aluminium flywheel / ring gear fit and materials

[carbonlife]

In constructing an aluminium flywheel with a steel ring gear, what should be taken into account to determine the inteference fit between the 2 components? I notice that many aluminium flywheels use bolts or pins to ensure the ring gear remains a good fit - is this really necessary (and wouldn't the pins be placed under unnecessary shear, likely to fail)?

The proposed ID of the ring gear is 285mm, the temperatures it will endure are typical environmental temperatures for a passenger car - i.e. from -35 degC (on a very cold day?) to around 80 degC (I'm guessing) but of course the surface temperature of the friction surface could be very much higher......

If assembled with a mild inteference fit, say 3 thou at a temperature difference of 220 degC, then, according to my simple calculations, the inteference fit is still an order of magnitude bigger than the differential diameter change due to cooling at -35 degC. Does that sound right? So why do people pin them?

Should the expansion due to centrifugal force be taken into account? (Up to 8, perhaps 9000 rpm). Seems trivial.

And what are the most suitable materials for the aluminium flywheel and the steel friction surface?

Thanks for advice and suggestions.

 

[evelrod]

About ~20 years ago I had Mueller Fabricators (before Lee Mueller's untimely death) build flywheels with sprayed on surface treatment for wear resistance and a pressed on ring gear for a Lotus twincam engine (Ford). The spray surface treatment worked in the short term but was crack and heat check prone and did not suffer well the Tilton button clutch. The pressed on ring gear continually came off with disastrous results. Design Products Racing machined the face of the flywheels for a steel insert (he used a Skillsaw blade!!!) and bolting the ring gear on was a positive cure for all our problems. No problems after the mods. As far as I know the present owner of the race car is STILL using one of these flywheels! It would have been great to have these 12 " dia. 4.25 lb. units function as designed---it just did not happen. Based on my experience I would reccommend sticking to bolts/pins, as a safety measure, at least.

Rod

 

[evelrod]

Make that ~30 years ago. Time does have a habit of slipping by, just not THAT fast! sorry for the error.

Rod

 

[kradicke]

Fidanza flywheels, which I am a big fan of, produce their flywheels out of either 6061 T6 or 2024 T3 heat treated aluminium with a replacable carbon steel friction surface. They also do pin and bolt the ring gears into place. I have an MGB and a TR6 Fidanza flywheel sitting beside me as I type, and if it wasn't solely for their cost, I would hang them on the wall as art.

For some cars positively securing the ring gears in place is a necessity. On more than a few of my TR6s, the ring gears have worked themselves sufficiently loose that the starter would no longer engage... man it sucks to pull the gearbox just to correct the ring gear's position.

 

[carbonlife]

Thanks for the replies Rod and kradicke. You both report problems with ring gear which is installed by the normal method - and that these problems disappear when the ring gear is pinned. I belive you - But I just don't get it! What's going on to make the ring gear loosen? I can't design something to stay on without understanding why it falls off!

If it's a temperature / expansion issue, it should be at really cold temperatures that the problem occurs. But like I say, according to my calculations, the change in inteference due to differential Al/Fe contraction is about a tenth of the magnitude of the designed inteference fit......... So it shouldn't move - designing a lip on the oouter edges of the flywheel (or a lip and a taper) could help keep its position if it's a centrifugal expansion issue (higher density of the steel ring gear - it will experience a greater expansion than the flywheel at high rpm) - but do you think that this is the issue?

Or is the issue to do with how these aftermarket flywheels are assembled? - I'll be using a heated press which certainly gives a greater control over assembley than heating and then quickly transerring a hot ring gear and positioning it in the cold press.

From your experiences it sounds like bolts / pins are needed but I don't see why. Is it really centrifugal expansion ? Any more ideas about the design criteria?


I had a look at Fidanza's site but it didn't tell me very much......

http://www.aluminumflywheels.com/flywheels.html

I notice some of the flywheels are anodised though. Do you think that's a good idea for preventing corrosion?

 

[evelrod]

I have had time to think about some of the problems that 'might' be causing the ring gear shift. On the Lotus twincam, MGB, TR-6, et al, the Lucas/Bendix can either hammer the ring gear on or off depending on which side of the gear the Bendix drive is on. Both types of Lucas starter motors have been in use for many years. On these three examples using a stock type clutch cover/pressure plate the mounting bolts often do double duty by also threading partially into the ring gear. Possibly, since my units were racing only and VERY light, some degree of flex along with expansion due to the high rpms these engines attained on a regular basis could be the culprit, at least in part. This much I do know---steel ring gears will come off lightweight steel flywheels just as easily as aluminium. My current ride, a 1963 Austin Cooper vintage racer with a 1380cc engine and extremely light steel flywheel has the ring gear WELDED to the flywheel. Should ANY problem arise, I'll scrap the whole thing. The car is right hand drive and guess where that puts the flywheel in relation to the driver!!! On one engine one of the local racers built a special aluminium flywheel with machined in ring gear teeth. Very light but also very expensive AND fragil.

