Repairs Kubota middle blade breaks

GentlemanFahmah

GentlemanFahmah

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OK this topic was very interesting to me, and I took the opportunity to send it to a friend who is a mechanical and metallurgical genius and here's his reply which is VERY interesting:

1)
From: Rob
Sent: Thursday, September 01, 2011 6:21 AM
To: Ken
Subject: Ok, this needs Ken's diagnostic brain

http://www.lawnmowerforum.com/general-mower-discussion/5418-kubota-middle-blade-breaks.html

This is very intriguing to me and I'm at a loss except to think that the blades originally on the mower were inferior and had a design flaw. If anyone can pinpoint a theoretical cause, I think it's you.

So, what do you think? I think it has to be a blade problem but you see stuff like this frequently and might better understand the physics of what would do this.

This is pretty interesting.

Rob
2)
From: Ken
Sent: Thursday, September 01, 2011 11:06 AM
To: Rob
Subject: RE: Ok, this needs Ken's diagnostic brain

The only way it could break in that short of time is from fatigue due to harmonics. His gearbox bearings / gears will be next in line for catastrophic failure.

I am not a member so I can't look at the photos of the breaks.

There was a good suggestion for measuring the blade at 180 degree rotations to see if the quill was bent.

Also, I know this is counter intuitive, but if he ran with a lighter blade this would push the natural frequency up and likely out of the range of the rotational frequency of the mower.

At present, he either needs to slow the motor speed down or speed it up to avoid the critical frequency of the blade.

Ken

-------I copied and emailed him the pics you posted and here's his final reply-------------

3)
From: Rob
Sent: Thursday, September 01, 2011 8:54 AM
To: Ken
Subject: RE: Ok, this needs Ken's diagnostic brain

[photo of mandril and remaining blade center with bolt looking up]
This tells the story (the guy takes pictures of all the wrong angles!!)

This is a classic fatigue failure from bending UP and DOWN. The original crack started in the dark colored areas and the final failure was a complete fracture (light colored areas).

The original crack had time enough to rust and looks like the crack started on the top surface and propagated downward with would mean the bending was oriented downward at the tip of the blade.

[photo of center of blade and bolt looking down]
This photo shows fretting corrosion.

Not enough clamping force to deal with the vibration of the blade doing the harmonic dance . . .

Also the bolt hole is huge compared to the bolt. Presumably there is a bushing that centers the blade on the shaft / quill . . . if not, that would explain the problem right there.

Ken
Click to expand...

I know that my Land Pride mower uses bushings around the bolt to nest the blade perfectly centered so that its balanced. I assume that this one does too but maybe not and perhaps that IS the problem right there.

I'd get a bushing for this as per original spec, and I'd make darn sure you have that blade properly centered and tight in this nest.

My mower blades are all hardened steel and if these are "el cheapo" aftermarket blades, then they might be prone to flex that could contribute too.

So #1, do you have that bushing spacer that goes around the shaft of the bolt to take up the space in the hole of the blade?
#2 - use original OEM Kubota blades, or a KNOWN safe equal. You hurl one of these babies off the mower and out the side chute when a car is driving by or a kid walking by and it's going to be a sad day.
 
JDgreen

JDgreen

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This thread has been one of the most interesting I have ever followed during my membership with LMF. Been thinking, however, since it is the middle blade that breaks, odds are any broken sections will be directed down, and the broken section will be heavy enough so that it is unlikely it will be thrown out of the discharge chute with enough force to become airborne...this assumes the discharge chute is NOT propped up or missing. OP never told us what resulted when a blade broke, ie, if anything was thrown out.
 
B

benski

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I await with bated breath what gets discovered.:smile: A dial indicator on a magnetic base might help. Crumb, this is scary.
 
GentlemanFahmah

GentlemanFahmah

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Here's what the washer and bushing combo looks like. Is this what you have, or do you have any bushing in there to center the bolt in the blade hole?
 

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BKBrown

BKBrown

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Interesting .... I should have noticed the blade hole being too big for the bolt.

I don't remember seeing an answer from the OP about him using a bushing or not.

If the blade was not centered and not mounted tight enough I could see that it would flex, but still believe the blades were not tempered correctly.

I guess (when you think about it) at speed, they would be flexing at quite a high frequency and would set up a harmonic that would eventually break the blade.
 
GentlemanFahmah

GentlemanFahmah

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Hey B7200 - would you please let us know what's happening here?
 
173abn

173abn

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I would also wonder if he's using fricton washers...russ
 
B7200HST

B7200HST

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Thanks for the interest in my mower problem. I got sidetracked by other projects, seems like everything around here's breaking at once. I hadn't imagined I'd missed so many posts here. Sorry to keep you hanging.

Attached are items I scanned from the manual.

ABOUT BLADE SPEED
My mower has two mid-PTO speeds indicated on a decal, 1546 and 2456 RPM. I run it at 2456 RPM. I run the motor at full throttle, somewhere around 2500 RPM, I think.

My deck manual says "Place the PTO lever in the HIGH (2nd speed) position.". I push the PTO lever down toward the 2456 indication. I hope that's what they mean.

The middle blade pulley appears to be about the same size as the outer pulleys. I won't be able to measure it until I pull the deck from the tractor.

Roger
 

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B7200HST

B7200HST

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ABOUT BLADE QUALITY
I think of three sets of mower blades: the "old set" of blades (that were on the mower when I started using it), the "black set" (they were painted black when I bought them in July 2010), and the "pink set" (I painted them pink when I bought them in July 2011, to hopefully keep track of this mess).

When I paid $54 for the "pink set" in September 2011, after paying $31 for the "black set" in July 2010, and the retailer saying the part number had changed, it crossed my mind that there was a manufacturing defect in the "old set" and the "black set" that was being corrected with the "pink set".

Although there could be some truth to that, I have disregarded it because the mower has broken all three of the "old set" (all of which lasted more than 89 hours after I started using the mower, and one of the "black set" (which lasted 21 hours). And it was always the middle blade that broke. I have not yet installed any of the "pink set".

BTW, due to the hot dry spell here in Missouri, there may not be five hours of mowing left this season.
 
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