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  1. #1

    Join Date
    Jul 2004
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    9
    Location
    Maryville, TN
    Tractor
    Kubota MX5000

    Default Rotary Cutter Shear Pin Set-up?

    I just started using a rotary cutter [KK standard duty 6'] with my kubota MX5000 over some rough terrain. My newly acquired property has 5-6 year old pines which I am removing with my FEL and then cutting with the rotary cutter. The KK gearbox is 40HP and the tractor PTO is 44HP. I'm using shear pins, and I've gone through several. In my experimentation, I found that if the shear pin is loosely attached [I did by accident 1st when the threads were damaged during insertion then I tightened the nut into the damage] the shear pin lasts much longer. My dealer gave me bolts with a lock washer, so I assumed to tighten fully, and these would break quickly. Well my question is why does the loose shear pin work better? and is this "technique" recognized by any others?
    Any feedback would be appreciated.
    Thanks.
    Britt
    [img]/forums/images/graemlins/cool.gif[/img]

  2. #2
    Elite Member Gary_in_Indiana's Avatar
    Join Date
    Apr 2002
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    3,388
    Location
    Fort Wayne, IN
    Tractor
    John Deere 4200 MFWD HST w/ JD 420 FEL w/ 61" loader bucket & toothbar & JD 37 BH w/ 12" bucket

    Default Re: Rotary Cutter Shear Pin Set-up?

    I have the same KK in the 5' version and haven't notice any particular difference in whether I use a lock washer and tighten down a 3" bolt or use a 4" or even a 5" with a couple nuts locked together.

    The biggest thing I notice is that my shear bolts don't need to be replaced nearly as often if I throttle down my tractor to idle to lock in my PTO.

    My shear pins seem to break in bunches, too. For no apparent reason I can mow without replacing one for so long I almost forget about them but, when that one finally does break, it seems like I'll end up not only replacing it but replacing two or three others in a relatively short period of use.

  3. #3
    Super Member Henro's Avatar
    Join Date
    Jul 2003
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    5,294
    Location
    Few miles north of Pgh, PA
    Tractor
    Kubota B2910, BX2200, Yardman 20HP pos...

    Default Re: Rotary Cutter Shear Pin Set-up?

    <font color="blue"> when that one finally does break, it seems like I'll end up not only replacing it but replacing two or three others in a relatively short period of use.
    </font>

    My guess would be that the variabilty in the third world steel making process is showing...Most grade 2 bolts on the weak side but an occasional stronger batch of steel finding its way into the bolt manufacturing plant...causing them to turn out a batch of stronger than normal grade 2 bolts...which get mixed in with the standard bolts...

    Just a guess though... [img]/forums/images/graemlins/smile.gif[/img]

  4. #4
    Elite Member Gary_in_Indiana's Avatar
    Join Date
    Apr 2002
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    3,388
    Location
    Fort Wayne, IN
    Tractor
    John Deere 4200 MFWD HST w/ JD 420 FEL w/ 61" loader bucket & toothbar & JD 37 BH w/ 12" bucket

    Default Re: Rotary Cutter Shear Pin Set-up?

    Bill,

    We look at the same thing from different angles. My theory is that occasionally I get grade 2 bolts that really ARE grade 2 and the rest of the time it's even inferior foreign production that isn't even grade 2 strength.

    Again, just a guess on my part, too. [img]/forums/images/graemlins/smile.gif[/img]

  5. #5
    Veteran Member
    Join Date
    Jan 2002
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    1,927
    Location
    Home-1+ acres New Hope, TX / 24 acres-Fannin County
    Tractor
    JD 950

    Default Re: Rotary Cutter Shear Pin Set-up?

    <font color="blue"> why does the loose shear pin work better? </font>
    Just a guess, but when you tighten the bolt (shear pin), it puts tensile stress on that bolt. Then when putting shear stress on the same bolt when using the implement may cause it to fail faster. Just a note. You don't want to tighten the bolt to the point it squeezes the collar to the implement shaft. That would partially defeat the function of the shear pin.
    As far as failing, I went 4 yrs on my Howse 6' before I broke my first. Then I broke my second the next weekend. [img]/forums/images/graemlins/confused.gif[/img] I was beginning to be concerned the the original pin was too hard as I had hit a lot of things I thought would have broken it.

