Straight Cut Drop Gears
Turbo Mini Forums

Hi All,

I know there is already a lot of stuff on this site about s/c drops and boxes and the general concensus is that the standard helical gears are the major limiting factor on tuning the blown aseries, however, I was just wondering how much better s/c drops are and why?

I am currently rebuilding my gearbox and am not sure whether to go for straight cut drops or not, my main reason for not doing it is because of the noise they produce. I also wonder however why everyone says that s/c gears are stronger, as I was under the impression that with helical ones there are always two teeth in contact with each other, making them stronger?

I am not saying anyone is wrong, to the contrary I would just like someone with more knowledge than me to explain why s/c drops are stronger and how nessecary they are. I am planning on running at around 12psi and have upgraded to a Minispares 4 pin diff and a few other bits in the engine. - is this going to die pretty quickly without s/c drops?

Thanks for any response,

Ash :cool:

Edited by AshP on 22nd May, 2007.

Turbo power here I come!

you get rid of the sideloads when using straight cut drops instead of the helical ones. So you will have less chance of destroying the transfer housing due to an idler gear that wants to get out the sides.

The reason why I use SC gears is because they are available in ratios much better suited for my usage. That simple.

I think that the general consensus of opinion is that the helical drops and gears will be OK if driven carefully up to about 140hp.

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."

its the side loadings, helical cut gears as you say are actually stronger (in most cases) but produce side loadings. The ideal would be (fook, cant remember the name) dual helical cut 90degrees apart to cancel out the side loadings.

i'd say at 12psi you'll be borderline.

depends on how well you look after it and how often it wants to come to bits.

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1/4mile in 13.2sec @ 111 terminal on 15psi

thanks for the response guys. I have just come across this article which is quite helpfull too and basically outliines what has been said:

http://www.minispares.com/Article.aspx?aid=109

Not sure what to do now though (s/c or standard??)...really dont fancy the noise (or the £200 its going to cost), but really want to be able to boot it a bit and not worry about it!

...maybe I should start a poll

Edited by AshP on 22nd May, 2007.

Turbo power here I come!

I drove to Newquay a few weeks back (270 miles each way) with SC drops and 'box it really isn't that bad when cruising or at all come to think of it! Lol

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1/4mile in 13.2sec @ 111 terminal on 15psi

straight cut gears dont last long!!! i had to replace some after 5000miles

get the drops in ash if not i can assure you you will shredd the head set in the box

I am pretty sure the correct name is double helical gears we use these in massive epicyclic reduction gearboxes on our Boiler feed pumps at the power station. When I say big these are the size of the whole mini lol

Can you not keep the helical gears and modify them for the better stronger bearings? (idler gear and nose bearing)

I don't know anything about this mod , but it could be the perfect solution.

I personally wouldn't want straight cut anything on my daily drive.

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

I have had one casing welded up and machined out to take a big thrust ball bearing, not tried it out yet but was an idea I came up with before I bought straight cut set.

The ball bearing would take the axial and radial loading and save the casing getting badly worn by the high thrust laods and the stupid washer on to plain ali situation which is the normal set up.

Or another idea was to machine back the ali face in the housing and insert phozy bronze washer into the casing so the steel thrust washer would be thrusting against a material (phozy bronze or aluminiabronze) better suited to resisting wear and with a lower co-eff of friction

Edited by mikemph on 22nd May, 2007.

lol sounds like i need a set of them mike!

Hmm...still not convinced s/c is the way to go - i am agreeing with you will h

i am liking these alternative options guys, I think this is something I may look into further (strengthening the bearings) if anyone could expand upon this further it would be most helpful......

Edited by AshP on 22nd May, 2007.

Turbo power here I come!

iv got nothing to add except to say what a relief it is to have such a polite nicely worded question ,and appreciation for the answers .well done ash !

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

The ideas above I never tried in practise, I thought it was just easier to stick to the tried and tested.

I also thought maybe if the thrust washer was a larger diameter and machine the whole of the trust face in the housing flat that the larger area would handle the trust loading better!!!

All ideas which might work help if you did want to stick with helical gears. The bigger washer idea is probably the most easy option. Most machine shops could machine the casing thrust face and make up a big washer and send it to be hardend or nitrided or whatever.

well heres the answer gents

here a not so good pic of a timkin SC idler the TIMKIN type which sports the centre machined out of the gear and a timkin roller thrust bearing fitted

this is then supported by two large bosses which then fit in where the old torrington bearings fit in the gearbox casing and the clutch housing

now do this on a helical setup and the problems cured!

iam going to see my pet machinist next week and get him to do one for me and fit it to my lads motor

i think there will be a demand for these

i would like TURBO DAVES thought on this

Edited by BENROSS on 23rd May, 2007.

Haven't we been here before? I'm sure this was asked back in the first few months of TM???

Anyways... The 'timken' SC Idler i have experineced with has a single cylindrical roller bearing in the centre. This one looks to be the same?

First thing about helical gear trains that is mis-understood:
They do indeed impart forces other than radial. The axial force is as a result of the helix, and as it is applied to the tooth, generated a bending along the axis of the shaft, which because the shaft (gear) is supported on bearings, translates to an axial force.

So, what happens with idlers?

Well, think about the way the idler train is laid out. You have three gears, with the idler in the centre. The driver and driven either side of the idler are the same size and helix - and as a result, the magnitude and direction of the force on the (top) of the idler (against the driver gear) is equal and opposite to the magnitude and direction of the force on the driven gear.
In short - if you have a helical idler geartrain with the same driver and driven gears either side of the idler, and with the idler axis symetrical about the driver and driven axes (so you could hold a straight edge between all three axes), then you would see zero axial forces.
That's right, ZERO axial resultant force.

However, the shaft axes in a mini are not in a straight line - hence you do see a resultant axial force about the idler axis, which reverses in direction between drive and coast.

If you want a little more evidence, just look at the thrust washers against the alloy casing - do you REALLY think this could take all the thrust of the helix if it wasn't in an idler train??? It is not a huge force.

Back to SC's - of course they eliminate ever last little bit of this axial resultant.

Ben's idea of installing a bearing into a helical is certainly do-able, but I am pretty sure a cylindrical bearing is not going to work in this application, because a cylindrical will happily take a thrust, and / or a radial, but the bending moment from the idler application in a transverse A-series is not going to cut it...

All is not totally lost; if you can fir two narrow (back-to-back) tapers in there, then you have about as stong a bearing as you could want.

Can you fit them in there? I don't know... There is also the small problem of keeping everything perfectly concentric.




:)

Edited by turbodave16v on 23rd May, 2007.

Oh. My reply was to SC gears in the box, not drops.

Hi,
According to the mini spares site that gear Benross has posted up has got taper roller bearings in it.
I would be very interested in this setup, Roger talked about the same thing last week.
I'm a member of the no S/C club too!
Cheers,
Gavin :)

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