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Wire Rope

01/09/2013 12:37 AM

Having trouble with type of wire rope to use on an operation here. The wire rope has frayed after only 20 days . The 5 metre x 6mm rope has a counterweight attached and runs back and forth over a 150mm pulley 1300 times a day.

Need recommendation of best type of wire to use to decrease strands cracking and get a longer life.

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#1

Re: Wire rope

01/09/2013 1:01 AM

Maybe wire is the wrong way to go, have you tried carbon fiber?

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#2

Re: Wire rope

01/09/2013 2:59 AM

Right diameter for the pulley? Right type of material? Lubrication? Deburr pulley?

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#3

Re: Wire rope

01/09/2013 3:21 AM

Wire rope is used in traditional railway mechanical signalling. Where the signal pull run passes round a mechanical pulley for a significant change of direction, the rope is made off onto an eyelet and then attached with a D-shackle to a piece of chain that runs around the pulley itself, and then the chain is attached to a second piece of wire rope with a second D-shackle and a second eyelet for the remainder of the pull. Rope frays are rare with this arrangement.

Try that, perhaps?

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#4
In reply to #3

Re: Wire rope

01/09/2013 4:11 AM

Also, look at terminating the wire in an eyelet and a D-shackle onto an L-shaped crank, then onto another D-shackle and eyelet onto another wire on the other part of the L; it works well for 90deg changes of direction.

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#11
In reply to #4

Re: Wire rope

01/09/2013 3:39 PM

The wire run 180o on the pulley. It is for a counterweight

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#12
In reply to #11

Re: Wire rope

01/09/2013 5:30 PM

What do you think about comment #3? That sounds like a good suggestion.

Is there some reason that would not work?

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#28
In reply to #11

Re: Wire rope

01/21/2013 7:10 AM
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#10
In reply to #3

Re: Wire rope

01/09/2013 8:19 AM

I have seen a lot of equipment with the counter balance just slung with the chain.

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#5

Re: Wire rope

01/09/2013 5:05 AM

20 days x 1300 cycles = 26,000 cycles, that may be asking a lot of a wire rope (but I am no expert).

You don't say the actual rope travel? If it is almost the full length then look at another method of movement, or different rope. If it is just a short distance travel then a crank may work.

If it is a short travel, and you can keep extra length on the rope. Advancing the rope a length equal to the length of travel every 17 days? will ensure that no one section gets fatigued.

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#6

Re: Wire rope

01/09/2013 5:13 AM

Ahhh I was wrong, that's not asking a lot!

http://www.pythonrope.com/wireropes/wr_techinfo_6.shtml

so more outer wires?

I am sure there is also a minimum bending radius and your pulley should be greater than that, but at 150mm, gut feel is that it is.

http://www.pythonrope.com/wireropes/wr_techinfo_8.shtml

sheave contour?

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#7

Re: Wire rope

01/09/2013 7:21 AM

It maybe the that the wire rope chosen is stranded wrong for the pulley diameter. Since this info was not given here is a guide to help.

http://www.mcmaster.com/#8912tac/=kyjfdb

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#8

Re: Wire rope

01/09/2013 7:55 AM

You might consider using elevator cables. They're designed for long term operation carrying heavy loads while riding over pulleys, without fraying.

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#9

Re: Wire rope

01/09/2013 7:56 AM

Your D/d ratio, at 150/6 is marginally okay. Depending on the answers to some of the questions below, you should consider moving up to 180 or larger, especially if you find you have to replace the sheave for any of the reasons mentioned below.

If the system moves fully 5 metres back and forth 1300 times per day - that's only 2600 x 5 = 13000 metres (approx). With (approx) 500 mm sheave circumference gives approximately 27000 revolutions per day. Is this a continuous process over 24 hours? If so, that's only about 20 rpm.

If this all happens in an hour or less, or intermittently at high cyclic rate, then we need to look at very good bearings and lubrication and possibly even a brake to stop the sheave from skidding (overspin) if the load and line stop faster than the inertia of the sheave. That gets back to some loading/weight questions.

