Snatch block sizes

[ramdough]

Is it “OK” to run a 9/16 or 5/8 snatch block on a 1/2 SRW wire rope?

I imagine the cycles are so small it would make no difference.

What are your thoughts?

Thanks!


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[98G]

Not really. The inadequate side support on the cable will allow it to flatten out and cause premature failure.

Probably not an issue on a rarely used SRW.

 

[Jbulach]

Depends on the block, what you are doing, and your definition of OK.

Most good blocks have an 1/8” range listed on their capacity label. If your lifting, stay in the capacity and rope size range. Winching, use your safe rigging and operating practices, 1/8” under the low end of the range probably wont damage anything, unless you overload.

 

[Awesomeness]

Aren't the blocks that are issued with these trucks the Skookum Model 62?

 

[ramdough]

Aren't the blocks that are issued with these trucks the Skookum Model 62? View attachment 800975

That one says 9/16” on it. Hence the confusion.


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[simp5782]

Might as well get a big one just in case

 

[Awesomeness]

Might as well get a big one just in case

Doubles as a garden hose pulley.

 

[Curtisje]

Aren't the blocks that are issued with these trucks the Skookum Model 62? View attachment 800975

This one says 5/8 inch. Also a model 62.

 

[Awesomeness]

This one says 5/8 inch. Also a model 62.

Looks like the part numbers are slightly different.

 

[ramdough]

So I called Skookum and asked them about the snatch blocks.

They said that they only made the 9/16 and 5/8 size snatch blocks for the army.

So, my interpretation is that either these trucks used a different brand snatch block for these trucks, or they were using a larger pulley than the cable size.

I asked if using a 1/2 cable in a 9/16 block was ok, and they said for this type of application, I could run it, but it is not ideally sized.

Does anyone know specifically which snatch block they issued for these trucks?


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[Awesomeness]

Does anyone know specifically which snatch block they issued for these trucks?

There are standard equipment lists in the back of some of the TMs, as well as a separate hand receipt PDF, available. You could look through those and see what they say.

 

[fasttruck]

I recall, from reading rigging manuals, the ratio between the cable size and diameter of the sheave is 12:1. 1/2" cable requires as 6" sheave, 5/8" cable a 8" shave and so on. Using a bigger sheave than the indicated size is OK, too small and the cable will breakout the sides of the sheave and make it unserviceable.

 

[Awesomeness]

I recall, from reading rigging manuals, the ratio between the cable size and diameter of the sheave is 12:1. 1/2" cable requires as 6" sheave, 5/8" cable a 8" shave and so on. Using a bigger sheave than the indicated size is OK, too small and the cable will breakout the sides of the sheave and make it unserviceable.

Using a bigger DIAMETER sheave is ok, but what he is asking about is the size of the groove in the perimeter that the cable sits in. You do not want to use a groove that is too big, or the cable will flatten and become damaged. So he's asking how much bigger of a groove you can get away with.

 

[Swamp Donkey]

So I called Skookum and asked them about the snatch blocks.

They said that they only made the 9/16 and 5/8 size snatch blocks for the army.

So, my interpretation is that either these trucks used a different brand snatch block for these trucks, or they were using a larger pulley than the cable size.

I asked if using a 1/2 cable in a 9/16 block was ok, and they said for this type of application, I could run it, but it is not ideally sized.

Does anyone know specifically which snatch block they issued for these trucks?


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Let me clear up some misinformation here. I don't know FMTV winches other than what's been posted here, but I collect block and tackle, civilian and military. I know blocks and rigging very well.

I'll start with Skookum, since they told you wrong. Skookum makes high end blocks, and some of my favorite blocks are Skookum, but whoever you talked to told you wrong. Skookum has made all different sizes of blocks for the military, from 1/4" all the way to 1-1/4". I've seen them in surplus lots. I had a military 7/8" Skookum block a couple years ago and I still have the military 3/4" block in the picture below. There is also Ulven which is the parent company of Skookum. Some Ulven blocks are double marked Ulven/Skookum and some are only marked Ulven. Regardless, Ulven and Skookum are the more common blocks the military used, with Skookum being the higher end blocks.



