Gear Strength
Turbo Mini Forums

TD recently made a very valid point that the strength of a helical gear setup depends very much on it previous use before putting them under high stress.

The point was that a straight cut gear set is going to be stronger than an 80,000 mile helical set, mainly because the straight cut set is brand new rather than a stronger design.

The tendency with a turbo mini is to buy a complete Metro Turbo engine and box. These are typicaly high mileage and have probably suffered at the hands of a few chavs before being broken.

So, question. If you wish to stick with helical gears for a daily drive, as the Metro gearboxes are DAM5626 as are the 998 gearboxes, would a 998 gearbox be a better starting point than refurbing a Metro Turbo box?

The point being that the 998 box has probably ran along rarely using its 40hp, whereas the MT box has probably been abused.

Question for TD. Is there a mathematical rule for comparing gear life and utilisation? For example, does a gear running at half the power have double the life or is the relationship exponential.

Edited by Paul S on 8th Nov, 2007.

Saul Bellow - "A great deal of intelligence can be invested in ignorance when the need for illusion is deep."
Stephen Hawking - "The greatest enemy of knowledge is not ignorance, it is the illusion of knowledge."

Firstly Axel - I'm not a gear engineer. I'm a transmission design engineer. I know enough about gearing by roughing out gear layouts, and my previous life as a tech testing gears to hopefully know enough to answer this however!



Anyways - the question in the last paragraph... It depends how close you are running to the design limit of the gears - and how much duty you will be putting - or have already put into the gears. If you are running at 50% of the design torque (assuming the case of a low mileage 998 gearset) life will be way more than you could use - you may as well say infinate for our applications.

At 100% design torque, design life will be whatever the duty for that gear was specified by the OEM to meet the warranty period plus whatever 'safety factor' they put in there. This is measured in hours typically, for example, for each gear, you'd have hours at 100% torque - automotive will probably have a rating hours at 50% or 60% as well.

For automotive applications, you can pretty much assume 1/3 of the torque would give infinite life given the yeild of heat-treated steel; this is the stuff that you'll really need to plot on an S-N curve for the material specs being used, to get the exact figures. 1/3 is a safe figure to assume I'd say.

Automotive applications are all about making the thing as cheap as possible, such that they 'just' last for the warranty period. I think it's safe to argue that a transmission manufactured by an automotive OEM could be slightly less aggressive on duty cycles (or a lower safety margin put simply) than an external transmission supplier.

Of course, other factors such as oil will make a difference to the gearing as far as wear goes - but i suspect you were looking more at a fatigue based question?

So, again (!) answering your last question - if you imagine an exponential curve of hours vesus torque, and have full hours, full torque at a fixed point, and at 1/3 the torque would be infinite hours; You can pretty much make some assumptions from there forwards.

Of course - it's not that simple, as granny in her 998 who lived up a 2 mile hill and drove down the hill in 2nd gear using engine braking all the way, then back up the hill fully laden in 2nd could be getting close to mr metro turbo guy who accelerated for a short time period but at a higher torque before shifting into 3rd...

So yes - the lowest mileage 998 box you can find will be your best chance of having a decent life from a helical set.
what i can't comment on is the effect of the cryo treatment on used gears - but its definately something i'd like to try.

Is that any help?

That is a great help Dave, Many Thanks.

The reason I ask is that Sturgeo wants to build a K100 headed 1380 which is going to be on the limit for a helical gear set for a daily drive.

The base engine and box would be my NA 1275 from an 80,000 mile MG Metro.

But I also have some 998 gearboxes laying about that may be a better bet.

Saul Bellow - "A great deal of intelligence can be invested in ignorance when the need for illusion is deep."
Stephen Hawking - "The greatest enemy of knowledge is not ignorance, it is the illusion of knowledge."

