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ROVER WHEELS: Monitoring changes over time, NOTE: Read back through the thread to avoid repeating misconceptions
DeanM
post May 16 2013, 08:35 AM
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[MOD NOTE: This thread follows on a post by Ed Truthan containing a MAHLI mosaic of MSL wheels taken on sol 275.]

Ed: zooming in on that marvellous underbelly MAHLI montage, there appears to be further (and previously noted) deformation of the wheel surfaces.

Damage is most evident in the view of the front-left wheel inner surface.

Such wear has previously been discussed and concerns allayed.

Nevertheless, the front left wheel surface *appears* to have been punctured.

Attached Image


We've only done 700 meters, and have 7000 more to get to Mt Sharp.....

Given that more odometry has now accumulated, is any (re-)new(ed) concern valid?

DeanM
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jamescanvin
post May 16 2013, 10:15 AM
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No, I wouldn't be concerned.

Yes there is a bit of damage, yes there will be 10x as much when we get to Mt Sharp, but the wheels will still be fine to drive on, this was expected.


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MahFL
post May 16 2013, 11:12 AM
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Your going to have to regularly reassure us.....it looks bad to the layman. Of course at least the wheels can be regularly photographed as MSL roves accross this amazing landscape smile.gif.
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djellison
post May 16 2013, 02:05 PM
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QUOTE (MahFL @ May 16 2013, 03:12 AM) *
Your going to have to regularly reassure us.....


If the rover is still roving, such reassurance is utterly redundant.
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jmknapp
post May 16 2013, 02:56 PM
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Isn't the "puncture" merely a glint of light? Could there be anything on Mars hard enough to make a narrow puncture through that much metal?


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mcaplinger
post May 16 2013, 03:03 PM
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QUOTE (djellison @ May 16 2013, 07:05 AM) *
If the rover is still roving, such reassurance is utterly redundant.

That's a nice empirical observation.

Somewhere in a review package I expect there's a detailed analysis of how the wheel was designed, with allowable levels of damage, predicted damage and margins. I haven't seen that package and if I had, I couldn't post the details anyway. So any assurance I could offer on this forum wouldn't really be fact-based, but I assume the designers had it under control.

Over on nasaspaceflight.com someone proposed a drinking game based on how many times this comes up smile.gif


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mcaplinger
post May 16 2013, 03:06 PM
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QUOTE (jmknapp @ May 16 2013, 07:56 AM) *
Could there be anything on Mars hard enough to make a narrow puncture through that much metal?

I won't speculate on these images, but the wheels are pretty thin (I don't know what the actual number is) and the loads can be pretty high on a sharp rock.
Aluminum isn't that hard.


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fredk
post May 16 2013, 03:13 PM
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QUOTE (DeanM @ May 16 2013, 08:35 AM) *
the front left wheel surface *appears* to have been punctured.
We've also discussed this before. I still see no clear sign of puncture. The dents may be reflecting bits of bright sky which makes it look like there are punctures.

About reasurrances, it would be interesting to hear from someone who knows something about the engineering of the wheels. Does the thin skin covering the circumference of the wheels, which is dented, actually provide significant structural strength to the wheels?

Or does the skin just prevent sand and dust from getting into the wheel mechanisms inside, and the raised, narrow, treads are actually providing the main structural strength? In this case the skin could be severely punctured and torn and we could still drive.

Edit: scooped by Joe, and thanks mcaplinger for the details.
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Ant103
post May 16 2013, 03:59 PM
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QUOTE
The dents may be reflecting bits of bright sky which makes it look like there are punctures.


It can't be. We are seeing these things in shadow, and from where there are, impossible to have the sky in sight. Because of the wheel itself, and the rover body.

I'm wondering about the material used to build theses wheels. At first, I thought they where designed in the exact same way as the MER's : a metalic monobloc.


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fredk
post May 16 2013, 05:33 PM
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Being in shadow means they couldn't reflect the Sun, but they still could see some part of the sky and reflect it. The sky is a big thing so it's hard to block all of it!
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PaulH51
post May 16 2013, 07:56 PM
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I am sure this wheel debate will roll on for some time (pun not really intended). I have not found the specification for the material for the rover wheels, but judging from the number of dings in the rims we know the material is very ductile, but it appears to be a high performance aluminium alloy. Reassuringly aluminium remains ductile even at extremely low temperatures, but good to see that drives have occurred at mid day when the temperature ranges reduce any fatigue issues created by very low temperatures.

I would not be concerned with small punctures in the rims, after all we already have the 'Morse Code' cut outs, nor would I be concerned with a plethora of additional dings that we can expect during the remainder of its mission, but am concerned about possible 'work hardening' of the material which over time could reduce the ductility of the material that could lead to undesirably issues.

I am sure the engineering team and drive planners will remind the science team from time to time that the rover wheels are not designed as rock crushers and that we would like them to last as long as the power supply smile.gif


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mcaplinger
post May 16 2013, 08:02 PM
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See slide 32 in http://trs-new.jpl.nasa.gov/dspace/bitstre...4/1/12-0690.pdf

QUOTE
Wheel cleats and surfaces thoroughly tested for odometry, wheel impact, functionality -- localized denting & rupture, ovalization permitted



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djellison
post May 17 2013, 09:36 PM
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QUOTE (Ant103 @ May 16 2013, 07:59 AM) *
It can't be


It is. That 'hole' is a dent, the stress of the dent has popped the anodized coating off the inside of the wheel and we have bare aluminum reflecting the sun. That dent isn't in shadow. Those that are can reflect the bright martian sky off them. Just because something is in shadow - it doesn't mean it can't 'see the sky'.

Look at it - it's not even the same color as the terrain behind it.

And even if it IS punctured - it doesn't matter.

Once of the test-bed rovers at JPL had flight like wheels whilst dealing with 3x the effective weight of a real rover. The wheels were punctured, dent ridden, ripped, torn, dinged, bashed, smashed, crunched. You could put your finger thru the holes in places - you could see clean thru them.

And they still worked absolutely fine.

That testbed now has tougher wheels simply to deal with terrestrial gravity. The lightweight scarecrow rover has flightlike wheels.

I'm not sure how long it's going to take until saying 'the wheels are fine' before it gets boring. Infact, I think it might already have passed.
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djellison
post May 17 2013, 10:01 PM
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QUOTE (mcaplinger @ May 17 2013, 01:50 PM) *
AKAIK the "tire" is one piece of NC-machined aluminum, the hub is titanium, and the two are connected together with titanium flexures.


That's exactly what I've heard from the team - and what it looks like having held a few spare wheels.
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Ant103
post May 17 2013, 10:01 PM
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Okay Doug, thanks for this very clear explanation smile.gif


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