why straight cut gears in gearboxes?

bajachris88 The international telephone dialing code for Antarctica is 6

6,661 posts · joined Apr 2005 · 22 Jun 2011, 8:01 pm

#847718

After this semester at uni, i'm a little curious (we did a gearbox design project this semester).

Why do people choose straight cut gears in transmissions instead of helical? It seems to be a common choice, and common place for aftermarket performance gears seem to be spur/straight cut gear.

We were taught helical could withstand greater loads as the helical angle allowed more teeth to be engaged at once compared to spur gears, plus for the same yield load on a gear, helical gears could be made smaller to take the same load as a larger bevel gear with the same yield strength (and thus can reduce weight by being smaller). Also the helical gear is said to be able to have a module up to two times smaller than that of spur gears, again, all because of the greater number of teeth engaged at once.

I understand that helical do induce thrust loads, is that why straight cut are used instead? because greater power through the transmission with helical will create greater thrust loads and perhaps exceed the tolerances not only of the thrust bearing but of the bearing journal in the transmission case?

Is it simply that they are just easier to produce perhaps? and thus after market gears of greater performance (yield strength) material are more affordable if you go straight cut?

thanks,
Curious Chris.

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Birdman Wolfsburg Wizard

428 posts · joined May 2010 · 22 Jun 2011, 9:22 pm

#847758

I always understood it was to eliminate thrust loads which cause all sorts of nasties but hopefully DB will chime in and educate us all.

dangerous 23 Windows of Awesome

5,901 posts · joined Jan 2005 · 23 Jun 2011, 5:53 am

#847805

I think Chris can educate us much better,
but he is correct about the higher strength of the helical design.
That is all that Albins makes now,
although in the old days straight cut gears were easier to make.

The only advantage I know of is the reduced end loads.

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vw54 Administrator Always Waiting 4 Friday

16,723 posts · joined Aug 2002 · 23 Jun 2011, 7:04 am

#847808

i think the straight cuts were made as its a cheaper process you can also get a chunker base on the tooth which makes it stronger again

I read it on FACEBOOK so it must be true

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Thinker Seriously Crusin Dubber

162 posts · joined Oct 2008 · 23 Jun 2011, 7:54 am

#847811

it is not the cost as it is the same machining process to make straight cut over helical gears.
to my understanding, yes helical gear are stronger than straight cut due to contact area of teeth but with contact comes friction and with friction comes heat.
so you lose power and generate more heat with a helical gear over a spur and then you got the end thrust,
another reason gear trains are helical is the noise spur are very load.

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lauzboy Seriously Crusin Dubber

177 posts · joined Mar 2005 · 23 Jun 2011, 10:10 am

#847834

yes you're right about the thrust loads. Because of the greater gear meshing with helical cut gears you also get greater frictional losses however theyre far quieter than straight cut

depends on what sort of use you want your engine for. racing - straight cut yes, road - helical (unless you like unnecessary noise and yes they are significantly louder)

bajachris88 The international telephone dialing code for Antarctica is 6

6,661 posts · joined Apr 2005 · 23 Jun 2011, 3:09 pm

#847871

I see now! thanks guys 🙂

so friction and thrust loads the two biggies as to why straight cut the go!

The lecturers are great for the academic stuff, but at least i can get tru worldly experience/advise in application from you'se, something they can't provide.
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ttriebler Officially Full-On Dubber

321 posts · joined Jan 2008 · 23 Jun 2011, 4:22 pm

#847881

The other key point about straight cut gears for hi po cars is they seem to be able to go non-syncro so you can do flat changes (in turbo cars no loss of boost in gear changes).
Although I wonder if it's more to do with good rugged dog design than the actual gear teeth themselves.

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waveman1500 Also rides a Honda

1,346 posts · joined Dec 2009 · 23 Jun 2011, 9:54 pm

#847954

In terms of upgrading for race gearboxes, one of the biggest gains is in using dog engagement instead of synchro. The dogs can be made much narrower than synchro cones, hence allowing the replacement gears to be made wider and fill up the extra room. This increase in width obviously gives a significant increase in strength.

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1,418 posts · joined Mar 2005 · 24 Jun 2011, 9:16 pm

#848134

vw54 wrote:i think the straight cuts were made as its a cheaper process you can also get a chunker base on the tooth which makes it stronger again

The chunkier tooth at the base comes from an increase of the pressure angle in the design of the tooth. Low pressure angles increase the undercutting of the teeth on small number of teeth gears thus making the tooth weaker.

Helical cut teeth are quieter and stronger and just as easy to produce with planing, shaping, hobbing and grinding methods of production. Yes axial loads increase but how much power it robs ????????? For general use it wouldn't matter one bit to the end user when helical gears are used. Most likely there would be more radial loads due to the larger pressure angles and modified addendum gears in many of the boxes.

Some people think it's cool to drive around with all that whining from straight cut gears in either gearboxes or cam drives regardless of whether it's better or not. One thing for sure is that a helical gear setup is stronger for the same tooth size.