Doesnt help and I have no tech knowledge what-so-ever, but my gearbox is a very low mileage rover unit - 2000 miles when I bought it IIRC.
done about 2000 miles with the turbo motor, between 12-15psi boost, and is still in perfect condition. Makes no funny noises, easy to get in gear, and nowt seems to have broken yet.

what occurs to me is , if a box is fatigue lifed ,and it goes on for ages ,
then recieves a shock to the gear train ,eg 4k clutch drop on a sticky burnout pad or glue sprayed track ,
will that shock have the same effect on the tooth or gear in q ,as it would on a new gear ,?

that is ,to say , is an older work fatigued but also work seasoned gear ,more or less resistant to shock loadings than a newer gear .

might be waffling here ..

Medusa + injection = too much torque for the dyno ..https://youtu.be/qg5o0_tJxYM

I understand your point Robert. A bit like BMW using old blocks for their F1 turbo monsters.

Fastest 998 mini in the world? 13.05 1/4 mile 106mph

Medusa + injection = too much torque for the dyno ..https://youtu.be/qg5o0_tJxYM

I don't think that gears get any stronger with use, the opposite in fact.

There is a definitive relationship between the stress and life. Highly stressed and it wont last long and vice versa.

BMW used to leave the F1 blocks in a field to settle. I dont think they went down the scrappy to get used old car engine blocks.

At my old firm, we used to cook our engine blocks in a furnace overnight to artificial age them. They were then more stable in use.

Saul Bellow - "A great deal of intelligence can be invested in ignorance when the need for illusion is deep."
Stephen Hawking - "The greatest enemy of knowledge is not ignorance, it is the illusion of knowledge."

All the A+ gearsets I chewed up, were 1000cc items.

1275 boxes back in 94 werent so common, but 100occ stuff was plentiful.
Although no idea of milage on them, except the 3000 or so they lasted for me.

9.85 @ 145mph
202mph standing mile
speed didn't kill me, but taxation probably will

Hi,
I think the way it has been previously used and the way you use it is going to have the biggest effect.
As Robert says the shock loadings are what will kill a box quickly.
My 1380 box is an old 998 box, which I changed the final drive in.
I've ran it for 10years, no idea of it's condition before, but I do tend to drive very smoothly, but not slowly!
Cheers,
Gavin :)

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I'm with TD on trying cryo treatment. I bought a set of gears for the portal hubs on my Unimog that had been treated because I kept breaking the stock ones and haven't broken the treated ones.

How much is Cryo Treatment?

1/4 Mile 14.3secs 96Mph Terminal 10psi of boost.


Fibreglass Parts? - http://www.tdkracing.co.uk/
Split Rims? - http://www.force-racing.co.uk/

Hi,
TDs point about design limits ... I would imagine the design limit to be the torque for the standard MT, as they put the boost modulator system in there to stop gearbox failures.
Cheers,
Gavin :)

VEMs Authorised Installer / Re-seller. K head kits now available!

WB/EGT gauges. Click here for customers write-up

Visit www.doyouneedabrain.co.uk

My Mini build diary

cryogenics treatment i have read up about for gearboxes as i had the concern that when my turbo is built it will eat gearboxes, even though my plan is only for 8psi boost i may still concider it as there is a company which do it for quite a reasonable price.

i also have this website i hope it helps
http://www.frozensolid.co.uk/motorspo.htm this company has done gearboxes as well as i have seen it in a few magasines but enjoy

sorry for any waffel

Drives
-Ford S-max Mk2 Ecoboost
-Rover 100 VVC #2 - track project

Searching is all you need on TurboMinis

Maybe we should get our resident expert gear man to have a look, seeing as they are both in the same town!

Saul Bellow - "A great deal of intelligence can be invested in ignorance when the need for illusion is deep."
Stephen Hawking - "The greatest enemy of knowledge is not ignorance, it is the illusion of knowledge."

@TD and all: whats your opinion on the strength of an A-box compared to an A+-box?

I've been told that the A-boxes have different gears than the A+-boxes. I think it has something to do with the shape of the teeth on the gears.

Im planning to use my old A-box on my 1000 turbo 2b engine since I know this gearbox for more than 20 years and to my recollection it shifted more subtle than my current A+-box.

Offcourse a rebuild will be done before installing, but do you think that it can handle approx 130 BHP (if I can make that happen, duh)