Aramid fiber ropes in dynamic applications? 1

[RichLeimbach]

In one of the projects I am working on, we need to have a very high strength, high flexibility connection between a rotating pulley and a linearly moving slide mechanisim. We are currently exploring the use of aramid fiber ropes for this connection.
My question is: Has anyone used aramid fibers in dynamic applications? Are there any big drawbacks vs. steel cabling? Any tips on terminating the ends?

 

[GregLocock]

Yes, I used a kevlar (aramid) rope as the strength member in a dynamic application. The old technology used steel. Off hand I can't think of any particular disadvantages - watch out for kinking, and the stretch can be higher than you'd like.

Most people settle for 60% termination efficiency, but we consistently exceeded 95% on our production line. If your application is a one off or not weight critical then settle for 60%, the extra 35% cost a lot of time and research.

Fatigue can be an issue - what diameter is your pulley? what diameter is your rope? How good is your alignment? There's no published data on this that we could use - we ended up building a simulator that torture tested our product.

Cheers

Greg Locock

 

[pauljohn]

Check out the sailing literature, they often test kevlar, spectra, T900, etc. well past the limit. I understand the limitations are the termination, the pulley diameter and the cross- section of the pulley channel. There are special splices to be used on the exotic fibres (check with the manufacturers or your local rigging shop if you live on the water) that are different to the regular ropes. I have seen kevlar break with tragic consequences under a relatively light load because it was not spliced properly.

A pulley that has been designed for steel cable is not suitable for kevlar because the v-shaped groove crushes and damages the fibres, a rounder groove is better. I think the diameter of the pulley has to be a certain multiple of the rope diameter.

Watch out for chafe and sharp bends, there are some new ones like vectran that can be bent around tight turns. the cover of the exotic ropes are there solely for anti-chafe reasons and contribute nothing to strength. Many racing crews remove the cover entirely to save weight aloft.

regards
Paul

 

[RichLeimbach]

Thanks for your inputs. We have been seeing some good results so far with the material (with the exception on the terminations, which are proving to be a real pain).

Any tips on this? I would love to be getting ~ 60% of ultimate strength from the terminations (so far I'm only to about 700 LB on a 2400 LB test line). I've been trying to do this by potting the line inside a cone shaped steel piece with epoxy, but have not been having too much luck since the epoxy tends to crush and pull through the fitting (probably because the epoxy does not fully impregentate down into the fibers). Crimping and other mechanical connection strategies are giving me even worse results.

Any input on termination strategies is appreciated.

 

[vonlueke]

Is your Kevlar rope twisted, braided, or parallel-lay? If parallel-lay, have you considered a spike-and-barrel termination (i.e., a tapered spike inside a matching tapered barrel)? The slightly smaller barrel opening points toward the rope; rope is inserted into the smaller barrel opening, then the spike is inserted inside the rope from the back (larger) barrel opening. (The spike point points toward your rope, not toward the end of your rope). My understanding is this is the strongest type of termination for Kevlar rope.

The larger barrel opening has a "forked" end, which is actually a clevis, for pin and clevis connection. I've heard claims that this type of termination has near 100% rope strength capacity, for non-cyclic loading, if the geometry, etc., in the barrel and spike design is optimal (though I don't know the optimal geometric parameters nor optimal material properties). For cyclic loading, the capacity will be less than 100%, though I don't know how much less, and it might(?) still be high enough for your application or worth investigating.

 

[RichLeimbach]

vonlueke.... Good suggestion. Just found out some info on this yesterday. Even though we are using a braided construction (because of fears of kinking, etc. found in parallel lay construction in this application), I think we can still look at this type of termination. I'll keep you posted on the results.

 

[pauljohn]

If your using the braided kevlar that I am thinking of your best bet would be to splice it around a hard thimble, this should give you 80% of the breaking strain of the line. If the line is old it could be difficult to splice.

Kevlar is not used much on boats anymore because it has been surpassed by the new lines like Vectran and t-900.These are far superior to kevlar for low stretch, virtually no creep, ultimate tensile strength, and durability.

T-900 will only give you 0.8% stretch at 15% of breaking strain which is 4400 pounds for a 6mm diameter line. This line splices easily. You should get close to 90% breaking strain with a proper splice.

Check with the New England Rope company.
Hope this helps.

 

[vonlueke]

RichLeimbach: It'll be interesting to see what you find. If you run the braiding itself up into the spike-and-barrel compression zone, off-hand I would suspect it will create a sharp stress riser on your aramid strands, as Paul is probably alluding to here. If, on the other hand, it's possible to unbraid the rope to try to simulate parallel-lay before it enters the termination, hopefully it might evenly tension the tows by the time the spike becomes seated.

 

[RichLeimbach]

Pauljohn... Actually, we are focusing most efforts on the Technora and Vectran fibers that you have suggested.
One of the termination ideas we have come up with is to splay the fibers into a cone shaped pocket in the steel end and impregnate the fibers with epoxy. We're having trouble finding any glues or epoxies that will wick into the cone, but hold up to the stresses generated when the fibers are placed under tension. Any ideas?

 

[pauljohn]

Sorry I did not mean to mislead you but the splice that I suggested has no similarity with the one you are using. The eye splice is very simple and does not require any glue.

