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Bob
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United Kingdom
1263 Posts

Posted - 26 Mar 2006 :  16:32:31  Show Profile  Reply with Quote
Anybody any UK experience of working with polyurethane elastomers for casting sticky tyres for minisumo?

I have found a website http://www.polycil.co.uk/ for the manufacturers of the systems, but there seems to be a range of different compounds, so which is best?

It referes to hardness measured on a scale with shore A units. What is this and is a low value hard or soft?

Bob

slurp
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Posted - 26 Mar 2006 :  23:52:01  Show Profile  Visit slurp's Homepage  Send slurp a Yahoo! Message  Reply with Quote
Dave Hylands' website has some details on how he's moulded tyres:-

http://www.davehylands.com/Robotics/Marauder/Making-Tires/

I'm going to look this up tomorrow... not used it in a while. It's common to see shore A thu' D and it relates to a ball side pressed in to the surface. when rubber gets very soft they up the ball size to get the scale reasonable again. I think there's a bit of an overlap but I'll have to check which is softer thru' to harder.

looking at the 2 part cold compound with shore A 60-80 I think this is seen as soft for engineering rubber but maybe on the hard side for a tyre... must check scale and comparison

regards,
colin


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Edited by - slurp on 26 Mar 2006 23:57:01
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slurp
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United Kingdom
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Posted - 27 Mar 2006 :  00:02:56  Show Profile  Visit slurp's Homepage  Send slurp a Yahoo! Message  Reply with Quote
http://www.materials.co.uk/rubber.htm

try this too

regards,
colin



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JamesC
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United Kingdom
398 Posts

Posted - 27 Mar 2006 :  00:37:27  Show Profile  Reply with Quote
I've cast up my tyres using Silcone rubber, made bt Trylone Ltd, Wallaston, Horthants - Phone 01933 664275 not sure if they have a web presence. The rubber is intended for making moulds but seems to work well. I basically used the same method as Dave Hylands except I made up a mould in aluminium, with a perspex lid.

A couple of tips:

  • Get a very accurate set od scales. most of the casting rubbers I've used are mixed by weight. it can be quite critical & you will only be using small quantities.
  • Make sure that the lid clamps down tight onto the wheel as well as the edge of the mould, this rubber is designed to pick up the finest detail and belive me it goes everywhere
  • Mix up more than you need, as you will inevitably lose some in overspill and mixing a second batch and hoping it will bond well with the first is a non starter
  • Use a parting agent very sparingly or you will have a tyre with marks from the release agent (usually vaseline)
  • Incorporate someway of centreing the wheel in the mould or you will have a wobly wheel
  • Leave the rubber to set for twice as long as the instructions say, it has very little strength untill fully set and seeing your perfecly formed tyre rip into shreds as you pull it out can be most disheartening
  • Finally don't do it - I want all my oposition to have nice slippery wheels when I meet them


Regards

James
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Bob
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Posted - 27 Mar 2006 :  19:48:19  Show Profile  Reply with Quote
Thanks Colin and James

I found this reference http://www.performance-plastics.co.uk/pdf/relationship_shoreA_ShoreD_hardness.pdf which is quite clear.

Which do you think best, a silicone rubber or a polyurethane? I remember using a two part silicone rubber to seal bacteriological culture vessels in the distant past. The cured rubber was pure white. I think it was RTV (room temperature vulcanising) from Radiospares. I remember being told that a) it was expensive and b) the uncured material/catalyst was toxic or worse.

Bob
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JamesC
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United Kingdom
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Posted - 28 Mar 2006 :  11:58:43  Show Profile  Reply with Quote
Hi Bob

The stuff I used was an RTV rubber, and yes it was expensive (around 18 for a 250gm pack) but I got a lot of tyres out of it, somewhere in the region of 10 pairs, I scrapped a few - thats how I learnt some of the above, so I'm not abosultely sure.