Sheave diameter is important but isn't the only critical factor. Groove size is just as important, if not more so. As stated above, there is a minimum diameter sheave that can be used with each diameter of wire rope. A larger diameter can be used without any issues. Larger diamter sheaves actually increase wire rope life but at some point huge sheaves become impractical. Sheaves that are too small cause the strands in the wire rope to bend too far causing premature fatigue of the strands.

Groove width on the other hand can cause damage if too wide and too narrow. If the groove is too narrow, the walls of the groove can be broken or the outer strands on the wire rope can be cut. If the wall is too wide then the wire rope is not supported. The wire rope flattens, causing fatigue from excessive internal friction between the strands. The wire rope is braided so strands cross each other. In an wire rope where the sides are not properly supported, these crossings become friction points and damage many strands all at once. This damage will mostly be internal and not apparent from a visual inspection. This is why most people say that flattening the rope "isn't that big of a problem". The damage just isn't apparent.

Something else to remember while looking for blocks is that there are lifting blocks and there are snatch blocks. There are also different braids of wire rope for both of those categories as well. Some blocks are sized specifically for the size, braid and use for their intended application. I say this because there is not a precise comparison that can be made for sheave diameter to groove size, or even the weight rating for that matter. Different safety factors, ratings and manufacturers further muddy the waters. An example in the picture below:




Both of the above blocks have 8" sheaves. The one on the left is a Warn 36,000# with a 3/4" groove for use with 1/2" wire rope. The one on the right is the one with the tag in the picture above, a Skookum 35 ton with a 1-1/4" groove for use with 3/4" wire rope. There is not a correlation between sheave size and groove width.

So to answer your question, you need a minimum of an 8" sheave with a ~3/4" groove. A larger diameter sheave is fine. The Warn block in the picture would suit your needs just fine. It's sized correctly and uses double ball bearings. They were issued with the BII for the MRAPs with the Warn 18k winches. They can be bought through surplus or on the civilian market. There are a couple companies making knockoffs of this style. The knockoffs all use brass bushings. The Warn is the only one using ball bearings. All my Warn blocks came through surplus.

If you want a stock block, the TM will have a BII list. Take the NSN and run it through Parttarget. You'll get a list of all the manufactures of that particular block and all the dimensional measurements you could want to help in your search.

 

[Awesomeness]

The one on the left is a Warn 36,000# with a 3/4" groove for use with 1/2" wire rope.

It just says Wire Rope 3/4", so how did you go about determining that it is for 1/2"?

 

[simp5782]

It just says Wire Rope 3/4", so how did you go about determining that it is for 1/2"?

The warn is the black one in the picture and does not have a tag pictured. The warn 36k in either version of snatch block is 1/2" cable by spec with a 3/4" sleeve

WARN 83086 8" Snatch Block 36,000 lb. rating, for Winches up to 18,000 lbs.

WARN 8" Heavy Duty Snatch Block Rated at 36,000 lbs. For Winches up to 18,000 lbs rating. Doubles the pulling power of your winch Aids in sharp angle and three point rigging 1 1/8" opening Buy with confidence from Montana Jack's, we are your WARN Authorized Warranty and Service Center.We are a...

www.montanajacks.com

WARN 93195 Epic Snatch Block, 36,000 lb Capacity, for Winches up to 18,000 lb.

WARN Epic Snatch Block, 36,000 lb Capacity Use either wire or synthetic rope, Maximum rope diameter 1/2" Reduces heat buildup and amp draw of winch during use Double pulling power Change pulling direction without damaging rope up to 18,000 lb. winch capacity Buy with confidence from Montana...

www.montanajacks.com

 

[Swamp Donkey]

It just says Wire Rope 3/4", so how did you go about determining that it is for 1/2"?

