Flail mower vibration

[coastlogger]

Have a Ford 918h 92 inch flail mower that I have adapted to an excavator. Use it about 50 hours a year for brush cutting. Have had no issues in 5 years other than changing cutters,about 1 or 2 cutters per hour of cutting which is ok.This year everything has gone to hell. It vibrates,in truth I can't notice it's much worse from sitting in cab,but there's a definite problem.bolts coming loose,welds cracking,cracks in body. Basically if I use it for a couple of hours I have a few hours welding to do. If I put hand on it while it's spinning there's a vibration there.
I figured out how to check the drum for true and it's out 100 thou ie seems to be bent by 50 thou so one side is 100 higher than opposite side .Hump seems to be midpoint on the 92 inch drum.
I wonder if that's enough to set up a destructive vibration like I have.i think I could straighten it in place with a jack and some heat but maybe its within spec .Other possibility I guess is that the cutters are seriously out of balance which I doubt.i religiously change them whenever I see one or two broken.So there's a mix of old worn ones and new ones but I don't think they're biased to one side or anything . Guess I could remove all cutters and see if I still have a vibration.

 

[John C.]

Does the vibration come and go with changing the speed of the engine?

 

[Tones]

Remove the belts and blades and spin it by hand, mark where it stops. That's where the bend is. Now depending on the way it's manufactured if it has the shafts bolted to the rotor then it can be corrected in a lathe or milling each shaft and both ends of the rotor to zero runout.

 

[coastlogger]

The vibration is the same at all speeds.
I think I know where the bend is. Right in middle. Found this by using a rod and feeler gauge from drum to housing. Confirmed with my longest straightedge which is 48 inches.
There are balance weights on the drum,near the ends.
Do we think the 100 thou difference at middle needs correcting or might adding balance weights do the same thing?
I'm thinking I should be able to correct the bend using a jack and a strongback,maybe some heat.
This is a low budget project so hoping by a bit of trial and error I can bring it back into tune.

 

[Tones]

If it has a one peace shaft heating may be a problem because you can only get heat on the rotor. Balancing maybe good but check the run-out at the bearings. A bent shaft even balanced will shorten bearing life.
A forestry mulcher I had bent the shaft, 12 thou one end and 14 thou the other, destroyed the bearings and a bearing housing. Was not amused.

 

[coastlogger]

There's minimal or no runout at ends of drum so I figure the bends in the middle.will check runout on pulley see what that is.. I'm guessing a shaft runs thru but maybe it has no support except at ends so it might not have bent?

 

[Bumpsteer]

Drum is bent. Remove it, strip it bare and take it to a really, really, really good drivshaft shop for straightening.
Otherwise, scrap it.

Yes, this is the voice of experience......I got lucky on my 7' model 907. Took in, kid looked over...yep, I have to call dad in out of retirement, he'll fix ir. The 'ol boy got it back within .010.
Still use it.

Ed

 

[Bumpsteer]

Take to your best driveshaft shop, if they can't straighten it, nobody can.

Yes, this is the voice of experience....

Ed

 

[Welder Dave]

It has knives copied from the original Mott/Alamo design. Interesting is if you don't have them at a high enough speed the bend in the knives can actually stretch and move. It's more prominent on the lighter duty units with narrower knives. 100 thou out is a lot. You need to take it apart and take the cutter shaft to a driveline shop and not only have it straightened but get it dynamically balanced as well. This is where they might have to weld weights on to balance it. You'll have to take all the knives off. Missing a few knives can also throw it out of balance. Probably want to install new bearings as well. My dad set sales records for Mott Interstaters. Kept a spare cutter shaft in stock after some bozo hooked a tow strap to the 4 1/2" cutter shaft on an 88" rear unit with 74" right and left side hydraulic cutting units. It's a pretty good pull when an MF275 tractor with duals on the back and 3 flail mowers gets stuck in the mud. Definitely in the running for one of the best Darwin awards. Spent a few weekends as a kid putting knives on Mott flails. Used a greens mower tire as a stool to sit on.