Today’s article looks at worm gears. These clever little screw-and-wheel partnerships have been quietly moving the world for over two thouuussaaand years. Yep, with three zeroes.
Worm gears, or worm drives as they’re also known, are the unsung heroes of many heavy lifts, delicate adjustments and clever engineering hacks. So, we’re going to wriggle our way into a worm hole and learn how they work, who invented them, the different types that exist, where you’ll likely find them, and all other kinds of earth-moving goodness. Helmets on. Let’s go digging.
What is a worm gear?
We all love speed. You. Me. Everyone. It’s wired into us. But every now and then, it’s worth remembering that slowing down has its perks too… you know, like catching the early torque… or… err… something like that…
Anyway.
A worm gear is made up of two parts:
- The worm – a screw shaped shaft with threads spiralling it
- The wheel – a toothed gear that meshes with the worm
When the worm spins, its threads push against the teeth of the wheel, causing it to turn. The clever bit, though, is that for every full turn (of a single-start worm, at least), the wheel only moves forward one tooth. This allows for huge gear reductions and massive torque multiplications.
What’s even cooler is that the setup is almost always arranged so the worm’s axis is at 90° to the wheel’s axis. This right-angled direction change combined with a big torque increase makes worm gears very popular choices for small-spaced applications.
Who made it first?
Credit usually goes to Archimedes, around 250BC. The ancient Greeks had already mastered gears for things like hoisting loads. But Archimedes is thought to have formalised the concept of a screw driving a toothed wheel (hopefully after many worms of encouragement…).
From there, they cropped up in all sorts of ancient mechanisms – from rudders on sailing ships to early winch designs. Fast forward into the Industrial Revolution, and worm gears became staples (not literally) in machinery, manufacturing equipment, and lifting devices.
And the reasons for choosing them haven’t changed much: they’re compact, strong, and can lock in place without extra breaks.
“How?”, you ask. Let me tell you.
How does the worm gear work?
It’s pretty simple. There are a few steps:
- The worm rotates (powered by a motor, crank, or other input). This is usually held in place so it can rotate but not move axially. Duh.
- Its threads mesh with the worm’s wheel teeth, pushing them along and causing the wheel to rotate. The teeth and worm are designed to perfectly match the helical profile of the worm.
- The worm wheel turns, usually at a much slower speed but with much more torque than the worm.
Think: big force, tiny movement.
Now, there are also some other characteristics of worm gears that you might like to better understand. Squirm and learn.
Gear ratio reductions
The number of continuous threads that wind around the worm is known as the “number of starts”. So a full turn of the worm moves just one tooth on the wheel. A two-start worm would mean that for every worm rotation, the wheel moves two teeth. And so on.
Common sense tells us that if the wheel has 60 teeth and we’re using a single-start worm, we’d need 60 full turns of the worm to rotate the wheel once. That’s a 60:1 reduction. Common sense is right.
More ‘starts’ would lead to a smaller reduction and faster speed. Or, if you had more teeth on your wheel, you’d increase the reduction (100:1 is doable!)
Self-locking
Many worm gears can’t be back-driven. In other words, the power can only be driven in one direction – the wheel can’t make the worm turn. This is because of the pitch geometry and steep friction between their sliding surfaces.
Because the worm threads have a very shallow pitch, the sliding action creates too much resistance for the wheel to push the worm backwards. Single-start worms – with their smaller lead angles – are the most likely to self-lock. Multi-start worms, however, have larger lead angles and can be reversible if needed.
Their non-reversing effect is used in two different ways. There’s the static self-locking, where the gear simply won’t move when the motor stops (great for preventing broken toes), and dynamically self-locking, where the gearbox naturally slows and stops when motion is cut.
Of course, it’s not completely foolproof. Materials, lubrication, and speed will affect how self-locking the gear will be. For example, iron paired with phosphor bronze has less friction, so the self-locking is less locking. Or if your gear is dynamically self-locking, your friction will be lower (because its dynamic friction) so vibrations or shock loads can bust through the locking-ness.
Types of worm gears
There are a few types that exist, and they all work in a similar way.
