Worm Gear Differential Explained: One Input, Two Outputs That Turn at Different Speeds
When a car turns a corner, its outer wheel has to travel farther than the inner one, so the two wheels must turn at different speeds. Yet both are driven by a single engine. A differential is the mechanism that makes that possible: one input, two outputs, free to turn at different speeds.
This article walks through a worm gear differential, a design that puts a worm and wheel in front of a bevel-gear differential. It follows the interactive 3D lab from 3D Mechanism, where the model and its motion come straight from the CAD file.
What Is a Worm Gear Differential?
It combines two well-known gear ideas in one assembly:
A worm and worm wheel: a screw-shaped gear (the worm) that drives a toothed wheel at a right angle. It turns the fast input into a much slower, stronger motion.
A bevel-gear differential: two bevel gears facing each other, with small spider gears between them, all riding on a carrier.
The lab lists the parts in the order the motion travels: Worm → worm wheel → right bevel gear → spider gear (top) → spider gear (bottom) → carrier → carrier shaft → left bevel gear
Worm: the input, a screw that spins fast.
Worm wheel: the toothed wheel the worm pushes, tooth by tooth.
Right bevel gear: turns together with the wheel, as one piece.
Spider gears (two): small gears that pass the motion across the assembly.
Carrier and carrier shaft: the frame that holds the spider gears and turns on its own.
Left bevel gear: drives the second shaft on the other side.
How It Works, Step by Step
The lab has seven steps:
Worm. The motion starts at the worm, a screw that spins fast.
Worm wheel. The worm pushes the wheel tooth by tooth, so the wheel turns much slower and with more force.
Right bevel gear. The wheel and the right bevel gear turn as one piece.
Spider gears. Two small spider gears pass the motion across, riding around the center.
Carrier. The spider gears sit on the carrier, which turns on its own.
Left bevel gear. On the other side, the left bevel gear drives the second shaft.
All together. Every part moves at once.
Why a Worm and Wheel Come First
A worm gear is a compact way to get a big speed reduction in a single stage, with the shafts at a right angle. One turn of the worm advances the wheel by only a few teeth, so the wheel turns much slower, and the force it delivers goes up in return. Worm gears can also be designed so they do not run backwards (self-locking), but that depends on the lead angle and friction of the specific design, so do not assume it for any given model.
The Core Idea: Spider Gears Let the Outputs Differ
Picture the two bevel gears facing each other with the spider gears between them. The spider gears can do two things at once:
Ride around with the carrier, carrying both bevel gears along.
Spin on their own axis, which lets one bevel gear turn faster than the other.
If both outputs meet equal resistance, the spider gears do not spin, and the two sides turn together. If one side is held back, as in a corner, the spider gears roll, and the speeds split: one output turns more and the other turns less. This is the standard rule of this kind of differential: the average of the two output speeds is set by the carrier, while the difference between them is free.
The lab's headline puts it this way: one input, two outputs, free to turn at different speeds.
Reading the Numbers in the Lab
The lab shows numbers that are measured on the 3D model while it moves. The headline figures are:
Worm: 1,120°
Left bevel gear: 238°
Motion ratio: 1 : 5.3
The live panel shows the worm turn and the left bevel gear turn rising as the model moves, and a running output-to-input ratio. These are measurements of this CAD model's motion, not ratings of a real product. The technical view next to them draws the parts in a front, back, left, right or top view.
What to Try in the Lab
Go through the seven steps and watch which parts light up at each one.
Use Auto to let the sequence play, or Manual and press Space for the next step.
Turn on X-ray to see through the housing and watch the spider gears roll between the bevel gears.
Use CAD colours to tell the parts apart.
Switch the views (3D, Front, Side, Top) to check how the worm sits at a right angle to the wheel.
The motion plays forward and back (ping-pong), so you can watch the same movement in both directions.
Where Differentials Are Used
Road vehicles, where the two driven wheels need different speeds in a turn.
Machines with two outputs from one drive, where the outputs may face different loads.
Mechanical drives that need a large reduction and a compact, right-angle layout, which is where a worm stage helps.
The lab does not name a particular product, so treat these as typical uses of the ideas, not as a claim about this model.
Designing a Worm Gear Differential in CAD
These are general design points, not specifications from this model:
Define the input, the reduction and the two outputs first. Everything else follows from them.
Choose the worm and wheel together. Their tooth profiles, lead angle and center distance have to match.
Fix the wheel to one bevel gear's side. In this model, the wheel and the right bevel gear turn as one piece.
Place the spider gears symmetrically on the carrier. Two (or more) gears spaced evenly keep the loads balanced.
Check backlash and clearance between the bevel gears and the spider gears, and use a motion study to confirm that nothing collides.
Designing for 3D Printing
A printed differential is a good way to see the idea at work.
Print the bevel gears and spider gears with a little backlash so the teeth do not bind.
Print the worm and wheel with generous clearance, and expect more friction than in a metal pair.
Use metal pins or rods for the carrier shaft if the load matters.
Print gears with the tooth faces flat on the bed, so the layers run along the teeth.
Common Problems
The outputs do not turn at different speeds: the spider gears are binding, or friction is too high on one side.
The gears grind or skip: the center distances or backlash are wrong.
The worm jams: the worm and wheel are too tight, or misaligned.
The carrier wobbles: its shaft is not supported at both ends.
Frequently Asked Questions
What is a differential? A gear mechanism that takes one input and divides it between two outputs, which are free to turn at different speeds.
What do spider gears do? They sit on the carrier between two bevel gears. When both outputs have the same resistance they stay still relative to the carrier, and when one side is held back they spin, which lets the output speeds differ.
Why add a worm gear to a differential? A worm and wheel give a large speed reduction in one compact, right-angle stage, so the differential receives a slower, stronger motion.
What does 1 : 5.3 mean in the lab? It is the motion ratio that the lab reports, measured on the CAD model. It compares the movement at the input with the movement at the output while the model moves.
Are the lab numbers real specifications? No. They are measured on the model while it moves and describe this model only.
Related
Four-Jaw Self-Centering Gripper Mechanism Explained — https://www.3dmechanism.com/post/four-jaw-self-centering-gripper-mechanism-explained
Stirring Mechanism Explained — https://www.3dmechanism.com/post/stirring-mechanism-gear-linkage-explained
Double Universal Joint Explained — https://www.3dmechanism.com/post/double-universal-joint-mechanism-explained
Mechanical Movements — https://www.3dmechanism.com/3dcadsketches/categories/2d-sketch
3D Mechanisms — https://www.3dmechanism.com/3dmechanism
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