Rotary Cutter Rotary Cutter Shear Pin Set-up?

[Britt]

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]

[Gary_in_Indiana]

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.

[Henro]

<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]

[Gary_in_Indiana]

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]

[BB_TX]

<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.

[Villengineer]

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.

[rcaretti]

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.

[Soundguy]

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

[DiskDoctr]

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

[Britt]

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]