So, celebrating that just today after a long time trying to get it, i finally fully understand the functioning of manuel transmision, i wanted to revisit the advantages vs disadvantages of helical vs straight gears. However, no matter how many places i searched about the answer, no one could give me an actual solid logical answer, so i want to know if anyone here could enlighten me, even if it is for a little.
Taking the sound factor aside, Straight heads are able to place the same amount of torque over another rotating gear over a bigger amount of space, right? Doesn't that mean that straight head gears can be lighter and weaker without sacrificing reliability? because ive also seen people saying the opposite its true, and that helical are usually more reliable, even if they use the same material and weight. I also still cant understand if straight head transmisions are easier to move around than helical, even if they weight the same and are the same material.
There is no reason to not go with helical cut gears.
I did the whole straight cut gear thing when I was younger to a WRX I had because I thought it sounded cool. It was a waste of money and did not give me any advantage, I should of just rebuilt it with helical gears and the helical gears were cheaper because of less demand.
Straight gear the same and material as a helical gear can't stand as much torque. The helical allows for smoother torque transfer and distributes the torque over a larger area. That saps more energy though. Race cars don't run straight cut gears for strength (although an aftermarket straight cut gear is usually stronger than a cast OEM gear). They run them because they sap less energy.
>>14137568
I'm probably getting this wrong, but I remember someone saying that straight cut puts less stress on the case. It's more advantageous to a point to do that than to reinforce it.
>>14137604
>They run them because they sap less energy.
Explain please, i dont get it
>>14137619
No you're right. Strait cut gears don't exert an axial force on the case which means you can design a case that doesn't need to be as strong in as many places which saves weight/simplifies the design.
>>14137604
that and its also easier to slam it into gear if you loose the clutch mid race if you have straight cut gears.
>>14137678
It's also pretty common for straight cut gears to be combined with dog ring engagement in a race transmission as opposed to synchro engagement as well.
>>14137678
Um, the gear design is irrelevant, they are always meshed. "Smashing into" a gear means running the syncros hard, or even better running a dog box which can be opperated without a clutch for the most part. The normal syncromesh setup (oem style) has many many teeth for a short lash and smooth engagement, where the dogleg setup has significanly more lash, but can be "slammed" into gear or even opperated without clutch disengagement.
Straight cut or helical cut gears does not youe shift speed change.
>>14137631
>helical allows for smoother torque transfer and distributes the torque over a larger area. That saps more energy though. Race cars don't run straight cut gears for strength (although an aftermarket straight cut gear is usually stronger than a cast OEM gear). They run them because they sap less energy.
part of the power sent through the gears is dissipated into the thrust load on the gears. because they contact at an angle.
>>14137773
So basically straight-cuts are slightly weaker but have less energy loss when the power goes from the engine to the output shaft?
straight cut lets you use a lighter case and driveshaft doesnt it. due to less axial loads and whatnot.
>>14137781
assuming same contact angle material etc yes. helicals have the advantage of more area of contact (assuming helical vs spur gears are same thickness) so less maximum stress on the teeth for a given load.
Well fuck... That explains the reverse whine. Never knew my rear was different than the forwards. Always thought I'd botched it by screwing up reverse the first time (was rolling forwards, tried to put it into reverse and got coffee grinding without meaning to - first time manual).
>>14137568
I remember a few months ago we had a giant thread about this and it ended in a massive fucking shitstorm.
I hope it happens again.
>>14137906
No lol you didnt fuck it up, reverse has straight gears thats why its so loud and different than all the other gears.
>>14138916
Only shitboxes and fossils have straight reverse.
Proper cars have helical and synchronised reverse.
>>14137916
i literally was there br/o/
>>14137815
Don't the helicals have a theoretical single point of contact that moves along the touching teeth, instead of the full width of the teeth being in contact like in straight cut gears?
Stupid question:
What wears quicker: An OEM helical, or a straight cut?
Assuming both have /o/tistic gearbox oil change intervals of 800 miles or less.
![Double-helical_bevel_gear_Citroen[1].jpg](https://i.imgur.com/Fnom5eI.jpg "Double-helical_bevel_gear_Citroen[1].jpg")
>tfw no herringbone
Straight cut or straight tooth. Not straight head OP. Please take note for future reference.
