2021-06-27 - Prototype 1 Design Review (With RoCoMo - Cory Molloy)

Feedback:

You'll want to have a member that can hold the heavier components
Be cautious of if the motors will be hitting anything when going over obstacles
Have wooden tracks that can simulate the conditions the rover might see in the field
Paper print or 3D print templates to make manufacturing parts easier
Think about how assembly will happen
Make sure to loctite fasteners (242 blue loctite)
Do proper torque calcs to be able to select correct reductions
Don't need to be picky about what weights you use, can just use anything
Careful about fatigue and stress concentrations for things shearing (Use online shearing calculators, those can be useful)
Cross bracing would be useful
Get ready to iterate on the spot
look at yield strength of the material relative to the stress on the keys
Marks (textbook)
Maximize the hub length (shear calcs will drive things here)
Oversize by factor of 2 or three
We have basically 2 by 4 stretching between that, and we're cantilevering it off everything. We're only really off 1 bearing in particular.
Take a look at shaft deflection standpoint wrt weights and stuff.
We don't have a separate bearing support on the end wrt the gear box (scary/bad)
Bevel set reduction from versa planetary (90 degree offset), motor sits flush with 2x4 https://www.vexrobotics.com/217-6293.html?___store=vexroboticsca&___from_store=vexrobotics
Try testing with some wheels removed/broken
Try to 3D print or paper print templates so you can just mark holes and go (instead of trying to utilize drawings, make it a 1 to 1 sample and cut with the saw)
Make sure we have enough planetary sets to do smaller increments (1.5 margin over what we have, and get a 0.75 margin below) customizability is key
Tough time turning, why? : When you turn ,you're running with a drivetrain that has friction points close to the ground, you're dragging all of that. Drivetrains themselves drop the centre wheel to drop the wheel base. (he'll send whitepaper), shorter wheel base affects turning radius, and
affects lever arm to the wheel. Shorter wheel base = better turning elements, accomplish that by dropping centre wheel in that case.
High reflected torque through friction needed to turn
recommended back wheel be slightly higher, from rocker standpoint, if our centre of gravity is forward, and we touch on the front and middle wheel. That will adjust the position of itself.
Things won't be perfect the first time, we spend so much time on the analysis etc, but then things don't work and you'll need to fix these small marginal things to make things way better.

Everything said in the meeting: (initials are who said what)

Watch scrubbing force (RM)
Move forward with prototyping quite quickly, do not get stuck in analysis (RM)
Add mass to simulate loads (LB)
Concerns of MDF board breaking due to the weight of additional mass (RM)

Ex. arm (7-10kg)
Plate weights suggested for testing, if not other raw materials can be used
Add more structure to belly pan-suggested flat 2 by 4s for cross bracing

Slide: materials and parts (RM)

Double check shear calcs for keys and shafts
Key can have a stress concentration
Shigleys and marks - online calculator
Key way shear strength calculator

Slide: Wheel Selection and connection design (RM)

Length of shaft making contact with wheel is
Maximize key length to decrease shear, oversize by factor of 2 or 3
Key both sides of shaft (inner edge) - RM says yes, increase length that shear force is across
Might want to look at shaft deflection in terms of loading on each of the tires

2 supports, rest is cantilever, the shaft in coupling could bend too

Don’t have separate bearing support on end relative to the gearbox

Tough to couple into versaplanetary
Eventually basically have transmission and shaft -2 bearings on either end

Slide: Differential Bar

Approximate various load cases in relation to the movement of the differential bar
Rigidly couple (possibly a 2 by 4 going across with 4 inch side down to hold the bar instead of coupling directly to belly pan, radial loads covered) (RM)
Use bevel set reduction to help protect motors (to sit flush with 2 by 4) (RM)
https://www.vexrobotics.com/217-6293.html
Simulate with wooden tracks (spray adhesive, attach surface), stagger obstacles
Test with some wheels non-functioning (EA)
Adjust horizontally - slotted plates with bearings, wood screws

Final: slots in aluminum tubes

Slide: Pivot Joints

Splintering, wood delamination can occur, RM says likely not an issue
Calculate reflected loads

Slide: Motor Mounting

Mount motor to gusset
Mount gusset to wood
When making, 3D print/paper print templates to mark holes and go (LM)
1:1 sample, cut with bandsaw/chop saw, punch holes and go in with hole saw

Slide: Questions

3d print clamps for wiring
Access bolts between wood and tire - enough space for wrench (MH)
Loctite fasteners in (use 242 blue loctite)
Torque calculations: static torque, expand on motor calculations for transmission, grab different combinations of gear stats to build to 100:1 or 60:1 or whatever we need/choose

Then go outside of those bounds and change limits after testing
Increase speed, help performance

Why may there have been problems turning

Friction points close to ground leads to dragging, difficult to drag
Drivetrains drop center wheel to have shorter wheel base
Affects turning wheel, lever arm to wheel
Difficulty in turning, high reflected torque through friction need to overcome to turn
RM will send paper?
Suggested: front wheels, middle wheels always on ground, back wheels (off the ground)
Lift one set of wheels off the ground
Understand problem efficiencies, use old drivetrain to determine these/for testing