The number of passes is probably not the problem. There are rope(cable)/sheave systems around the world using 1/4 inch wire rope for hundreds and thousands of moves per day for years, even decades, in between replacements. There are line sets in a theater where the load is 400 pounds on 1/4 inch cable and that system moves at least 40 times a day, and has been moving for over 40 years and the sheaves are just fine. The wire rope has been replaced twice. That said, these systems must be lubricated, inspected, cleaned and adjusted, but the wire rope very rarely frays unless there is a failure of the sheave or overloading.

Now, on the other hand, if the rope only travels 300mm in each cycle, then you could have a condition where you are simply wearing out the rope. If it is heavily loaded you could even be work-hardening the steel to the point of brittleness. In this case we need to look into a system that will progressively index the rope so that it wears on a new spot, perhaps changing as often as once per day. But, for that short of a movement, a pivot arm would be a far better solution and eliminate the rope and sheave altogether.

Back to the existing issues -

Is the sheave rated to handle the load? Is it a bushing or bearing roller? How is it lubricated? Are you certain that it is not jamming, at all, at any point in the movement?

What is the speed of the movement?

What is the sheave profile? It must be made for the 6mm wire rope. If it is for 6mm-1/4" fiber rope (nylon, poly, hemp, cotton, etc) the profile will be wrong. If it is for 8mm or larger the rope will flatten and the d/D ratio is destroyed. If it is for 5mm, well, there's your problem!

What is the type of steel in the rope? Galvanized, Stainless, Bright, other?

What is the stranding of your rope? 6x25, 6x37, 16x9 or 16x36 or 19x7 or other?

To be exact, is it cable by definition, and not rope at all, and therefore stranded perhaps in 7x7 or 7x19? And again, your sheave is not the right profile?

Is there a rotation imparted on the rope as well as the bending around the sheave? Are you possibly using a rotation resistant stranding like 8x25 and yet forcing it to rotate?

What lubrication are you applying to the wire rope? How are you applying it? How much, and how often?

What is the environment - wet, dry, humid, dusty, ice?

What is the load and is the system possibly exerting a load factor greater than one on the sheave, as would happen if the sheave is at the "top" and the load and the counterbalance are vertically opposed? This means your 1000 pound load, plus the 1000 pound counterweight is applying a 2000 pound load to the sheave.

Have you discussed the fraying issue with the manufacturer? What did you learn from that likely dead-end, other than "it can't be our fault..."???

Knowing these answers will help point you in the right direction.

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#13

Re: Wire Rope

01/09/2013 10:21 PM

How much force is on the wire rope? As with txmedic, I'm also curious about the stranding. Metal or fiber core?

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#14

Re: Wire Rope

01/09/2013 10:39 PM

It is not the wire rope. It is the pully size causing the problem

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#15

Re: Wire Rope

01/09/2013 10:48 PM

Dear Mr.whezmabeer,

You have not mentioned the gauge of the wire used and how many strands in the rope. No.of Strand and Gauge plays a vital role for increased life of the rope.

Pl. post the data.

DHAYANANDHAN.S

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#17
In reply to #15

Re: Wire Rope

01/10/2013 12:29 AM

The rope is 6mm 18x7 Non spin. It carries a counterweight of approx 10kg. I have checked the sheave and it is OK. Looking at the old rope, not too much lube there, so maybe that didn't help.

Looking at the broken strands, they are all broken at the same place (like it had a neat cut across a couple of the bundls of strands).

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#18
In reply to #17

Re: Wire Rope

01/10/2013 12:40 AM

This is the source of your problem. Please look closely at the rest of the wire rope. I believe you will find a repeating pattern of cuts and approximately equal spacings. Most likely caused by dropping the spool on a sharp corner while in transit, or stabbing the spool side with a lift fork blade. 10 kg is essentially no weight at all on the 6mm. The self-weight of the cable is more than your load at slightly longer lengths... I think we can scratch overloading as the issue. Carefully inspect the replacement rope assembly and I will bet your problems are solved.

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#16

Re: Wire Rope

01/09/2013 10:48 PM

I would find out where the elevator folks get cable from and use that kind of cable. It will cycle for years and years. Jimmy

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#19

Re: Wire Rope

01/10/2013 1:12 AM

Ask a recognised rope supplier. He is the expert and has access to his manufacturer's knowledge. He will advise on the suitablity of your setup and which rope to use. There are many types of wire rope for many different tasks.

