Concept Crank II

Disclaimer: This is prototype work in progress, not certified for medical use. Specifications, parts and instructions may change rapidly.

Description

This concept imitates the movement of the human hand originally intended to operate the bag. The bag is compressed between a lever and two bag holders. A crank disc translates the rotational motion of a gear motor to an oscillatory pushing movement of the lever. Therefore, it is possible to use a simple geared motor without any electrical circuitry.

Video showing V1 of the Crank II concept.

Required Components

  • Bag valve mask
  • Printed parts from Thingiverse
  • Electric motor and power supply: e.g. HG37-300-AA-00 and 12 V / 0,3 A PSU
  • Wooden baseplate (also possible to print): ca. 120 x 115 x 20 mm
  • Standard parts: 1 pc. M5x35 screw, 2 pc. M5x25 screw, 5 pc. M5 nut, 7 pc. M5 washer, 2 pc. M5x18 washer, 3 pc. M3x10 screw, 5 pc. 4x17 wood screw, 4 Pc. 4x35 wood screw
  • Required tools: wrench/allen keys according to screws and nuts, screwdriver for wood screws, small wood drill, pliers

Thanks to Andy Triboletti for creating a shopping list on his blog.

Build Instructions

  • Print plastic parts. At 20 % infill, 112 g PLA is required. Print time total is about 10 hours with an Ultimaker Original.
  • Craft wooden baseplate
  • Use wood screws to attach motor bracket, bag clamps and bearing support to the baseplate.
  • Attach motor to motor bracket.
  • Push disc onto the motor shaft. This may require some force.
  • Assemble the rotational and linear bearings from M-screws, washers and nuts.
  • Attach bag stamp to the end of the lever.

Electronics

Currently a HG37-300-AA-00 DC gear motor and a 12 V / 0,3 A PSU are used. The device uses the rotation directly for ventilation, so a motor speed of about 15 rpm seems advisable. Adding a motor control circuit it is possible to set custom ventilation frequencies.

Lessons learned

  • The bags sits securely in the bag holders, even when pushed by the lever.
  • In V0 the printed plastic parts (bag holder, motor bracket) were not stiff enough to counter the resistance of the bag being compressed. A truss design was implemented successfully to stiffen the structure.
  • The shaft-hub joint presents another challenge: The shaft was slipping in the 3D-printed disc under high torque. After tightening the clearance and printing the disc with 90 % infill the torque transmission works much better. Still, improvements have to be made for long-term use.
  • In V0, a M5 bolt was used as a linear bearing. After working for some time in the device, the PLA bearing face of the lever was ground down by the bolt thread. Hence, in V1 a bearing bush is used. The bushing is made from aluminum, but may be printed as well. Furthermore, a metal surface was added to the bearing face of the lever.
  • Challenges to work on: The design requires relatively tight tolerances in assembly, otherwise there is a lot of friction between moving parts. Wear of the linear bearing and the shaft-hub joint have to be observed in long-term testing.
  • Further reduction of print time is advisable.