Mini-500 MRGB Thoughts/Findings/Solutions

[bryancobb]

Here's the starting point and my goal.

Starting point is...

I have two complete Mini-500 gearboxes. One is a brand new, unmolested, never spun under load, factory assembled one. This one is an early Alpha that was shipped with Kit S/N 117. I assume it is identical to the one I installed in my other Mini-500 in 1997 since it was S/N 129. It is well known that factory gearboxes assembled during this time-frame had the preload set way high and they ran hot. Also the pinion has a 4-pack of bearings, one of which is an angular contact bearing (rearmost one). This setup was problematic. All of my 100 hrs of Mini-500 time was on helicopters that had this setup and I had no problems except the case got a little warmer than I would have liked.

The second one is out of my Texas parts helicopter. It was originally shipped in the kit (S/N 417) which was a factory Bravo model that was originally shipped without the Mast Support Upgrade. Mr. Dave Mealy, the original builder, flew it for 152.9 hours between March 1998 and Feb 1999 when he modified it per the Mast Support Upgrade. Between then and 10-20-2002, he flew it to a TTAF of 226.6 hours in the western PA area. Now there is 245.4 on the Hobbs and I have no idea about the circumstances surrounding those 19 hours on it. Bravo gearboxes had the correct preload and ran a lot cooler. I suspect it still has the pair of dual-row, angular contact bearings on the pinion. I have the complete maintenance records and for the first 153 hours, logbook entries are almost zero. Because of the frequency of entries in the flight log, and the nature of the flights (many short cross countries) with no long gaps, I can assume this gearbox was trouble free because the only logbook entries are lubricant flush and changes. After he did the Mast Support, logbook entries were very thorough and include several AD c/w entries were made. I feel like he flew this gearbox for the whole 227 hours without any major problems. The scab on the frame tube tells me that his frame DID crack like Alphas did, at about the 153 hour point, and then he purchased the expensive Mast Support Upgrade and installed it.

 

[StanFoster]

Bryan- On gear box flushes.....my first TR gearbox dump was after 5 hours. I was alarmed to see the oil looking like it had metallic paint in it. I mean I was expecting a little, but not so much. I called the factory and was told it was normal, and it might take 40 hours to clear up. After that I changed the TR gearbox oil every 10 hours, and each one became clearer. I still dump my oil every 10 hours, as it only takes just the bottom 1/4 of a very small plastic cup.

Now my main transmission was totally clean each oil change.

Was this the experience you had with your Mini-500 TR gearbox?

Stan

 

[bryancobb]

My goals are...

* A safe gearbox that is properly set-up
* Properly manufactured ring and pinion gears, with documentation to prove it
* A trouble free, properly engineered team of bearings with paperwork from Timken and Fafnir that proves it
* Assembled with all the Mast Support Upgrade modifications.
* Case drilled and tapped for a temp sensor so that the EIS engine coolant channel can be used to warn me of MRGB overtemp
* Built from as many brand new, never used parts as possible.
* End up with a serviceable spare gearbox for reserve or to sell to whittle down how much I am "into" my new helicopter.

My next goal is to "forensically" disassemble both gearboxes and compile the autopsy findings so that it can help any other Mini-500 people, gain more understanding myself, and perhaps answer the "proper heat-treat or not" question.

I'm certain the new one has never been touched or disassembled. I'm pretty sure the Texas one has never been disassembled and has only been "touched" as needed to c/w the Mast Support Upgrade.

 

[bryancobb]

Thanks Stan,

My 1997 Mini-500 gearbox ran pretty warm. I could NOT leave my hand on it immediately after a flight. The factory told me that was "normal" so I believed them and kept flying. I got chip lights for the first 10-20 hours and then it stopped. I changed the oil once, very early, maybe 5 hours? (whatever the factory advised) and then every 25 hours, with either factory supplied "Revlube" or Mobil synthetic...I can't remember which.

I remember some talk about the old-style 4 pack of bearings having "issues" with the cages coming apart because of the "hammer" effect when abruptly going from engine-driving to coasting autorotation. For that reason, I never rolled off the throttle quickly, even during the sprag-check before takeoff and during shutdown. This may have helped, maybe not? But I never had ANY issues with the MRGB other than shreds of a cogbelt ruining the pinion seal once.

 

[bryancobb]

I am going to attempt to get Arrow Gear or their design counterpart, Gleason Gear to accept both sets of my gears and do a battery of non-destructive tests on them, and provide paperwork documenting dimensions (new and worn), hardness test results on base metal and surface hardening on the applicable portions of the teeth, and Nitaletch results showing surface hardened depth on new and worn gears.

If they won't do it, I WILL find someone who will. The paperwork is very important to me here.

 

[StanFoster]

My gearboxes are all felt after each flight, and the TR gearbox even in the summer can have my hand held on it. .....its very warm , but not hot.

