Rigging Kit Question

[Luckyeleven]

I have the stein invert-a-wrap friction device, Working load of 750kg.

 

It has been my assumption that in order to use it effectively I need a bottom sling to tie it to the tree of equal rating (750kg working load).

 

In my logic this would lead to the use of a pulley block of 2x the rated capacity (150kn) and comparable top sling (1500kg) for the anchor point.

 

Assuming this to be correct, although I stand to be corrected....

My small friction device would require a a top sling of around 22mm in diameter which is about the largest size commonly available on most online arb shops.

 

So really my questions are:

1. Is my maths correct?

2. Doesn't it seem a bit bizarre that one of the smallest friction devices requires one of the largest ropes available?

3. What are people who are using much larger friction devices using to attach their block?

[TIMON]

I'm not quite sure I understand you correctly but I'll have a stab at your questions. (Albeit in a round about way according to the best of my ability)

 

Most ropes and slings have a rating of 22kn and above (or 2200kg)

 

To calculate the SWL of:

 

Textiles: MBS divided by 10 = SWL

 

Metal hardware. MBS divided by 5 = SWL

 

(This is the general rule of thumb and there are many variables e.g negative rigging when blocking down a stem)

 

We use a ratchet strap rated at 5000kg to fasten our RC3001 to the base of the tree, backed up with a large Yale dead eye sling.

 

Hope this is helpful.

 

Without being condescending to you, if you are considering doing rigging work it is best to get some tuition.

There are quite a few publications out there that are worth reading as well.

 

https://g.co/kgs/XELcU0

 

http://www.hse.gov.uk/research/rrhtm/rr668.htm

 

In any rigging system the final SWL is calculated at the weakest link in the system. Usually a textile item as the metalware components generally have higher MBS values than the textile counterparts.

 

Hope this helps (and is understandable).

 

Stay safe [emoji106]

 

 

 

 

Sent from my iPhone using Arbtalk

[Marc]

I have the stein invert-a-wrap friction device, Working load of 750kg.

 

It has been my assumption that in order to use it effectively I need a bottom sling to tie it to the tree of equal rating (750kg working load).

 

In my logic this would lead to the use of a pulley block of 2x the rated capacity (150kn) and comparable top sling (1500kg) for the anchor point.

 

150kn is roughly 15000kg if that's written on your block it's a WLL in straight pull but there are many variables hence the WLL instead of SWL you will half that to 7500kg per leg when used in rigging. Most rigging blocks are massive overkill this is because the block has to be able to tackle repeated loading (cycles to failure) and will probably outlast other parts of your system, the reality is the block is bomber.

 

Assuming this to be correct, although I stand to be corrected....

My small friction device would require a a top sling of around 22mm in diameter which is about the largest size commonly available on most online arb shops.

 

A rough rule of thumb is to double your top sling, so if your using a 16mm line then you need a 32mm top sling, again there are variables - how you tie your top sling etc your essentially basketing but it's the final choke that becomes the weak point.

 

 

So really my questions are:

1. Is my maths correct?

2. Doesn't it seem a bit bizarre that one of the smallest friction devices requires one of the largest ropes available?

You have to consider that your friction device and rigging pulley will be over rated as these will outlast the soft components of your system and be in service for a greater length of time.

3. What are people who are using much larger friction devices using to attach their block?

We have a GRCS it's SWL is only around 1.5ton we also have another friction device with a SWL of 500kg

As a rule the largest dead eye sling we have is 22mm but most of the time we use 16mm, work to these as your weak points and just consider the other items such as the pulley and friction device as long service items.

 

There are so many variables in rigging it's hard to judge, you have factors such as shock absorbtion in your rigging line - so when snatching as you get closer to the ground and the bits get fatter you will have less absorbtion in your line because there is less of it being loaded.

 

Edited February 20, 2017 by Marc

[Marc]

I'm not quite sure I understand you correctly but I'll have a stab at your questions. (Albeit in a round about way according to the best of my ability)

 

Most ropes and slings have a rating of 22kn and above (or 2200kg)

 

To calculate the SWL of:

 

Textiles: MBS divided by 10 = SWL

 

Metal hardware. MBS divided by 5 = SWL

 

(This is the general rule of thumb and there are many variables e.g negative rigging when blocking down a stem)

 

We use a ratchet strap rated at 5000kg to fasten our RC3001 to the base of the tree, backed up with a large Yale dead eye sling.

 

Hope this is helpful.

