- You want the tooth?
You can't handle the tooth of these gears.
Friends, Romans, countrymen,
lend me your gears?
That's right, gang.
This week, we're talking about
my second favorite thing to grind, gears.
Gears are super old.
The first ones we know of were on a chariot in China
from maybe the 27th century BC
under mythical Chinese emperor Huangdi.
But that's not to say they're not older than that.
Before they were invented,
if you told somebody to get it into gear,
well, they had no idea what you were talking about.
These days, there's all kinds of gears.
There's miter gears, worm drive gears, herringbone gears,
rocket peanut gears, reingears that help Santa fly,
gears of war, gears for fears, craft gear lovers,
Happy New Gear, New Gear's Eve, pierced gears,
gear in the headlights, John Gear, power gearing,
legendary Seattle Supersonic Geary Payton, and bevel gears.
There's more types of gears than there's types of people
in the world, left-handed and right-handed.
Today, we're focusing on two very special types of gears,
straight cut gears, sometimes called spur gears,
and helical gears.
And I guess, we're also gonna be tackling some tough issues,
having a discussion about race.
No?
Alright, everybody's shaking their heads no and mouthing
racing.
Alright, we're going to be talking about racing.
Right now, you're probably asking yourself,
what the heck are straight gears and helical gears.
That's right, I knew what you were thinking
because I can read your thoughts.
Now you're thinking how did he read my thoughts.
Well, it's a genetic mutation I got
because my mom used the microwave when she was pregnant.
Now, you're thinking that you should tell your dad
that you love him.
Don't, he might not say it back.
Helical gears are the type of gears found
in the transmission of almost all vehicles on the road,
and I say, almost, because there's a few cars out there
that prefer straight cut gears.
These are a special type of car that are used to race
other cars in races to see who can go fastest
within the confines of a race.
I think those cars are called cars that like to go fast.
The teeth on a helical gear are angled,
and the teeth on a straight cut go straight out.
So a helical gear can absorb contact smoothly
and spread it out.
And straight cut gets it all at once.
It's the difference between jumping in in an ice bath
or getting in slowly,
or ripping a band-aid off real quick
or taking your time,
or building up to telling your mom
you're never gonna be an astronaut doctor
or just blurting it out.
Sorry Mom, I'm a garage scientist.
So why would some cars want straight cut gears
instead of helical?
Man, if only there was some sort of weekly show
presenting information like that
in an entertaining fashion that people can complain about.
Wait a minute, I have a CGI creation
that lives in a program just like that.
So helical gears are cylindrical gears
whose teeth are not parallel to the axis of rotation.
They're cut at an angle to the face of the gear.
The teeth are angled and appear as a segment of a helix,
so they have helical teeth.
The angle between the teeth and the axis of the gears
is called the helix angle,
which normally ranges from 15 to 45 degrees.
On these gears, there's two pressure angles,
transverse plane pressure angle
and normal plane pressure angle.
The module and circular pitch of helical gears
are defined on the transverse plane.
They're designed to distribute pressure gradually along
the whole tooth.
When two teeth on a helical gear system engage,
the contact starts at one end of the tooth
and gradually spreads as the gears rotate
until the two teeth are in full engagement.
This gradual engagement makes helical gears operate
much more smoothly and quietly than other gears,
and that's the reason helical gears
are used in almost all car transmissions.
They make enough contact to transfer power,
and they're quiet.
Helical gears can also be mounted on perpendicular shafts.
You adjust the rotation angle by 90 degrees.
They're quieter there too,
and they can transmit energy between parallel
or perpendicular axes.
In any running system, with a shaft and a bearing,
there's usually loads in two directions,
one in the direction of the axis of rotation
and the other, radially.
Thrust is the net force along the axis of rotation.
Because of the helix and the teeth of helical gears,
while the gears mesh, thrust force is developed axially.
