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

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

Jul 01, 2021

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