Gearbox Modification.

[gidge348]

I am not sure this fits into a DIY project, but I hope this is of interest to some people.

I have had a fairly low hour Nimbus engine for a few years and have been trying to work out how to make it work in a streamliner salt lake racer. This may take some time to load all the posts, so please be patient.

As those familiar with the Nimbus will know that the power output comes out underneath and faces forward with a reduced output speed of a governed 2250 rpm.



In a streamliner this poses a lot of problems, you want the drive to the rear, as small a frontal area as possible and wheels need to do between 4200 & 4800 rpm so stepping this up from the 2250rpm causes huge torque loads on the drive line.... All in all a fairly bad layout so that is the first thing I need to do something with.

I have gone through probably 30 different layouts and thought I have it all sorted until I got onto this web site and had a few extremely informative conversations with John Wallis who I owe a great deal of thanks to. The plan ""was"" to drive directly off the FPT along a drive shaft into a speedway type quick change diff where a total 8.5:1 reduction would take place to reduce turbine speed of 35K to a wheel speed of 4200 rpm. I had seen a similar arrangement on the net where someone used a speedway quick change as the mast gearbox on a small helicopter.



So I merrily ordered about $1000 worth of bearings, machined down the FPT gear to suit the Greek coupler and though that was all fine. After talking to John he suggested I do some calculations for critical shaft speeds and sure enough the drive shaft would have made a beautiful explosion long before the top speed was achieved....... Bugga!!!!

I have since gone back to the standard Nimbus gearbox to see what could be done.

[gidge348]

So I disassembled the Nimbus Gearbox to see what I have.





As you can see the gearbox has a 2 stage reduction, the first reduction has 19 tooth pinion and 85 tooth first stage reduction 4.473:1 ie 35,000rpm to 7800rpm (a reasonably manageable speed), obviously turning the wrong direction but I will get to that.

The free power governor pinion (spur gear number 13 on the drawing) turns aprox 600 degrees to 360 degrees of the first stage reduction gear, then in the governor it is stepped down by a 27:46 tooth sun gear (about 1.7:1). So 600 degrees divided by 1.7 = 352 degrees therefore the governor shaft spins at close enough to 1:1 with the first stage reduction, once again more on that later.

Now I have a pickup point and a 1:1 governor speed the only problem is bottom of the gearbox is way too long and would be under ground in its standard form so I need to remove the redundant section,





for the part to be removed. The two bolt up faces are flat & parallel so I was intending use a piece of 12mm mild steel plate as a jig and bolt the front to one side using all the mounting points and the opposite side of the same jig for the rear so theoretically they should line up & be straight. Note the I am not changing the relationship between the pinion and first stage reduction, all I am really doing is moving the sump.

A the moment the first stage reduction also operates the piston for the torque sensor, unfortunately this will need to go, but I will use all the standard mounting points and bearings. Currently the first stage reduction is held radially by 2 roller bearing and deep groove ball bearing in the piston of the torque sensor to handle axial loads.




The only parts that need to be manufactured are numbers 13,15 & 18 on the drawing, everything else is as it came from the factory other than cutting splines in inside of the gear (see second gear assembly). There is a shaft in there at the moment but it only held by a slot at the end of the shaft and only 12mm dia.

The designed shaft at the spline is 32mm dia at the root & 35.5mm dia at the tip, it is also positioned directly under the tertiary stage gear that would normally take all the torque to the rest of the gearbox. I have done a quick FEA analysis of the shaft theoretically?? it looks strong enough. More on that later.....

Lastly the diff, I have checked and can get a 3.89 reverse direction ring and pinion and housing for the 8 3/8 winters quick change. Because the drive shaft and governor will spin at the same speed I will mount it on the rear diff cover

The problem is that depending on the tyres the wheels will need to spin at between 4200rpm and 4600 rpm. I can change ratios to get a final drive of 1.8: 1 all the way to 7.95:1 so plenty to choose from but if I need the wheels to be doing say 4200 rpm the pinion will be doing 3.89 times as fast ie over 16,000 rpm........AAAAAARRRRRGGGGHHHH back to silly speeds.

Any way more later in this saga

Cheers
Ian...