These forces push the gear toward the bearing,
hence a thrust bearing has to be placed
in the direction of axial force.
The axial thrust bearings have to be able to resist
the developed thrust forces,
and this restricts the amount of torque
that can be applied through the gears
before failures occur.
The axial load produced by helical gears
diminishes the output energy from the transmission.
They also have a slight increase in friction,
so we get to lose some power as heat.
Now, let's contrast that with straight cut gears,
but before we do, why don't you guys
hit the subscribe button so that we can keep working hard
to bring you stuff like this?
The name explains it all really.
The actual teeth of the gears point straight out
from the center of the gear.
They go straight out.
A lot of people think that straight cut gears are stronger
than helical gears,
and that's why they're used.
Well, a lot of people don't drink Mr. Pibb
think that good deeds are rewarded
and think it's a good idea to get involved
in a land war in Asia
and all these people are wrong.
Race transmission with straight cut gears
are usually strong,
but it's not because of the teeth on the gears.
Straight cut gears are actually slightly weaker
than a same sized helical gear.
Shut up, let me finish.
Helical gears have more teeth meshing,
so now you got a greater load bearing surface area.
And also, difference in efficiencies are pretty negligible.
Both spur and helical gears have about 98 to 99% efficiency.
Technically, on a tooth versus tooth comparison,
a helical gear can carry a larger load
because it's diagonally positioned on the gear,
assuming tooth size is equivalent.
And that means it distributes the forces applied on it
much more efficiently compared to a vertical tooth.
So why are straight cut gears used in race transmissions?
Because of the axial load from the helical gears.
We don't want no axial loads,
and straight cut gears don't produce thrust loads.
Okay, let's pretend I'm the gear.
There's a y-axis going from my head to my feet.
That's radial force.
There's an x-axis going from elbow to elbow,
and that's tangential force.
And there's a z-axis going from my chest to my back,
and that's axial force.
In a straight cut gear, I'd be super skinny.
The teeth would just hit my head and mesh instantly,
so there is no axial force.
Basically, since they don't produce thrust loads,
you don't need all the bearings
and a big box of heavy crap to absorb
and deal with all that axial load.
So the transmission is gonna be way, way lighter.
Heavy duty transmission casings and shafts
gotta be used with helical gears
for the transmission to cope
with all of those additional loads.
So straight cut gears save heaps and heaps of weight,
and that is important in racing.
The straight cut transmission's generally gonna be
more efficient.
So is it strength that makes straight cut gears better
for racing?
Not really.
It's easy to build strong gears either straight or helical,
so strength isn't the issue.
The problem is when you start putting huge amounts of torque
through helical gears, the axial loads get so high
that everything else, from transmission case
and shafts and the axles,
they gotta contain them loads and that adds all the weight.
Straight cut gears simplify almost everything,
from the transmission case
to the design of the bearings needed
and that makes things leaner.
And because there's less stuff there,
you get a larger power train that you can place in a vehicle
without risking output shafts and other bearings
tearing themselves apart,
which is actually making a larger safety factor
within the transmission itself.
A negative to straight cut gears
and it's why they're not in most street cars
is they produce a lot of noise.
You're trading weight for all that noise.
And that's no problemo if you're racing.
Straight cut gears are also easier to assemble.
They have less catastrophic results when things go wrong
because of the simple structure.
They're not gonna fly out this way.
They're just gonna break.
Whoo, now, the gear box with straight cut gears
is not necessarily a dog box.
Usually, they are, but they don't have to be.
Dog box refers to how the gears are engaged,
and straight cut refers to the type of gears themselves.
You want to learn more about that,
check out the video on dog boxes,
or just buy some Donut merch.
That's the quickest way to learn, through osmosis.
Helical gears.
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One of my favorite shows is Miracle Whips,
and it's new, check it out here.
You like gears, check out this Up to Speed on Land Cruisers.
Don't tell my wife I'm responsible
for the weird sounds in the car.