3D render of the Car Differential: a graphite housing cut open along the axle, an orange ring gear and pinion, a blue carrier with the spider gears, silver axles to two white wheels with black tyres, on a white stand
Free 3D model

Car Differential

The rear axle of a car with the top of the housing cut away, 25 cm across the wheels. The crank stands in for the propeller shaft: it turns a 15-tooth pinion, which drives a 45-tooth ring gear and the carrier bolted to it, three crank turns for one turn of the wheels. Inside the carrier two spider gears sit on a cross pin between the two side gears, one on each axle. When both wheels turn at the same speed the spiders ride round with the carrier and don't spin. Slow one wheel down, as the inner wheel does in a bend, and the spiders start to turn on their pin while the other wheel speeds up by just as much. Hold one wheel still and the other turns twice as fast: that is why a car with an open differential gets stuck with one wheel on ice. All the gears are straight bevel gears with real involute teeth, and every part prints without supports.

  • 27printed parts
  • 23 × 25 × 14 cmassembled (L × W × H)
  • ≈ 0.8 kgfilament in total
  • 210 mmbed length for the housing, which prints upside down; every other part fits a 190 mm printer
Download all files ZIP · 0.8 MB

Free for personal use under CC BY-NC 4.0: print it, remix it and share it with credit to Shapekit. Please don't sell the files or the prints.

Take it apart in 3D · Parts · How to build it

In 3D

Turn it, take it apart

Every part as you print it. Pick colours to see what your filament will look like.

The 3D model (2.5 MB) loads when you get here.

Preview colours

Housing and caps
Gears
Carrier
Axles, shafts and crank
Wheels
Tyres
Stand

Drag to rotate · scroll or pinch to zoom

In the download

19 parts, ready to print

Every STL is already placed the right way up. Download them all at once, or one by one.

Housing

The lower half of the axle housing: the round centre with the nose for the pinion and the two axle tubes. The top is cut away along the axle, so everything inside stays in sight. Square pockets take the flanges of the bearing sleeves, and the bores for the pinion shaft and the axles are V-shaped, so each shaft rests on two lines.

Size
204 × 181 × 67 mm
Print
Upside down, the cut face on the bed. No supports: inside, every surface slopes at 45° or more. Needs a 210 × 185 mm bed.
Filament
≈ 156 g
STL 0.3 MB

Bearing cap × 2

Clamps the flange of a bearing sleeve into its pocket, with two M3 × 16 screws.

Size
58 × 22 × 12 mm
Print
Cut face down. No supports.
Filament
≈ 14 g
STL 29 KB

Nose cap

The top half of the nose, over the two journals of the pinion shaft. Four M3 × 16 screws.

Size
56 × 52 × 26 mm
Print
Cut face down. No supports.
Filament
≈ 19 g
STL 86 KB

Tube cap × 2

Closes the journal at the end of an axle tube. Two M3 × 16 screws.

Size
36 × 18 × 13 mm
Print
Cut face down. No supports.
Filament
≈ 8 g
STL 45 KB

Bearing sleeve × 2

A Ø26 mm tube with a square flange. The carrier turns on the outside of the two sleeves, and each axle turns inside one. The flange can't turn in its pocket.

Size
33 × 32 × 32 mm
Print
Standing on the flange. No supports.
Filament
≈ 23 g
STL 62 KB

Ring gear

45 teeth, 114 mm across. Six M3 × 20 screws fix it to the carrier, with the nuts inside the carrier wall.

Size
114 × 114 × 16 mm
Print
Back face down, teeth up. No supports.
Filament
≈ 56 g
STL 0.5 MB

Pinion

15 teeth, on the hex at the end of the pinion shaft. Its back face runs against the rear journal in the nose.

Size
44 × 43 × 17 mm
Print
Small end down. No supports.
Filament
≈ 9 g
STL 0.2 MB

Carrier

The wall that the ring gear bolts to, with two arms that hold the spider pin. The spider and side gears go in from the open side. Nut slots in the arms lock the pin.

Size
80 × 80 × 49 mm
Print
Wall down, arms up. No supports.
Filament
≈ 33 g
STL 0.1 MB

Carrier cap

Closes the open side of the carrier. Two M3 × 35 screws go through it, through the arms and through the spider pin, so they also stop the pin from sliding out.

Size
71 × 54 × 20 mm
Print
Flat face down, boss up. No supports.
Filament
≈ 15 g
STL 0.1 MB

Spider pin

Ø10 mm pin for the spider gears, with a cross hole at each end for the carrier screws.

Size
68 × 10 × 10 mm
Print
Standing up. No supports.
Filament
≈ 4 g
STL 40 KB

Spider gear × 2

12 teeth. The two spiders turn on the pin only when the wheels turn at different speeds.

Size
35 × 35 × 10 mm
Print
Small end down. No supports.
Filament
≈ 9 g
STL 0.1 MB

Side gear × 2

18 teeth, with a hex bore for the axle. Its back face runs against the carrier wall.

