Repairs Kubota middle blade breaks

B7200HST

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I have a circa 1989 Kubota B7200 HST with an "RC60-72H" 60-inch belly deck. I started using it April 2008 when it had 846 hours. In Sept. 2010, at 935 hours the middle blade broke. Then again at 956, 960, and 965 hours. So the middle blade has snapped four times - 3 x with old blades, once with a new blade.

The breaks always occur about two inches from the center mounting hole. They are clean breaks, and almost look at though they had been sawed off. Sometimes the blade breaks in one place, sometimes in two places (on either side of the center hole).

I see nothing that the blades could be hitting. The blades are clearing each other, unless a blade shaft is distorting a lot, and I detect no wobble in the shafts.
The middle blade is driven directly by a gearbox, while the outer blades are belt-driven. I wonder if vibration is fatiguing the metal blade. (The outer blades would be cushioned by the belt.) The U-joints from the PTO to the gearbox seem OK, but the U-joints aren't a really tight fit on the splined PTO shaft. I can see the PTO shaft wobbling, but perhaps that's normal due to the action of the U-joints turning. It's not a very long shaft.

Any ideas what's wrong? Anyone ever heard of this?
 

SteveG

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If you see the PTO shaft wobbling, then I'd argue (without even knowing the specific mechanicals on this) that you've got a basic problem. There's no way that's part of the design regardless of the u-joints or shaft lengths.

I think you're probably right about the blades wobbling and getting metal fatigue, and that the outer blades are cushioned by the belts.

I also wonder if the U-joints or PTO shaft wobble is triggering huge forces when the blades are at speed, which are hard to see when you move them by hand. A U joint that's gone bad (in cars at least but just in general I think) can cause enormous forces when a torque load is applied I believe.

It all makes sense to me.

I'm sure some others with more familiarity with this particular deck and drive will comment of course....

Steve
 

benski

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With the blade breaking about every 5 hrs., it's obvious you've got an issue here.:wink: Here's my idea; Buy yet another blade, install it, and with everything shut off (obviously!) measure the distance to the ground from the tip of one blade. Then rotate the blade 180* and take another measurement. If those two numbers don't match, you've probably got a bent quill. As that blade travels in an elliptical path, it is probably stressing the blade right at the failure point you've been seeing.:eek: Also obviously check for broken/missing bolts that hold all that jazz to the deck, and any cracks or damage in the mounting area on the deck.
 

B7200HST

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Thanks for the Replys. I took a movie of the PTO shaft while rotating at idle, & posted it at Kubota pictures by Roger_deMO - Photobucket

The wobble I see doesn't seem any worse that I've seen on some tractors pulling implements, but maybe it is the culprit.

I guess I'll remove the deck this week & take another look at the situation.
 

BKBrown

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Hard for me to understand how any blade would just break like you describe (I believe you, just don't understand it) . Those blades should be heat treated (tempered) to withstand quite a jolt and maybe bend, but not break.
Where do the pieces that break off go ? :eek: They could kill someone ! :eek:

What blades are you using ? I have bent a couple Oregon Blades and they are like spring steel - they are nearly impossible to straighten because they spring back when you bend them.
 

B7200HST

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Three of the broken blades were on the mower when I started using it.

Purchased new blades from a Case/IHC implement dealer that sells Kubotas.

9/24/2010 $31.30 for three mower blades. I asked for Kubota P/N 70722-3433-3. I've broken one of these; it lasted 21 hours.

7/29/2011 $54.27 for three blades - P/N 70000-25006 I haven't tried these blades yet.

Let's see if I can post some photos of the broken blades...
 

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BKBrown

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I don't see any big dents or chips in the blades and it doesn't show that the spindle bolts are moving (might not see that under the grass anyway). You have a mystery here, but breaking in the same place suggests a defect in the blade manufacture.

I'd probably take more good pictures and send the pictures to the blade manufacturer and Kubota. Keep the actual blades for evidence.
This is a dangerous situation and I would think that at least Kubota would want to help you solve the problem.
 

JDgreen

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I have a HD 60-inch 7-Iron MMM on my JD and have been trying to think of a cause of your blades breaking since you began this thread....appears most others have ideas while I don't, but just curious...what rpm are you running the deck at? PTO speed, or higher, or lower? Might be a stupid question but I am wondering if you are running the mower spindles at a speed greater than the blades are rated for. My understanding is that walk behind push mowers have their engines limited to a certain rpm to the blades so not exceed a certain FPM speed.
 

BKBrown

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Is there any reason to believe that the center spindle is rotating faster than the two outer spindles ? Are the 3 pulleys on the spindles the same size ? :confused3:
 

SteveG

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I don't see any big dents or chips in the blades and it doesn't show that the spindle bolts are moving (might not see that under the grass anyway). You have a mystery here, but breaking in the same place suggests a defect in the blade manufacture.

I'd probably take more good pictures and send the pictures to the blade manufacturer and Kubota. Keep the actual blades for evidence.
This is a dangerous situation and I would think that at least Kubota would want to help you solve the problem.

I agree it's a dangerous situation (VERY dangerous), but I don't see how blade defects alone would cause this type of failure unless he hit something vertically each time the blade breaks.

I'm much more suspicious of metal fatigue due to a vertical vibration at speed under load. It could be a mfr defect that's exacerbated by a vibration, but the loads of a spinning blade that's balanced are not going to break the way these are unless they're so freakin' weak or cracked to begin with! Without a vibration, the stresses are less vertical and more horizontal I believe.

I can't wait to hear what the real cause is.

One thing I was wondering: The bolt has a gap to the blades, and there's usually a "washer" that usually locks the blade in sealing the gap. Is that washer's "lock" feature possibly worn out allowing the blade to slide from side to side or front to back? That would cause enormous stresses I'd think.

In other words, is the lock mechanism that holds the blade in place (not just the bolt) worn out?

Steve
 
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