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SiWhite
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Right......little calculation for someone who knows lots about maths and engineering......I don't pretend to be any good at this sort of thing - in fact, I haven't the first clue about how to begin calculating the answer to this.

I'd like to know the strain that would be exerted on a snatch block when changing the direction of pull by, say, 20 degrees. For example, you're doing a winch pull with an 8,000lb winch. Straight pull isn't possible, so you rig a snatch block to 'tweak' the direction of the winch line by about 20 degrees. How much pull would be exerted on the snatch block and the strop? Would it be 8,000lbs? Or would it be 888lbs? (cause 20 degrees is 1/9th of 180 degrees, at which the full pull would be exerted, and 888lbs is 1/9th of 8,000lbs). Told you I'm c**p at this sort of thing!

I would ASSUME that as the angle of 'tweak' is increaced, the force on the snatch block would increace as well. Is there an easy formulae for working out how much pull would be exerted that a mere engineering mortal could use?

Oh, and there is definitely Land Rover content - I just don't want to reveal just what yet for fear of getting flamed (even more) for being a muppet!

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OK i found this it might help.

"Snatch block" is a specific type of block. A "block" is a "sheave", which is a wheel with a grooved edge to hold the rope or cable, combined with an axle or pin, side plates and a means of attaching it to something, typically a hook or shackle. Blocks are used in "block & tackle" combinations using multiple sheaves and two block assemblies, to increase mechanical advantage. A "snatch block" is a specially-designed block which is used to "snatch" loads or anchors, and is designed so that the side plate can be opened or swung away for the easy insertion of a loop of rope or cable without having to thread a bitter end through the side plates, and can thus be inserted anywhere in the cable without having to remove the load first to change the direction of the cable pull. Snatch blocks are one of the most mis-understood and misused items in the RV winching field, and can cause enormous harm and death if not properly rigged. Remember, if you overload a cable/snatch block, and the block parts from the anchor, you have a 10 or more pound missile flying at great speed, which can kill instantly. The most important thing to remember about snatch block winching, as used in the typical 4wd sceneario, is that there is tremendous force applied to the block/anchor even when using a single-block system for a simple change of line direction.

> = vehicle

O = block

# = anchor

- |= line

1. >-----------O|

|

|

|

#

No mechanical advantage, 1 pound of pull required for each pound of lift/tension.

2. >----------------O|

>-----------------|

Mechanical advantage = 2/1 1 pound of pull results in 2 pounds of lift/tension.

Now for the IMPORTANT part; The amount of force applied to the *block* is dependant upon the angle between the incoming line (to winch) and the anchor line (to tree)! The total load on a block can be as much as *2 times* the load applied. The maximum force on the block comes when the angle of the two lines is 0 degrees, or are parallel to each other as shown in #2 above. The actual load varies with the angle between the legs.

Here is the table from the Crosby Group catalog:

Angle is the measured angle between the legs of the line as they pass over the block sheave. Factor is the multiplier for the applied line load to calculate the block load.

Angle... Factor

0.......... 2

10........1.99

20........1.97

30........1.93

40....... 1.87

50........1.81

60........1.73

70........1.64

80........1.53

90........1.41

100......1.29

110......1.15

120......1

130......0.84

135......0.76

140......0.68

150......0.52

160......0.35

170......0.17

180......0

In #1 above, the angle is 90 degrees, so the factor is 1.41. So a 10,000 pound load will place 14,100 pounds of force on the block/anchor.

And in #2 above, if the bitter end of the line is *not hooked to the winched vehicle* but is hooked to a tree or rock next to it, for each pound of winch force applied, the block will be under *twice* the load, while the line strain will be equal to the line pull. So if you apply a full 12,000 pounds of pull, you are putting 24,000 pounds of stress on the block/anchor. If, however, the bitter end *is* attached to the winched vehicle, the block becomes a "traveling block", even though it is attached to a solid object, because the vehicle itself is actually doing the "traveling", and the load on the *line* is halved, since you are using two lines to "support" the load. In this case, the mechanical advantage is 2/1, so you get 2 pounds pull for each pound of winch effort. Now, since you have two lines splitting the load, let's say it's 10,000 lbs. (you're *really* stuck), the line load is halved to 5000 lbs per line, but the *block* load is still 10,000 lbs because of the parallel line angle factor of 2. As you can see, while the load on the winch is cut in half, as is the line speed for winding, the load on the block is *not*, and is equal to the total line load.

This is why using a properly rated and carefully maintained block is of the utmost importance when winching. In most RV winching situations, "double-lining" is the best method to reduce stress and strain on the lines and winch motor, at the expense of speed, but may be the *worst* thing you can do from the safety or block standpoint, if your block is not adaquately rated or is in poor condition or if you don't know the strength of the anchor or the stall-pull of your winch. The block is being subjected to twice the force of any of the other components, and consequently the winch operator may never know he is exceeding the capacity of the block until it parts and takes his head off. This is especially true when using very powerful winches.

