Week 11 - Analysis on the Linear Lifting Sub-assembly

Note: This section was updated after week 12&13 to include the appropriate sub-assembly drawings for visual reference,

This is the final sub-assembly of the linear lifting system with a BOM (see Week 12&13 page for more info).

To reiterate, the purpose of this sub-system is lower and raise the top gripper sub-assembly (this is what applies the torque to the lid to open the jar/bottle). Originally, we had decided on using a pulley system to achieve that purpose, however we realized this "CNC-style" system was more efficient for the following reasons:

• Based on our original torque calculations, the pulley system required a certain width and height to achieve the desired torque - it would take up more space.
• A pulley system would require the use of some kind of elastic material e.g., a belt or some type of rope, which would wear out faster compared to using a threaded rod on the CNC setup.
• A pulley would not be able to raise and lower the top gripper sub-assembly as steady as the CNC setup.
• Lastly, a pulley system would require analysis on belt design and pulley systems, which seemed like a lot of unnecessary analysis compared to the analysis for the CNC setup.

To determine how to adapt a CNC z-axis rail system to our sub-assembly, we referenced various CNC blogs and YouTube videos on homemade CNC machines. Here is a list of references we used:

• https://buildyourcnc.com/torquemotion.aspx
• https://www.cnczone.com/forums/diy-cnc-router-table-machines/200872-cnc.html
• http://www.nookindustries.com/Content/image/LinearLibraryItem_Ball_Screw_Design_Considerations_LinearLibraryItemImageGallery_5.png
• https://www.cnczone.com/forums/stepper-motors-drives/354208-z-axis-heavy-stepper-choice.html
• https://youtu.be/IVpZtrmzP8s

We also found a few kits that detailed what parts are required for a CNC z-axis rail system, however some of the ones that we found were very exepnsive, except for this one:

After seeing that this system could be viable, we then wanted to understand what type of motor we would need for our lifting mechanism. To calculate the necessary torque for our system, the following equation was used:

T = (P*L)/(2𝝅e)

Source:http://www.nookindustries.com/Content/image/LinearLibraryItem_Ball_Screw_Design_Considerations_LinearLibraryItemImageGallery_5.png

Where:

T = Torque (lb*in)

P = load (lb)

L = screw lead thread (in/turn)

e = ball bearing screw efficiency = 0.90

From the top gripper group, it was told to us that the load that needed to be moved was 7lb (3.2kg). We selected a lead screw size of 2mm, with a 2mm/turn screw lead thread (converted to 0.0787in/turn). As shown above, the ball bearing screw efficiency was found to be 0.90. The necessary torque was found as:

T = 7lb * 0.0787in/turn(2(0.90))=0.097lb*in

With this torque value, we then went to see what motors we could select. From this selection and from videos we saw online, we decided to use the website servocity.com as our parts distributor. From there, we collected the bill of materials shown below: