I’m not sure if ‘sliced thread’ even makes sense. But helical gears are great – to look at and to use. And you’ll find them in a lot of places. So let’s get our butts into gear and explore these smooth criminals of the engineering world. First, though, the obvious:
What is a helical gear?
A helical gear is one of four main types of cylindrical gears. So before we define a helical gear, let's define a cylindrical one first. And this is easy: It’s a gear with a cylindrical pitch surface. In other words, it's a gear that's round. *Looks around in amazement*
So, what do you think names cylindrical gears? Here's a clue: Say "Cheese".
That's right. The names of cylindrical gears primarily refer to the shape of their teeth. For example:
A helical gear has teeth angled diagonally relative to the gear axis.
A double helical (and Herringbone) gear has two sets of opposing helical teeth. A double helical gear, however, typically has a gap between them (i.e. "\ /"), while a Herringbone doesn’t (i.e. "V")
A spur gear has teeth that are straight and parallel to the gear axis. And there are two types: internal (teeth on the inside) and external (teeth on the outside).
A worm gear looks a little different. It has a shaft with a spiral thread (the worm) that moves a toothed wheel. So the worm's teeth look just like a screw. Imagine that.
Now, because you, my friend, are an engineer – or at least find engineering interesting – you have a question hardwired into you. And over the years, it means you've probably taken a lot of stuff apart. Some of which didn't find its way back together again...
In fact, I'm sure you're asking it right now. So allow me to answer it for you:
How do they work?
Unsurprisingly, each type of cylindrical gear works a little differently. So go ahead. Sink your teeth in. One by one.
How a helical gear works
Because a helical gear's teeth are angled, they engage progressively. All their coolness stems from this. For example, they're smoother. They're quieter. And they're capable of handling a lot of load. But because of their angle, they also generate a "side force" (axial thrust), meaning they require a helping hand from the ol' trusty, ol' reliable: thrust bearings.
The helix angle can be anywhere between 5° to 45° but is usually between 15° to 30° with helical gears.
A lower angle – i.e. making it more “spur-y” – will lower the axial forces and thus the need for thrust bearings. This reduces contact/friction between the teeth, also lowering heat generation and wear. It's no surprise, then, that this hones the load transfer and makes them more efficient.
On the twist, if you raise the helix angle, you increase gear smoothness and decrease its noise and vibration. One of the main benefits of helical gears is their load capacity. And the higher the angle, the more load they can bear – thanks to increased teeth-on-teeth contact.
Helical gears are usually made through a process called "Gear Hobbing". Here’s a video that shows you how. And if you want to get mathsy with their mechanics, read this.
How double helical (Herringbone) gears work
So unlike single helical gears, which produce an axial thrust, double helical gears eliminate it. Their opposing teeth are cut so these forces cancel each other out. No more big, chunky thrust bearing.
Fun fact: Double helical gears were made by Andre Citroën, the Founder of the Citroën and the beloved Citroën 2CV…. The gear even inspired the logo too.
Double helical gears, like their single-toothed cousins, also engage progressively for minimal noise and vibration. The tooth contact is near-continuous, making them very stable, very smooth, very nice. And because the contact area is higher, they're also capable of handling more load.
And while these are great benefits, the greatest comes to the eye. Just look how magnificent they look here!
The only downside, however, is their difficulty in manufacturing. They're much harder to make. So only find themselves in really demanding spots - no spur of the moment thinking there…
If you’re one of the 3D printers among us, here’s how you could make one yourself.
How spur gears work
There's no helix angle. Just straight-up power transmission between straight-cut teeth.
Spur gears are the simplest design and look like your stereotypical gear. (Search "Gear icon" on Google, and you'll see hundreds of them!).
But simplicity and noise don't go hand in hand. Unlike the soft, tender connection of helical gears, spur gears gnash together. Their abrupt meshing and involute profile mean the gear teeth experience a lot of stress via one point of contact. And you'll hear it – especially at high speeds.
One point of contact does make them efficient, though. There's very little sliding between the teeth or lateral forces applied to supporting parts. So, spur gears are, in fact, the most efficient of all cylindrical gears.
How worm gears work
The last of our cylindrical gears are the squiggly, worm ones. They're made of two components: the worm gear (a bit like a screw) and the worm wheel (its helical-gear counterpart). What’s special about these are two things:
1) They transpose motion by up to 90 degrees. In other words, you can connect perpendicular shafts (versus parallel like other cylindrical gears). As the worm gear rotates, the threads push against the worm wheel causing it to turn in a perpendicular direction. This is great if you want a little worm wiggling about in small spaces - a space too small for parallel shafts.
2) Worm gears also have great reduction ratios (especially in small spaces). For example, a full rotation of the worm gear might lead to only a 1/30th of a turn of the worm wheel.
