Tethers

Any news abot the tether i.e what is the best thing to use how to fit em etc

Am I correct in thinking that a teather is basically a bit of string keeping the flipper attached to the robot if it breaks away from the robot.

Now I Know string would not hold it (5-10mm wire rope would do the trick) but am I correct in what I am thinking a teather is?

Regards
Ian Mc Donald

yeah, its mainly due to an incident that happened during S7. Someones piston blew through the end of the cylinder, although nothing flew off, it could of! But it was a dam big bang when it went!

They shouldnt have used a 16bar RAM at Full pressure (50bar+) then should they. :)

Mr Stu

Spawn Again, and it was a 10 bar cylinder with a LONG spring to cushion the endstroke. I warned them a half a year ago. They didnt listen.

Tethers.. a aramide rope will do the trick.

A possible alternative to Aramid / Aramide rope....

http://www.dsm.com/en_US/html/hpf/products.htmhttp://www.dsm.com/en_US/html/hpf/products.htm

http://www.englishbraids.com/products/marine/ya.htmlhttp://www.englishbraids.com/products/marine/ya.html

Spawn eh...pity it wasnt spotted earlier:sad:

Tom

Ive been thinking about this and forgive me for being thick, but if the ram bursts on a flipper, the thing will simply continue to rotate around its hinged pivot. I cant see that tethering anything will stop this, unless the tethers reinforce the pivot point. If you try and stop the sepearting ram you are trying to stop a tremendous force, if you simply allow it to continue in its arc, its likely to flip the robot but not to send any shrapnel anywhere???
Please let me know if Im missing the point.

From what I gather the tether is in place to stop the flipper going further than it should past the rams stroke especially these new breed of robots that use static rams eg gravity where the flipper is not connected to the ram, this way if the cylinder end did blow off the tether would be designed to stop the movement of the arm to say just above its highest point rather than all the way over and possibly hitting someone behind if they are loading the robot into the arena or powering up. Apologies if I have the wrong end of the stick would not be the first time :P

Francis

http://www.ceros.org.ukwww.ceros.org.uk

ahhhhhh
now i see

Does a bungie cord in a single acting system count as a teather? I pressume it will, but better safe then sorry!

regards
Dave moulds
Team Turbine/PLF

dont know, but hearing the latest discussion at fra meeting, I think it maybe not.

Id not worry too much for now though, if you enter the robot early and explain the mechanism to the evnt orgainiser they can advise you what they will need for that event.

Is there any more information on tethers particularly with the FRA event coming up.

Rob,

My understanding is that you cant have a new rule without specifying what is required. It hasnt been done yet. If you volunteered to produce a draft rule, Im sure that the executive committee would bite your arm off.

The current suggestion, as I understand it, is that individuals are urged to come up with their own solutions to the problem. From that a rule change will emerge.

There would need to be some time to fit a tether before the rule became operational. In my opinion I dont think that we will have a new mandatory rule for tethers on flippers in 2004 - but I may be wrong!

Jeremy

i believe there is testing being carried out on different materials, including seatbelts. I havent seen any robot with them in yet.

oh am i correct in thinking, that if seatbelts are to be used as tethers, then natrually people will go and buy them cheaply from a scrapyard? In which case they may have been in a crash and now unsafe to use? Just raising a possible saftey issue as i saw on a program the other day that seatbelts are unsafe to use in a car crash more than once.
Grant

Most cars in scrap yards have not been in crashes. In fact, in the case of most Rover Metros, they went from factory to scrapyard with suprisingly little in between.

Weve got a seatbelt as a tether in the new Kronic. Havent tested it yet though.

Dave

I have been tasked by the committee to attempt to find a solution to this problem, now there have been several tests done, the results of which are inconclusive. I can catergorically state that steel cables are not acceptable unless they are in a relatively straight run and are correctly terminated with a thimble of the correct radius for the size of cable and the cable is of certified lifting quality. Ubolt type clamps are not acceptable due to the stress levels generated by them in a point contact.

