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assassin



Joined: 27 Jun 2004
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Location: SunshineCoast


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How would a needle bearing go running on aluminum(hard 7075), guys?
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Post Wed Aug 12, 2009 8:56 pm 
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Nick
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It wouldn't last as long as a steel shaft but since you are only running it for 3 minutes at a time, it should last quite a while. Even though it increases friction, a roller bearing with seals on both sides would be a good idea to keep grit out of the bearing surface. a 1.5" od by 3/4" wide bearing looks like a good size.

Post Wed Aug 12, 2009 9:33 pm 
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Valen
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If your after bushings, have a look at SKF, they have some teflon based ones that are designed to work on anodized aluminum. (some I think on non anodized as well).
They are fairly cheap as well (Few $ per item kinda thing)
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Post Thu Aug 13, 2009 12:32 am 
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Glen
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are they those fibreglide ones? i hear good things from the US peoples about em
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Post Thu Aug 13, 2009 12:45 am 
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Valen
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Those are different to the ones I was thinking of, but I think they are also Al compatible, and really light too.
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Post Thu Aug 13, 2009 10:37 am 
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assassin



Joined: 27 Jun 2004
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Interesting guys, thank you. You wouldn't have any links to the products you are mentioning?
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Post Thu Aug 13, 2009 5:04 pm 
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assassin



Joined: 27 Jun 2004
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Just a quick update. I did a bit of machining tonight, the other side of the ram is done.



And I think I'll try nylon for the large outside bearing.
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Post Tue Aug 18, 2009 10:15 pm 
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Nick
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I looked up this comparison of plastic bearings and Nylon is right down the bottom - PTFE or MDS Nylon looks like a better bet: http://www.mcmaster.com/#sleeve-bearings/=38pj86

The ratings for plastic bearings are specified the same as for bronze bearings, the actual numbers for nylon are WAY lower than for metal bearings. Have a look at these design formulas: http://www.mcmaster.com/#about-plain-bushings/=38pszl .

For your bot with Dewalts in high gear and using four 1.25" ID by .75" wide bearings, I get the following:

Regular driving around, V=569.85, P= 6.67, PV=3,799. Nylon bearings can handle the load but not the velocity. the PVmax is too low as well.

Driving with another bot on the wedge: V is the same, P = 13.33 (double) and PV also doubles to 7,598. Nylon will wear out pretty quickly with a PVmax of only 3,400

Firing the weapon will put a huge shock load on the front bearings, could easily be 5 to 10 times the normal running load. This could easily deform plastic bearings.

In contrast, Bronze bearings have PVmax ratings of around 50,000 to 75,000, which looks like it will cope well in all situations. Needle roller bearings also exceed the load and speed requirements.

Post Wed Aug 19, 2009 12:45 am 
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assassin



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Thanks Nick. I still haven't really got to the bottom of which way to go for the bearings. The reason I stated nylon as the choice was for two reasons. One being a flanged bearing would seem to make a simpler/basic design and two if I used nylon it would be the first to wear(not the aluminum). The other choices for bearings ether seemed to hard(wear on the aluminum) or very expensive. The nylon I could purchase locally and machine bushes easily and quickly. And yes your correct I was looking @ 1.25" ID hahah.

So I'm still up for ideas and stuff.

I'm sold on a fatter outside bearing and will look to regain the width lost by moving the inside bearing into the team delta dewalt mount.

The aluminum grade of the timing pulley stock I can't control and I guess it wont be super hard(like 7075), and that's what the outside bearing would run on.

So I'm still on the flanged bearing design and the bronze bush seems like it maybe too hard on the aluminum, which was discussed from the start.

So while I'll keep looking @ it when time permits, if others have there solution please post in detail and keep money and weight in mind too, or just post whatever you want, it's all helpful. hhehe

Thanks.
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Post Wed Aug 19, 2009 8:16 pm 
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assassin



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This one is pretty close from Mcmaster.

http://www.mcmaster.com/#2803t35/=39vuz9

I have done a bit of reading about fiberglide, but haven't found a supplier.
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Post Thu Aug 20, 2009 9:32 pm 
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Rotwang
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Joined: 15 Jun 2004
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Couple of thoughts, first of I would use suspension on the front wheels.

This would allow lighter construction as the bot can just bottom out when the weapon fires.

Also if grippe rubber pads are used this will counter the tendency of the bot moving backwards due to the recoil from the weapon firing.

This will reduce recoil stress on the timing belts that could snap belts or strip teeth.

Might even help in throwing your opponent harder. Smile


Long wheelbase + relatively hard cholson tires + very ridged frame + 4 wheel drive + typical arena surface = lucky to have 3 wheels touching at any one time.

Front suspension improves traction; turning is probably still highly unpredictable.

Omni wheels on the back wheels would improve predictability in close maneuvering.

Reduce battery drain to.
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Post Fri Aug 21, 2009 10:08 am 
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assassin



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Thanks for the input. A flexible(suspension) wheel assembly @ this stage would screw with my head too much, the design concept so far has been a super rigid frame. Many other areas of the bot have been designed around this and they would all need to change too, so maybe next time. I'm a fan of suspension and will keep it in the back of my mind.

I will be using my gyro to help with driving precision and I have had thoughts about changing or cutting the rear wheels decrease there sideways grip.

I've had brief thoughts about when the weapon fires how the drive train will hold up if its still driving @ the time. And large power spikes might be seen as the motors gain instant load. Ideally the power needs to be cut to the drive @ the time of the weapon firing. It will probably be handled by me @ the controls.


This is the latest wheel assy:




From a earlier post, the main bearing there is the:
http://www.mcmaster.com/#2803t35/=39vuz9

And the other bearings are off the shelf items made from 841 Bronze

The main bearing is 3/4 inch long and I wouldn't mind machining it down to 14mm long, I don't know if that's a good idea or not.
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Post Sat Aug 22, 2009 10:31 am 
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assassin



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Just motivating myself....

http://www.youtube.com/watch?v=sWQ_cf3Tp0U
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Post Sun Aug 23, 2009 3:59 pm 
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assassin



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I received some parts today. The wheels seem to be a decent size which I'm happy about and now with the wheels in hand I can finalize the wheel assembly design.


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Post Tue Aug 25, 2009 5:40 pm 
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assassin



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Anyone see any problems if I got my paintball bottles sandblasted and powder-coated?
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Post Wed Aug 26, 2009 6:13 pm 
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