>>14137601
tbqh i'd see benefit into having the first 2, maybe 3 gears straight, and the rest helical for cruising. The first few are gonna see the most abuse, after all.
>>14137794
Yup.
tfw u got a new transmission and drive it and wonder if it has straight cut gears for a second before you realize you forgot to put oil in it.
>>14137604
>The helical allows for smoother torque transfer
This is correct
>and distributes the torque over a larger area.
This couldn't be farther from the truth.
>don't run straight cut gears for strength (although an aftermarket straight cut gear is usually stronger than a cast OEM gear). They run them because they sap less energy.
And this is the exact opposite way around.
>>14137604
the teeth are also stronger, but only because they can be wider.
>>14139056
This.
>>14137815
This should be disregarded.
>>14139164
In theory, straight cuts wear quicker, because they exhibit greater friction when meshing, as opposed to how helicals gently glance across a singular contact point, but can spread the load across more than one tooth at a time.
But while the straight gear can wear faster than a helical under normal driving conditions, they are far stronger.
>>14139397
>they are far stronger.
No, they are weaker. The only advantage of spur gears is a lighter gearbox.
Spur gears mesh with each other more harshly than helical. Helical have a smoother mesh. However, because they're helical, they produce axial forces, which means you need much stronger bearings and support (IE thicker, more expensive gearbox).
Spur gears are easier to engage with one another, easier to manufacture, easier to replace (though if the gears are disengaged, then both are easy to replace), can take more short term abuse, do not waste power through axial force (and thus reduced bearing friction). For identical physical dimensions, they are also stronger since the individual teeth are thicker.
Helical gears are MUCH quieter, wear less quickly since they have more tooth wear surface. They survive high speed wear better as well, since the contact point moves slower on a helical gear profile.
>tl;dr spur gears require less power and are simple/cheap to make and replace, simpler/cheaper gearbox design overall
>helical gears are quiet, good for high speeds, wear less for identical use
If you wish to contradict anything said here, source or shut the fuck up.
>my sources: Shigley's Mechanical Engineering Design, 4 years of mech eng, hours of research for a project on this exact topic
>>14139659
>can take more short term abuse
How do you figure?
>>14139665
Because they can transmit more power without failing. Both gear sets can withstand about the same input force all things considered, but spurs can put all of that power to the output, so less input is needed. They can also handle higher shock without busting a tooth or shattering a bearing. But will wear faster, hence short term (short being relative).
>>14139612
because thats the exact reason CAT's are full of them
>fucking mong
>>14139699
CAT whats?
>>14139674
So spur gears are more efficient. Why can they handle higher shock?
>>14139715
because the gears aren't constantly meshed
>>14139659
Then why do racecars uses them?
>>14139731
No axial load means lighter transmission case.
>>14139731
Why do they use spur gears?
They don't care about noise and they make for a lighter gearbox. Also easier to work with when they have to rebuild it every race. Usually they're also machined gears in racing, not cast, and the spur profile is a lot more simple to machine than the helical.
>>14139729
That doesn't mean what you think it means.
>>14139731
Did you read what your quoted or what?
>>14139738
What's the weight difference with the cases then? is it that much of a difference to even care?
>>14139746
a case designed purely for straight cut gears would explode fairly quickly if you used helicals in there
>>14139746
Can't give you an exact number, but yes. My guess would be maybe 10-20kg?
You have to factor in larger bearings (+thrust bearings), thicker supports for those bearings, more complex engagement (dog vs. synchro, or just needing less robust synchro). 10-20kg is out of thin air but it must be significant enough for racers to want to remove it.
>>14139746
Race car gearboxes have WAY less components than your average street legal car's gearbox.
Everything is lighter, so what you may consider negligible (which is definitely non, btw) is just part of an overall lightening process that yields very appreciable results.
>>14139746
In a race car it is.
>>14139765
>>14139758
Right, thanks, i just wanted to know if there was any advantage to use straight cuts in a sports or DD car, but seeinng how its a matter of lifespan vs weight, i think i will keep the helical
>>14139788
Not unless you replaced the transmission case.
>>14139715
Each tooth is generally thicker. Since a properly designed spur mesh always has at least 1 engaged tooth, the thickness of the thing taking the force is really all that matters.
