Thread: Hand vs electric winch: solo travelling

Originally Posted by Tank

Dave I don't know where you get your figuring from, mine comes from over 20 years as a Qualified Class1 Ticketed Rigger, Riggers guide and any reputable recognised source on the operation of pulley blocks, my ticket is recognised world wide.
I am telling you and anyone else interested, that to get a Mechanical Advantage (MA) from using pulleys or Snatch Blocks the formula is and has always been worked out by the number of parts of rope supporting the Moving block, the operative word here is the moving block.
A block or SB that is stationary is only re-directing the path of the cable, explain to me how you get any MA from a stationary block, and if you believe the stationary attached to an anchor point is MOVING, well.....
A set of snatch blocks set up as in previous post is pulling to advantage, i.e. pulling motion is in the same direction as the load is moving.
A single snatch block anchored ahead of the load is pulling in the opposite direction, in other words to disadvantage.
Now if you want to argue this point further, first, go and read a Riggers Guide and come back and show me how a single grooved wheel sandwiched between two plates of metal riding on a solid axle can impart magic energy to the cable rolling over it.
An example, an industrial accident (true story), a worker on a building site was killed when a rope holding a snatch block broke while a load of around 300lbs. was being hauled up to a floor below the snatch block.
When the DLI inspector and Police arrived they were told by the dozen or so workers on the site that the man that was killed was pulling the load up by himself and when it was about 20' in the air the rope holding the snatch block broke and the load fell on the worker and killed him.
The DLI inspector told the police that the workers were lying, because if the load was around 300lbs. and the worker killed looked to weigh around 140 to 150lbs then it would take at least 3 men to lift that load. DLI inspector explained to the police that one man could not possibly lift that load(even with the magical single pulley snatch block, my comment, not DLI).
He explained that the rope attached to the load side of the pulley would've had 300lb on it while the lead rope being pulled would've had around 360lb pulling on it, so it was impossible for a 150lb man to haul 360lbs, the rope holding the snatch block would have been subjected to around 660lb. which was a higher load than the guaranteed breaking strain of the snatch block supporting rope, which resulted in failure of the rope supporting the snatch block.
The Foreman was arrested and he immediately confessed that he and 4 others were hauling on the lead rope when snatch block rope broke, dropping the load which hit the unfortunate worker.
Single pulley does not offer any MA whatsoever when pulling to disadvantage it actually increases load on lead rope to overcome the up to 20% friction of the wheel rubbing on the axle.
Please read the riggers guide or this will go on forever, Regards Frank.

I doubt that everyone, including Warn have gotten the use of snatch blocks wrong and how they work.

The example you gave is showing the force applied to a rope holding the snatch block, not the force required to lift said object.

I am no expert, and only going on HSC physics, but I am probably going to go with what the experts like the guys at Warn are prepared to put on their website in this litigious world we live in.

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Go for it, no skin off my nose, seems you didn't read the Riggers Guide, or are they all bull****, and Warn are the be all end all, before you make ridiculous statements do some research.

Originally Posted by JoeFriend

I doubt that everyone, including Warn have gotten the use of snatch blocks wrong and how they work.

The example you gave is showing the force applied to a rope holding the snatch block, not the force required to lift said object.

I am no expert, and only going on HSC physics, but I am probably going to go with what the experts like the guys at Warn are prepared to put on their website in this litigious world we live in.

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33






How to work out the load in a single part of a purchase








The greatest load on any rope in a purchase is the load in the lead rope to the winch. This is due to


the friction between the rope in the groove of the sheave and the sheave pin. Friction is estimated at


between 3 per cent and 5 per cent per sheave (ie up to one twentieth of the rope load that would occur if


there was no friction).


The effects of friction, acceleration or deceleration are not usually included when dealing with work of a


temporary nature unless a number of falls are used or the rope velocity is high, ie. 0.6m/sec.


When a load is at rest, suspended from the lower block, the becket load (the load in each part of the rope


purchase) is found as follows:






Becket load = Total load on lower block ? No. of parts of rope supporting load








Note: The total load on the lower block includes the load to be lifted plus packings, slings, shackles,












blocks etc.


For example – (including frictional effects)


Total load on the lower block = 10t including gear


Number of parts of rope = 5 supporting the moving block






Becket load (BL) = 10 + 5 = 2t








However as lifting commences friction causes the load in the rope falls to increase by up to 5 per cent for


each sheave the rope passes over, including lead sheaves (if any).