Regards

James
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slurp
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United Kingdom
497 Posts

Posted - 28 Mar 2006 :  14:18:14  Show Profile  Visit slurp's Homepage  Send slurp a Yahoo! Message  Reply with Quote
quote:
Originally posted by JamesC

I've cast up my tyres using Silcone rubber, made bt Trylone Ltd, Wallaston, Horthants - Phone 01933 664275 not sure if they have a web presence.


are these the people?

http://www.trylon.co.uk/ Northants but 01933 411724

Seem to have a lot of related stuff.

Not had opertunity to look up other references.

regards,
Colin

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slurp
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Posted - 28 Mar 2006 :  22:06:10  Show Profile  Visit slurp's Homepage  Send slurp a Yahoo! Message  Reply with Quote

Re-found W.P.Notcutt Ltd (www.notcutt.co.uk) who do a range of mould-making materials and casting resins, their site has a _lot_ of info on it (but no prices).


  • Polysulphide Rubber, requires 25 deg C curing but gets down to Shore A 7

  • Polyurethane Rubber, Room Temperature Cure (RTC) Shore 65 down to 30 with two part mix. With suggested softener to Shore A 20 or less.

  • Silicone Rubber, RTC Shore A about 22.



Generally the softer the material becomes the larger the coefficent of friction. Typical coefficents of friction for rubber compounds are in the region of 0.9, I've not tracked down the comparision between these rubbers.

It's easy enough to test for coefficents of friction if you're wanting to push this to the limit... it's just the moulding of sample mixes that's the pain.

hope this help,
colin

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Bob
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United Kingdom
1263 Posts

Posted - 28 Mar 2006 :  22:19:47  Show Profile  Reply with Quote
quote:
Originally posted by slurp


Re-found W.P.Notcutt Ltd (www.notcutt.co.uk) who do a range of mould-making materials and casting resins, their site has a _lot_ of info on it (but no prices).


  • Polysulphide Rubber, requires 25 deg C curing but gets down to Shore A 7

  • Polyurethane Rubber, Room Temperature Cure (RTC) Shore 65 down to 30 with two part mix. With suggested softener to Shore A 20 or less.

  • Silicone Rubber, RTC Shore A about 22.



Generally the softer the material becomes the larger the coefficent of friction. Typical coefficents of friction for rubber compounds are in the region of 0.9, I've not tracked down the comparision between these rubbers.

It's easy enough to test for coefficents of friction if you're wanting to push this to the limit... it's just the moulding of sample mixes that's the pain.

hope this help,
colin

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www.minisumo.org.uk



Hi Colin

Have seen reference to materials with shore A of 10 having coeficient of friction greater than unity (Reason for expressing it this way is to avoid peeps telling me that mu cannont be greater than 1, I know that already, so save your breath) Suggested figure is 2 for some really sticky stuff, but what about the rule that if you put your sumobot on a sheet of paper and it lifts the paper the tyres are illegally sticky? will this rule be enforced in June?

Bob
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slurp
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United Kingdom
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Posted - 29 Mar 2006 :  11:17:26  Show Profile  Visit slurp's Homepage  Send slurp a Yahoo! Message  Reply with Quote
Hi Bob,

I can't really comment on what will be enforced in June, as the competition is run by others. While I've offered to help Gary he's in a better position to comment as organiser.

My thought are that the paper test is an enforcement of not "sticking your self to the surface", could be likened to other test where the 'bots weight or size is checked. There should be no reason why you can't get your rubber cured to Shore A 10 and not have a sticky surface.

I'd think that if you're wheels are still tacky/sticky then they've not actually cured properly. You may end up with mechanical problems, that is the material breaking down under load. Our shear loads are small so that would be a little extreme!

I know of people in RC racing using stuff like lighter fluid to make tyres tacky but it may be argued that this can in some way mark or effect the dohyo surface so it's always going to be better to avoid this - back to paper test.


I don't doubt the greater than unity. On dry concrete and asphalt you get values from 0.35 thru' to 1.2, as ever generalised with out hardness. Wiki quotes a range of 1 to 2 on some surfaces. Accepting wedges can get under, I believe that friction can give you the best edge in static push - just make sure the motor/gearbox is up to the torque!