>>14139729
See above.
I'm having a hard time finding a helical gear animation similar to pic related, but it's very similar. A standard helical gear still has only 1 line of contact at some points I think.
If the teeth are the same size, then helical is actually stronger since the load is transferred partially tangential to the tooth and partially along its axis (effectively making it a thicker tooth in the direction of the force). But for the same weight/size of gear, spur teeth are larger. A helical gear with spur sized teeth would be pretty big.
>>14139715
Because the physical size of the cog is much thicker. You dont as fine teeth compared to oem helical gears. Also the surface load distribution is much better on a straight cut gear. Helical has a moving contact face that rides the length of each cog. While a straight gear has one non moving contact face. The input force is transferred across a larger surface area.
ie breaking a thin twig vs breaking a thicker stick. One has much more mass to break through.
>>14139788
The "losses" are actually fairly negligible. Thrust bearings are pretty good at what they do. It's mostly due to the weight and simplicity that racecars use them, so putting them in your DD is probably the most expensive possible way to lose 20lbs. A good diet will get you further than a DD-able straight cut box.
Plus, the noise would be UBERKEWL, but only for the first 15 minutes.
>>14139796
why do straight cuts chatter
>>14139809
What do you mean by chatter..? Do you mean the whining noise?
When a helical gear first engages, only the very tip of one tooth touches the tip of the other. They gradually continue to turn until more and more of the line of action (the parts of the teeth that touch) are in contact. Key word is gradually.
When spur gears engages, one tooth mates with the other entirely. The full width of the tooth is immediately in contact with the other.
The whining is essentially the sum of all those little impacts with each gear tooth mesh. If you were to slow it way down and amplify it, I guess it would sound like chatter.
>>14139824
Also should be noted that since we don't give a shit about noise in a race box, the gear tooth profile on a race transmission is pretty aggressive. They are made thicker, with less of a rounded profile (literally to keep them thicker, hence stronger). This type of profile has an even harsher mesh, so it's even louder.
>>14139824
i assume they mean the slight clicking/thunking when rapidly pumping the throttle or anything else that would highlight the lash in the gears as they wiggle back and forth.
>>14139824
does that fully explain the increased wear?
>>14139835
Oh. In that case yes, it's exactly that: the lash. Spurs are not as "precise" as helical gears so their mesh has more play in it, hence more lash.
>>14139845
In conjunction with the above, pretty much. They also have less surface engaged on average, meaning the full force of the engine is pressing on 1 part of 1 tooth most of the time (more sliding friction), whereas helical gears I believe are meshed with more than 1 tooth for the majority of the time, and their teeth are longer (more contact area, less pressure, less wear on that part).
>>14139854
is it possible to design a spur gear to mesh in more than one contact point
>>14139861
no that would cause either a very loose gear mesh where power would be lost. or a complete lock up.
>>14139861
>>14139874
on an external gear i mean.
usign an internal gearing design (cogs on the inside face of a ring gear) there are multiple touching face. much more power can be passed through this arrangement. thats why transmissions have planetary gears toward the output shaft
>>14139861
I was actually just thinking about this.
I think you could design it to have more than one contact line more often, but I don't think it's possible to ALWAYS have more than one contact line with a spur gear. I'm not even sure if that's possible on any type of single-start external gear, can't find a source one way or the other. I would think not.
>>14139861
Like, a gear that has 3 lines of spurs, all 3 (or more) in different places? so as to make it more like a staircase of gears?
thank you for taking time to respond to my dumb questions gearanon
>>14137604
This man is correct. Helicals are smoother, quieter, but slightly less efficient. The difference is small, but there is one. Unless you actually race and are close to winning, don't bother with straight gears
>>14139699
You're retarded. Straight cut gears distribute the same load over less area than helical gears. There is literally no possible way for a straight cut gear made with the same amount of the same material to be stronger than a helical gear. You can't win this argument, because you're arguing against physics.
>>14139922
>>14139930
>made with the same amount of material
But they're not. The thickness of the tooth makes up for the smaller contact line, and then some. If they use the same amount of material in total, then helical gears will have more teeth, but they're smaller. At some point in the mesh, they still only have 1 tooth engaged. 30degree angle doesn't make up for half tooth thickness.