Rod

 

[kradicke]

As Rod stated, the Lucas starters tend to hammer off the ring gears... which on one of my TR6s was definitely the problem, as that car was solely used for street driving (back and forth to my office). All that hammering probably took its toll on the starter as well, because about 200 miles after I fixed the ring gear, the starter itself failed.

Other considerations, with the TR6 at least, is that the flywheel deviates from its at-rest plane by as much as 1/2" when running above 7500RPM. The crankshaft flex is horrible, and I can only imagine what that does to a flywheel when combined with the help of the Lucas starters in loosening the ring gear.

 

[GregLocock]

I like the suggestion of crankshaft bending - the gyroscopic forces from that on the ring gear would be huge. Let's see roughly speed= 7500 rpm=125 rps= 750 rad/s, acceleration=750*750*.006= 300g or so (That is not the way to work it out but it should be in that order).

At that loading, at that speed, low cycle fatigue of the interface could be an issue.

Also, a 3 thou press fit might be enough to locally yield the aluminium, so you don't have the interference you thought you had. What is the push-off load?


Cheers

Greg Locock

 

[patprimmer]

I have had a shrink fit ring gear on an aluminium flywheel on my boat with a 350 Chev since 1970. No problems

It appears to fit against a shoulder in the aluminium, so the ring gear is pressed towards the shoulder as the starter engages.

Also, being a boat, there is no clutch on the flywheel, and as it has not corroded, I presume it is marine grade alloy.

I expect that problems can arrise if there is no supporting shoulder to resist end loads from the starter engagement. I also expect that clutch heat will expand the flywheel a lot more than the ring gear, causing yeilding by the increased interference, exasivated by the heat reducing the compressive strength of the aluminium.

Also, I believe that unless very well heat treated, the silicone in aluminium continues to dissolve into the aluminium during thermal cycling, causing the aluminium to shrink. At least that is how a metalurgist once explained it to me when I had a project that required aluminium parts with exceptional dimensional stability, while undergoing repeated thermal cycling.

Bottom line, machine the aluminium with a shoulder, carefully calculate the interference at room temp and at say 300 deg C (or whatever a clutch is likely to reach), consider the compressive strength at that temp for the alloy you use, and use a heat stable alloy that has been heat treated to the extent that it will shrink no further.

I might still have details of the grade I ended up useing successfully, but the info will be buried dep if i still have it. The bar stock i used was over twice the price of the normal T6 high strength grade.

Regards
pat

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[carbonlife]

Aluminium yielding by plastic deformation........ Well clearly increasing the inteference fit isn't going to help here....... and all the other effects such as vibration (harmonic or otherwise) and hammering from the starter will only speed up the process. If this is principally what's happening, I'm surprised that pinning the flywheel to the ring gear offers such an improvement. Wouldn't the holes tend to deform and the pins become loose? The holes / pins are not subject to centrifugal expansion and this inteference fit would of course reduce with increasing temperature but surely the aluminium walls of the hole would be subject to similar forces compared to the flywheel itself. And the pins will be taking a lot of shear force..... would have to make sure they stay within elastic range and preferably relieve them at the flywheel / ring gear interface to ensure no fretting / local stress riser and to increase elasticity.

Pat, your comments about the dimensional stability and compressive strength are well taken. If you trip over the material spec of the aluminium you used, please let me know. In the meantime I will have a look at the compressive strength of various alloys and see what theoretical yield there might be at the temperature cycle and dimensions I'm using. Can anybody tell me where data on shrinkage due to thermal cycling might be found? And how much heat treating improves this?

 

[idano]

Flywheel ring gears are a shrink fit onto the flywheel....whether Al or steel...and butt against a register.

Flywheel ring gears are not perfectly concentric before they are shrunk on the flywheel.

For the dimension parameters you might call Design Products (714)892-1513 or McLeod industries in Anaheim, Ca USA.

it is a bitxx of a job....wheel in the freezer for 12 hours and CAREFUL heating of the ring gear with a rosebud tip. Too hot and there goes the ring gear. About .015"-.020"

Three or four button head screws with red loctite only serve to keep things in place just as you would safety wire,clip,and clamp everything used for other than driving to and from the supermarket.

 

[evelrod]

idano, your new here so you may not have picked up on the fact that Don did my flywheel and he bolted the ring gear on. Perhaps on certaain engines and applications the standard shrink fit may work indefinately---NOT on any race engine I have ever built. Like I said, it's cheap insurance.

