INTRO

Please read the safety and other notes on this page before starting: Introduction.

This is the starting point. The upper and lower crank assemblies are the first things to be put together and then you work outwards from there.

Two such assemblies are needed: one top, one bottom. They are the same except for the shape of the ‘crank arms’ (5, 6). The CAD design is shown below.

Below is listed all the parts that will be needed on this page.

Nb regarding part: ‘motor holder brace (7a) [x4]’ …This was added from Page 7 as the motor holders started to crack. This is explained further below.

For each of the parts listed above, for example: “crank wheel, socket side (1) [x2]”, the number in curved brackets, e.g., ‘(1)’ is the number allocation of the 3D printed parts; and the number in the square brackets, e.g., ‘[x2]’ is how many are needed. The number allocations are shown in the second image below (line drawing).

A couple of minor points regarding the fasteners used…

Also note, the lower lug screw -just visible, bottom right in image above- will be 16mm in length not 10mm long -this is explained in subsequent pages). (It is not necessary to worry about these points here -it is easy to change the screws later).

The designs and 3D prints of these components are shown below.

These are printed in PLA at 0.15mm layers and 40% (or higher) fill.

The smaller hole should be drilled out to 3mm and the larger one to 4mm.

The crank wheel designs are shown below. These have a simple slip-clutch design. A screw and lock-nut cause the wheel to tighten on the motor spindle and can be adjusted to allow slippage to occur to prevent the motor-gearboxes from being overloaded. There is a ‘socket side’ with a round recess for the head of the M3 socket screw, and a ‘nut side’ for the side with the nut-shaped recess, as shown in the two images below.

It is essential to trim excess material off the slip-clutch slot edges otherwise these could stop the gap from closing when being tightened around the spigot shaft later.

Drill out the crank holes in each crank wheel (to the right of the photo above) to 3mm, remove the support material (not visible above) and trim edges where necessary. The clutch screw hole also needs to be drilled out, but to 4mm, for an M4 socket screw.

The ‘crank wheel spacer’ design is shown below.

The edges should be trimmed with a knife or file, especially the spigot hole.

The motor ‘spigot’ design is shown below.

3D print images shown below:

This was 3D printed as previously in green PLA at 0.15mm thick layers and 40% fill. Support material was used (printed flange side down). The spigot has a rectangular hole to engage with the spindle of the motor. Clean up with a knife / file and remove all support material.

Below are some pre-assembly photos of the parts so far made…

ASSEMBLY

Attach one crank wheel spacer (3) to a crank wheel spigot (4):

These should fit together easily but if necessary trim off any residual lips from the 3D printing with a scalpel.

Add a crank wheel.

Two such sub-assemblies are needed: one with the nut-side crank wheel (2) and one for the socket-side crank wheel (1; shown above).

The next stage is to prepare the four motor-gearboxes. This means soldering on wires and the protecting the soldered connections with hot glue.

Various views of the motors are shown below. These are 12V, 200:1 gearing ratio economy motors: one per crank wheel assembly.

The small lug should be filed off (bottom left hand corner in photo above) and a red wire and a black wire should be soldered to the tabs on the motor.

Once soldered (and tested) I normally use a bit of hot glue to stop them getting yanked free or from work-hardening and fracturing. It doesn’t matter which way round the wires go as this will be worked out later when connecting to the terminal blocks.

The wires should have plenty of length say 250mm. This will be trimmed later.

The motor is held between two ‘motor holder’ and inside one and an outside one

This should printed out with support as there needs to be a gap between the screw lugs when the two are holding the motor.

Remove the support material and drill the holes out to 3mm. Make sure that the support material under the lugs (between the lugs and the print bed) has been removed as there needs to be space to attach to the space frame lugs (red arrows below). This can be done carefully with a scalpel and a fine-toothed file.

Add 5 x M3 nuts to each of the four motor holder pieces.

The two motor holder pieces can then be ‘sandwiched’ around the motor as shown below and the M3 socket screws, washers and nuts added (nb: not tightened yet as the lugs from the space frame need to be added).

(* NB: now using captive nuts so less washers are required than shown here).

Nb: the three socket screws on the left should be 12mm long not 10mm. This is to accommodate the the motor holder brace (#7a) which was introduced later because the motors are ‘stressed members’ and suffered some cracking see further below and associated blog post.

When attaching the M4 socket screw to the crank wheel to create the clutch, a 30mm M4 socket screw and M4 locknut is used. But initially the M4 locknut needs to be drawn in to the hexagonal recess and for that an M4 screw with 50mm thread length can be used, as shown below (nb this is included in the kit).

Once the M4 locknut has been drawn into the recess, the 50mm long socket screw can be removed and the shorter one used (30mm thread).

When the crank wheel is ready and is in position on the spigot shaft (as shown further below) the M4 locknut should be tightened until the wheel grips around the shaft but will slip under excessive loads (a simple slip-clutch effect). The idea is to protect the plastic gears and the motor from overloading. This can be achieved by trial and error later when the whole device is assembled. It is usually advisable to keep it rather loose and then tighten it up slightly if it slippage is happening to easily.

Nb., adjust all four crank wheel clutches simultaneously.

Once two such crank wheels have been finished, they are joined along with the con-rod and two M3 washers, a 20mm long M3 socket screw and an M3 locknut as shown below.

The motors and other components can now be added to complete the assembly as shown below.

The next step is to build up the space frame around this (see next page please).

(NB The three screws on the left should be 12mm not 10mm to accommodate the ‘motor holder brace (#7a) added later).

Next page is about building up the top space frame around the crank assemblies