Cylindrical worm gears
This is the most common type. The starter worm. They feature a straight, cylindrical worm that meshes with a matching worm wheel. They’re relatively straightforward to manufacture, which helps keep costs lower. Hence why they’re widely used, especially for applications demanding high torque and a small footprint.
Globoid worm gears
In a globoid design, the worm is shaped like an arc that partially wraps around the worm wheel. This increases the contact area between the worm and the wheel, allowing the gear to transmit higher torque and handle heavier loads. The downside, though, is that they’re more complex and pricey to produce.
Dual-lead (duplex) worm gears
These have different leads on their left and right tooth surfaces, causing the tooth thickness to vary along the length of the worm. By moving the worm axially, you can fine-tune the backlash – the small clearance between meshing teeth. This makes duplex designs ideal for high-precision applications. But again, a little pricey.
What are they made from?
Material choice is critical. These worms and their toothed accomplices live hard lives. Lives full of endless sliding friction. On the one hand, friction is necessary – they wouldn’t work without it. On the other hand, it’s kind of sad. Poor lil’ wormy. Although it’s probably worse for the wheel…
You see, the wheels are generally designed to be the sacrificial component of the two. So, in the classic combo of steel worm (tough, wear-resistant, and expensive to machine) and brass wheel, the wheel will wear away, protecting the more expensive worm. What a friendship!
Of course, these aren’t the only pairings; you will still see some steel-on-steel action. This is a super strong gear, and if one component fails, it generally ruins the other. As you’d expect, it’s rather expensive to replace if things go hats up.
You’ll also see plastic on metal (in light duty robotics) and plastic on plastic in some low-load uses.
The friction addiction
With lives so friction-focused, lubrication is important. Because the worm gear slides across the wheel tooth, it slowly rubs the lubricant away. If you’re doing it correctly, the worm should pick up more lubricant before its next go-round – and protect both pieces. But it’s not easy.
Very high viscosity oils (ISO 320-1000) or specialised synthetics are ideal options to prevent the worm and wheel touching (they can get a bit feisty if they get too close) and handle the constant sliding contact.
Without it, they’ll run rather hot and wear away faster than you can react to their screams. Thread lightly.
There’s a brilliant article on lubricating worm gears here.
Advantages of worm gears
Time to get your worm’s worth.
BIG speed reductions
One of their standout advantages is their ability to achieve massive speed reduction in a single stage. While most gear systems would require multiple stages of gearing to reach ratios of 100:1 or higher, a worm gear can accomplish this in one go. No gear trains necessary for Mr Wormy.
BIGGER torque output
This reduction means they deliver huge torque outputs too. Because each turn of the worm only moves the wheel a tiny amount, the mechanical advantage is enormous. This makes them ideal for heavy lifting, clamping, and holding loads in place.
They’re self-locking
In many designs, the worm can turn the wheel, but the wheel cannot turn the worm. That means when power is cut, the gear holds its position, acting like a built-in brake. In applications, where you don’t want movement when the system is powered down (e.g., lifts and winches), this is a particularly valuable safety feature.
Compact design
Their small design is also a huge plus. Because worm gears combine high reduction with a 90-degree change in direction, they pack a lot of punch in a small footprint compared to their straight-toothed or helical cousins. Worm gear, meet space-constrained applications.
Quiet operation
Our wriggly little worm gears are also known for their smooth and quiet operation. Unlike spur gears, which rattle like my nan’s old teeth, the sliding contact between the worm and wheel produces far less vibration and noise.
Disadvantages of worm gears
They’re cute. But they’re not perfect. Their not-so-good characteristics looks like this…
Low efficiency
The most obvious drawback of worm gears is their low efficiency compared to other gear types. Because power is transmitted through sliding (vs rolling contact), friction losses are high, particularly in self-locking designs.
Heat build up
That lost energy turns into heat (and broken teeth), which must be managed for the gear to perform reliably. Without adequate lubrication or cooling, heat generation accelerates wear – and failure. Another worming.
Tricky lubrication
We touched on this earlier. But lubrication is vital with worm gears. It’s just rather difficult. To avoid some weird form of wormaggedon, folks use high-viscosity lubricants (usually ISO 460 or 680) to keep the components apart… but these can be harder to filter and pump.
