Central blade breaking - Kubota B7100HST-D 4WD mid-deck rotary mower RC70-71H

[tatejones]

Hi all,

I have a Kubota B7100HST-D 4WD with a mid-deck rotary mower RC70-71H. Three blades with the two-outer driven by a single belt and the central one off the gear box.

For the past 10 years it has operated without braking any blades. Recently the central blade is breaking regularly (every time is it used now) next to the cup washers. There is no visible damage to the broken blade.

What I have done to date is:

1. Replaced the seals, bearings and spindle holder.
2. Different brand of blades including a blade holder with standard mower blades. This would indicate that the blade is not hitting anything as a blade holder is half the size of a normal blade.
3. Visual inspection of deck shows no signs of damage
4. Raised the desk to reduce the chance of the blade hitting the ground.

I have stopped the tractor, checked and compared the temperature of the blade bolts. They are warm, but certainly not super-hot. Moments before the blade breaks there is a loud clanging sound and vibrations. Inspecting the half still remaining on the spindle, the bolt is tight and secure.

I am now out of ideas why this is happening and was wondering if anyone has come across the same problem or has any advice for troubleshooting this issue?

Is it possible I am tightening the bolt too much or the cup washers need replacing?
What purpose is the friction washer for?



Thanks in advance
Tate

 

[Peter 315]

Could the spindle be bent and the blade is wobbling and vibration is cracking it.......:confused3:

 

[tatejones]

Thanks Peter,

Would I lift the gear box out with the spindle attached and do a visual inspection?

 

[Peter 315]

If you take the blades off and use a Dial indicator to see how far out of round the shaft is......https://www.amazon.com/Magnetic-Base-Adjustment-Indicator-0-0005/dp/B01JU18A0Q?ref_=fsclp_pl_dp_2

 

[Peter 315]

Here you go read this ...........https://www.lawnmowerforum.com/threads/kubota-middle-blade-breaks.5418/

 

[tatejones]

Here you go read this ...........https://www.lawnmowerforum.com/threads/kubota-middle-blade-breaks.5418/

Some very helpful comments.. This is exactly what is happening to my central blade. I shall study the link tonight to come up with a plan.

 

[Dave M7040]

I can only find two mid mowers for a B7100 and neither match the RC70-71.

The two are RC60-71 & RC54-71.

In any event I would make certain your assembly is using the special shoulder washer Kubota calls a JOINT. See attached.

This JOINT will center the blade on the spindle. If it is not present, the blade hole is too large and will not be centered on the spindle leading to lots of vibration and failure.

Dave M7040

 

[tatejones]

Correction. Typo RC60-71H

 

[tatejones]

Thanks Dave for your suggestion,

Rc60-71H doesn’t have this part as the bolt is performs this action. See original diagram attachment

My plan is.
1. Get new cup washers as I believe mine are “squashed”
2. New fiction washers.
3. Make sure the bolt is tighten to the correct specification
4. Inspect for vibrations due balance and alignment issues.

I shall get back with updates. Hopefully this weekend as the grass is overtaking us

 

[Peter 315]

Thanks Dave for your suggestion,

Rc60-71H doesn稚 have this part as the bolt is performs this action. See original diagram attachment

My plan is.
1. Get new cup washers as I believe mine are 都quashed
2. New fiction washers.
3. Make sure the bolt is tighten to the correct specification
4. Inspect for vibrations due balance and alignment issues.

I shall get back with updates. Hopefully this weekend as the grass is overtaking us

Sounds like that will work, cheaper then buying blades and the ER trip......