 

Without being condescending to you, if you are considering doing rigging work it is best to get some tuition.

There are quite a few publications out there that are worth reading as well.

 

https://g.co/kgs/XELcU0

 

RR668: Evaluation of current rigging and dismantling practices used in arboriculture

 

In any rigging system the final SWL is calculated at the weakest link in the system. Usually a textile item as the metalware components generally have higher MBS values than the textile counterparts.

 

Hope this helps (and is understandable).

 

Stay safe [emoji106]

 

 

 

 

Sent from my iPhone using Arbtalk

 

The 5000kg ratchet strap is that WLL or break point?

It will of course be set up in basket configuration around the tree doubling its strength. And the way the bollard is designed it will impart less force.... probably giving the RC3001 it's 3000kg WLL

 

I ask as we have 30kn slings that we use on a 300kg SWL

We also have 3ton slings which are 3000kg SWL

So it's important to know.

[carlos]

I have the stein invert-a-wrap friction device, Working load of 750kg.

 

It has been my assumption that in order to use it effectively I need a bottom sling to tie it to the tree of equal rating (750kg working load).

 

In my logic this would lead to the use of a pulley block of 2x the rated capacity (150kn) and comparable top sling (1500kg) for the anchor point.

 

Assuming this to be correct, although I stand to be corrected....

My small friction device would require a a top sling of around 22mm in diameter which is about the largest size commonly available on most online arb shops.

 

So really my questions are:

1. Is my maths correct?

2. Doesn't it seem a bit bizarre that one of the smallest friction devices requires one of the largest ropes available?

3. What are people who are using much larger friction devices using to attach their block?

 

if you want a pulley thats tated twice what your friction device is then thats not 150kn its 15kn .

check your figures again against the 15kn pulley and id say it will make a bit more sense.

i would go for somthing pretty chunky for the base (portawrap) anchor because it takes a bit of wear n tear.

carl

[Luckyeleven]

if you want a pulley thats tated twice what your friction device is then thats not 150kn its 15kn .

check your figures again against the 15kn pulley and id say it will make a bit more sense.

i would go for somthing pretty chunky for the base (portawrap) anchor because it takes a bit of wear n tear.

carl

 

I assumed it was because the portawrap has a 10:1 SWL ratio? As even the large bollards are only rated about about 2000kg which would give a pulley of only 40kn?

[Marc]

I assumed it was because the portawrap has a 10:1 SWL ratio? As even the large bollards are only rated about about 2000kg which would give a pulley of only 40kn?

 

A pulley of 80kn, but why work on the limit with an item like that we usually go through 4-6 ropes per pulley.

[carlos]

with that kinda lowering device id get a impact block and 19/20mm dead eye sling as main anchor then get a swing cheek rigging pulley with either a small dead eye sling or cheaper purple 1 ton endless loop sling for a second positional anchor and probably a 13/14mm lowering rope.

you could also look on the arb shops at their complete kits for ideas on compatible gear.

carl

[NFG]

If it were me, I'd select a fairly meaty pulley, such as the 150Kn one you mentioned, as someone rightly stated theyre going to last a lot longer.

 

Now the ISC blue one will take ropes upto 19mm rig rope & upto 29mm on the strop.

A 24mm English Braid strop is rated at 14000Kg you could go slightly bigger.

 

The pulley dictates the rope size of 19mm so 16mm or 18mm is 6300Kg & 7700Kg MBS - Teufelberger, but others are similar.

 

So you have WL of (150/5) 30Kn (around 3000Kg) on the pulley, (14000/10) 1400Kg on the strop & (7700/10) 770kg on your rope if you have 18mm, Im sure 16mm would be fine.

 

Be aware of the relationship between Kn & Kg. A Kg is a static force, a Kn is a dynamic one the resultant of weight, time of fall & force of gravity.

 

Say a stump weighs 750Kg it exterts a force of 7355 Newtons at rest or 7.355Kn (thats a fair huge chog tho!)

 

If this were to fall for say, 2 seconds, the force would be 750*9.81*2 = 14.715Kn - where 9.81 m/s/s is acceleration due to gravity. Your block & strop should be fine, your rope not so, but shouldnt part.

 

A 300 Kg lump falling for the same 2 seconds would be well within specs - 300*9.81*2 = 5.886Kn

 

I think the maths is right as I said, its just my take & Im sure there are far more knowledgeable than me who will pitch in.

 

N

Edited February 20, 2017 by NFG

[Marc]

What NFG said:biggrin:.