Updated 15 April 2020
This section documents the mechanical design of the MIT E-Vent.
Note: Any mechanical design must meet the specifications outlined in the Key Ventilation Specifications page.
We are in process of continually testing and refining our prototypes to increase robustness. The basic concept consists of two arms that gently close in sync to compress the bag. This must be coupled with a closed loop control system. Major mechanical design requirements include:
- Be nice to your bag and its hoses – Up to 7 day ✕ 24 hour ✕ 60 minute ✕ 30 bpm ✕ 2 stroke = 604,800 cycles will be needed for 7 day usage. Any design must secure the bag and gently grasp and squeeze it from both sides to reduce the risk of material fatigue. The grippers must be smooth and shaped to maximise air expelled without damaging the bag. The bag must be supported with flexibility to allow motion during operation.
- Fail-Safe operation – If the machine fails, a clinician must be able to immediately shut down, open the device manually, remove the bag and convert to manual bagging.
- Keep It Simple – Empower and support others to fabricate. We are focusing on the lowest specification system and open-souring our design information for adaptation to local supply chains.
- Multiple drive motor and sensing possibilities! Enable multiple configurations to meet local supply chain capabilities.
The overall dimensions and operation are now set and any skilled mechanical designer will be able to execute this design and adjust it to suit locally available materials and fabrication technologies. We have ready access to waterjet and laser cutters and 80/20 components, however we are now focusing on designs that can be CNC milled, stamped, molded, welded and bolted as per your supply chain and capability.
Version 3.1 herein is our most recent prototype design. Older prototypes are available in Past Designs.
Version 3.1 – Testing / Pre-Production
For more information on Version 3.1, including CAD files, please visit the Downloads page.
- Big gear (bottom of arms): 16 pitch, 48 tooth, 3 in. pitch dia., 14.5° pressure angle, 0.25 in thick.
- Pinion dear (driving): 16 pitch, 30 tooth, 1.875 in. pitch dia., 14.5° pressure angle. 0.5 in thick – this is to accommodate axial misalignment with the arms’ gears.
- Gear ratio: 1.6 (arm/pinion)
- Based on the estimated torque (τ) of 10 N-m per arm, given in Power Calculation, divided by the gear ratio, we arrive at 12.5 N-m applied to the pinion of diameter (d) 0.0476 m (1.875 in) with pressure angle (φ) of 14.5° the net radial load (F) on the pinion is given by: F = 2τ/(cos(φ)d) = 2*12.5/(cos(14.5)0.0476) = 550 N.
- Also, for a handy diagram see Engineer’s Edge.
- This radial load is applied to the pinion approximately 2 cm from the face of the gearbox which results in a bending force on the on the gearbox shaft that must be withstood by the gearbox bearings. Consult your motor manufacturer.
- We have created a Gear Torque and Speed Estimator Spreadsheet, available in Downloads.
- Material choice is extremely important – we prototyped, based on the materials readily available in the shop. Arm gear and driving pinion life must be checked for wear and fatigue, as a function of your material selection and width of parts. (Note: this is an oscillating load with force on the in stroke, while the return stroke is nearly unloaded.)
- Aluminium is not recommended. We recommend steel gears, but not stainless as this will gall/spall. Hardening the steel gears and adding lubrication will increase life.
- We have created a Gear Stress Estimator Spreadsheet, available in Downloads.