– 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. Thanks to Honey for sponsoring this video. Honey is a browser
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joinhoney.com/sciencegarage. Honey. Hey, you guys, click on this link. That means you love us. We got merchandise at shop.donut.media, and we got new stuff
coming out every week. Follow us on Twitter and
Instagram @DonutMedia. Follow me @BidsBarto. 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.

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