I am ashamed to admit that I still after 3.5 years of flying that I don't have telatemp stickers on it.


Stan

 

[bryancobb]

I have ALWAYS feel every gearbox when I first exit the helicopter after a flight. That was not so easy on a Huey or Chinook.

I don't remember any T/R boxes running very warm, but MRGB's seem to all heat-up. Do you see any problem I don't see about using the EIS engine coolant channel for the MRGB?

I feel the upper mast bearing is getting adequate oil supply but I am going to confirm that. It should be easy to see with a borescope when a bench grinder motor is used to spin it up to 550 RPM.

 

[Vance]

Some thoughts on gearbox temperature

Some thoughts on gearbox temperature

I have been fortunate to work with some very smart gear people and I will try to share a little of what they shared with me.

Energy can be changed in form, it does not just go away.

I have been told that the number to use for calculating such things is two percent per set of meshing gears. Right angle gearboxes tend to be slightly less efficient; sometimes as high as three percent.

The design and surface of the gears can affect the efficiency slightly but two percent of the power being transferred is still generally a good number to start with. Very little heat is produced if no power is being transmitted as long as nothing is binding.

In this application if there was 50 horsepower running through the main rotor gearbox then it produces a little over 2,500 British Thermal Units of heat per hour. If the output for the tail rotor gearbox was a part of the main rotor gearbox it would add a little over 250 BTUs per hour to the assembly if five horsepower was being transmitted by the gears.

When running 5 horsepower through the tail rotor gearbox it would probably be making a little over 250 BTUs of heat per hour.

In my opinion because everything is trying to become equal the surface of the gearbox cools the oil more the hotter it gets but does not cool very well unless the oil is flung against the surface because the cooler oil on the surface tends to insulate the surface. I have found temperature felt on the outside of the gearbox is significantly cooler than the oil.

I have found running the oil through an oil cooler will reduce the temperature in the gearbox.

If things are nicely designed the gearbox will run at a temperature that the lubricant is well suited for.

Most gear box oil is still happy at a little over two hundred degrees F (100 C).

Most hands touching something over 140 degrees for five seconds will sustain a third degree burn.

If some part of the gearbox is not working correctly it becomes less efficient and makes more heat.

Thank you, Vance

 

[bryancobb]

I have been fortunate to work with some very smart gear people and I will try to share a little of what they shared with me.

Energy can be changed in form, it does not just go away.

I have been told that the number to use for calculating such things is two percent per set of meshing gears. Right angle gearboxes tend to be slightly less efficient; sometimes as high as three percent.

The design and surface of the gears can affect the efficiency slightly but two percent of the power being transferred is still generally a good number to start with. Very little heat is produced if no power is being transmitted as long as nothing is binding.

In this application if there was 50 horsepower running through the main rotor gearbox then it produces a little over 2,500 British Thermal Units of heat per hour. If the output for the tail rotor gearbox was a part of the main rotor gearbox it would add a little over 250 BTUs per hour to the assembly if five horsepower was being transmitted by the gears.

When running 5 horsepower through the tail rotor gearbox it would probably be making a little over 250 BTUs of heat per hour.

In my opinion because everything is trying to become equal the surface of the gearbox cools the oil more the hotter it gets but does not cool very well unless the oil is flung against the surface because the cooler oil on the surface tends to insulate the surface. I have found temperature felt on the outside of the gearbox is significantly cooler than the oil.

I have found running the oil through an oil cooler will reduce the temperature in the gearbox.

If things are nicely designed the gearbox will run at a temperature that the lubricant is well suited for.

Most gear box oil is still happy at a little over two hundred degrees F (100 C).

Most hands touching something over 140 degrees for five seconds will sustain a third degree burn.

If some part of the gearbox is not working correctly it becomes less efficient and makes more heat.

Thank you, Vance

Thanks 10,000% for that Vance! That corroborates my experience in 1997 that the outside of the gearbox case was too hot to lay your hand on but I had no gearbox problems.

The idea that it was running too hot at that temperature got planted in my mind when Fetters (the designer) said in several posts all over the internet, that the early MRGB's from S/N 001 to about 190 or so, had the Timken bearings on the mast installed with too much pre-load. They started lessening the pre-load and those folks said case temps were noticeably lower. I never did that to mine.

 

[Vance]

Back to Basics.

Back to Basics.

Your welcome Bryan.

I have found that sometime going back to the most basic principles helps to clear things up.

In my experience even over tightened tapered Timken bearings don’t produce a lot of heat.

To reduce movement on a Harley Davidson Sporster crankshaft we used to run the tapered Timken opposed bearings tighter than the factory recommendation. They had a Timken supplied spacer between that we would have Blanchard ground to have multiple choice for how tight the bearings were.

I would often go to races without first starting a reassembled engine. Jim Belland who used to build the engines always wanted to let me know after I had started the engine because either they would seize quickly or he had set up the bearings about right. If they were set up “about right” there was very little indication of wear.

Jim told me if they were too tight they would seize quickly and turn blue from the heat. He handed me an example so I could see what it looked like. None of mine ever seized.

The point of this story is; if your tapered Timken bearings have not seized I would look elsewhere for the source of the higher temperatures in the gearbox Dennis reported.

I raced a Moto Guzzi Lemans for three seasons that had a shaft drive and a hypoid gear to transmit power to the rear wheel. I set up five rear ends so I could change the final drive ratio quickly at the races. I set them up as carefully as I could and yet no two rear ends ran at the same temperature even though they were roughly transmitting the same horsepower. The difference was perhaps 30 degrees and we could not find a trend based on ratios or gear finish.

Regards, Vance

 

[bryancobb]

...The point of this story is; if your tapered Timken bearings have not seized I would look elsewhere for the source of the higher temperatures in the gearbox Dennis reported...

OK, that's good information. A lot about what approach I will take is going to depend on how the 240 hour bearings look when I get the used gearbox apart. I will post pictures.

Then I will compare the preload amounts on the old vs. new transmissions, realizing that the 240 hour one has probably "loosened-up" because of wear.

I wish I had an OEM unmolested, new Bravo box around S/N 350+ to disassemble and measure but THAT ain't gonna happen.

This is the one area on this helicopter that I will make absolutely everything has passed muster with Timkin engineering, Fafnir engineering, and Arrow Gear or Gleason Gear design folks.



Heat treat and hardness specification from Fetters' factory drawing he posted here a couple of years ago

ALPHA Configuration (What's inside my NEW box and the arrangement I flew for 100 hours in 1997 withot problems)

BRAVO Configuration (Probably what's inside my 240 hour box)

Proposed VOYAGER Configuration (some Mini-500 owners did this upgrade themselves)

 

[bryancobb]

Latest & Greatest

Latest & Greatest

I don't know the purpose of this drawing because it had no Title Block, but it appears to have the case alignment tubes that were introduced at Mast Support Upgrade time. This is absolutely needed because rotor loads at the top of the mast tries to tilt the mast which makes the two case halves move around no matter how tight the bolts are. That pinches bearings and changes gear mesh so my helicopter WILL include them.

The tapered Timkins on the Pinion leads me to believe that this is a Voyager 2-seat gearbox?

 

[bryancobb]

I want to see with my own eyes how much splashed or slung oil is getting to the upper bearing. If I conclude it's not enough, I will probably incorporate an oil pump of some type to supply cooled, filtered oil to the upper bearing and right onto the gears where they are about to mesh.

I don't want to do this because I want a ship that is as stock as possible, but if indicators say it's needed, I'll do it.

 

[Blue Chips]

Bryan,
On the heat treat specifications you posted a few post back, do you know how close they may be to say Robinson gear specification?

Ken

 

[bryancobb]

Ken,

To answer your question...I don't.

But knowing Fetters, and looking at an R-22 Illustrated Parts Breakdown, I can clearly see that a large percentage of then Mini's parts are part-for-part, an equivalent similar part, serving the same purpose.

I suspect he got his hands on drawings from a set of R-22 and just followed that trail. I also know that ring/pinion design, hardening, and tooth geometry has not changed for decades and there's enough known about the field and enough specialized software that Gleason Gear (Dennis said this was who designed the Mini gears if I remember right) could "knock out" the design and a set of drawings in a couple of hours.

I would trust my life to that printed spec. and feel pretty certain it came from Robinson/Arrow/Gleason.

 

[Blue Chips]

He could have done a lot worse then mimic Robinson, why reinvent the wheel.

That is certainly the proper steel to be used.

I am definitely interested in seeing how you make out with the non-destructive test of your gears and how they compare to the specs.

Thanks for the reply.

Ken

 

[bryancobb]

Ken,

Do you or did you have the Timkens on your pinion?

 

[Blue Chips]

Bryan,
Have never owned a Mini-500.

 

[bryancobb]

Don't know why... I THOUGHT you were one of the 4 that were flying buddies with Don Fairbanks?

Sorry.

 

[NoWingsAttached]

Do you know of any Mini-500s for sale? I am in the market. I think there might be one in Ft Lauderdale, do you know anything about that one? No big secret as to my intentions for buying one... Seems like a suitable platform for a Mohawk GT4 engine with sufficient mods and upgrades.

Living so close to you, that could be a plus for both of our projects. I can fly to Cartersville from CTJ at slow cruise in about 35-40 minutes, I think.

You should have come to the Peach State meeting December 7th in Rome, we missed you...again.