Excessive vibration/shudder with flail mower

[super55]

I have an old Mott 60 flail mower that I had picked up off craigslist that shudders and has excessive vibration whenever operating at PTO RPMS on the tractor.

It originally had knives that were too large for it and were slapping against the housing but I have since replaced all of them with new correct size finish blades. Lifted it up on 3 pt with a couple of jackstands and crawled underneath to inspect. Not slapping the housing anymore, the bearings on the shaft seem tight and as far as I can tell the main shaft does not appear to be bent.

It doesn't do it all the time but seems to occur more often in heavy grass. I took the safety cover off where the drive belt is and when it really starts to shudder the spring loaded tension pulley is really bouncing back and forth. Is it possible that the lack of pressure on the tension pulley is causing the flail to slip and grab at a rapid pace causing the shudder? Its the only thing immediately that I can think of besides grass being wrapped around the flail shaft is putting it out of balance.

Anybody else have any similar experiences and/or remedies? I'm about to concede defeat and swallow my pride and admit that I may have gotten hosed on this purchase.

 

[greg_g]

Mowing efficiency is directly proportional to blade tip speed. I suspect your PTO revs are dropping in the thick/wet stuff. The likely solution is either slow down or throttle up.

//greg//

 

[leonz]

I have an old Mott 60 flail mower that I had picked up off craigslist that shudders and has excessive vibration whenever operating at PTO RPMS on the tractor.

It originally had knives that were too large for it and were slapping against the housing but I have since replaced all of them with new correct size finish blades. Lifted it up on 3 pt with a couple of jackstands and crawled underneath to inspect. Not slapping the housing anymore, the bearings on the shaft seem tight and as far as I can tell the main shaft does not appear to be bent.

It doesn't do it all the time but seems to occur more often in heavy grass. I took the safety cover off where the drive belt is and when it really starts to shudder the spring loaded tension pulley is really bouncing back and forth. Is it possible that the lack of pressure on the tension pulley is causing the flail to slip and grab at a rapid pace causing the shudder? Its the only thing immediately that I can think of besides grass being wrapped around the flail shaft is putting it out of balance.

Anybody else have any similar experiences and/or remedies? I'm about to concede defeat and swallow my pride and admit that I may have gotten hosed on this purchase.

Mowing efficiency is direly proportional to blade tip speed. I suspect your PTO revs are dropping in the thick/wet stuff. The likely solution is either slow down or throttle up.

-//greg//

AH!!! so you finally got to mowing the brush eh??

I hope you are in creeper gear, I know its boring but that is what fender radios are for.


Adding to what Greg has already told you, If this belt came with the mower,
change it out with a new Kevlar belt from NAPA as a good belt should not do that
period and I doubt your tension spring is shot.

I would also raise the cutting height of the mower by lowing the rear roller
position if you have not done that already for the first few mowing sessions
and be sure to keep the mule in creeper gear till its all knocked down and
you can mow over it again the same day to shred it some more.

Happy Mowing

 

[Arc weld]

It is important that the PTO does not operate below 540 RPM. OEM belts are the best. They're pricey but specially made for Mott/Alamo flails.

 

[super55]

That seems to be the issue for me. My mower seems to like to the shake, rattle and roll the more I have it at 540 PTO rpms with the tractor. Actually I would estimate that it seems to run best at around 400 rpms. It almost feels like I have something out of balance below. I almost feel like I need to put a straight edge on the shaft and see if it isn't bent. Also are these shafts by chance factory balanced? Is there by chance a possibility that there could have been balance weights on these that I could have lost or thrown off?

 

[redman135]

The shudder when operating the mower will be due to something out of balance.
1)check you have all flails on the shaft,ie each flail has a counterpart on the opposite side. On another brand, I had a bad vibration like you describe . This was tracked to a missing flail. When the owner was told they said "yeah, been like it for a while" and were not worried.
2) If you can swap the pto shaft. This will tell you if the vibration is in the pto or flail shaft
3)Check the flail shaft for true.

 

[leonz]

That seems to be the issue for me. My mower seems to like to the shake, rattle and roll the more I have it at 540 PTO rpms with the tractor. Actually I would estimate that it seems to run best at around 400 rpms. It almost feels like I have something out of balance below. I almost feel like I need to put a straight edge on the shaft and see if it isn't bent. Also are these shafts by chance factory balanced? Is there by chance a possibility that there could have been balance weights on these that I could have lost or thrown off?

Yes the flail mower rotor shafts are factory balanced. there may be a missing weight that has become detached from the flail mower rotor . If you do not have any missing mounting stations you are either missing a weight or the flail mower rotor shaft is bent the bearings are bad which is a good possibility as well because of the previous owners installing the larger heavier knives .

You will have to roll the mower on its back to lay a 4 foot straigt edge on the rotor shaft to check it for straightness-be doubly sure that the area is clean-use hot water and a wire brush to clean the 4 rotor sections to remove any gras dust before you place the level on the rotor to check it for straightness.

If the shaft is straight at all four 90 degree positions and there appear to be no missing weights its the bearings.

 

[greg_g]

The rotor on mine employs balance weights similar to those used on automotive mag and alloy rims. They're flat, slightly bigger than a postage stamp, and self-adhesive on one side. Application is trial and error, but the weights are stackable. Just keep adding weight till the vibration stops. In your case, you don't know how many weights it started with. If you're sure the bearings are good, might be a good idea to remove what's (still) on there - and try balancing from scratch.

Because operating under load at 400 PTO revs is not good for either the mower or the tractor. Produces inconsistent mowing results as well. Both machines are designed to mow most efficiently at 540 rpm. It's in the best interest of both, to apply whatever fix is necessary to recover your 540 rpm operating speed.

//greg//

 

[leonz]

Adding a bit more to what Greg and I have mentioned.


It might be time to remove the rotor

take it the rotor minus the knives and pins to a machine shop and have them run it on a
lathe with a dial indicator to see how true it is as this will be a big help to you.

The low speed shaft balancers are really expensive in the $30,000+ dollar range these days.I would be doing flail mower and snow blower repairs if I had the means to do so but I would need a complete machine shop

I will suspect the bearings are the issue by all that you have mentioned in your posts as
these bearings will work at low rpms until they sieze.

 

[jimmysisson]

My Mott SHD88 flail shudders only when it's bogging under load. I slip the shuttle into neutral and let the mower digest the clippings that are balling up under it. This only happens first mowing on a brushy field. Second mowing of the season never.
#2 on slow speed is critical in thick growth. We have a grass here in Mass called June Grass, thick and fine and hard to cut with ANY implement. Sickle, rotary, scythe, clippers.
Does your Mott have the 4-5" diameter drum the knives mount to? The 88 does and it looks like a beast to handle out if the machine.
Good luck and let us know how you make out.
Jim