Size
47 × 47 × 10 mm
Print
Back face down, teeth up. No supports.
Filament
≈ 15 g
STL 0.3 MB

Axle × 2

Ø12 mm, 103 mm long, with a hex at each end: the inner one for the side gear, the outer one for the wheel. The collar stops it at the journal in the end of the tube.

Size
103 × 18 × 18 mm
Print
Standing on the short hex end, with a brim. No supports.
Filament
≈ 16 g
STL 65 KB

Wheel × 2

Six spokes, 93 mm across. It slides onto the axle's hex and is held by an M3 × 12 with a washer.

Size
93 × 93 × 34 mm
Print
Spoke face down. No supports.
Filament
≈ 93 g
STL 0.2 MB

Tyre × 2

120 mm across, with a grooved tread. TPU stretches over the rim; a PLA tyre is a snug slide fit and gets a drop of glue.

Size
120 × 120 × 30 mm
Print
Lying flat. No supports.
Filament
≈ 140 g
STL 0.2 MB

Pinion shaft

Ø12 mm, with hex ends for the pinion and the crank. It turns in the two journals of the nose.

Size
83 × 12 × 12 mm
Print
Standing up, with a brim. No supports.
Filament
≈ 6 g
STL 26 KB

Crank

45 mm crank on the front of the pinion shaft, held by an M3 × 12 with a washer.

Size
64 × 22 × 14 mm
Print
Front face down. No supports.
Filament
≈ 7 g
STL 45 KB

Grip

Turns freely on an M3 × 35 screw, with the nut in a pocket at the back of the crank.

Size
28 × 13 × 13 mm
Print
Standing up. No supports.
Filament
≈ 3 g
STL 45 KB

Stand

A 184 × 140 mm foot with two saddles that hold the axle tubes, so the wheels turn freely just above the desk. An M3 × 16 goes down through the floor of each tube into a saddle.

Size
184 × 140 × 78 mm
Print
Foot down. No supports: the windows in the saddles have pointed tops.
Filament
≈ 143 g
STL 61 KB
Also needed

Parts you buy

  • M3 screws with nuts: 6 × M3 × 20 for the ring gear and 3 × M3 × 35 for the carrier cap and the grip, with 9 M3 nuts.
  • 14 × M3 × 16 for the caps and the stand, and 3 × M3 × 12 with washers for the wheels and the crank. These cut their own thread in the plastic.
  • A little grease or PTFE spray for the gears, the sleeves and the journals.
  • Optional: TPU filament for the tyres.
Print settings

Slicer settings

0.20 mmlayer height (0.16 mm for the gears)
3walls
20%gyroid infill
PLA or PETGmaterial

For a 0.4 mm nozzle. Filament amounts are estimates for these settings.

Assembly

How to build it

Print the parts

Nothing needs supports. Print the axles and the pinion shaft standing, with a brim. Clean any elephant's foot off the gears, the sleeves and the shafts: the bearings have 0.3 mm play and the gears 0.15 mm per flank.

Build the carrier

Put a nut in each of the six pockets inside the carrier wall. Lay a side gear on the wall, back face down. Hold the two spider gears between the arms, teeth in the side gear, and push the spider pin through the arms and both spiders. Turn the pin so its cross holes line up with the screw holes in the arms, then put the second side gear on top.

Close the carrier

Drop a nut into the slot in each arm. Put the carrier cap on, so the ends of the arms sit in its pockets, and screw it down with two M3 × 35 through the cap, the arms and the pin.

Fit the ring gear

Lay the ring gear against the outside of the carrier wall, teeth towards the arms, and fix it with six M3 × 20 from the back.

Sleeves and axles

Slide a bearing sleeve into each side of the carrier, flange outwards. Push an axle through each sleeve, short hex first, into the side gear. Hold the carrier and turn one axle: the other turns the opposite way.

Pinion

Push the pinion onto the short hex of the pinion shaft and lay the shaft in the nose of the housing, pinion inside. Screw the nose cap on with four M3 × 16.

Into the housing

Lower the carrier with its sleeves and axles into the housing, flanges into the square pockets, so the ring gear meshes with the pinion. Screw a bearing cap over each flange (2 × M3 × 16) and a tube cap over the end of each tube (2 × M3 × 16). Turn the pinion shaft: the carrier should turn freely.

Wheels and crank

Push a tyre onto each wheel from the inside. Slide the wheels onto the axles and fix each with an M3 × 12 and a washer. Fit the crank on the front of the pinion shaft the same way, and the grip with the M3 × 35 and a nut.

Put it on the stand

Set the axle tubes in the saddles and screw an M3 × 16 down through the hole in the floor of each tube into the saddle.

Try it

Turn the crank: both wheels turn the same way, three crank turns for one wheel turn. Now slow one wheel with your fingers and watch the spider gears start to turn while the other wheel speeds up. Hold it still and the free wheel turns twice as fast.

The model has small parts and screws: keep them away from small children.

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Open the makers