Double Universal Joint Explained: How Two Joints Let Shafts Bend and Shift at the Same Time
Sometimes you need to send rotation from one shaft to another that is not in line with it. The two shafts may sit at an angle, or side by side with an offset, or both, and one of them may even move while it spins. A rigid shaft cannot do that. A double universal joint can.
The idea fits in one sentence from the interactive lab: one joint can only bend, but two joints in a row let the shafts bend and shift at the same time.
This article explains how the mechanism works, step by step, using the 3D lab from 3D Mechanism, where the model and its motion come straight from the CAD file.
What Is a Double Universal Joint?
A universal joint (also called a Cardan joint) connects two shafts so that rotation passes through even when the shafts are not aligned. A double universal joint puts two of these joints at the two ends of an intermediate shaft.
The model in the lab has these parts, in the order the motion travels through them:
Input shaft → cross → center shaft (two halves) → cross → output shaft
And, so the whole assembly can also move:
Height slider → output bracket → swing arm → swing gear
Input shaft: where the rotation starts. In the model it turns at a steady speed.
Crosses (two): the pivoting heart of each joint. Each cross connects two forks at right angles.
Center shaft (two halves): carries the rotation between the joints, and its halves slide in and out to change length.
Output shaft: the shaft that keeps turning at the far end.
Height slider: lifts the input end up and down.
Output bracket, swing arm and swing gear: let the output end swing side to side.
How It Works, Step by Step
The lab has eight steps. Each one highlights one group of parts:
Input shaft. The turn starts at the input shaft, spinning at a steady speed.
First cross. The first cross pivots on two pins, so the turn can change direction.
Center shaft. The center shaft carries the turn, and its two halves slide to change length.
Second cross. A second cross bends the line again at the other end.
Output shaft. The output shaft keeps on turning.
Height slider. The slider lifts the input about 46 mm, and the joints follow.
Swing bracket. The output bracket swings side to side on its gears.
All together. Every part moves at once.
Why a Cross Makes a Joint Bend
Each joint is built around a cross with four arms. Two opposite arms sit in the bearings of one fork, and the other two in the bearings of the second fork. Because the two pairs of pins are at right angles, each fork can tip around its own axis while the cross carries the turning force from one to the other. That is what lets the shaft change direction and keep turning.
Why Use Two Joints Instead of One?
A single universal joint can bend, but that is all it does. It cannot shift one shaft sideways relative to the other, because both its shafts meet at a single point. Putting a second joint at the other end of an intermediate shaft solves this: each joint takes care of one bend, and the center shaft covers the gap in between, so the two end shafts can now sit at an angle, at an offset, or both.
There is one more reason that engineers know well. A single universal joint that is bent does not deliver a perfectly steady output speed, even when the input is steady. With two joints at equal angles and the right alignment of the forks, the speed variation of the first joint is cancelled by the second one. This is the reason many drive shafts use joints in pairs. The lab does not show a speed graph, but it does show the result: the output shaft keeps turning.
Why the Center Shaft Slides
When the input moves relative to the output, the distance between the two crosses changes. If the center shaft were one rigid piece, it would jam. The lab's center shaft is made of two halves that slide to change length. This is a slip joint, usually a splined connection that transmits torque but allows axial movement. It is the same idea used in vehicle drive shafts and many machines.
Reading the Live Numbers
The lab shows two numbers that are measured on the 3D model while it moves:
Input shaft turn: in the model's 24-second loop, the input shaft turns up to 7,200°, which is 20 full turns.
Output bracket travel: the output bracket moves up to 75 mm.
These are measurements of this CAD model's motion, not ratings for a real product. The lab also includes a technical view that shows the same angle and travel as a diagram, which you can switch between front, back, left, right and top.
What to Try in the Lab
Click through the eight steps to see each group of parts highlighted in turn.
Switch to Auto and let the sequence play on its own, or use Manual and press Space for the next step.
Use the 3D, Front, Side and Top buttons to look at the joint from different directions.
Turn on X-ray to see through the housings, and CAD colours to tell the parts apart.
Slow motion is on by default (×½) so you can follow the parts.
Where Double Universal Joints Are Used
The same idea appears wherever rotation must pass between shafts that are not in line:
Vehicle drive shafts, where the gearbox and the axle are at different heights and angles.
Agricultural and industrial machines, where a power take-off shaft connects to moving equipment.
Steering columns, which route rotation around obstacles.
Machine tools and test rigs, where an offset between a motor and a load has to be absorbed.
Designing a Double Universal Joint in CAD
These are general design points, not specifications from this model:
Start from the shaft positions. Fix the input and output axes and the range of angle and offset they must cover.
Mirror the joints. Placing the two joints symmetrically on the center shaft keeps the bend angles equal.
Model the slip joint explicitly. A splined slide with enough engagement at maximum length prevents the halves from pulling apart.
Use assembly joints to test the motion. Move the slider and the swing arm through their full range and check that nothing collides.
Check the bearing clearance on the cross. Too tight and the joint binds, too loose and it rattles.
Designing for 3D Printing
A printed double universal joint is a good teaching model and works for light loads.
Print the cross arms upright or use metal pins, because layer lines are weak across thin arms.
Leave clearance on the pin holes so the joints move without sticking, and test the fit.
Use thick walls on the forks.
Print the sliding halves of the center shaft with a looser fit, since printed surfaces are rough.
Common Problems
The joint binds at large angles: the forks touch each other. Reduce the angle or increase the clearance.
The output shaft shakes: the two joints are not working at equal angles, or the forks are not aligned correctly.
The center shaft jams: the sliding halves do not have enough clearance or are not aligned.
The shaft pulls apart: the slip joint does not have enough engagement at its longest length.
Frequently Asked Questions
What is a double universal joint? Two universal joints joined by an intermediate shaft. Together they let two shafts that are not in line, whether angled, offset or both, keep turning together.
Why use two universal joints instead of one? One joint can only bend. Two joints let the end shafts bend and shift sideways as well. With equal angles and the right alignment, the second joint also cancels the speed variation of the first.
Why does the center shaft slide? The distance between the two joints changes when the shafts move. A sliding connection lets the center shaft change length so it does not jam.
What does the number 7,200° mean in the lab? It is the total angle the input shaft turns during the model's 24-second loop. That is 20 full turns, measured on the 3D model.
Is a double universal joint the same as a constant-velocity (CV) joint? Not exactly. A double Cardan joint is a related design that can give a nearly constant output speed in certain conditions. A real CV joint uses a different internal construction.
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