Rod

 

[idano]

Rod:

Copy the "bolted the ring gear on"....kinda rewriting my last paragraph....

I always use screws when using a iron or alloy wheel for other than driving back and forth to the supermarket. (grin)

its just common race prep.....(not an option).

 

[PrecFab]

Rod - I have a Muller Fab Aluminum flywheel still on my TR-6 racecar that hasn't been run since the mid 70's, but the last time I had it out I noticed some conical wear (center worn more than outside) and was wondering about the special spray treatment that these had. I don't remember how much wear, but it was significant. I presume worn through the coating. Is it possible for me to resurface and recoat or is it best to make a steel insert? I never experienced the ring gear working loose, but also can't remember how it was attached (30 years!). It was one of the best performance adds I did to the engine and would like to keep it...assuming I ever drive the car again. PS - When did Lee pass on? He was quite a character.

Jim

 

[evelrod]

In a nut shell, Don Oldenburg at Design Products Racing of Costa Mesa CA (714) 892-1513 can insert the Mueller flywheel as he did mine years ago. It is a simple operation that only requires a re check of balance. Don does excellent work and has my highest reccommendation.

Lee developed heart problems after a trip to Japan and required a heart transplant (Carrol Shelby and Lee were at Cedar Sanai while I was doing some work there). I don't know the cause of death which was some years later but he survived the transplant for years. His son John was still racing with Cal Club last I looked.

Rod

 

[MaxRaceSoftware]

while dyno testing a Chrysler SS Hemi engine
the flywheel ring gear loosened to a point where when we
went to start it to make another test , the starter just spun the ring gear not the engine

we had to pop the engine off the dyno and drill / tap for
(8) 1/4 inch bolts where i tig'ed welded the ring gear to
bolts

then a few weeks ago with a steel flywheel ..same thing
i was testing a NHRA A/ND Nostaglia front engine SBC 393 cid engine to 9400 RPM , and ring gear loosened and walked
backwards ....started hitting the starter bendix gear / sparks at very end of test

what was pretty strange ..i expected the ring gear to be loose ?? ...but i had to heat up and drive it back in place with hammmer blows ...so we wound up tig-welding
8 spots ring gear to flywheel

these are acceleration tests and i guess theres enough ring gear inertia at 9300 to 9500 to loosen ring gear as i either snap the throttle back closed at that RPM limit or the SF901 grabs the engine and tries to bring it back to starting RPM

Larry Meaux (maxracesoftware@yahoo.com)
Meaux Racing Heads - MaxRace Software
ET_Analyst for DragRacers
Support Israel - Genesis 12:3

 

[MaxRaceSoftware]

http://www.maxracesoftware.com/Spot_Tig_Aluminum_Flywheel.jpg

that pic link shows starter ring gear spot tig welded to 1/4 NC button head bolt.

its not pretty, but a quick fix so we could resume dyno testing

the 1/4-NC button head bolt was installed after drilling and tapping for it in the aluminum flywheel

this was a Weber aluminum 8-Bolt Chrysler Hemi flywheel

Larry Meaux (maxracesoftware@yahoo.com)
Meaux Racing Heads - MaxRace Software
ET_Analyst for DragRacers
Support Israel - Genesis 12:3

 

[liampope]

Hi guys.

Just stumbled in here looking for information on aluminium flywheels. Some very useful posts so far. I'm building an aluminium flywheel (in 7075-T6) for a custom car project and am trying to finalise my ring gear attatchment method. I have a few queries I hope some of you may be able to help with.

1. I had intended to have the ring gear shrunk on and use the clutch bolts to hold it in place (as well as holding the clutch on of course). But looking at it closer, the clutch bolts are m8 (6 of them) and drilling/tapping such big holes into the ring gear will leave it quite thin around the holes. I haven't done any calcs but it seems possible the ring gear may want to fall apart at high speed being so weakened. What size bolts/pins do people use to bolt on a ring gear, and how many?

2. Is it necessary to have a shrink fit if you're going to bolt the ring gear, or would an exact/interference fit be ok?

3. I'm a bit unsure about the terminology used in this thread. When people mention a '0.015" shrink fit', for example, does it mean the flywheel shoulder OD is machined 0.015" larger than the ring gear ID at room temp? Or is this the difference in diameter when the ring gear is heated ready to be fitted? Or are we talking radiuses or something else entirely?

4. Bearing in mind I will be bolting the ring gear on too, would a sufficient shrink fit allow the heated ring gear to drop onto the flywheel and grip as it cools, or would you always need a press? Any idea what sort of difference between flywheel and ring gear diameter I would need to achieve such a fit?

Right, that should do for now. Thanks for your time guys.

Liam