Arborist Block Ratings

N

NailerB

New member
Location
illinois
Pardon my being very green but I do not understand the MBS to WLL on some of these.
CMI Compact 3/4 MBS, 51,000 WLL 5,100
DMM 200KN 5/8 MBS, 44,960 WLL 8,992
CMI 178KN 3/4 MBS, 40,000 WLL 6,000

Why does the CMI that has the highest MBS have the lowest WLL? The sheave is equal to or more than the other two.

Need a lil schooling

Nailer
 
It’s just how they are quantifying what a swl is. Many manufacturers rate at 1/5 of break strength, CMI is just doing a little math for consumers with the rp160 and rating at 1/10, as the load is doubled in a complete bend scenario at the rigging point.
I’m not sure why the last block is rated at a different percentage of MBS, but the critical concept is to never exceed 10% of MBS as your base load weight in the system.
If you are shopping, the rp160 is a great block. Mine had the spring issue for a minute, but they fixed it for me, and I’m sure they have worked out that issue in subsequent production runs (purchased mine when they first came out).
 
It is strange that cmi doesn't use the same rules to rate two of their own blocks. 1/10th or 1/5th, I understand why they are used, but the block rated 40,000 mbs 6,000 swl is strange. That is a 6.667 to 1 working load. I assume that is the stainless steel cmi block? Maybe its rated differently because its stainless steel which has different properties than aluminum or non-stainless steel. It would be nice if they made it clear why they don't use the same rules for the rating. That being said, I have the orange cmi block rated 51,000 mbs. I only got it a few weeks ago, but I have made a good bit of use of it and like it a lot. And 5,100 swl is fine with me, no rope I have is rated that high for working load, nor am I likely to cut a piece of tree that wieghs 5,100 lbs
 
From experience, the DMM takes more abuse and outlasts the CMI orange block. I’ve used a dmm block that’s 8 yrs old and still runs smoothly. The cmi that we got in the fall is already rattling after being used in a double blocking scenario on 1,500 lb pieces, and regular duty since. Remember, the working load isn’t just about the weight of a piece, but must factor in the forces generated. My guess is that, while the cmi boasts a greater capacity before failure, the dmm’s internal components are sturdier, giving it greater longevity.
 
Crimsonking said:
From experience, the DMM takes more abuse and outlasts the CMI orange block. I’ve used a dmm block that’s 8 yrs old and still runs smoothly. The cmi that we got in the fall is already rattling after being used in a double blocking scenario on 1,500 lb pieces, and regular duty since. Remember, the working load isn’t just about the weight of a piece, but must factor in the forces generated. My guess is that, while the cmi boasts a greater capacity before failure, the dmm’s internal components are sturdier, giving it greater longevity.
Click to expand...

Interesting. Out of curiosity, what type of rope were you using? How long was the piece? I am just a really big rigging nerd and want to know more about the forces in the situation.

I went and checked out my rp160, and it has play where the axle seats into the spring cheek. Not sure how much rattle it had originally, it’s a small enough degree to be on the broad side of a manufacturing tolerance. Anyone else?

Can the DMM fit a 7/8” or 1” sling?
 
I use polydine (or something with similar construction/properties) for almost all of my rigging. In this particular situation I was using DBR to add rope to the system and reduce loading on the spar. Adding rope was due to the tight drop zone and heavy loads coming down, allowing for a running stop instead of a sudden catch.

The pieces were between 4 and 5’ long, ranging from 1-1.5k lbs per the log chart. The cmi block rode the pieces, while the dmm was on the spar.

The bearings on the cmi didn’t like the rig, but they’re still working within tolerance.
 
I love all cmi gear but I often wonder about their ratings for example basically all of their micro pulleys are rated 31 kn while other manufacturers have different ratings for each of their micro pulleys. I've noticed this even with their bigger stuff too
 
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