9 replies to this topic

[03jkirk]

this is a general question about straight-cut gears, in a fairly recent artical it showed how its myth that straight-cut gears are inherantly stronger than the standard helical type. so with this in mind why do people always insist on changing to SC gears wen buildin high horsepower motors??? is it to do with side loads on the casing or is it jus due to the myth of them being stronger???

[jamiestevenbell]

lower transmission losses

[brad-the-bear]

as above

[03jkirk]

this still doesnt explain the reason for everyone telling other people that they MUST use straight-cut on high power motors and surely this would be more suited to lower power motors where the power gains would be even more noticable??? or does that power sapping from the gearbox get worse at higher powers?

[rawky]

The loss would be a percentage of the power, so the greater the power, the greater the nominal loss in the gearbox.

[03jkirk]

thats makes sense but the reasoning behind straight-cut gears does seem to have really been distorted by many people, most people do seem to think they it is because of the extra strength and many companies market them as this being the case

[Tahiti Joe]

Partly to do with the materials and processes used too make a lot of the 'good' straight cut gearsets. True that helical can be as strong as straight cut, but a lot of the good straight cut sets, such as those produced by MED, swiftune.... Will of been produced to a much higher level and with tighter tolerances and sometimes using better materials than Morris/Austin/Rover ever did....

As already said though, it's mainly to do with the vastly reduced transmission loss!

[oltonlad]

some people just love the noise that they make...............

[Cooperman]

Straight-cut gears were originally introduced for competition use. When a race or rally engine is built the hotter cam and high compression ratio will cause the high power to be produced over a much narrower power band. So closer ratio gears are necessary as it is no use changing up at very high revs only to find the revs are not then high enough to pull cleanly and powerfully in the next gear. It is easier and cheaper to produce strong close ratio gears in straight-cut form.
So with high powered motors with a narrow power band it's the closer ratios which are required, but these are only available in straight cut form. If I could buy a helical gear set with ratios the same as the SC ones I have, then I would buy them and enjoy a quieter drive line.
Of course, with SC gears there are no end loads which is a bonus.

[tiger99]

What you really need, but will probably never get, is "herringbone" gears, which are double opposed helical, so no end loads, and they run quietly. They can be stronger than straight cut, which is why they are used in certain very highly loaded industrial applications.

There are two reasons why you will probably never see them in a Mini, firstly they are difficult and expensive to make unless there is a big gap between the two opposing helixes, or they are made in two halves and are joined by something like electron beam welding. Secondly, they are wider than standard gears, so there would be no room in the box.

It might just be possible to make the troublesome drop gears double helical, and that would be very worthwhile, as the idler has hideous side loads, but not in the main box. If someone knows otherwise, please go and do it, and put them into production. I may want such a box myself one day. Oh, and make it 6 speed, please......