DMM Impact Block Load

[chris_girard]

I’ve visited the DMM website and downloaded the spec sheets for the impact blocks and I’m surprised by some of the numbers. I could not find a MBS (Minimum Breaking Strength), which would have allowed me to develop a 10:1 SWL or WLL for Tree Care operations (not the 5:1 Industrial Rigging app). I did find the following numbers:

IMB-S (small block)
Working Load Limit (WLL) – 40 Kn = 8,992 lbs = 4 tons
Maximum rope diameter – 16 mm = 5/16”

IMB-L (large block)
Working Load Limit (WLL) – 60 Kn = 13,488 lbs = 6 tons
Maximum rope diameter – 20 mm = 3/4”

What really surprises me is the 4-ton load rating on the small impact block. That’s DOUBLE the rating that you see on blocks that size used in tree work and that’s why I think the WLL listed on the DMM spec sheet is a 5:1 industrial rating, not our 10:1.

There is no way that I would want to block down that much weight on the line sizes that they’ve listed. Manufacturers have to start being careful when they list WLL on their equipment, as tree work IS NOT the same as industrial work. Yale Cordage does the same thing with their rigging lines listed for tree care. Their new catalog shows WLL of 5:1, not 10:1.

Am I missing something?

Here’s their link:
http://dmmprofessional.com/productsDetails.asp?id=13&id2=57&id3=401

 

[tomthetreeman]

Talk about the tree becoming the weak link!

 

[chris_girard]

I hear you Tom and was thinking the same thing. Equipment these days are so much stronger than some of the trees that they are used in that it is down right scary!

Maybe Mark Bridge can share with us a little more technical info on these blocks and how they came up with the numbers.

 

[Colin]

in practical application you would only want to take a two ton block with it as the force applied to the rigging point will be double the weight of the piece rigged. we have had to hang some very large storm damaged trees and i was very glad to have blocks and rigging with 20000lbs mbs. it makes me feel a lot better knowing for sure that the block or rigging isn't going to break and the durability is even more important. we just bought an isc 3/4" spring block after one big job with mostly 750lbs to 1200lbs pieces the block is showing signs of being worn out; the pin on the main pulley is very loose and the cheek plate will hardly open, we've only had it a month, the paint isn't even wore yet!

 

[chris_girard]

Jeff, if you don't want to wait for the new DMM blocks to come out, I would purchase a 3/4" HOBBS Block from WesSpur while they last, since they no longer are making them.

That's not a good sign that the ISC block is showing that kind of wear after only 4 weeks.