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Micro Magnum, Team Magnum, NSW
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MoonSet416



Joined: 25 Sep 2016
Posts: 436
Location: Sydney


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Yeah will be using 3M VHB for that.

EDIT: to be exact it's VHB 4932

Post Sat Sep 23, 2017 12:57 am 
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maddox



Joined: 21 Dec 2006
Posts: 786
Location: Belgium


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I would suggest using a load of bolts too.

Post Sat Sep 23, 2017 1:37 am 
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MoonSet416



Joined: 25 Sep 2016
Posts: 436
Location: Sydney


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Bolts are certainly stronger than tape... However it is possible that they will get grabbed by a spinner and transfer a lot of force into the bot (even countersunk screws). (for example the first fight of King of Salted Fish vs Unicorn in the August FMB tournament, where the screws in the side armor cost KSF a match)

Post Sat Sep 23, 2017 3:26 pm 
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MoonSet416



Joined: 25 Sep 2016
Posts: 436
Location: Sydney


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Update:

Changed weapon mount design, now 30g lighter, a bit shorter (height wise, for better clearance when upside down), and uses a slot instead of a swing arm for belt tensioning (one less part, yay!).

Post Sat Sep 23, 2017 3:37 pm 
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MoonSet416



Joined: 25 Sep 2016
Posts: 436
Location: Sydney


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Hey I just found out that this has been a whole year since I started this project... Big thanks to all of you guys, Without your help I couldn't have been here.

I will continue to take Contraption to competitions and make improvements, and maybe make some new robots in the distant future...

An overview of all past versions of Micro Magnum/Contraption IIC:









Post Sat Sep 23, 2017 4:15 pm 
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MoonSet416



Joined: 25 Sep 2016
Posts: 436
Location: Sydney


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Question:
The disk part of my spinner is 16mm wide each, and the bearings are 7mm wide each. What would be better? 2 bearings with 1mm left off either side or 3 bearings with 2.5mm of the bearings sticking out on either side?

Post Sun Sep 24, 2017 9:25 pm 
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Nick
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Joined: 16 Jun 2004
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Location: Sydney, NSW


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The 3rd bearing has a good chance of breaking or not really supporting the disk due to being so far out of the disk. I would still leave it in there as it can't hurt and should add some support. 7mm is thin for a bearing supporting a KE weapon - what is the bearing number?
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Post Sun Sep 24, 2017 9:37 pm 
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MoonSet416



Joined: 25 Sep 2016
Posts: 436
Location: Sydney


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Yeah if two bearings were covered and only the third one was sticking out, it's doing pretty much nothing.

What I was thinking about was to have one bearing right at the center, and two bearings each with 2.5mm sticking out from each side. The thought behind this was that since 2.5 is less than half of 7mm, the bearing balls might still be supporting the disk. Does that count as three bearings supporting the disk?

Sorry not sure where to look for bearing numbers... Here's a screenshot of the Ebay page.http://www.ebay.com.au/itm/Aussie-Bearings-Precision-High-Speed-Bearings-Heat-Resistant-Proven-Quality-/151628723903?var=&hash=item234dc6b6bf:m:mK8gpe5V2RiWmub40LbMREw

Post Sun Sep 24, 2017 9:50 pm 
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Nick
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OK, I had the wrong idea about the set-up. With both the outer bearings partly out of the disk, I think you have extra risk of the outer ring deforming or cracking. The bearing industry has a standard numbering system so that each size of bearing can be identified. There should be a 4 to 6 digit number with a couple of letters engraved on the side of the bearing, along with the manufacturer's name. Using that number, you can look up all the performance ratings on the bearing and find alternative suppliers if needed.
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Post Sun Sep 24, 2017 10:05 pm 
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MoonSet416



Joined: 25 Sep 2016
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Location: Sydney


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OK I see, thx. I guess I might go with two bearings + one on the side then.

Post Sun Sep 24, 2017 10:29 pm 
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Nick
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The bearing is a 6804 or 61804 any extra codes at the end describe the type of seals or special features. all the tech data for a similar model are here: http://www.skf.com/au/products/bearings-units-housings/ball-bearings/deep-groove-ball-bearings/deep-groove-ball-bearings/index.html?designation=61804

The load ratings for those bearings are rather low. I don't know what other people use but Mr Mangle's bearings have four times the dynamic load rating and have still been known to break.
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Post Sun Sep 24, 2017 10:33 pm 
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MoonSet416



Joined: 25 Sep 2016
Posts: 436
Location: Sydney


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Thx for the info! Wow I will probably change to some other bearings for the new version. Glad that I haven't sent the drawings yet. Very Happy

I'm thinking about a needle roller bearing or a cylindrical roller bearing, they seem to have better numbers in terms of load. What do you think would be better?

Post Sun Sep 24, 2017 11:03 pm 
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Nick
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Most needle rollers have very thin outer shells that can easily deform. Some people have had good experiences with them but I have also seen plenty fail. The other problem is that they have almost zero strength in the axial direction, so you need some thrust bearings to stop the drum being forced along the shaft. Needle rollers are much better in low impact situations and where you need the smallest possible OD. Cylindrical bearings are usually much tougher and have far better axial strength, so I definitely recommend them over the needle rollers.
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Post Sun Sep 24, 2017 11:11 pm 
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MoonSet416



Joined: 25 Sep 2016
Posts: 436
Location: Sydney


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Hmm... Just looked up some more roller and needle bearings, and you're right, roller bearings have higher impact loadings. I will see if I could get away with the weight added by the larger bearing.

Also, I noticed that there are some needle bearings with thick, machined outer ring instead of drawn sheet metal ones. (I only found data on the machined one, but 19.3kn seems good) Could that be the reason for the inconsistency you mentioned just now?

And would it be possible to get roller bearings without inner rings? I already have thrust bearings so axial load shouldn't be a huge problem.

Post Sun Sep 24, 2017 11:36 pm 
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Nick
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Location: Sydney, NSW


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The needle roller bearings with the thin shells are called 'pressed cup' bearings due to the way the shells are formed. They are usually sold without an inner race and you use them straight on a shaft, which has to be hardened and polished. You can also buy inner races separately if the shaft isn't hardened.

The other type of needle roller is generally just called a precision needle roller and has that thicker shell and a proper inner race. They can have extremely high load and impact ratings. their only drawback is that they are larger and longer than the drawn cup bearings and you may have trouble finding one to suit.

Are you getting new disks cut or do you need to find a bearing with the same 32mm OD? If you are getting new pulleys made, perhaps they can have a 32mm recess cut on the inner face to hold part of the bearing for better support.
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Post Sun Sep 24, 2017 11:54 pm 
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