on this thinking when it comes to the subaru.
 

Vance

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I am fine with that Stan.

I am fine with that Stan.

I beleive you'd be in the minority on this thinking when it comes to the subaru.
It would not be the first time I don’t agree with the majority opinion.

In my opinion if too tight is the problem it is an indication that the outboard end of the sprocket is not properly supported.

In my opinion bending loads and harmonics are what breaks the crank still pointing to poor outboard sprocket support.

I don’t have a Subaru with a toothed belt driven propeller and I am no longer directly involved with the maintenance of one.

Thank you, Vance
 

StanFoster

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I was not comfortable with my SpartowHawk crank support. They came out with a much better design and offered it as an upgrade kit. I bought it and was really impressed how it was a lot friendlier to the crankshaft, and inspection and maintenance was much simpler.



Stan
 

dinoa

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Another thing is avoid operating at rpm where the drive system resonates.

Dino
 

RayPierce

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Tensioner

Tensioner

Has anyone investigated a separate constant pressure belt tensioner?
Extra weight I know .Spring...gas?

Just curious.

I got one on my Chevy that seems to work. Same belt that came on it back in ought 4. It still looks good. I had to replace the battery last week for the first time. Sounded kinda different/weak and sure enough it tested below norm.
I have never had a battery last over 4 years. That one almost lasted 10!
 

brs

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I agree with Vance on the reason for crank breakage. That's why my redrive got a spline-shaft and a second bearing. There is no longer any bending force on my crank. Just torsion.

An engineer friend just called me and had some great info. I'll summarize here but I think he is going to send me the documentation where it came from. In which case I'll post it.

This is for the 880-8-85 belt:

  • RPM: expect greatly reduced service life above 5500 rpm. Our top cog-pulley runs right around that on max power.
  • Power: This belt seems to be rated at 118 HP. I think my EJ22 is putting out at least 130 HP on takeoff. The prop pitch is set for only 5300 rpm.
  • Tension: This was my original question. Using a flat stock across the entire belt it should be tensioned to deflect 1/64th" per span/inch at 19 lbs. Span being the pulley to pulley center distance. The 19 lbs is specific to the belt with (85mm width and 8mm pitch). For high pulse applications (our situation) the tension can be increased up to 25%. So that would be 1/48th" per span/inch.
  • Split Belt: A 2" belt has exactly double the HP rating as a 1" belt. Thus it seems to reason that splitting the belt will not reduce or increase it's strength. Though I'm hoping it will have other benefits.
 

scottessex

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The thing that people do not realize is the tension is critical.
Not enough tension and the belt try's to climb the teeth on the pulley/sprocket, if it is too tight as the assembly heats up it will expand increasing the center lines distance between the pulleys and make it even tighter, then pow broken belt or crank.

How wide is the belt? The one on my CAM100 is 390mm wide almost 4" probably similar to the RAF belt.
 

WHY

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A cog belt drive is simple "only in appearance" . The factors that will influence the belt tension are many more than what you get with a simple speed reduction arrangement setting on the ground on a piece of industrial machinery, been there done that, was a real education.

Tony
 

eddie

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I have 260 hours on the same belt on my 230HP RAF,no problems yet. My belt is a Goodyear.

I swing a 68" 5 bladed warpdrive pitched at 14.5 degrees at 5,050 on take-off.

I have 1/8" deflection on the belt when it is cold.

Best regards,eddie.....
 
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