The black Warn block is rated by Warn for winches up to 18k. The 18k winches use 1/2" wire rope. I physically measured the groove at the leading shoulders to come up with the 3/4" groove since I don't see where Warn lists groove size. The groove tapers in towards the bottom to create the fit for 1/2" wire rope. You have to have a slightly open tolerance at the top of the groove to make sure the wire rope is guided smoothly to the bottom.

The green Skookum measured 1-1/4" at the top shoulders and Skookum rates it for 3/4" wire rope. The same applies as above.

This is the point from my previous post though. There is a big difference of block specs between vertical blocks and horizontal blocks. Horizontal blocks, or snatch blocks, are usually designed by winch manufacturers. They leave tolerances and specs on the loose side, or just sometimes throw them out the window. Nobody worries about this stuff while they're stuck in the mud. Notice in Wes's link to the Warn block that Warn say UP TO 18k winches. Some of the smaller winches use smaller wire rope but they're saying it'll be fine in this particular block. Some of their newer design blocks are rated for 1/2" wire rope but are only using a 4" diameter sheave. In the winching industry, wire rope is cheap and is replaced often. It gets dragged through the mud, wrapped around trees and pulled through rocks. They know this so they don't worry about the specifics that much because they know most end users won't. Many winch manufacturers use the breaking strength to rate their winches, ropes and fittings. There is no safety factor built in. There isn't much regulation there. It's just a mix of marketing tactics and the nature of the environment for the end user.

On the other hand, vertical blocks, or lift blocks, are very specific. They're regulated, rated and very specific to the task. When replacing crane wire ropes can cost several thousand dollars, wire rope life is extended as long as possible. The wire rope is what keeps things in the air. Everything about anything that touches that wire rope is done to protect it. Manufacturers maintain very tight specs on sheave diameter, groove size, safety factors and many more critical areas. That is a highly regulated industry and for good reason.

Both of the blocks in the picture are horizontal blocks. I was showing the difference in the sheaves to show the major variations that horizontal blocks have from the "rules". Most of the "rules" came from the vertical industry anyways. Point being, just get close to the proper sizes for winching and you'll be fine. Wire rope on winches has a far better chance of getting damaged by environmental factors than it does by blocks that are slightly off on the specs. The sheave and groove measurements I gave for the Warn block are fairly common for blocks for 1/2" rope. I listed those to open some options for finding blocks that'll work for Ramdough.

Here is a link to a PDF sizing chart from Gunnebo Johnson showing wire rope/sheave diameter for lifting equipment, along with a whole lot of other information pertaining to this. Take a look and you'll see the differences I'm talking about for the horizonal blocks above compared to lifting blocks. Start looking for blocks in the sizes the chart recommends and prices start getting pretty steep. I'm just saying, don't get hung up too much on the numbers for winching. Stay within reason of the sheave diameter and groove size and you'll be fine.

https://www.google.com/url?sa=t&sou...FjAIegQICBAB&usg=AOvVaw1sd8iQ4g0n60gpGVuLVmUt

[/URL]

 

[Awesomeness]

https://www.google.com/url?sa=t&sou...FjAIegQICBAB&usg=AOvVaw1sd8iQ4g0n60gpGVuLVmUt

[/URL]

Link got chewed up by the forums, let's see if this works: http://www.gunnebojohnson.com/wp-content/uploads/Rigging-Combined1.pdf

 

[ramdough]

Aren't the blocks that are issued with these trucks the Skookum Model 62? View attachment 800975

I found a deal on the 9/16” Skookum snatch blocks that look like the one in [mention]Awesomeness [/mention] picture. So, I think I am set based on everyone’s feed back. I don’t have the safety latch for the hook..... anyone know where to get them?


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[ramdough]

Thanks everyone for your informative responses!


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