What caused the bolt to shear off?

Geo-TH,In · May 6, 2016

What sheared off the bolt holding the electric mower clutch off my old JD275 garden tractor? The direction of rotation is ccw, same direction you would turn the bolt to loosen it. The small part of the bolt looks like it has some blue lock tight on it. It backed out with of crank with ease, Thank God. Something caused the bolt to snap, what? I took the dust cover off the bearing, it looks and feels good. There is a pic looking down the clutch. Notice the broken part is level with the bottom of the key, So bolt had to snap before the clutch could fall off. Looks like the OEM bolt. It has JD 109 stamped on the head. Looks like a hardened bolt too. Never seen anything like it have you. Any theories how this happened?

Plan to go to town tomorrow and buy a case hardened bolt. Engine is a Kawasaki. So I'm guessing the bolt is metric.

ericlb · May 6, 2016

thats a high load crack, if its on the mower blade clutch time could do it or hitting something but what else i see is that bearing, i believe that should be a sealed bearing and yours has been very hot and melted the plastic seal out of it, i would sure replace that while you have it out,

Bob Bancroft · May 6, 2016

It's metric alright. 10.9 is the hardness. Roughly equivalent, I think, to a grade 8. some metric bolts are 8.8, which I think are close to grade 5.

GordoSD · May 6, 2016

The bolt is 85% fatigue crack through. That little spike of metal was the last straw on the camels back, Amazing the bolt held with only 15% of it still intact. It failed in torque mode, that is rotational. Impact sudden complete failures always show rough jagged pieces. Fatigues takes thousands of minor flexing all within design limits, and displays smooth edges at failure location.

Geo-TH,In · May 6, 2016

Yes bearing had a dust cover, which I removed to show condition of bearing. Bearing is smooth. No signs of overheating, the plastic dust cover. Plan to put some white lub on it when I put it back together.

Geo-TH,In · May 6, 2016

Thanks for the tech info. Plan to go to NAPA and see what they have.

Geo-TH,In · May 6, 2016

GordoSD,
Tend to agree, fatigue cracked it. Only things that makes sense.
geo

gtractorfan · May 6, 2016

One thing I've noticed about a neighbor, he constantly stopped and started the mower deck (for instance if going back and forth and turning around on the driveway, he'd turn off the deck every time turning on the drive, I keep mine high enough not to worry about hitting anything) My thought it's extra wear and tear on the clutch and belts.

gtractorfan · May 6, 2016

BTW, good luck getting a specialty bolt at NAPA, don't think it's going to happen.

RusselAZ · May 6, 2016

That is a tension break. Pulled apart with the key being the small piece came out easily. Why it broke is because the clutch and the crankshaft don't fit each other. Rust and use has taken their toll. The 10/9 on the bolt head says it is very strong.

Northvale - PA · May 6, 2016

And what scares me is fatigue and airplanes

Mike M · May 6, 2016

I was going to look on the JD site for the bolt specs. but it just crashed ? Got s JD dealer near ? I'd get the bolt they call for and torque it to spec. 50 ft lb on F725 with 20 hp V twin water cooled kaw. The service guys should be able to help with that. IIRC my book says to use never seez on the shaft. Many clutches have an adjustable air gap. If yours does check it. My F725 book calls for .015" to .025" air gap. I'd use a good high temp wheel bearing grease in that bearing. To hold the crank from turning if you can't easily jamb the flywheel with a screw driver remove a spark plug bring piston up to top dead center on compression so valves are closed and then back off some. Then feed some rope into the cylinder and then it should hold it from turning. Tighten clutch bolt. Then back up crank and remove rope.

Mike M · May 6, 2016

JD site back up. Your bolt is the same dia. and strength as mine so the torque value I posted should be it. Your GT275 use bolt M124721 M10 x 1.5 x 90 grade 10.9 if your key is worn you might want a new one ? 1/4" x 1/4" but they don't show a length.

LarrySTN · May 6, 2016

Have a gt275 which has broken 2 bolts already, now it's only a back up mower.

Bob M · May 6, 2016

Backdoor neighbor has a relatively new 18 HP rider from a "big box" store. It's set up so blade automatically stops whenever the shifter is moved to neutral or reverse - a CPSC requirement I presume.

The blade starts and stops 100+ times every time he mows. I wonder how long the clutch and belt is gonna last...

T.R.K. · May 6, 2016

Did you clean the grease off the bearing? I don't see any.

