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 bajachris88
A.k.a.: Chris Leete
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 posted on June 22nd, 2011 at 08:01 PM |
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why straight cut gears in gearboxes?
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.
(ô_!_/ô) (ô_!_/ô)
69' baja: kombi box, thing spindles, irs, disc front, type 3 rear drums, 2 inch lift kit, 31x10 rears. 
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Birdman
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| posted on June 22nd, 2011 at 09:22 PM |
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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.
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dangerous
A.k.a.: Dave Butler Muffin Man
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| posted on June 23rd, 2011 at 05:53 AM |
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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.
| Quote: | Originally
posted by westi
That's mad Alan.
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 vw54
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| posted on June 23rd, 2011 at 07:04 AM |
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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
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Thinker
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| posted on June 23rd, 2011 at 07:54 AM |
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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
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| posted on June 23rd, 2011 at 10:10 AM |
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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)
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bajachris88
A.k.a.: Chris Leete
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| posted on June 23rd, 2011 at 03:09 PM |
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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.
Unless its guns... my Ukraine professor has an alarming obsession....
(ô_!_/ô) (ô_!_/ô)
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New engine in process: 94mm p&bs, 74mm C/w chomol Crank, 35.5x39 SP heads, turbo. Wierd combo, hopeful torque monsta! |
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ttriebler
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| posted on June 23rd, 2011 at 04:22 PM |
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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.
Reversed gears for Subaru transmissions.
Fit a Subaru 5 speed to your rear engined car
www.subarugears.com |
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waveman1500
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| posted on June 23rd, 2011 at 09:54 PM |
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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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1500S
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| posted on June 24th, 2011 at 09:16 PM |
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| Quote: | Originally
posted by vw54
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
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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.
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