Mowing Breaking BX Blade Bolts

[logdog]

I have a BX1860 with a 48 inch deck. It has been flawless the two previous mowing seasons but it is giving me fits this year. I have had four mower blade bolts shear off.

In the spring my wife took the deck in to the dealer to have the blades sharpened because I was out of town with work for awhile and couldnt get to it. The first bolt broke the first mowing after it came back. She was using it and said she didnt think she hit anything. I figured they didnt torque them right. They gave me a new bolt and washer. I replaced them and torqued all the blades to the manual spec.

A few weeks later another one (different blade) broke on me when I hit an ant hill. Replaced that and have broke two more under seemingly normal use. The heads just shear off the bolts!

I have looked and looked at the deck and dont see anything out of the ordinary. Nothing seems bent and the blades seem to spin as usual.

Has anybody else had any issues like this? The dealer hasnt heard of anything like this.

 

[Tollster]

Sounds strange, I ground mine out on a stump and stalled the tractor and never snapped a bolt. Are all of them from the same place? I would suspect cheap imported bolts. Check the old bolts over visually and see if you notice any metallurgical issues or improper machining that would create a "Stress riser". Typically this area is the first area to fail when over torqued if there is a manufacturing flaw in this transition area, If no flaw is present, I have seen them "neck down" on the shank.
(I saw what 45,000 LBS on a hydrotorque does to a 2.5" nuclear grade bolt, necked it down to a 2.0" bolt outside the thread engagement area)
If its a cheap import, then I would say the grade of material used is not the one specified (Falsified grade stamping).
I would also check the one that has not broken if it came from the same store increasing the likelihood they are from the same batch number.

 

[SPYDERLK]

Are they the 12mm bolts with ~ 30mm heads? I have a BX1500 with 54" mower and havent had a problem in 2000 hrs - some of it pretty ruf. I broke one myself tho when I tightened it too tight with the impact wrench. Its head popped off just like you say. Your problem puzzles me. Its almost like the bolts are spinning tighter if the blade slips. But there should be a washer with internal hex that keys to the spindle to prevent that.
larry

 

[logdog]

All the replacement bolts have been OEM Kubota ones.

 

[logdog]

Are they the 12mm bolts with ~ 30mm heads?

Yep. Ive hit stuff the previous years and never had an issue

 

[dex3361]

Have you replaced the washers on the deck? I think the set up of the washers is supposed to allow the blades to slip on the spindle if you hit something. The washers are cupped and I think they are called a bellville spring but I could be wrong. Here are the PDFs showing the spring plate the goes between the blade bolt and the blade.
View attachment A00701.pdfView attachment A01001.pdf

 

[dex3361]

I could see the bolts breaking off if someone replaced the springs with washers. Just a thought.

 

[SPYDERLK]

Yep. Ive hit stuff the previous years and never had an issue

Is the thick washer with the internal hex stripped out? It should not be able to slip on the spindle -- that could wring the bolt tighter.

 

[Grandad4]

Could someone have weakened them with heat when removing them from the spindle?

 

[Scooby074]

Thats weird.

Ive eaten rebar with my MMM and never broke a bolt.

You sure torque is right?

Triple check that the blades arent hitting. Im wondering if there is a particularly rare situation where things line up to create contact.

The bolts arent bottoming out are they?