All that said I'm not turning up with moulded wheels... yet

Regards,
colin

Just to add for comparision - http://www.jameswalker.biz/docs/jwco/o_rings_full_doc.pdf this is the James Walker O Ring guide, Table 3 gives typical materials and hardness ranges. Shore A readings are usually one to three degrees higher than IRHD readings.

I've a bit more work to do but it'll give you a flavour of where O Rings might be in comparision to moulded tyres, bands appear to be some where between.

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Edited by - slurp on 29 Mar 2006 11:39:05
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slurp
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Posted - 05 Apr 2006 :  19:31:54  Show Profile  Visit slurp's Homepage  Send slurp a Yahoo! Message  Reply with Quote

I had some other thoughts on tyres that are not moulded rubber, mainly on the basis of improving friction without the complexity of moulds.

First was expanded EPDM (or similar), a foam rubber that can be bought in sheets or rolls with a self adhesive back.

One of the other was self amalgamating tape, I've just picked some up from the local hardware shop. While it can be used to seal pipes and electrical connections it's also aimed at grips for sporting equipment.

Bonding with itself it leaves no sticky residue, my aim it to use an irregular surfave so that it keys to the wheel with a couple of wraps to give a smooth finish. Hopefully with better results than bands or o-rings.

regards,
colin



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Bob
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United Kingdom
1263 Posts

Posted - 06 Apr 2006 :  21:24:48  Show Profile  Reply with Quote
quote:
Originally posted by slurp


I had some other thoughts on tyres that are not moulded rubber, mainly on the basis of improving friction without the complexity of moulds.

First was expanded EPDM (or similar), a foam rubber that can be bought in sheets or rolls with a self adhesive back.

One of the other was self amalgamating tape, I've just picked some up from the local hardware shop. While it can be used to seal pipes and electrical connections it's also aimed at grips for sporting equipment.

Bonding with itself it leaves no sticky residue, my aim it to use an irregular surfave so that it keys to the wheel with a couple of wraps to give a smooth finish. Hopefully with better results than bands or o-rings.

regards,
colin



--

www.minisumo.org.uk



Hi Colin

I remember a sort of transparent blue plastic mat that we had at school for a dyspractic pupil. I was very 'sticky' so that when he put equipment down on it it was not easily dislodged. That would have made good tyres, but I don't know the details of the product. It felt unnaturally heavy, but I don't think it was lead loaded.

I have been reading a book on minisumo, not many insights into sticky tyres, but some good ideas for strategies other than 'push harder than the other robot'.

Bob
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slurp
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United Kingdom
497 Posts

Posted - 06 Apr 2006 :  23:24:22  Show Profile  Visit slurp's Homepage  Send slurp a Yahoo! Message  Reply with Quote
I've seen an interesting minisumo that had a white strip across the front of the 'bot, forces the opponent into edge detection mode usually reverse :)

what's the book? I've got the offical guide but not picked it up in a while, loads in there

colin

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Bob
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United Kingdom
1263 Posts

Posted - 07 Apr 2006 :  07:21:55  Show Profile  Reply with Quote
quote:
Originally posted by slurp

I've seen an interesting minisumo that had a white strip across the front of the 'bot, forces the opponent into edge detection mode usually reverse :)

what's the book? I've got the offical guide but not picked it up in a while, loads in there

colin

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That's the one. By Pete Miles. Quite a lot of the technical stuff on motors is straight from his book on Build Your Own Combat Robot, but it was worth getting for the later chapters on specific tactics.

Bob
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slurp
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Posted - 07 May 2007 :  01:24:42  Show Profile  Visit slurp's Homepage  Send slurp a Yahoo! Message  Reply with Quote
here's another interesting pointer... http://brooksbots.com/ExSpurt%20Tires.htm

Rick Brooks got that co-efficient up to 2.5 for over a kilo of push!

regards,
colin

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