Wear on sacrificial wheel
It’s generally done as a necessity, but you still have to deal with it. In the common steel-worm/brass-wheel pairing, the brass wheel is deliberately used because it is softer – to protect the steel worm. So the wheel will need replacing. It’s an accepted but unavoidable maintenance cost, unfortunately.
Manufacturing cost
Lastly the cost. Not only do you need special lubricants and pump, worm gears can be pricey to manufacture. You need precision machining to prevent losing teeth like hair – and ensure proper meshing. Of course, for more complex forms, like globoid worms, you’ll need preciser machining.
How do they compare to spur and helical gears?
Now, I know some of you might be thinking about comparisons. So here they are in table format. Nice and easy. Figures are a guide, of course. (Oh, and learn more about helical gears here)
|
Feature |
Worm Gear |
Spur Gear |
Helical Gear |
Herringbone Gear |
|
Teeth |
Screw-like thread |
Straight (parallel to axis) |
Single diagonal |
Double opposing diagonal |
|
Reduction ratio |
Very high (up to 100:1 in one stage) |
Moderate (1-10:1 per stage) |
Moderate to high (up to~20:1 per stage) |
Moderate (similar to helical) |
|
Torque output |
Very high |
Good but limited per stage |
Higher than spur (more tooth contact) |
High |
|
Efficiency |
Low(er) |
Very high |
High |
High |
|
Noise |
Low |
High |
Medium |
Low |
|
Self-locking |
Yes |
No |
No |
No |
|
Load handling |
Excellent for high loads |
Good for moderate loads |
Excellent for high loads |
Excellent for very high loads |
|
Complexity/cost |
Some precision machining required |
Easy and cheap to make |
Slightly more complex, higher cost |
Most complex and highest cost |
Out of the can and into the world
Big torque, compactness, and self-locking are the aims. Worm gears are the game. Some typical worm drive applications include:
Elevators and lifts: This is probably their most famous use. Worm drives prevent cars from falling when the motor stops, thanks to their self-locking feature. Without worms, the only way would be down…
Tuning pegs: They hold strings in tune on instruments like guitars, violins, and cellos. Their biggest perk is their precision. Small movements of the peg translate into fine adjustments of string tension – and again, their self-locking stops them creeping back out of tune.
Automotive steering: Yep, cars use worm gears too. Older ones, at least. They’re used to convert rotational motion (from you on the steering wheel) into controlled movements of the steering linkage. The torque multiplication helps you turn heavy front wheels without needing superhuman strength.
Industrial conveyors: Conveyors often depend on worm drives to deliver controlled, powerful motion. Because they can handle high loads and offer precise speed reduction, they’re perfect for moving products along in an orderly flow.
Rudders on ships: For centuries, our squirmy gears have helped steer ships – big undercurrents stand no chance against the almighty worm. They hold the position of the rudder in strong forces, so you’re not being dragged about by the sea.
Hoists and winches: As with elevators, worm gears hold heavy loads securely without needing an extra brake to help. They house huge torque in compact spaces. They might be the only worm to not wriggle under pressure.
Power tools: Circular saws, drills, and grinders use worm drives because they allow high-speed motors to be wormed into usable torque – and not take up lots of space.
Medical equipment and robotics: Worm gears excel here with their precision, compactness, and holding power. They have to sometimes make millimetre-accurate movements and not slip mid-job. No trouble ol’ wormy.
Wriggling into the future
The worm gear is proof that you don’t have to be fast to make a big impact. By trading speed for torque and adding a built-in safety brake through self-locking, worm drives have become a mechanical staple – from the sea to shopping centres to guitars.
They may run hot from time to time (who doesn’t, eh?), and wear out their wheels if they’re not looked after… but when you need compact, quiet, reliable power, the worm gear rightfully earns its spot.
So, the next time you hitch a lift in a lift (without one of our creations in your hands), remember somewhere in there, a little screw and wheel are doing the heavy lifting.
Speaking of MetMo creations, have you seen these?
Helico
Our beloved fidget gear. Have a look here.
Fractal Vise
Our most successful Kickstarter campaign yet. Take a look at why here.
Thanks for reading. Hopefully you know more about worm gears than you did before! Come join our discussions over on the MetMo subreddit and CubeClub forum. We’d love to chat about all things gears – or whatever else tickles your fancy.