Tethers

[grant_ploughbot]

Tethers

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

[julian]

Tethers

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.

[kronic]

Tethers

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

Dave

[arfamortis]

Tethers

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.

[arfamortis]

Tethers

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

[julian]

Tethers

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.

[arfamortis]

Tethers

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?

[mark_m]

Tethers

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

[arfamortis]

Tethers

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.

[jamesb]

Tethers

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

[m2xt]

Tethers

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

[shredderteam]

Tethers

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

Sam

[spinnerboy]

Tethers

Thats the beauty of discusion forums.

[julian]

Tethers

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.

[Guest]

Tethers

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