Integrated Coil Spring Car

Some time ago whilst browsing 3D printing sites, I would notice model cars which all appeared to have one thing in common and this was a wind-up mechanism. Nothing un-usual in that respect but the spring was wound with a key. Not only this but the spring was a separate element.

However, looking at this I set about to simplify it, creating a keyless mechanism that can still be wound.

The first area to look at was the spring, rather than this being a separate element why not integrate this with a cog, the result being a integrated cog spring.

As for winding the spring, a pull back mechanism which simply requires the user to pull the car back (with the wheels in contact with a surface), in the opposite direction to the direction of intended travel.

The integrated cog spring (engine), would be 3D printed and the remaining elements will be made up from LEGO pieces. A list of parts used in this specific design is included.

3D Printer

Suitable Filament

Studio 2.0

BlocksCAD

Cura

Lego parts:-

1 x 16 brick * 2

1 x 12 brick

Wheels * 4

16 teeth gear * 2

8 teeth gear

Axle Peg

3M Beam

Half Bushing

Connector pegs * 3

10M Connector Axle

8M Connector Axle

6M Connector Axle

4M Connector Axle

Bushings * 5

The integrated cog spiral spring engine, was designed using BlocksCAD

Once complete an OBJ file would be loaded in Cura for 3D printing.

This consisted of the main cog with integral spiral spring, a smaller drive cog and supporting housing.

It was decided early on to make the mechanism LEGO compatible in respect of spacings and fixings.

Both ends of the spring will remain stationary the centre of the spring is to remain held in place by a small axle (in cross section a 4 arm equilateral cross), and this passes through the centre cross hole and into the support housing being in both cases a interference fit requiring no other fixing elements.

The outer end of the spring is attached to the inner perimeter of the cog.

Therefore the outer edge of the cog moves around in relation to the fixed centre winding the spring.

The smaller drive cog also has a centre cross hole but this cog needs to rotate freely therefore the drive axle fits though the cog and in to a round hole in the supporting housing.

A centre cross hole is placed in the surrounding housing to improve stability.

Making a spring consideration needs to be given to the modulus of elasticity (Young’s modulus), the ratio of stress to strain in the elastic deformation. Steel alloys are in the range of 200 GPa and in comparison PLA 3.5 GPa a larger number indicating its stiffer.

This indicates PLA is at a disadvantage against a equivalent steel alloy spiral spring.

However, there are a number of options available to overcome this.

1: Increase the width of the spring material.

2: Increase the thickness of the spring material

3: Increase the number of coils

4: Increase the spring diameter.

Spring dimensions.

Material width = 5mm

Material thickness = 1mm

Number of coils = 4

Diameter 50mm.

Based on this information we can calculate the spring length and the torque.

Length = Pi * n * (Ro + Ri) = 3.142 * 4 * (2.5 + 0.8) = 41.47 cm

Torque =(Pi * E * b * t^3 ) / (12 * 180 * L) = (3.142 * 3.5GPa * 0.2 * (0.04^3)) / (12 * 180 * 16.32) = ~4 lb in/deg.

An equivalent steel alloy spring of a similar design would have ~ 5 times the Torque.

Considerations need to be taken into account that the spring will be 3D printer and too tight a coil will result in glued or incomplete coils and or the inability to wind the spring tight enough to be useable.

Additionally as this is a spring cog the number of teeth can be increased and the number of teeth on the gear cog can be reduced increasing the gear ratio and the number of turns of the drive axle.

Spring cog = 30 teeth

Drive cog = 9

Gear ratio = 3.33 to 1

Load the OBJ file and apply the required slicer setting.

Settings applied in this case, although these can be changed if required.

Layer Height 0.15mm

Infill 80%

Infill pattern - Tri Hexagon

Build Adhesion - Brim

Weight - 40g

Filament length - 13.29m

Size - 156.9 x 85.6 x 7 mm

Print time 7hrs - The infill can be reduced to speed up the print time.

Assembly starts with the engine.

Push an 4M connector axle through the centre of the spring and align this with the corresponding hole in the centre of the surround. You may need to apply a needle file to widen the opening subject to printing variation.

Push the axle through the surround and press the spring in place, make sure that the spring cog can move freely within the housing but that the axel is locked in place.

In the course of making this Instructable I evaluated Lego design applications Studio and Mecabricks finally settling on Studio and used this to layout the design.

Of course this is minus the Integrated Spring Cog engine.

In to the surround push an 8M axle. Again adjusting with a file if required.

Leave 35mm protruding from the front.

On the end protruding from the back of the surround take a 1 x 16 brick and push the axel through the 4th hole on the left place a bushing over the axle end ensuring it stay on.

In to the left most hole on the 1 X 16 brick place an connector peg and onto this attach a wheel.

In to the 9 tooth drive gear push a 6M axle ensuring that this goes through the hole in the surround and into the 5th hole on the right in the 1 x16 brick placed previously.

On the remaining length of the axle place a bushing then a 3M beam using one of the end holes then slide a 16 tooth cog on to the axle.

In to the other end of the 3M beam inset an axle peg on the other end of the peg place a 16 tooth cog ensuring this is aligned on the same side as the previously placed 16 tooth cog.

On to the free end of the 8M axle previously inserted push on a bushing approximately 1.5 cm and onto this attach a 1 x 16 brick at the 4th hole on the left and attach a bushing.

On the 1 x16 brick in the first hole on the left fit a connector peg and then attach a wheel.

The drive gear should fit through the 5th hole on the left of the 1 x 16 brick.

Fit a connector peg into the 7th hole on the right, this is used to prevent the clutch dragging on the surface.

Fit the 10m axle partially through the third hole on the right of the 1x 16 brick and fit a bushing and the 8 tooth cog.

Adjust the bushing and the cog to align with the clutch gears and preventing too much sideways movement whilst maintaining an equal protrusion either side of the two 1x 16 bricks already placed to allow attachment of the 2 from wheels.

Place a 1 x 12 brick across the front of the two 1 x 16 bricks on the first peg.

Assembly is complete.

The Engine is linked to a 16 tooth cog which then links to a 8 tooth cog = 2 to 1 gear ratio.

The total gear ratio being 6 to 1.

For a spring cog diameter of 60mm the circumference is188 mm but the wheel size in this case is only 30mm the circumference is 94 mm. Meaning the travel is 56 cm.

This on the face of it does not seem like much and would be an issue if the gears were connected directly to the wheel axle and would stop once the spring had unwound.

In order to prevent this a slipping clutch is incorporated to disengage the gears when the spring has unwound enabling the car to freewheel. In this case on a smooth surface the car can travel 160 cm

The drive train has high a degree of friction that has to be overcome which could be reduced considerably by the use of bearings, gearing adjustment, weight reduction and the rolling surface are other considerations for improved efficiency.

Holding the car, place it on a flat surface and roll the car backwards this will engage the clutch and will wind up the spring. Do not try to force the spring at this stage as it may result in it breaking.

Another clutch could be incorporated to negate this issue.

However, once the spring has been wound up release the car and watch it go.

In it's current incarnation it works most effectively on a flat hard surface.

Thanks for reading.

Hope you found it interesting.