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  1. #1
    Veteran Member
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    Kubota b2920

    Default pulling force of a B2920

    I was wondering if anyone ever estimated the pulling force of their tractor. I'm trying to size some rope / chain / cable for pulling trees (i.e. putting pressure on them as they are cut to get them to fall in the right direction). I have 5/16" chain now, which is labelled as having a working load of 4,700 lbs. I pulled on a tree on a level surface with great traction (paved and I have loaded turf tires) and spun all 4 wheels without breaking or stretching the chain. The shock load of going forward and being stopped by the chain also did not effect the chain. I"m not sure if my chain has a max load of 4700 lbs or something greater.

    I'd like to get a chain / cable / rope that is far greater than the pulling strength of the tractor so that I cannot break it and create a dangerous situation. Anyone have any ideas what the largest pulling force a tractor of this size can apply is?

  2. #2
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    Kubota B2620 (2012)

    Default Re: pulling force of a B2920

    I'm no tree expert, but.......it's my understanding that a rope...cable...chain etc. is used to simply guide the fall of the tree; not literally pull it down. A chain long enough to do the job would be too heavy in my estimation. I use 3/4 inch nylon rope that has a "work load limit" of 1,420 lbs. I've had no rope failures using my F-250 as the power source. On some ocassions I use the bucket of the B2620 to push an easy one over. I doubt that a B2920 could break any decent size rope..cable or 5/16chain....
    Kubota B2620 HST

  3. #3
    Gold Member locknut's Avatar
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    Default Re: pulling force of a B2920

    Breaking strength on that chain is very high.You won't break it unless you take a running start with a lot of slack,hooked to something totally unmovable.

    http://www.1st-chainsupply.com/chain/SpecsCh70.pdf

  4. #4
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    Mahindra 4035HST purchased 2013 - John Deere 42" Geardrive w/bagger purchased 2012 - Craftsman 42" HST purchased 2006

    Default Re: pulling force of a B2920

    Most chains are underrated for wear and abuse. 4700 pounds "work load limit" basically means with 4700 pounds hanging, it will never stretch or break. The actual point that it will stretch or break would be significantly higher depending on the type of force put on the chain (jerking, binding, etc.). A chain made to work with 4700 pounds of weight would likely take up to 6000 pounds of hanging weight before any stretching occurs, and a break weight would likely be closer to 10k. I've seen what it takes to stretch an old "unrated" 5/16" chain, and 10k isn't even touching it...

    Before the safety police jump in, none of this is any sort of recommendation to overload a chain or anything else; my point is only that "working weight ratings" are underrated to allow for the typical wear and uses for such things.

    I would agree that a B2920 will never break a 4700# rated chain without adding a lot of weight to the tractor. At 30ish horsepower, I think there is the power to break such a chain, but at 1500ish pounds, it'll never put enough of that power on the ground...

  5. #5
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    Kubota b2920

    Default Re: pulling force of a B2920

    Thanks, that is an eye opening link to the strength, i had no idea breaking force was 18k lbs. i'm looking to get some rope so i can use pulleys, i was looking at 8,000 lb breaking strength on that. Think that will be a safe enough margin?

  6. #6
    Super Member
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    JD2010, Kubota3450,2550, Mahindra 7520 w FEL w Skid Steer QC w/Tilt Tatch, & BH, BX1500

    Default Re: pulling force of a B2920

    My 12K# 7520 setup will stretch a 5/16 Grade 43 chain but will not stretch the Gr70 - which is the one rated for 4700 working load. This is steady pull w/o intentional jerking. Inevitable jerks are what the ~ 4X breaking strength safety factor covers.
    larry
    This side of 40
    JD2010, Kubota L3450/FEL w SK QC, L2550 w FEL
    Mahindra 7520 [Pinky] /FEL w Skid Steer QC/w Tilt Tatch & BH, BX1500 [Mighty Mouse]
    IH37 Baler, CCM165 Drum Mower, JD Rake
    JD 127 bushog, Flail, SK Tilt Tatch , KK tiller, Rhino rear blade, Post driver, post auger, chipper, pallet fork, Grapple/Loader Buddy, Homemade Splitter/DC Welder

  7. #7
    Elite Member TomSeller's Avatar
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    Default

    Remember, you do NOT want to pull hard on a tree that is being cut. As stated above, you want to GUIDE it. Pulling too hard could create a very dangerous situation for the person behind the saw. Effectively making the tree into. to a "leaner" which could barber chair on the cutter.

  8. #8
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    Default Re: pulling force of a B2920

    My memory seem to recall that the safe working load of chain is typically about 1/4 of its breaking strength.

    But being a boater, I've used 150' anchor ropes and a come-along to pull trees in the direction I want them to fall. Leaning an extension ladder up against a tree can get you high enough to apply some real good leverage without needing tons of force. Using my tractor as the anchor point also allows me to stretch the ropes before tugging any with a come along. If I need more pull I can walk over to the tractor and work the come along without even putting the saw down, all while staying out of the tree's fall path.
    "Contentment makes poor men rich, Discontent makes rich men poor." Benjamin Franklin

    "...The conflict between believers and non believers is not about the beliefs..." IslandTractor

  9. #9
    Veteran Member xring100's Avatar
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    Kubota M8540

    Default

    If your not jerking the chain just a dead pull then you could guesstimate your tractive effort on level ground by knowing the operating weight and mu level Ag tires have about 0.7 mu on hard pack or gravel. Meaning if your rig weighs 4000 lbs you can only pull about 4000 lbs .7 = 2800 lbs is the maximum cable tension you could hope to sustain. However if your jerking it loads go through the roof and i would switch to a snatch strap. As you don't want to be on the receiving end of a broken chain or cable

    Dave

  10. #10
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    Default Re: pulling force of a B2920

    Thanks. In pulling configuration the machine is probably 3500 lbs, the BH almost doubles the weight. I usually tense it up, make a partial cut then pull to get the tree to pivot on the hinge if it's a bad leaner then make the final cut when it's leaning the right way. It does load the tree a bit, so safe positioning is huge.

    The biggest forces will be from skidding or from pulling a leaner's butt end out to get it on the ground.

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