  6. #6
    Gold Member
    Join Date
    Dec 2003
    Posts
    364
    Location
    OH
    Tractor
    Ford 851/ JD 770 MFWD

    Default Re: Rotary Cutter Shear Pin Set-up?

    You have to remember that the SAE Grade identification is based on MINIMUM requirements. So, to a certain degree you are correct in that it probably would be common to see variation in the actual strength of any given batch of bolts. I would expect this variation in Grade 4 and lower no matter where the bolt was manufactured. I would think that once you got into Grade 5 and up that the variation would be less, especially in those made in the US.

  7. #7
    Bronze Member
    Join Date
    Sep 2003
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    66
    Location
    Frederick County, MD
    Tractor
    New Holland TC40D

    Default Re: Rotary Cutter Shear Pin Set-up?

    I just broke my first shear bolt this evening. My mower has a slip clutch, but I now know there is a grade 8 bolt after the clutch. I thought the clutch was supposed to replace the bolt?

    I'm not sure why it broke, I didn't hit anything , I was turning around, probably a little tighter and faster than I should have been. I'll re-check the clutch to make sure it's working properly when I replace the bolt.

    If I just would have read the manual a little better I would have known there was a bolt, and would have had one on hand... from now on I'll have a few extra around.

  8. #8
    Epic Contributor Soundguy's Avatar
    Join Date
    Mar 2002
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    Central florida
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    ym1700, NH7610S, Ford 8N, 2N, NAA, 660, 850 x2, 541, 950, 941D, 951, 2000, 3000, 4000, 4600, 5000, 740, IH 'C' 'H', CUB, John Deere 'B', allis 'G', case VAC

    Default Re: Rotary Cutter Shear Pin Set-up?

    I wish my slip clutch to gear box connection was smooth, and that I could use a shear bolt on it.. unfortunately it is splined...

    Oh well.. hope the slip clutch 'does'.

    Soundguy

  9. #9
    Platinum Member
    Join Date
    Jun 2004
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    546
    Location
    Western PA
    Tractor
    Mahindra 3510

    Default Re: Rotary Cutter Shear Pin Set-up?

    Britt,

    As you can see from the responses, the general advice is to do whatever works for your setup and carry extra shear pins [img]/forums/images/graemlins/cool.gif[/img]

    Resist the temptation to put a much stronger bolt in the hole. This only breaks more expensive parts [img]/forums/images/graemlins/frown.gif[/img] Better to shear a $ .10 piece than a $40 universal joint [img]/forums/images/graemlins/grin.gif[/img] (I don't know the actual prices, it's been quite a while since I've bought one- fingers crossed [img]/forums/images/graemlins/tongue.gif[/img] )

    Rcaretti,

    Tight turns force the universal sometimes beyond its limits. If you must turn tightly, slow your RPMs or even disengage PTO first. Better to shear a pin than to break a universal joint. Watch out for flailing PTO shafts and bending shafts, especially on turns if the pins shear.

    -JC

  10. #10

    Join Date
    Jul 2004
    Posts
    9
    Location
    Maryville, TN
    Tractor
    Kubota MX5000

    Default Re: Rotary Cutter Shear Pin Set-up?

    I appreciate the feedback. A problem with the loose shear pin is you cannot remove it after some work. It gets bent due to the random orientation and the periodic shear force, so eventually it stops to slide. I do think that the tensile stress induced during tightening is a factor. I also know that the usage of a stronger bolt would eventually trash my gear box (or PTO). This good advice was given also by my local TSC manager when I purchased the KK rotary cutter. I will continue my loose bolt technique and report any further updates.
    I will put this standard duty KK cutter to the test. My option was to purchase a heavy duty for the price of three standard duty cutters. I'll post some photos of my three clearing job next weekend for those who are interested. We'll see how the KK with the loose shear bolt holds up to clearing the trees.
    Thanks again for all comments and information.
    Britt [img]/forums/images/graemlins/cool.gif[/img]

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