Weekly Brainware
Week 10
Part 1: Lead Screw Mounting, Bearing Life & Mounting, Misc Connections
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Previously, my plan was to support the lead screw at two ends such that it was a Fixed-Simply supported beam (see Brainware 7). I planned to achieve this by using a moment connection at one end using two bearings suitably spaced apart and by having a self-aligning bearing at the other end. I primarily wanted to have this configuration as it allowed for a greater load capacity before bucking, given that I was using a 10mm lead screw of length 700mm. After discussing my design with Professor Slocum, he recommended going with a simply supported configuration given that it is more forgiving of angular misalignment. Given that my bearing blocks would be mounted directly to a wooden backing, there is a good chance of having misalignment as the wood expands and contracts.
This also lowers cost of the desk as I require 1 lesser bearing. However, I soon realised that (from leadscrew design spreadsheet) the lead screw would now buckle for a lower load. In order to rectify this, I increased the diameter of the lead screw to 12mm. My results are summarized below.
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Note: Shaft whip is not a major concern in this case given the speed of the desk.
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Next, I calculated the load on the bearings due to misalignment. This was to ensure that the bearings do not overload ​during operation. It was also necessary to determine the load life of the bearings.
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I first estimated the reaction forces at the supports. This was estimated to be 190N for maximum load operating conditions. Next, I determined the reaction forces due to shaft misalignment. This was estimated to be 6N using the bearing stiffness alignment spreadsheet. I went with a loose fit on the shaft with h7 tolerance based on the application.
With a combined load approximated to 200N and with a safety factor of 2, the L10 life of the bearings was estimated to be 213 million revolutions. If the lead of the screw is 3mm and the length of travel for my desk is 800mm, I would be able to move my table top all the way up and all the way back down, for nearly 400,000 cycles. The bearing would probably corrode before they wear due to loading.
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Bearing Block Redesign: UPDATED REFER WEEK 11 BRAINWARE (THIS DESIGN NOT VALID)
I redesigned the lower bearing block from housing two bearings to a single bearing. +
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While the calculations specify 1 bolt is needed, the design will factor in 2 to constrain the parts together.
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Previously, I envisioned milling out the block from aluminum barstock. If I am unable to secure this barstock, my backup plan is to waterjet the block as 2 separate plates and fasten them together. For this design, I ensured that the stress cones would overlap( shown below) to create an excellent connection.
The updated CAD can be downloaded below.
The final version of all my design spreadsheets can be downloaded below.
First Order Analysis
Final Error Budget
Detailed joints and support designs
Leadscrew Design
Bearing Alignment Stiffness