[Johansson]

Wow Ian, just reading about it makes my head spin.

I can but applaud your effort, good to see another gas turbine land speed project coming to life!

Cheers!
/Anders

[cursorkeys]

Great to see what you're up to

Making my head spin too. So your new first stage reduction shaft is fine it's just the diff that won't be able to take the input speed?

Cheers,

Jon

[turbochris]

how are you planning on splining the gear? This will make the nimbus power turbine a lot smaller and closer to useable rpm.

[ozbooster]

Hey Ian thanks for linking me here and Hi everyone
Looks like it will work if you can reverse the drive direction
chasing overdrive transmissions for my liner came up with a few options that may help
there is a drive reverser gearset, not sure if it would cope with the torque , will try and find the link, but i was pointed to it by Nova racing transmissions in the UK
My end result with the liner was to use a B&J transmission (planetary used in drag racing ) i have contacted Mike to see if a custom gear set could be made in ~6:1

Interacting here should fire me up to drag my Nimbus out of mothballs , now i just need a few more months in a year

[gidge348]

Any way, back to the saga..... and sorry for the information overload and head spinning mentioned

Firstly where I left off, and the pinion spinning at 16,000 rpm well John came to the rescue again and after measuring the pinion bearings:




He cross referenced the dimensions and came up with rated speed of 11,000 rpm.... well that's close enough for me may need some improved oiling for heat rejection, but that can be worked on later.

Then I remembered a conversation I had with Winters (the diff manufacturer) in regards to their monster 3000hp diff and he said that they have had no problems running 350mph ++ regularly with no oil coolers, pumps or anything and 2500hp+ nitro motors. They have a similar setup to mine (bigger of course) and they use either 2.0 or 3.08 ring & pinion and similar tyres so..... 350 mph means tyres doing 4900 to 5400 so their pinion speeds are a minimum of 10,000 rpm up to over 17,000 rpm........ any way still looking good.

For those not familiar with a speedway type quick change here is simplified pic.






As you can see in a lot of ways it is a lot like a normal diff facing the opposite direction, drive comes in through a shaft at the bottom into two spur gears in the back covered by a plate and then into a ring & pinion. So to change ratios all you do is drop off the rear cover put in a couple of new gears and off you go. This layout also helps with another problem, going the wrong direction. To reverse direction you simply put the crown wheel in from the opposite side with a small amount of machining on the housing. So that's the change of direction sorted.

Next thing is to start ripping the gearbox apart to do some accurate measuring.

[gidge348]

Ok, first thing pull the torque sensor apart.

1. Hang the gearbox from a rope so I can get to all sides.




I have taken off the pressure sensor mounts and all the bolts on the cover.

2. The steel item with the "C" shaped grove is the actual piston that has a range of 15mm from full in to full out and is on a bearing in the centre and brass piston rings on the outside.




This makes it difficult to grab hold of.

3. This is a bit brutal but did not leave too much of a mark



4. Then to extract the bearing I needed to make a puller.




5. Then cut some lock wire and remove 8 bolts





6. Hey presto drive shaft





7. The "drive shaft is held in by a cross key on the opposite end on the shaft, lock plate & nut.




The shaft itself is way too puny to hold any real torque and a single key at the rear wont do it either. Any way sorry about the crappy pics and more on the saga later.


Ian...

[gidge348]

Oct 2, 2013 5:09:56 GMT -5 cursorkeys said:

Great to see what you're up to

Making my head spin too. So your new first stage reduction shaft is fine it's just the diff that won't be able to take the input speed?

Cheers,

Jon

Hi Jon, my original input shaft has been machined down to fit the Greek coupler (second pic first post) now I know that the drive shaft I was intending to use will blow up I need to revert to the standard shaft and gear so that is why I needed your shaft and thank you very much for that. Without your help as well I would be stuck.

ps. I have fixed the damaged thread now and all is ok,

Thanks again for your help

Cheers....
Ian...

[turbochris]

Make a new shaft w good thrust bearings. Then spline the gear w edm or something.

Are there stops on the torgue meter piston? I probably smashed the MAX stops out a long time ago....