Seat belt webbing may be acceptable, again consideration should be given to the terminations, I would reccomend using the web adjuster buckles that tighten on impact to form a loop, the ends should be stitched, not heat sealed.

Aramid rope (Kevlar) is acceptable, again providing it is correctly terminated. I have a lot of experience with Kevlar cables and the only way that you can guarantee that the end will not let go is by using a cone or wedge anchor fixing. Knots would not be acceptable as they will loosen no matter how tight you make them.

Our official stance at the moment is that tethers will remain advisory only. This may change in the future.

Personally I am not even sure that they are needed at all, as all arenas should be able to contain failed weapon missiles, otherwise that weapon should not be permitted to run in an arena of insufficient strength.

Arthur Chilcott
FRA Chief Judge.

PS
A thimble is the tear drop shaped eye that you see in spliced steel cables in case you ask.

You were never in the Scouts, Arthur? If you have a hole just larger than the size of the rope, you pass the rope through and put a figure of eight knot in the end. Other knots and arrangements might work, round turn and two half hitches etc.

Having said that I would have thought that some steel arm arrangement alongside the ram might be a better deal, the arm not being fully extended during the normal operation of the ram.

Julian,

Yes, I was in the sea scouts (waits patiently for laughter to subside from those that picture me in a sailor uniform) I also come from a family of six generations of seafarers (my father was the master of a 160K ton ship until he retired) and I suspect I have forgotten more knots than you have ever heard of, how about a reverse, square sennet?
Knots dont work in Kevlar, due to the elongation characteristics of aramid fibres, its the same reason that they are so good at dissipating enrgy, it also has a very low coefficient of friction, typically 0.03 and knots rely entirely on friction to hold.
As to the steel arm, steel does not like shock loading, if one component has suffered catasrophic failure what hapens if it goes into a violent spin as it leaves at high speed and puts the rigid tether into a bending moment against a corner on the robot? Or jams under commpression inside the robot?

Arthur,
Whats a cone or wedge anchor? Ive done quite a lot of sailing and regurlarly use lots of different joining techniques and Ive never heard of them. Wouldnt whipping or splicing be simpler, cheaper and lighter? I realise sailing isnt going to vibrate knots as much as in a robot but wouldnt these methods be just as suitable?
I dont know what the coefficient of friction is for fishing line but thats also very low and quite stretchy and the knots never come undone. Incidently the dynema lines used in kite flying are normally joined using knots very similar to fishing knots e.g. tucked blood knot.
Ive never load tested any of these techniques though and I assume youve looked into them, but I just wondered why you considered they werent suitable?
cheers
Mark

Cone and wedge anchors are designed specifically for cables of multi strand construction, pass the cable through a tapered fairlead, narrow end first, splay out (or unlay) the strands, insert a cone of identical taper (usually with an axially grooved pattern). When tension is placed on the cable the cone is pulled in with the fibres, the greater the force the tighter the grip. Wedges are used for flat sections, i.e. webbing. A similar method can be used on steel cables, fit into a cone, splay strands in the wide end and fill with solder, or lead on larger sizes, this forms the inner cone. This is the traditional method for securing lift and crane cables, it is also the correct method to use when fitting nipples to Bowden cables (brake and clutch cables on bikes etc). I you have ever had a cable pull through the nipple on a bike cable it is because it wasnt done this way.

I worked as chief technician at Cambridge University Engineering Department for 15 years, during that time we carried out thousands of experiments on terminating aramid cables, flat and round, mainly for use on bridges, many using fancy knots, the only type used currently are based on tapers. Another comment I should make is that aramids should only be used sheathed as they are prone to attack by oils, solvents and UV. Also, the only reason aramids are used on bridges is that their weight/strength to volume is so much lower than steel that they can span greater distances without any special techniques. The belief that steel would not support its own weight over larger spans is a myth, all that is requred is to make the cables thinner towards the center of the span, but due to the complexity of the proccess this has never been adopted.
BTW. I was also part of the team that modelled the Millenium (wobbly) Bridge to solve its problem.

and if you have ever seen arfas beard, you will realise that he must be good with knots to keep that as neat as he does
:)

The last I heard was that tethers were not required for flippers that had 3 point fixings (ram and 2 pivots). Gravity style with 2 point fixings did need a tether.