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#20

Re: Wire Rope

01/10/2013 5:28 AM

Don't forget to lubricate the rope slightly. If the rope ins't stainless, it protects against rusting too. Fibre cores are already lubricated.

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#21

Re: Wire Rope

01/10/2013 5:37 AM

Please clarify following:-

1.Weight being lifted.

2.Construction of wire rope. Is it 6x36 construction normally used for such application. This means that rope has 6 strands and 36 small dia wires in each strand.

3. Wire rope/ pulley dia seems to be o.k. 150/6=25 which is in permissible limits.

4. For such extensive use rope need daily lubrication with recommended oil.

5. There are basically two types of cores provided. Fiber core or steel core, you can try steel core as use is heavy.

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#22

Re: Wire Rope

01/10/2013 3:19 PM

The best suggestion that I have is to use Kevlar rope. Kevlar is extremely strong yet very flexible. The pulley size may be a problem as well as the wire rope. Reduce the friction by having a machinist make a pulley for you out of Teflon.

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#23

Re: Wire Rope

01/10/2013 4:13 PM

For the short term can you protect the (a) wire rope?

A very well greased sleeve could prolong the life of a rope on the pulley.
(which can we assume is running freely?)

Are you using the correct grade? To my knowledge there are at least two grades
of this type of "steel rope" You need the high tensile (springy) one which refuses to
break even with cutters, not the softer wire rope which breaks upon multiple bending.
If you have the wrong wire (rope) you will never cure the problem. (flexing=breaking)

If it is breaking (as you state) at regular "cuts" can you apply to your supplier
for a new piece of this "sub-standard" damaged rope? Get a free replacement?

Is there any possibility of sabotage? i.e. "Help" from a disgruntled soul "nicking" it?
If so, try a new rope and covertly video it when you are away?

I write because I have never experienced the breakage you describe, (using high
tensile rope) and would also like to understand the cause, if possible.

jt.

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#24
In reply to #23

Re: Wire Rope

01/10/2013 4:43 PM

My experience is with aircraft cable. I have owned air planes that have wire rope cables running to the various control surfaces. In all the air craft I have flown and those that I have owned, there were no fraying or breakage of the wire rope. Sometimes the connector pieces had to be replaced due to wear, but not the cable itself. The aircraft cables, however, probably did not sustain the use in their lifetimes that the wire rope in this inquiry did in a month. So if wire rope is what is to be used, then aircraft cable might be the answer.

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#25

Re: Wire Rope

01/10/2013 4:46 PM

I do not work with wire rope alot however.

Aircraft pulleys for 1/4 wire rope run in the 4 to 5 inch diameter range.

The alignment of the rope to the pulley is critical.

Make sure that the pulley is made for wire rope service & not fibre based ropes.

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#26

Re: Wire Rope

01/10/2013 6:57 PM

Your sheave is too small. For 6mm wire ropes Federal Specification RR-R-571a recommends a minimum sheave diameter of 162mm for a 6x37 rope (6 strands of 37 wires each), 180mm for 6x29, 186mm for 8x19, 270mm for 6x19 or 6x25, and a 432mm sheave for 6x7 rope that is 6mm in diameter.

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#27

Re: Wire Rope

01/13/2013 6:06 AM

1)Enlrage the pulley (twice the diameter-should help a lot !!)

2)Use stainless steel wire rope with 6 strands and soft center !!

3)Impregnate the wire rope with heavy duty Molykote Graphite Grease

...Hopefully it will prolong the wire rope's life at least for several months...

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#29

Re: Wire Rope

09/16/2013 1:24 PM

Dear Mr.whezmabeer,

You have not mentioned the following. 1. Dia of the Rope. 2. Dia of the Pulley. 3. Whether the Rope has hemp.core.? 4. No. of Strands of the Rope - which decides the Flexibility.

Please try the Ratio of the Dia of the Pulley to the Dia of Rope as Minimum 15 and if 20 is adopted - it is good. If hemp core is not provided - pl. use a rope with hemp core as t is a MUST.

Pl. post information how did you solve the problem.

DHAYANANDHAN.S

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