Imagine the stress on a block (and it's anchor as well) when the SUV owner with the Warn 12,000 lb winch double lines to pull his vehicle over an obstacle and sticks a wheel behind a rock, locking the vehicle in place. The winch is already rated for 6 tons single line pull, and adding a traveling block doubles the pulling force to 12 tons. If your snatch block is (as many are) underrated for the winch/line combination, and you have a 6 ton rated snatch block, or even a "really big 8 ton-rated" block, which is commonly sold with RV winches, you still have the potential of block failure because you are using a 6 - 8 ton block with a system capable of exerting 12 tons of force under circumstances where you might not recognize the actual pulling force you are exerting, because you are not really stressing out the winch.

A recipe for disaster.

This is *even more true* of hydraulic winches, which, unlike electric winches, usually pull until something breaks. With an electric winch, there is a built-in "safety" mechanisim which is that near the maximum-pull winch-stall load, the motor draws enormous amounts of amperage and heats up quickly. This means that the motor will usually stall due to lack of available amperage, increased electrical cable resistance due to heating, or motor overheat long before one can actually reach the rated pull, which is determined under lab conditions.

With a hydraulic winch though, the hydraulic motor runs happily along, getting slower and slower, and exerting more and more force, until something snaps, and will *rarely* stall before reaching breaking load. So, one must be *extra* careful when using hydraulic winches, since the winch appears to be quite happy, and is just getting slower and slower, and the typical RV person expects it to stall, which it won't.

Please keep in mind that we are always talking about the "safe working load limit" here, and while the ultimate failure safety limit is typically 5 times the working load limit for commercially-rated hoisting gear, it may be less, or non-existent in cheap, uncertified blocks, *particularly* "surplus" blocks available at low price.

NEVER, NEVER, NEVER!!!!! USE AN OFF-BRAND, UNCERTIFIED OR SURPLUS BLOCK WHEN WINCHING.

Good blocks are expensive for a reason. You spent big bucks on your rig, and big bucks on your winch, don't, for gosh sake, scrimp on your block! I bought two 15 ton McKissick Light Champion snatch blocks last week, and they cost about $125 each, and are worth every dime.

Baz.

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Simplified view: There are 3 forces acting on the snatch block as shown in the below diagram.

snatchblock.jpg

F1 and F2 are the forces applied by the rope and as the rope is free to run through the snatch block they are equal, F1=F2.

F3 is the force the snatch block applies on it's anchor.

F3 is balanced out by the components of F1 and F2 that are in the opposite direction to F3.

This is where the old trigonometry comes in, remember sines and cosines from school? The component of F1 in the direction of F3 is F1 sin a

As F1=F2, then F3 is balanced out by 2F1sina

This is how that table above has been derived.

When a=45° (ie the total angle between the ropes is 90°) sina~0.7 so twice this is ~1.4, hence the load on the snatch block is 1.4 times the winch pull.

Simple eh?

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Mark

sorry but i would say that your F3 = 2 F1 COS a

its the adj angle hence COS not Sin. Sin would give you F2 in a vertical plan.

or F3 = 2 F1 SIN (90-a)

COS and SIN 45 are both 0.707 hence as you stated the 1.41

i'll do a pic for Si question

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angles2.jpg

ok i cant make it any bigger, when i upload it from A4 size photobucket resizes it then only gives options to reduce size, not enlarge.

i will have to have a play around with it.

Mark you win the computer skills front!!!

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F1 to F3 is not going to be 90° as in your diagram if the snatch block is free to move then it will rotate such that F1 and F2 are at equal angles from F3. If the forces F1 and F2 are the same (which they will be assuming no/negligable friction in the snatch block) then that angles will be the same.

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SOH CAH TOA

SOH = Sine : Opposite / Hypotenuse

CAH = CoSine : Adjacent / Hypotenuse

TOA = Tangent : Opposite / Adjacent

or

Silly Old Hitler Couldnt Advance His Troops Over Africa

No help to the question I know, but easy way to remember a bit of Trig basics.

Or just learn a CAD program and forget how to do proper work!

Rog

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would it not always end up as 90deg becuase yep the snatch block would move as the angle changed, as would the strop to the tree in this case changing the applied F3 force. but the force F3 would always be perpendicular to the anchor piont.

in other words if the angle changed to 40 deg from the straight line the block would move with strop when the force was applied from the 3 O clock position to the 4 O clock position (for example)

its a good while since i have worked with these!!!

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heres some other info we use at work for recovery:

Estimated pull = RR + GR + DR + SF

RR = rolling restance = weight of Vehicle(WOV) / ground factor(GF)

Where GF has various readings: hradstanding 25, grass 7, hard wet sand 6, soft wet sand, 5, dry sand 4, light mud 3, bog clay, deep mud 2.

