Lotus Elan

Gear Box Reliability

PostPost by: rgh0 » Wed May 19, 2004 9:55 am

Recently I have become aware of an ongoing problem with gearbox reliability in high HP racing situations due to a failure in one of my own boxes and discusion with suppliers revealing this failure is relatively common and I would like to know what others have experienced in this area and if they have developed any solutions.

The problem appears to be that the layshaft which is a light press fit into the gear box casing becomes loose under high load conditions due to elastic distortion of the iron gear box case. This results in the hardened steel shaft fretting on the iron casing and the resultant wear means the press fit becomes loose. This then leads to greater vibration of the layshaft with the locating shoulder on the end of the shaft which prevents the shaft spinning rapidly chewing out the sides of the locating groove in the tail extension housing. Once this happens the shaft is free to spin and this rapidly leads to enlarging further the locating holes for the layshaft and loss of alignment between the layshaft gears and input and output gears on the first and third motion shafts. In my case this ultimately affected the ability to select gears and resulted in brocken syncros.

The problem appears to occur more in Quaiffe gearset boxes than standard but this may be just due to people with big engines using Quaiffe gearsets. However the straight cut gears used in the quaiffe boxes impose higher radial loads for the same horse power transmitted than the helical cut sandard gears so this is pehaps why failure in quaiffe gearboxes is more expected.

No one of the major suppliers has yet addressed this problem that I am aware of but they appear to do a good trade in replacement gear box centre casings.

TTR 's latest alloy casings have a bronze bushing insert for the layshaft mounting which is obviouly aimed at helping in overcoming this problem. TTR also advertises an uprated layshaft with "antispin peg" but as far as I can tell from the shaft I have sitting on my desk it is no different from the standard shaft in terms of antispin design using the same shoulder on the end of the shaft to locate a groove in the tail extension housing.

I am currently trying to figure out what modification options I will try with the gearbox that suffered this failure while I am in the process of rebuilding it. Any help would be appreciated.

Rohan
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PostPost by: type26owner » Thu May 20, 2004 4:41 am

Rohan
Could you provide some pictures? I don't have any experience fixing the Elan gearbox but I'm currently up to speed with most of the free technical info related to corrosion fretting. Any chance you can assemble it clean enough to use an anerobic adhesive? If yes, would suggest you call Henkel-Loctite for advice.
<a href='http://www.loctite.com/int_henkel/loctite/entry.cfm' target='_blank'>http://www.loctite.com/int_henkel/loctite/entry.cfm</a>
There are probably several other solutions to this problem which are possible to apply. Here's one, on the cast iron case you can braze/solder two steel trusses on the outside of the cast iron housing over the female bores and apply a clamping force with a threaded fastener or a wedge distorting the housing by a few microns up against the layshaft to stop the fretting.
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PostPost by: type26owner » Thu May 20, 2004 3:28 pm

Hey Rohan,
Actually there is a cleaner, better way to preload the layshaft fit into the housing. Between the bearing and layshaft pockets in the case you bore a hole offset to the one for the layshaft so the wall thickness ends up being about 1/8". Either build or buy a device like a 'ball plug' as shown at www.mcamster.com on page 3480. Modify it to make it removable though from the outside only. Otherwise the gear cluster is in the way and you won't have enough access from inside the housing to complete the disassembly process. It could also be a simple tapered pin arrangement with an o-ring groove to seal the grease in. Another thought is a two piece expanding pin assembly with a male taper inside which can be drawn with a drawbolt into a matching taper female compliant collet with a cylindrical shaped OD. This would load the layshaft in the preferred direction.
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PostPost by: rgh0 » Fri May 21, 2004 10:18 am

Keith

Thanks for the input. I had been thinking of ways to pin the shaft to stop the fretting under load but your ideas on increasing the interference fit clamping via a post fit force within the casing are interesting to contemplate.

The gear loads mean that the shaft wears the casing at about a 45 degree angle from the vertical as its frets and then finally spins so an increase in clamping force aligned with those gear loads would be an elegant way to help prvent the problem.

I have a worn centre casing that I am aiming to have the layshaft mountings bushed with a case hardened steel bushing or maybe a bronze alloy bush ( As TTR do in their alloy casings) to carry the layshaft with something more substantial than direct into the cast iron casing. What ever I do I will build into the work some way to more positively fix the lay shaft than the current inadequate press fit and locating groove in the rear extension. If nothing else the casing can withstand a greater interference fit than standard but you need to get the tolerancing right and probably bore the case (or bushing in the case when I fit one ) to achieve an increased interference fit with an individually measured layshaft.

Anyhow the spare gearbox rebuild is a long term project, this weekend its a new set of front shocks and springs to fit and new billet steel rear hubs to replace the orginal cast iron ones that were cracking up and fretting on the shafts under racing loads. Racing next weekend so I need to get it finished !

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PostPost by: type26owner » Fri May 21, 2004 1:39 pm

Rohan,
If you want an elegant way there is one supposedly. The fretting process results from oxidation. If the oxygen is not present then the fretting can't happen. This seems counterintuitive but there are numerous papers which support this claim. You might try degassing the gearlube under a ~ 100 millitorr vacuum before filling the gearbox. I've done this before with mineral oil we used in the hydraulic actuators for the Keck Telescope. This process takes about an hour for the absorbed air to be removed at room temperature. The gearbox could then be modified by adding a gas purge inlet to displace the atmosphere with an inert gas. After every themal cycle of the gearbox when it would gulp in some more air through the breather the gas purge would have to be done again. It just occured to me you could add a gas bag to the breather making it a totally sealed system and let it deal with minimizing any pressure gradients. Wonder how much of a pressure gradient the seals can take without puking out the lube? Most likely this would not totally stop the corrosion process but there's chance it can be slowed down maybe by allot. Nobody has tried doing this on an actual gearbox that I can find documented so this would be totally experimental and could fail miserably.
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PostPost by: 1964 S1 » Wed May 26, 2004 4:52 am

What about the cranais pin and flutenary rod influences?
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PostPost by: rgh0 » Wed May 26, 2004 8:54 am

Unfortunately I dont think the normal cranais pins will fit in the space available and the cost of a special sized one is to much based on the quotes I have from the usual suppliers. The standard Quaiffe conversion does not use flutenary rods these are only on the standard ford gear set according to my parts manual, this may be a cause of the problem.

However if you have any idea how to incorporate the flutenary rods into the box with the Quaiffe conversion and if this may help with the problem I would appreciate any input or photos of what you have done.

As for lube oil degassing and nitrogen blanketting, while I have done it on 50,000 hp steam turbines and gas compressors and their gearboxes I think I will work on an easier solution first for my Elan !

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