Any metalurgists here? Snapped PTO

[Richard]

Rotary mower wasn't rotating.... long story short, found this. Main question is, was this a break waiting to happen, or did I snap it?

Tractor/mower are pretty new to me, only have 2-3 hours on it. Worked fine one day.... next time I went to use it, didn't work and this was there.

It's a hydraulic mower and has a speed increaser mounted to the PTO (metal plate mounts to back of PTO assembly, speed increaser mounts to plate and hydraulic pump mounts to increaser)

The speed increaser multiples the PTO speed by 4X which has me wondering, if I hit a stump (I hit some sapplings that might have been up to 3" diameter??)

Question: When you hit something the shock hits the blades. Does the shock go up the system, get multiplied by the 4x increaser and send 4x the shock to the PTO??

 

[ruffdog]

It does look a bit grainy to me. A quality tool steel that is hardened and drawn back normally has a tighter/finer grain to it. I don't see evidence that it had been cracked for a while though. Like you suggest, a speed increaser works as a torque multiplier in the opposite direction.

 

[jaxs]

Look's like cast to me.

 

[radios1]

Look's like cast to me.

to me too.. maybe it's cast to prevent too much damage, sort of like using shear bolts.. as to the OP's question, a 4X multiplier will give you only 1/4 the torque meaning the blades can be stopped much easier, and whatever is driving it(the PTO) will see 4 X the torque when the blades stall over a 1:1 setup..

 

[MHarryE]

Single impact ductile fracture, overload.

 

[dragoneggs]

Very grainy and those cracks are suspicious. I don't know what are the design requirements so really hard to comment.

 

[Darvish]

Brinell test? Put a small 1/16 bearing on the side and squeeze the **** out of it with a good vice. See if it dents.
More fun to do it with a hammer, but must wear safety glasses.

That does not look like a shear pin or similar. Almost looks like cast iron that's been sitting in acid and then sandblasted.

==============

Oh, I took another look. So that's the actual splined shaft connector..
My mower is the "flail" type, and if it encounters a "log" or similar, the drive belts squeal, and the shock is dissipated. I would think there would be some clutch, or coupling, that would protect your drive...??

 

[oosik]

Looks like a bad shaft. A break just waiting to happen.

 

[Richard]

I've got a new one in my hands now, along with a rebuild kit for the entire PTO (may as well update everything once I get in there...to get to this means essentially taking it all apart)


What I find interesting is "while I was using it", it worked 100% fine. It wasn't until the next time I went to use it that it didn't work..... almost like it snapped overnight while everyone was sleeping, except the mice.

As I reflect on it....and look at the shiny strip in there.... I'm presuming it must have been cracked while using it....BUT, though it was working, it was damaged and still holding.

The next time I went to use it, the start up torque was probably what finished it off (the shiny part). In other words, if I did it, then it was hanging by a thread... and snapped the next time I stressed it. (which means it would have finished the break had I gotten into some rough stuff)

Tis my logic...Personally, I have no clue and am tossing some spaghetti on the wall trying to piece the puzzle together.

 

[radios1]

you never did mention what brand the mower is, do you know what brand it is?. it's possible this is a common failure for that brand..