The load in the lead to the winch (given 5 per cent friction) may be calculated as follows:






Load in lead to winch (LL) = BL + (BL x number of sheaves x 0.05)


= BL + (BL x number of sheaves x 5 ? 100)


or for a given load in the lead, the maximum load that can be lifted is calculated as:


Load = LL x no. of parts supporting load


1t (number of sheaves x friction)


Example 1:


No. of sheaves in purchase = 5 (3 top block + 2 bottom block)


Number of parts of rope = 5 supporting lower block


Number of lead blocks = 2 (7 sheaves in total)


Total load on lower block = 10t


Becket load = 10 ? 5=2t


Load in lead rope to winch = 2 + (2 x 7 ? 20)


= 2.7t


Example 2:


Calculate maximum load for the above arrangement using a winch with a 2.7t line pull


Load = 2.7 x 5


1 + (7 x 0.05)


= 13.5


1.35


= 10t








The above calculations do not allow for sudden impact, acceleration and deceleration which can cause


very high loads in the rope. These should all be avoided.

[ame]https://www.youtube.com/watch?v=chZsmKZ9Rjw[/ame]

I see where your crossed up.

MA is the total advantage which is different to rigged to advantage and rigged to disadvantage.

a single block rigged to disadvantage is only redirecting the rope real work gives you an overall MA (including losses) of about .85-.9

a single block rigged to advantage has and overall MA (including losses) of about 1.7-1.8

just as an additional...

why dont you, as a professional rigger get your hands on a set of strain gauges and do a youtube vid for us of how snatch blocks actually work...

just as a by-the-by what you're talking about as whats needed to achieve a theoretical MA of 2:1 is called "gun tackle"

Keep up the good talk, It's all over my head at the moment, but I'm absorbing it.
I like it when you talk nerdy...

Right, figured this out - both sides are in fact correct.

You are right in that one pulley/snatch block does not increase or decrease the load, except for friction so it in actual fact increases the load.

The difference between a car and the situation above, is that the car (or pulling point) is both non static (which the other chap alluded to - sorry on the phone and can't be bothered scrolling) - the key point to keep in mind here is the winch drum, which in actual fact operates as a second pulley/snatch block in this situation, hence reducing the load. Because the drum is a cylinder, all pulling points are technically after a 180 degree point, hence giving you the mechanical advantage.

Sorry for saying you are wrong, its just we weren't comparing apples with apples.

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Originally Posted by Blknight.aus

I see where your crossed up.

MA is the total advantage which is different to rigged to advantage and rigged to disadvantage.

a single block rigged to disadvantage is only redirecting the rope real work gives you an overall MA (including losses) of about .85-.9

a single block rigged to advantage has and overall MA (including losses) of about 1.7-1.8

just as an additional...

why dont you, as a professional rigger get your hands on a set of strain gauges and do a youtube vid for us of how snatch blocks actually work...

just as a by-the-by what you're talking about as whats needed to achieve a theoretical MA of 2:1 is called "gun tackle"

Dave, I am 68 years old I haven't worked as a rigger for many years, a "Strain Gauge" was used on a thread about this on 4WD action forum a few years back, but to find out how much load was being pulled by the winch the Gauge should have been placed between the snatch block and the anchor point instead of one side of the cable.
This stuff I am posting about is not out of the deep recesses of my brain, it is from Government supplied documentation from respective DLI depts. and NSW TAFE colleges, it is from Riggers guides supplied by Work Cover Authorities, research from years of rigging and actual work in the field, then we get some ...... coming along and saying that all these organisations and people are all totally wrong and that Warn is the one to listen to, this same company that sells winches rated at some ridiculous level which is higher than the Guaranteed Breaking Strain of the cable fitted to it and this bloke says he don't know much about it, the only sense he has spoken.
This issue has been around for years, because years ago some bright spark pulled his bogged 4WD out by using a snatch block when he couldn't budge it with a single line pull.
So the GOSPEL becomes a snatch block will make your winch pull harder by 2, not the fact that he was winching from the lowest part of the winch drum which gave him probably a 2:1 advantage over a single line pull which was pulling from near the top of the winch at about half the advertised load rating.
Remember the Earth was flat once, only because most believed it so.
If you want to argue this subject either read some correct info or go and annoy someone else, not you Dave, but those blokes that have NO idea of what they are talking about, Regards Frank.

Have a look at page 31 of the 1995 version of Work Cover's Riggers guide.

[ame]http://www.riggingtraining.com.au/Rigging/rigging_guide_part_1.pdf[/ame]

I think the reason for the disagreement is that none of the images there represent what normally happens with a 4WD recovery.

In fact I can't imagine a rigger needing to use the setup that a 4WD recovery uses, which I suppose is the reason it is not illustrated in the Riggers' Guide.

The example of the man in the seat on this page is close to a lot of 4WD recoveries.

Simple Machines -- Mechanical Advantage

Here are examples where the fixed point is not obvious:

A man sits on seat that hangs from a rope that is looped through a pulley attached to a roof rafter above. The man pulls down on the rope to lift himself and the seat. The pulley is considered a movable pulley and the man and the seat are considered as fixed points; MA = 2.