02-21-2011, 04:40 PM #1
Broken sprag pics, old style sprag
How many of you guys have broken a sprag and have actually broken the elements in the sprag?
Mine isnt breaking the elements, but instead they suffer an inertia moment on the sudden dis-engagement of the sprag upon rev limiter activation at 9000 rpm setting.
Here is my take on what is happening to the sprags copied from my build thread in yamaha project section...
Those who have pics of their broken old style sprag i would love to see them and see if they are like mine.
Water tested Bertha today, rode her for over an hour... Here are the numbers.. 80 degree day.
Rotax pump, 160mm impeller, 83mm hub, 15-25 pitch, R&D 82.5mm nozzle, 8380- 8400 rpm = 84 mph on 3/4 tank of fuel.
Earlier in the day engine went to 8600 rpm on a test run but didnt have gps on.
Only speed run was done after a 20 minute ride with no cooldown period.
Everything is better, including handling at hi speed. Always had a slight bounce with OEM yammi pump and twin prop setup, but not now, very stable and solid at speed.
Holeshot seems to be great also, but i only attempted one..
First holeshot with trim full down and leaning over the bars it still hopped out of the water and hit the limiter, and you guessed it, broke another sprag.
Engine flared to 9477 rpm according to vipec log file. Limiter set at 9000.
Interesting that on close inspection the sprags are breaking on de-cell, not accell.
Seems the sprag element locator spring that fits between the inner and outer sprag cages is not strong enough and allows the sprag elements to tilt to the point they lock and wedge between the inner and outer cages from the sudden inertia of a 9000+ rpm rev limiter activation.
Pic of sprag in hub is a new sprag element..
Note elements are in a relaxed position/angle of approximately 12:30. Pic of 2 sprags out of hub are broken ones out of my engine. The one on the left i used a pocket screwdriver to un-lock 2 elements that were wedged between the cages from de-cell inertia.
The one on the right i took halfway apart and am using my fingers to simulate the way it was when i took it out. The elements had rotated to approximately 2-2:30 and rotating that far causes them to wedge and lock.
So the weak point in the sprag is simply the little element locator spring between the inner and outer cages.
At least in my situation it is.. I have never physically broken a sprag element, always this over rotation wedge lock action.
02-21-2011, 11:19 PM #2
So nobody has any pics.. Dammit man... How many know if the sprag actually had broken elements in it or was just rolled forward as mine are.
I actually fixed mine this evening and its as good as new, all it took was a pocket screwdriver to un wedge the 2 stuck elements from the cages.
Going to disassemble one completely and see if i can install 2 of the element locating spring bands in one sprag. they are thin enough i think it will work. If so it should keep the elements from over rotating in the cages.
02-21-2011, 11:34 PM #3
Good investigative work. Let us know how it works out. If more of you that has "broken" sprags before chime in, we can possibly find a solution to one of the reasons the sprag clutch may be breaking, or better yet, malfunctioning.
02-22-2011, 12:19 AM #4
My 2 sprag failures went in a similar fashion, they rolled over like they slipped or something. They both failed in a few hrs of hard off shore riding and were like they just slipped and rolled. Nothing broken. These bothe failed in 3 hrs at 13 PSI boost at 8500 rpm limiter but there were plenty of 8800 to 9000 rpm spkikes because of the ocean run. What we found as well is because I was running a high friction modified sysnthetic oil that this probally also helped the sprag teeth to slip. We are running a non friction modified oil now and the new sprag has lasted 30 hrs but with the motec laucnch control and a few other things trying to be carefull with the new sprag.
A well known engine tuner here has seen this type of failure with the use of friction modified oils plenty of times, it might be a coincidence but the stats point to it.
Sorry I don't have any lying around here, gunna might have one of my old sprags lying around but the failed similar to yours.
02-22-2011, 12:46 AM #5
The oil is coincidence. A friction modified oil would cause a slip on the drive cycle under sprag load when it is locked. This failure is happening on the over run cycle of the sprag, not on the drive cycle.
Have even a better idea than doubling the spring bands.
02-22-2011, 12:54 AM #6
- Join Date
- Apr 2007
Dont want to change the thread but I run synthetic motorcycle oil. My highest spike is 10200 so far I run my rpm limiter at 8900. No sprag failures on Fx 65 hours or Fz 20 hours, but because of the setup I do take care not to hit it from standstill. I think that is maybe worse than hitting the limiter when going wot.
02-22-2011, 04:10 AM #7
Some interesting reading here about the sprag clutches,,,,and some terminology of what is happening . Read the lubrication paragraphs also.
02-22-2011, 04:32 AM #8
02-22-2011, 04:37 AM #9
02-22-2011, 08:04 AM #10
Mine looked the same as well...I have a couple of them laying around...
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