If I understand you correctly you are using a type of 'Norseman' or 'Stalock' terminal that is generally used with wire rope. This uses the tension in the line to pull a cone, (that the core of the line is wrapped around)into a tapered body, (slightly smaller than the cone), that the terminal end is part of. If this is so then the compression between the cone or spike and the tapered body is what keeps the line in place. This crushes the fibres and is generally not used in the dynamic applications that I am familiar with.

Much better is the eye-splice. This consists of removing a portion of the line, from the core and unlaying the core into 3 or 4 or 5 strands, (it depends in the make up and material of the line). The line is then wrapped 180 degrees around a thimble, (egg shaped plastic or metal object about 5 or 6" in length) and the strands are interwoven back into the line. The cover is slid back on and sometimes sown into place.

This process is very difficult to describe without pictures, but is fairly easy and only requires a few special tools such as a fid.

The thimble then takes the strain usually from a d-shaped shackle. This removes any direct surface contact between the line and the load so all chafe is taken by metal parts.

Hope this explains it better.

 

[RichLeimbach]

PaulJohn.... Thanks for the info. Unfortunately, the small size of the ropes we are using (3/32" or 1/8" dia) and the rather limited room to work with makes the splice a little impractical for our application. That's why we are exploring other options like epoxy or different mechanical terminations.

 

[GregLocock]

The epoxy (or polyester resin) is amazingly unimportant, in theory. We sploshed it on the cone and the spike before assembling the joint. It gets squeezed out as you apply the load to the joint to assemble it. Surface finish of the spike and cone, and cleanliness, are important. We definitely had one join that slid apart during assembly - a $100000 mistake if it had happened on a real part.

The proper stuff is ridiculously expensive and is available from Jaques De Regt cables in Rotterdam in Holland. You use so little of it that there doesn't seem much point in searching for a cheaper alternative.

A conventional spike and cone joint with fluffed out fibres should give you 60% although I have no experience with such tiny joints - our spike was about 20 mm across at the small end. When analysing the joint for radial stiffness you might like to consider how the spike contracts and the cone expands under tension - and try and make sure that the fibre isn't getting cut in the process, as the spike moves into the cone. (That was a big hint).

There is an interaction between the spike's taper and the cone's taper and the relative radial stiffness of the components and the friction of them. Once we found a system that worked we stuck with it.





Cheers

Greg Locock

 

[TVP]

Sorry to resurrect a mostly dead thread, but I just came across an article that some of you may find interesting:

"Rigged For Success", pp.36-38 of December 2002 issue of Composites Technology.

You can find more information about the magazine, and maybe get a free copy, using the following link to their website:

http://www.compositestech.com

 

[RichLeimbach]

TVP ...
I saw this article as well. This is the same basic strategy we are working on for this. Keep the ideas coming. We're still learning about these materials as we go and any experience or ideas are appreciated

 

[Tofflemire]

Hi All

I have some good results terminating Kevlar in braided and parallel-lay. I use a machine part with an internal cone at an angle of 14 degrees which faces away from the load and extra counterbore well. I then spread the kevlar out using equal amounts in 3 directions approximately 120 degrees and pot the material using a 2 part epoxy by Emerson and Cuming (Part Number 2850FT and 24LV catalyst)The epoxy can be purchased in different quantities. These assemblies that I have used have given me better than 80% breaking strenght. I will add that I have also used a carrot (bend limiter) outside so the kevlar does not break from exiting the machine part.

Tofflemire

 

[RichLeimbach]

Tofflemire...

Excellent post. I appreciate your input.

- Rich

 

[Tofflemire]

Hi RichLeimbach

I was just wondering if you had any luck terminating/solving your kevlar?

Thanks

Tofflemire

 

[RichLeimbach]

Yeah, we came up with a pretty slick little MIM fitting that looked like a long flattened cone (with about a 3 degree internal angle). We splayed the fibers out and coated with epoxy(3M DP-460) and then pulled them into the cone. Once everything cured, we drilled a hole through the side of the fitting (through the steel, through the epoxy, through the LCP rope) near the top to act as an attachment point. The key for us was coating the LCP rope with a nylon type coating so that it could be more consistently propcessed with the epoxy (the coating was stripped off at the ends where the fibers were splayed and glued).
We got about 95% strength at the fitting. Results were about 3x better on durability than the steel parts of the same size that we had been using, but still didn't meet our specs. We ended up redesigning the system to allow us to increase the size and number of wraps that the cable takes and we ended up going back to steel as our cable material. Seems to work well with this new design.
In short, we found that LCP rope was the best bet for our dynamic application, and we came up with a pretty nice design that ended up getting shelved for now. We'll probably get back into it at some point, but for now, we're sticking with steel.

 

[Tofflemire]

Hi RichLeimbach

Thanks for the feedback, hope my info helped pointing in the right direction.

Tofflemire

 

[exit01]

This is an old thread, but I just discovered this site.
I'm looking for a rope terminal, similar to Norseman but not for steel wires. I don't have particular strength requirements, but I need it small and very quick to mount.
I'll be using it with simple polyesther ropes.
Anyone has a product to point to?

Thank you all.

Beppe