2013 Pivot Development

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October swerve module work

Over the past few weeks prior to 17-Oct-2012, we have been modifying DEWBOT VIII's swerve modules to test cost- and weight-saving design changes. Thus far we've made 2 changes each on 2 pivots and have begun testing them as the front (intake & bridge arm side) on DEWBOT VIII Prime.

  • #35 to #25 Wheel Drive Chain: the wheel drive chain has been changed from #35 to #25, maintaining the same gear ratio. The #35 9T Transfer Axle Sprockets and 24T Wheel Drive Sprockets were changed to 12T and 32T, respectively. The maintained an integer #25 chain link distance and thus did not require any new machining. 1640 has never used #25 chain, but given the weight savings we're interesting in investigating its practicality.
  • Bearing to Bushing Coaxial Drive Shaft: The 1" low-friction, high-durability ball bearings on the Coaxial Drive Shafts have been switched with bushings, one plastic-on-brass and the other plastic-on-plastic. (This did require remaking the cage top plate and module bottom plate.) The bearings have never failed on any generation of pivot, but at $18/ea (x8), they represent a major cost investment both in real dollars and Bill of Materials quota. (The entire FIRST competition generally cannot cost more than $3,500 + Kit of Parts). They're also represent a major weight-saving opportunity, as most of the rest of the module is running out of reasonable diet options.
Test Results
  • Coaxial Drive Shaft Bushing Failure: The 2 pivots were put to the test at driver practice on 18-Oct-2012, primarily in bridge-balancing and slalom runs (the shooter was malfunctioning). Unfortunately, one (the front left, plastic-plastic bushing) steering motor failed catastrophically in less than a hour, smoking visibly for almost 10 minutes and getting hotter than any steering (or other) motor we have ever had. The other test pivot (front right, plastic-brass bushing) was the second hottest motor we've ever had, though it did not emit smoke. Though the motors were checked periodically throughout the practice, the heat seemed to spike from mid-high normal to catastrophic in a matter of minutes, albeit during a rather difficult slalom course. Initial assessment attributes the failure to additional friction from the Coaxial Drive Shaft bushing.
    • Update: Upon disassembly, the plastic-on-plastic bushing module's failure has been attributed to steering motor gearbox lockup. However, based on the observed plastic-plastic stiction, we have decided to terminate this option anyway. The plastic-on-brass bushing module's failure is due partially to a re-assembly error causing undue vibration in the steering motor. This has been remedied and the module will be re-tested.
   11-27-2012 Update. The Igus Bronze bushing module failure was from poor assemble. Will be re-mounted and tested again.
  • We have not observed any issues with the wheel drive chain.

Steering Motor Mounting

Deaver steering motor mount with conceptual polycarbonate motor guards
In 2011 & 2012, the design for mounting the steering motors onto the pivot modules turned out to be unsatisfactory, due to the extreme effort required to change them when damaged. While usually not a competition issue (we just swap pivots), the shop repair time was too long. This problem was not so obvious in 2011 because the BaneBots RS540 motors used for steering in 2011 has internal cooling fans and a robust mount and as a consequence seldom failed. In 2012, we adopted the smaller, lighter RS395 motors for this job. In addition to their reduced size and weight, 1) they lack an internal cooling fan; 2) the front-face cooling vents are blocked by the P60 back plate; and 3) the front-face threaded mounting holes are in very thin metal and are easily stripped.

Gary Deaver modified the mounting by utilizing (4) 6-32 1/4" hex spacers into which both the front and back plates of the P60 gearbox are bolted (independently). The threaded holes in the P60 backplate need to be bored out (0.144") for 6-32 bolt clearance. This mount will allow quick removal and replacement of the steering motor/P60 backplate assembly.

Polycarbonate steering motor guards are also under consideration to prevent damage to the delicate motor mounts. In addition, we are investigating how this guard can 1) prevent undue vibration in the RS395s; 2) incorporate improved motor cooling measures; 3) incorporate a a more secure electrical plug (the tab connectors often become detached).

We also intend to mill away part of the RS395s' P60 backplates to allow better ventilation through their front-face cooling vents.

Steering Motor Cooling

Here follow a thread of concepts for improved cooling of the RS395 steering motors.

A common point to all of these concepts is milling channels into the rear face of the P60 backplate to open access for the cooling air openings in the motor's face. The current backplate blocks these openings. P60 backplates for the RS540/RS550 motor come with these milled channels for this purpose.

Pivot Module 9

Module 9a.jpg
Module 9b.jpg

We are looking at the following changes:

  • Replacing 35 chain with 25 chain - 9T & 24T sprockets replaced with 12T & 32T - 44 link 25 chain reduces pivot cage height
  • Change wheel to AndyMark 4" HiGrip Wheel (am-2256) - 32T sprocket secured w/ 10-23 x 5/8" SHCSs - No sprocket spacers needed
  • Narrow pivot cage from 4" to 3.184" - machined acetal spacer eliminated - shorter axles
  • Pivot tube designed to be welded into the pivot top
  • Drive miter gear shortened axially by 0.25" - further reducing pivot cage height
  • Elimination of (1) of (2) E-clips on the pivot tube
  • Reduce spacer length between module plates to 1.75" (2" in 2011; 1.875" in 2012)
  • Replace BHCSs securing module plates with FHCSs - shift bolt/spacer nearest steering drive pulley 0.05" for clearance
  • Use Deaver steering motor attachment
  • Clamp collar securing the 56T HTD5 pulley

17-December update

Module 9d.jpg
Based on feedback, a few changes were made:
  • Switched to BaneBots RS540 motor in lieu of RS395
Reliability & avoidance of complex cooling & guard schemes with vanishing mass benefits
  • Returned to BHCSs (in lieu of FHCSc) to secure the plates
Machining avoidance
  • Adopted the VexPro 32T sprocket
Reduced mass & cost; improved design
Bill of Materials is nearly complete. Provisionally, it looks like we save 0.21lb (2.4%) and $44.21 (13.0%) vis-à-vis the 2012 pivot module. Some of the cost savings is based on securing (2) CIM motors via FIRST Choice (the other two assumed to come in the Kit of Parts).
We are currently testing a modification in which the top 1" ball bearing race is replaced with metal and plastic bushings. This could nearly double weight savings, and save significant additional cost as well. This should work, but performance needs to be confirmed.