Paul

This ones just gonna go on and on me thinks :sad:

Sam

Thats the beauty of discusion forums.

Putting a ram in a middleweight, for reasons of geometry I may well fix the ram rigidly to the base of the chassis and work the flipper with an arm. Should the ram fail, it cannot escape the chassis. I may well need a new chassis though.

you can get webbing for winches thats got to be strong?
cat no WWS8

you can get 10ton webbing minimum. I think you can get more but I Definately know you can get 10ton. Contact Gunnebo. They are a very good company and will supply quality webbing (not the cheep chineese stuff). You can probably get away with the cheep stuff but it will not last and considering webbing is so cheep anyway (about 5euro for a 2ton 1meter web sling) your better off with the proper stuff. Also Arthur have you thought about chains?

Grabiq chain is top quality and incredibly strong.

Regards
Ian

Nevr understood this tethers thing to be honest.... Ill explain why.

a normal flipper (dantomkia for example) has 2 flipper arm attachment points. one at the hinge and one at the ram.

gravity style (gravity for example...) has 2 flipper arm attachment points. one at the hinge, and one at the movement restricting bungy cord.

Yes, I know bungy cords alone are not good enough to be tethers, but gravity runs aramid rope in addition to these to stop his flipper going 180 degrees. I would expect that similar designs cannot rely on bungy ropes for the same reason, so is it not logical to say that gravity type flippers are compromising their design by not having teathers anyway? In which case, do we need the rule?

Also, has anyone tested what happens if the arm leaves the robot with just bungy cords?

I almost got whacked on my head at a live event in rijswijk, cause I only had bungees on it. The flipper arm did do the full 180 degrees, and in doing so reached to about 1 metre behind the robot... swooshing thru a spot my where my head was half a second before.

In the series 7 gravity I had 8 strings of 10mm rope each capable of holding 2500 kg... the ropes started to bend a 30x30x3mm box section... So this type of rope will do fine.

Im having trouble picturing the three point attachment thing - Can anyone help me to visualise this? All the ones I can imagine have a single hinge, plus one or more rams. When we say 3 point - do we mean those where the flipper is physically attached to the ram, which itself pivots? Or is there more to it than that? If this is whats meant by three points, then presumably in the absence of tethers, gravity becomes a single point attached flipper whereas in this thread it keeps getting referred to as a 2 point....

Cheers

-- Kev

on ours we are going to have it fixed to the flipper by a ball joint type of thing and a tether from 3 lyers of seat belt (brand new for an old jag or from a fiat panda)

Out of interest, why a ball joint? I suppose it allows the flipper to flex sideways a bit without putting pressure on the ram, but Id have thought its a relatively weak connection if youre allowing more than a narrow range of movement.

Just curious, not trying to be critical. (After all, Im not exactly expert!)

--
Fluppet

@ Kev: I think a long hinge counts as two points - one at either end of the hinge. A single pivot such as that on most axes would count as one point, and can be more easily damaged by twisting forces. If the flipper is attached to the ram by a joint then that also counts as one point. It looks like Gravitys hinge counts as two points.

If Richards definition is correct, my next question would be: how long does a hinge have to be before it counts as two points?

Working towards my first CO2 flipper, I was assuming that hinging all along the edge was the way to do things.

No offence, but I dont think Richards defenition is correct.

Im just guessing here, but according to what Ive heard, the defenition depends on how many pivot points you have. For example, a flipper like Dantomkia have three points, one at the bottom of the ram, one at the flipper arm, and one that connects the ram to the flipper arm.

But as the ram in Gravity is stationary, I consider that a one point flipper, with the only pivot at the top of the arm.