GR = gradient = (WOV x degree of slope (over 45deg = WOV)) / 60

DR = drive restrictions = no really relevant for wheeled vehicles but 1 locked track = 1/3 WOV, and 2 LT = 2/3 WOV for wheeled half total number of axles locked 1/3 WOV, more than 1/2 axles locked 2/3 WOV

SF = safeyt factor = (RR + GR + DR ) / 4

Mechanical Advantage needed = EP/Winch cut out.

when calculating further weights depending how you got stuck:

sheer weight = RR + 2/2 WOV

any side over side = 2/3 WOV

and end over end = 1 1/4 WOV

these are in addition to the EP.

flat tyres

upto 4T = additional 1/2T per tyre

over 4T = additional 1T per tyre

some food for thought

or do you jsut connetced up and hope for the best?

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would it not always end up as 90deg becuase yep the snatch block would move as the angle changed, as would the strop to the tree in this case changing the applied F3 force. but the force F3 would always be perpendicular to the anchor piont.

in other words if the angle changed to 40 deg from the straight line the block would move with strop when the force was applied from the 3 O clock position to the 4 O clock position (for example)

its a good while since i have worked with these!!!

What I was saying is that F3, the load on the snatch block / anchor will always be at the same angle from both F1 and F2.

So if the angle between F1 and F2 is 160°, ie 20° from straight pull, as it appears to be in your diagram, the the angle of both F1 and F2 from the line perpendicular to F3 would be 10° each so you have F3=2F1cos10 and not 2F1cos20

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Just to confirm what has been said, if the block (sheave) is fixed in space, e.g. built into a RIGID structure and not allowed to move, just spin as the rope feeds through, then the angle of rope going into it (from winch) can be different to the angle of the rope coming out of it (to the fixed end).

If however, * as is most likely for our type of situation* , the sheave block is free to move in space - i.e. attached to a tree by something flexible like a rope or chain, then it will inevitably take up a position where the angle in = angle out. So in this case the rope could never go into the block at 0 degrees and come out at 30 degrees. The block would move so that input and output angles both settle at half the total - e.g. in this case 15 degrees each.

This is because ropes and other flexible things only support tension forces (pulling apart).

This could be altered by adding additional flexible links to support the block from different directions, but that's probably over-complicating things. (I just thought it's nice to know that it CAN be done if absolutely necessary).

Al.

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Er

Jeeez 2 neurophen later, :huh:

heres the

'HFH Simpletons' version special 'equation' for winch pulling.....

Take one " 'E' thats F BIG".... Big Snatch Block that is mate .... a la :

BBQ004Medium.jpg

Them take a HUGE Winch where people say " 'M' mmmmm'...... thats big... winch that is....."

FW.jpg

Attach Winch to 'Muppet'..... 'C 'here for example ....and pull making sure the winch is 'square' to the recovery...........

AWDC7.jpg

and this folks proves "E = MC Squared"

I'll get me coat...

Nige :ph34r:

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Yeah,

This was horrendous...

I arrived a the carnage to find a melted motor on one winch and a damaged 8274, they couldn't shift it, it had crashed into the bank crossing the ravine front and rear embedded, I was asked to "Get it out and NOW..please nige :lol: "

I shoved the winch on it, with 6x LRs and RRs tied to me and a ground anchor...it came out in bits, tore off the front winch with "Recovery (chocolate) mountings :

AWDC3.jpg

AWDC4.jpg

so having pulled off the front bumper assembly and the winch cradle (held on with studding), we pilled the bits on the bank, I was asked to "Bring it out in bigger bits Nige"...but, it WAS moving, just kept tearing lumps off it !

AWDC5.jpg

The passenger chassis leg cracked...so moved to drivers leg knew it could cause probs...... :

AWDC6.jpg

but this pulled it the wrong way, but it moved, mate then shoved another winch cable on and I wrapped strop around axle :

AWDC8.jpg

he'd been in nearly 2 hours and in fairness apart from cacking himself was immensely appreacative of being out...

TYhe old Hydraulic W has a bit of pulling power..... :D ...quite a few thought that after 2x winches earlier failed to move it how was 1x winch going to do it..... had a few people peering at it after the RR was recovered, some even asked me "what sort of Electric winch have you got theremate " :(

So, on a serious note to everyone, please do us Marshals a favour, have you got REALLY GOOD FRONT AND REAR recovery points, with HD bolts, spreader plates and siutable to be SERIOUSLY recovered ?....

Without these any recovery becomes so much more difficult for us, and dangerous for us, you and spectators, sorry to hapr but its one of my pet bug bears !.... PLEASE think and look, ask if your unsure SCL has done a post on this very subject...PLEASE

Pant Pant

Ok nurse ...sorry I'll be a good boy and ranting stop now :blink:

No, seriously ple.........argh gerrof................................................. :huh:

Nige

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