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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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Just figured that I can have a larger pulley and sort things out.

http://imgur.com/BO8B9jY
http://imgur.com/PWtvwfY

Post Tue Mar 28, 2017 10:43 pm 
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Cpnwolfe



Joined: 29 May 2012
Posts: 454
Location: Rockhampton/qld


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I know you have settled on where the weapon motor is, in an alternate universe, why not turn down the centre tube into a pulley and save some weight and space?
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Post Wed Mar 29, 2017 5:54 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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In that alternate universe, the weapon motor(s) could easily be mounted behind the drum - there is a heap of space above the battery
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Post Wed Mar 29, 2017 6:32 pm 
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Nick
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Location: Sydney, NSW


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Three more good reasons to put the motor behind the drum:

The motor can be on a movable bracket for belt tensioning. This will also make the belt and drum much easier to install.

The motor will be WAY easier to service without taking major parts of the bot apart.

With the motors out of the way, the front panels can be angled back to reduce weight and improve their resistance to impacts.
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Post Wed Mar 29, 2017 6:44 pm 
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MoonSet416



Joined: 25 Sep 2016
Posts: 436
Location: Sydney


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Good ideas...

Question... I don't quite understand how to convert a tube into a pulley. Also, what type of pulley are we talking about here? (timing pulley or V-belt or maybe something else?)

Post Wed Mar 29, 2017 7:47 pm 
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Nick
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Location: Sydney, NSW


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Pulleys mostly start off as a solid round bar; its hard to find tube with a thick enough wall. The main choices for belts are V (the most common), timing and micro V (super rare but my fave).

A non-toothed timing belt pulley is the easiest to make; its just s flat bottomed groove. The UK builders use them a lot and the thinking is that the slip between the pulley and belt reduces shock damage to the motor. While that may be true, the belt can develop too much slip if the frame gets slightly bent during a match and then the weapon is disabled. Its possible to buy timing pulleys and adapt them; you lose the slip and gain reliability. Large timing pulleys can be pricey.

V belts are extremely reliable and are very tolerant about tensioning. The pulleys are harder to make due to the slopped cheeks but a suitably sized lathe will have no trouble making them. The drawback with V pulleys is that their thickness means the minimum size pulley that they will wrap around is larger than for thin timing and micro V belts; too small a pulley will cause overheating, loss of grip, melting and failure.

Micro V belts combine the advantages of traditional V belts and timing belts. They work on small diameter pulleys, are tolerant about tension, light weight and compact. The problem is that there are few to zero commercial pulleys available and making your own is harder than the other pulley types. After learning how to make them, I am converting all my bots to micro V belts.

The real take away from all this - include a tensioning mechanism in the belt drive - its the biggest mistake I made with Mr Mangle!
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Post Wed Mar 29, 2017 8:28 pm 
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MoonSet416



Joined: 25 Sep 2016
Posts: 436
Location: Sydney


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Wow that's a lot of info... Thanks Nick.

The more I thought about it, converting the central tube/bar to a pulley of some kind might be a bit detrimental to the strength, which it may need to transfer energy from one disc to another.

I tried to put the weapon motor right above the electronics but it would interfere with the inverted drive, and if I put it right above the drive motors I will have to take the weapon motor off before I can undo the screws holding the drive motors in place should I need to replace them.

I think I have found a way to mount the weapon motor where it is right now without taking the side armor off. (basically it is the same thing as how I mounted the drive motors, a block of chassis will come off from the bottom to give way.)

Post Wed Mar 29, 2017 11:34 pm 
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MoonSet416



Joined: 25 Sep 2016
Posts: 436
Location: Sydney


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Did some more research on different types of belts... And I think I will go with timing belts due to the greater availability

Also I did some update to the chassis so that most of the panels (except the ones in the middle) can be used on either side, reducing the amount of spares I need to carry. Added a support plate for the side panel instead of two support screws.

Still thinking about the tensioning device placement, anyone has some ideas?

PS: Couldn't upload pics because apparently imgur doesn't like me...

Post Thu Mar 30, 2017 3:52 pm 
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Nick
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Location: Sydney, NSW


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There are a couple of ways to tension belts; you can mount the motor on slots so it slides away from the other pulley or you can make an idler pulley that mounts on a slot at right angles to the belt. Industrial machinery sometimes has spring-loaded tensioning pulleys but bots don't really have room for that.
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Post Thu Mar 30, 2017 4:42 pm 
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Knightrous
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Joined: 15 Jun 2004
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Location: NSW


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Just right click on the image in IMGUR and select "Copy Image Address".
The put it between some IMG tags like below.

code:
[img]http://i.imgur.com/PWtvwfY.png[/img]


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Post Thu Mar 30, 2017 5:18 pm 
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MoonSet416



Joined: 25 Sep 2016
Posts: 436
Location: Sydney


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The problem I had with imgur was that I can't even upload my pics... Any other similar sites where I can upload my pics?

Regarding the tensioning system, I think I will add a slot for an idler behind the "ears" besides the drum since it's hard to move the weapon motor with the current mounting system.

Post Thu Mar 30, 2017 7:16 pm 
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MoonSet416



Joined: 25 Sep 2016
Posts: 436
Location: Sydney


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More on the topic of belt tensioning: a belt length calculator warned me about a low number of teeth in mesh. Should I have my tensioner pushing in so that I can have more teeth engaged?

Post Thu Mar 30, 2017 9:58 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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Definitely, put the tensioner close to the motor pulley so you get as many teeth engaged as possible. The tensioner is almost always on the outside of the belt pushing in.
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Post Thu Mar 30, 2017 10:42 pm 
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MoonSet416



Joined: 25 Sep 2016
Posts: 436
Location: Sydney


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Added a tentative tensioning device: one end is fixed to the chassis by a screw, the other end has a ball bearing screwed onto it. The tensioning device will swing up and down to adjust tension. Will that work?

Post Sun Apr 02, 2017 4:28 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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That should do the trick, although with the pulleys so close, it won't have that much range. What ratio are those pulleys, it looks like a huge reduction?
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Post Sun Apr 02, 2017 4:59 pm 
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