Janicholson · May 6, 2016

That is (opinion) a fatigue failure, not a shear failure. A bolt tightened to 90% of yield strength and subjected to heat cool heat cool (that application) will fail exactly like that. A new titanium alloy bolt, or a grade 8 bolt will work. Case hardened might fracture as did this one. Look up the recommended torque on the bolt size and do not exceed it. good luck. Jim

Brad Gyde · May 6, 2016

(quoted from post at 17:01:17 05/06/16) What sheared off the bolt holding the electric mower clutch off my old JD275 garden tractor? The direction of rotation is ccw, same direction you would turn the bolt to loosen it. The small part of the bolt looks like it has some blue lock tight on it. It backed out with of crank with ease, Thank God. Something caused the bolt to snap, what? I took the dust cover off the bearing, it looks and feels good. There is a pic looking down the clutch. Notice the broken part is level with the bottom of the key, So bolt had to snap before the clutch could fall off. Looks like the OEM bolt. It has JD 109 stamped on the head. Looks like a hardened bolt too. Never seen anything like it have you. Any theories how this happened?

Don't want to say it's common, but it's common. Dad has a LX188 with the kawi and the first bolt lasted many years. The first time it broke was shortly after the engine came apart, a rod bolt had backed out and was ticking (at that time, we estimated close to 2500 hours on the mower) and it seems like we have to replace the bolt about every other year since. Dad gets em from JD and I think the one he got last year was about $5.00 (I picked it up when I got baler parts, they had it in stock at my local dealer). I've finally convinced him to keep an extra and the "washer" that holds the clutch on, because one time we couldn't find the washer, of course he only has the extra cause I bought 2 LOL.

Brad

JD Seller · May 6, 2016

George There was fix as fail warranty issue with the electric clutch bolts on the JD 275, JD 325 and the GT 262 when they were new. The bolts would shear like yours did. Go get the CORRECT bolt from JD. This was an issue until they changed the bolts supplied. Then make yourself a cylinder stop or use rope in the spark plug hole and then torque it to specifications. Many of the failures I have seen on these mowers, now that they are older, is when someone uses an impact wrench to tighten them. Use a Good torque wrench and tighten it to 60 Ft. Lbs. Also the factory bolts come with thread lock on the threads. IF you use an none OEM bolt you the medium strength Loctite on the bolt.

PS if you where able to get the broken piece out with out drilling it then the bolt worked lose and that cause the fatigue crack/failure you had. It was common on these when they where new.

Geo-TH,In · May 7, 2016

JD Seller and others, Thanks for your replies, been helpful.

If this issue is a warranty issue, I plan to order an OEM part from JD. Thanks for the torque tip.

Had the broken part been glued in place with lock tight on all threads, I see no way I could have gotten it out so easily. Secondly, I see no way I can drill a hile in the broken part and use an ez out. I would have been screwed. So I plan to get a new bolt and torque it to 60 instead of using an impact. No lock tight. So what if bolt falls out.

If this problem is so common, I may next time use a softer bolt that will flex instead of crack. I don't understand, I have a 30,20 and a 15 year old B&S mowers with electric clutches, NEVER had a bolt crack. Somethings is wrong. Design issue may be the only answer.

Thanks again to all,
Geo.

Geo-TH,In · May 7, 2016

Geo-TH,In · May 7, 2016

Mike,
Thanks, The key is a part of electric clutch, can't fall out. Doesn't appear to be worn. Nicked a little on the end when it fell out, nothing serious.
geo

Geo-TH,In · May 7, 2016

This unit is about 20 years. Bet it's been turn on and off many times. I usually leave mine on all the time. This unit also has a 30 inch tiller. That could have been rough on the bolt.

Geo-TH,In · May 7, 2016

JD seller recommends I get a bolt from JD.

farmer boy · May 7, 2016

I don't really understand your thinking here, JD seller gives you good advice you'd normally have to go to a dealer for and you ignore him. Do as you like, but if you don't want it causing trouble again, but the JD bolt and move on.

Geo-TH,In · May 7, 2016

I am buying an oem bolt, I will torque it to 60, just not using lock tight. If it's common dfor the bolts to break, then I need to have a way to get the broken part out. There wasn't enough lock tight on the broken bolt I removed to hinder my removing it. So I don't get your point if there is one.

Tell me, how do you drill a hole 109 grade bolt to use an easy out? And how many ez outs can remove a bolt that is lock tighted? A little common sense here. Can't remove the broken bolt, engine is worthless.

Geo-TH,In · May 8, 2016

I only use mine as a tirm mower, 30 minutes each time. Sounds like something is definately wrong with the design. Never broke the first bolt on a B&S, have 3 of them.

What caused the bolt to shear off?

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