Because it’s sliding and, in essence progressive, it's quiet and smooth like the other helical gears. However, the sliding contact between the worm and the wheel generates a lot of friction, making it very hot and unsuitable for heavy loads. As you'd expect, then, lubrication and cooling are very important.
To see one in action, watch this.
Comparing cylindrical gears
The next question you might be thinking is, well, which one is best? And like it always does, the answer depends. So, to prevent you from grinding your gears, here’s a table comparing the different types of cylindrical gears:
|
Gear Type |
Teeth |
Key pros |
Key cons |
|
Helical |
Single, diagonal |
- Smooth - Quiet - High load capacity |
- Produces axial load (needs thrust bearings to support) |
|
Double Helical (Herringbone) |
Double, opposing diagonal |
- No axial thrust - Smooth(er) - Quiet(er) - Even higher load capacity |
- Complex design - Expensive to manufacture |
|
Spur |
Straight, parallel to gear axis |
- Very efficient - Simple design - Cost effective to manufacture - No axial forces |
- Noisy, especially at high speeds - Limited load capacity |
|
Worm
|
Screw/diagonal |
- Smooth - Space efficient - Great reduction capacity |
- Lowest efficiency - High friction (and heat!) |
Where are they used?
So, the “best” cylindrical gear depends on the application and the constraints you’re facing. Obviously. To paint a clearer picture of where each prevails, here are some typical uses:
Helical gear uses
Their uses follow their advantages. So helical gears find themselves nestled in applications that require smooth, quiet moving gears and aren't afraid to work up a sweat with loads and speeds. It’s common to find them in…
Transmissions. Vehicle gearboxes demand smooth, quiet operation and at high speeds. Because the only time you'd want to hear your gearbox is when you're using it to nod off.
Manufacturing. Compressors, conveyors and rolling mills require robustness. Those heavy loads need stability. And stability lies in angled teeth...
Power plants. Turbines and generators often use helical gears for their efficient (and load-capable) power transfer. Usually, churning out mega power also means mega heat transfer. But the smooth, helical teeth engagement minimises it.
Double helical gear uses
Double trouble. Or... err... double delight? The difficulty in manufacturing double helicals is often outweighed by their elimination of axial loading and superior load capacity. So you might find them in:
Power plants. Like their simpler single-toothed sibling, turbines and generators often used double helical gears too. With the amount of power some generate, you don’t want broken teeth in your power sandwich.
Ship engines. If you saw our article on engine pistons, you might recall the Wärtsilä RT-flex96C – the biggest reciprocating engine in the world. As you’d expect, it generates a lot of power (its 14-cylinder variant produces a small 108,920bhp!). So you need a transmission that can handle it without causing a mini earthquake.
Offshore platforms. Heavy marine drilling requires a lot of power and uses heavy machinery. Double helical gears are keen to get their teeth into the work.
Spur gear uses
Spur gears offer great reduction (or multiplication) ratios when they're used in series. They’re also cost-effective to make. But compared to helicals, they don't offer the same load capacity, so they find themselves in applications they... maybe... shouldn’t.
So to protect their home – and your ears, here are some places you might find spur gears:
Washing machines. Mine is old and quite noisy. Nor bearing the loads of a power station. So, a spur gear fits right in.
Clocks and watches. Watch making is a fiddly, low-load business. So a component with a simple design is warmly welcomed.
Bicycles. The next time you’re riding to work, look down. "Oh, look, a spur gear..."
Aircraft APUs. Noise generally limits their use in cars. But not aeroplanes. Their high reliability and comparably low load makes spur gears great for APUs.
Worm gear uses
Wiggle wiggle. Their speed reduction, torque multiplication and ability to transpose motion by up to 90 degrees in small spaces make worm gears useful in specific applications. So, this little wormy went to mar… sorry, wrong song.
Guitars. Their ability to deliver fine adjustments makes them great for tuning. They’re used in other stringed instruments too.
Elevators. Worm gears are great at stopping and holding loads. There's nothing worse than getting off a moving elevator.
Security gates. Like me, some worm gears don't run in reverse (in fact, I don't run at all). So, depending on their lead angle, the wheel can't drive the worm. Security gates typically use two separate worm gears - one for each motion. Once you’re through, there’s no turning back.
Even better than sliced thread
It might not be load-bearing, but it has been engineered to keep your 'finger-gears' turning. The MetMo team have made Helico: the world’s first helical gear fidget toy. And it’s certainly a handful. Of torque. And fun.
The outer shell has been 3D printed using Polyamide Nylon 12 to make both a beautiful Herringbone and Spur profile. So, there’s no need for thrust bearings either...
The internals are made from machined brass and neodymium N52 magnets, making it a tactile and mesmerising delight. If you want to see how we made our prototype, watch this:
Once you've tried one, there’s no turning back. Because the fun just keeps on going. Round and round. Anyway, if you’d like to grab a pair or two of the Helico, you can here (whilst stocks last). So get ‘em now. In the spur of the moment… See you next time.