[gidge348]

Oct 3, 2013 10:17:02 GMT -5 turbochris said:

Make a new shaft w good thrust bearings. Then spline the gear w edm or something.

Are there stops on the torgue meter piston? I probably smashed the MAX stops out a long time ago....

There don't seem to be any stops on the piston I think it just relies on compression of the oil to stop it going too far. Worst case the secondary reduction gear would rub on the casing. The rings show a small amount of wear but the cylinder shows only maybe 1-2 mm movement total.



The bearing that runs in the piston is a thrust bearing so I was intending to use that, it handles the thrust now and I think the engineers at Rolls Royce are a lot smarter than I am I am trying as much as possible with this project to use as much original factory stuff as possible. Firstly because I am stingy and secondly because it works now and I hope it will still work in the same environment.

I was intending to have the gear spline cut with EDM as a friend of mine has one of these, I am not sure if it will work as the uncut area forward of the spline in narrower.





I think I may have to have these broached in I have spoken to a machinist that says he can do it.

[gidge348]

Oct 2, 2013 16:50:30 GMT -5 ozbooster said:

Hey Ian thanks for linking me here and Hi everyone
Looks like it will work if you can reverse the drive direction
chasing overdrive transmissions for my liner came up with a few options that may help
there is a drive reverser gearset, not sure if it would cope with the torque , will try and find the link, but i was pointed to it by Nova racing transmissions in the UK
My end result with the liner was to use a B&J transmission (planetary used in drag racing ) i have contacted Mike to see if a custom gear set could be made in ~6:1

Interacting here should fire me up to drag my Nimbus out of mothballs , now i just need a few more months in a year

Hi Greg, thanks for posting hope we can both get turbines out to Lake Gairdner....

I am not familiar with the B&J Transmissions but I have seen inside a Lenco and assume they would be similar they look like a nice compact unit. What sort of cost are they?

With the 6:1 gear reduction, I assume you are thinking of running that off the back of the FPT shaft? Are the gears in the planetary straight or helical? The reason I ask is that the standard Nimbus shaft has 2 bearings 1x roller and 1x ball (not annular contact) and no thrust bearing. Most of the "push" from the gas generator on the free power turbine is handled by the 19 tooth helical pinion gear pushing on the first reduction gear which counters this by pushing into the torque sensor. It is kind of a really delicate balancing act.

If the B&J gears are helical I am sure they can sort this out, if not you may need to find some type annular contact or thrust bearing that can handle the 35,000 rpm.

[gidge348]

Back to the saga....

Next thing is splining the gear, and picking a steel to make the drive shaft out of. Well this turned out to be an interesting exercise in itself. One thing I found out was that different steels and different hardness's "Don't play well together".

When I destroyed my first FPT shaft I just put it in the lathe and buzzed away so I know it will machine. It is hardened but very lightly. I tested the gear area to be splined with my trusty drill and no problems drills fine.

The only problem is that I do not know the exact chemistry or hardness of the area to be splined. So I thought to be safe I will drop it down to a local lab and they are going to test it. It costs $300 but I feel it will be worth it to make sure I make the shaft of the same materiel and temper.

Ok time to start making something.


Time to turn big bits of steel into swarf...



Partly done,



I don't have an indexing wheel to drill the holes so I will go out to see a friend of mine and bludge a favour.

One last thing I thought I would put the large final drive gear on some scales and see how much weight I am taking out



17 Kg that will not be spinning around the place. I guess this will mean quicker acceleration, but I will need to fairly spot on with governor to prevent an over speed?

I would be interested in what others might think of what may happen with all this weight removed?


More later.

Ian.....

[racket]

Hi Ian

Great to see the Build Thread progressing .

You could always use a turbocharger type thrust bearing for 35,000 rpm , all it needs is a bit of pressurised oil and some tapered lands.

Cheers
John

[gidge348]

Oct 3, 2013 23:45:55 GMT -5 racket said:

Hi Ian

Great to see the Build Thread progressing .

You could always use a turbocharger type thrust bearing for 35,000 rpm , all it needs is a bit of pressurised oil and some tapered lands.

Cheers
John

Thanks John, that may be the solution to Greg's idea for a planetary box.

Cheers
Ian...