slipping gearbox (b-box)?

[V]

I thought I would ask the forum's wisdom. First a little background: I have a Windlass 503 dcdi with a b-box. Not too long ago I had an engine out. The culprit appeared to be the gear pressed onto the crankshaft: it was floating and slipping. The crankshaft was heavily scored under the gear. There were a couple of other small things wrong and I ended up rebuilding the engine with a second hand crankshaft, pistons, new bearings and a full top-end overhaul. The gearbox checked out ok and was re-installed. The gear in question was pressed on and locked, torqued as specified in the manual.

After completing several hours of ground and flight testing, I experienced the following yesterday:

1. Reduced climb ratio. One up, I found myself going full throttle, 6500 rpm and very slow gaining altitude. Much less than on my previous flight 3 weeks ago and comparable to what I experienced on the last take-off prior to the engine out. Since the rebuild I climbed comfortably at 6200-6300rpm with bar in neutral position.

2. Sudden, slight shudder through the airframe 3-4 minutes into the flight. The best I can describe this as a momentarily loss of thrust that lasts a split second only. If the air wasn't as smooth as it was on Sunday, I would probably not have noticed it. The feel was very similar to that when my engine quit, just a bit more subtle.

We've done the following tests on the ground:

1. Set the hand throttle to run the engine at 6300rpm. Initially the rpm remained stable (+/- 20rpm change - the MGL rev counter operates in 20rpm steps). A few minutes into the test (high operation temp) 80-100rpm fluctuations could be observed.

2. Locked the engine and pulled on the prop by hand. No slippage could be felt, other than a minor backlash from the dog-clutch and the pin we locked the engine with. By this time the engine and gearbox were cool.

Engine mounts and prop bolts appear to be in order. We took off the gearbox, but haven't pulled the gear yet (no puller handy). The oil was clean, no metal filings and other bits visible.

Any ideas? Could the dog-clutch slip (jump) under operating temperature only? What else could slip in a b-box?

Thanks!

Cheers,

V.

[Duck Rogers]

What's your email address so I can send you a pdf file with the correct B Box maintenance procedure. I don't think you've checked everything.

[Duck Rogers]

You have PM

[nickjaxe]

When you say pressed on....on my 503s b-box its not pressed on....just the bolt pulling it on...you didn't use lock tight on the taper did you by any chance they go on dry....and did you re-new the removable gear as I presume its mating face/taper will not be perfect if it had that effect on the crank taper,

Rotax have recently upped the torque setting for the bolt holding this gear on...so I suspect there have been probs with them....I always amazes me that just the friction of the 2 tapers is enough to lock them together.

Nick.

[V]

Hi Nickjaxe,

thanks, the gear went on dry. Do you perhaps have the updated torque setting?

Cheers,

V.

[nickjaxe]

I will dig it out...got a brand new 503 a few month back...all the info came with it on a CD....I think its on line as well...leave it with me,

Did you fit a new drive gear...or was the old one still nice and undamaged.


Nick.

[nickjaxe]

Found it...its in the latest Rotax parts catalogue in table of amendments...torques on the very last page, but first you need to look up the part number for the bolt which I make item 10 part 841656 torque...59ft lbs or 80nm and you will find it listed in Accessories which the gearbox is,

This catalogue is free to download along with all the manuals,

I seem to remember the old torque was 45ft lbs...off the top of my head.

Nick in the UK.

[V]

Thanks Duck and Nick! We torqued the locking bolt to 45 Nm only as per original specification. This is going to be the problem.

Cheers,

V.

[Dobbs]

Hi V, I do not want to down play your concerns, you are understandably concerned after your first experience, and well done to the guys that have offered you the good advice, however, on the engine RPM vs climb performance, if you are getting the RPM, assuming that there is no slippage, and if there is, it will not last long before something breaks - so I am assuming that there is no slippage, so if you are getting the RPM with the same prop as before the incident, you will be getting the same thrust, and if your climb is not as before, maybe different conditions, hotter, lower QNH etc.

[nickjaxe]

Are both mating surfaces smooth and undamaged.

Nick.

[MADDOG]

Be Careful - I don't think you have found the problem.

The co efficient of static friction is more than that of sliding friction. This means that when the taper joint is held together without slipping it can take more torque than when it starts to slip. Once it starts to slip the engine will run away - not just increase a few revs. Once it has run away, the tapers will be blue and clear circumferential lines visible on both surfaces.

When you pull it apart one of two thing will be apparent. either the joint will fall apart - no puller needed to loosen it OR else the joint will be "welded". Huge difficulty to get it apart.

Show photos of both crank and pulley tapers to be sure.

[V]

Thanks - we're going to pull the gear tomorrow, will post photos. I think I have a photo of the crankshaft after my previous incident showing the heavily gouged surface. When I had the engine out, the rpm definitely dropped from the 5300 cruise to about 4500 then to 0 and didn't go up. Now it is still rather a drop, albeit a much smaller one. The engine has been rebuilt in the meantime, the gearbox not.

Cheers,

V.

[V]

We put the gearbox back together. Attached some pics of the drive gears before applying emery paper. Tightened the locking bolt to 80 Nm now. Run the engine on the ground, but weather did not allow a flight test.

The ground test was inconclusive and there might another problem. The engine develops full 6500rpm and after a short while the revolutions fluctuate in the 6440-6520 range. Switching on the auxiliary fuel pump changes this to a 6500-6560rpm range. I'm not sure whether the unstable rpm is significant. However, the change indicates that I might have a slight fuel starvation problem too. I'm going to run tests with pump on and off and check the difference.

The knuckle joint assembly is the newer 12 spring washer configuration. There is absolutely no evidence for any unplanned movement on the joint itself. I think, in a pusher set up it is impossible for the joint to slip without any markings, noise and other evidence.

Crankshaft PTO #1

gear1.jpg (37.57 KiB) Viewed 2464 times

Crankshaft PTO #2

gear2.jpg (42.54 KiB) Viewed 2464 times

Drive gear

gear3.jpg (38.6 KiB) Viewed 2464 times

Cheers,

V.

[V]

As a comparison, this is what the PTO looked after my incident. Even though the shaft is somewhat out of focus on the picture, evidence of the slipping can be seen clearly.

Cheers,

V.

Slip marks on the old crankshaft

pto-old.jpg (33.8 KiB) Viewed 2463 times

[V]

Looks like we nailed the problem with tightening up the drive gear to 80 Nm. Saturday's beautiful pre-frontal allowed a decent test-flight and the trike performed flawlessly. One up with the Aquilla 1 wing it climbed at 6300rpm at approx. 450fpm and at 6500rpm at approx. 550fpm. No shudder.

Auxiliary fuel pump on or off made no difference.

Thanks everyone for the assistance!

Cheers,

V.