Bush Hog violently shaking

[kneedeep]

Something is causing the mower not to spin smoothly, my guess is a blade is hung up and not freewheeling on the stump jumper.
Worst case could be PTO shaft in a bind and bent or the blade shaft.
What kind of mower is it? HD 10’ with twin pans or a finish mower?
Makes a difference, pictures help.

 

[POGuy]

Did you happen to notice one of the blades folded forward, but not stuck in place like with the sharpened side towards the pan when you checked the blades..?? If you had to pull the blade so it's straight out like it should be, then spun, that could have been the problem, knuckled ahead, and it makes it hard for centrifugal force to straighten out to cut and makes it unbalanced.

This is the answer to your question and problem. By some fortune you have been spared up until now from having the folded blade at start up.

 

[blehmann]

I bought an old brush hog that had a rather excessive vibration. The first problem that I found was that the nut holding the rotating assembly was loose and allowing a wobble. I used an impact wrench to firmly seat the nut, yet the vibration remained. After measuring the length of the cutter blades and weighing them, the only remaining possibility was that the output shaft on the gearbow was bent. I replaced the gearbox and the problem went away.

 

[Rustyiron]

A 10' bh will be dual spindles. What a bunch of guys are suggesting is that the blades, which are usually only about 20-24" long attached to a heavy bar or a round saucer shaped metal thingy and generally called a stump jumper, pan. Where an immovable heavy object is struck by the blade it's allowed to pivot out of the way and absorb some of the impact protecting the rest of the drive train.
All of your blades should be "radiating" outwards for the center of rotation.
Once you make sure of that, check for interference between the 2 disc's that the blades don't come in contact with each other. They generally are timed to be a 1/4 turn off from each other.
Always support the mower with something thicker that you are and when crawling around under it and slowly spinning it by hand.
Tractor key in your pocket.

 

[repete]

I HIGHLY suggest checking the blades on a regular basis. I failed to notice the hole in the blade for the bolt was elongating.
When the blade finally let loose it tore through the side of the deck and shot some distance across the field. When it came apart, it was so out of balance it nearly threw me out of the tractor seat.
I have a VERY HEALTY respect for these now and I never let anyone within 100' or so of the unit. I am glad it went through the side wall of the deck as it likely would have thought nothing of shredding a tire or me....

 

[Ridersupfarm]

I just got the seal on my gearbox replaced & brand new blades. Woods 10’ cutter with 3 point hitch, took it out today and worked so well, until I crossed a ditch that had washed out during the winter and was deeper than I thought. My husband had to get the shovel to dig me out. Them when I turned on the PTO to start cutting that field, the cutter shook VIOLENTLY. What happened and how do we fix it? Please!!!

My 7' Kodiak heavy duty will shake like it's out of balance if I hit something larger than the deck can handle. Nothing is bent or damaged. To remedy I disengage the PTO, wait for the blades to slow down, reengage the PTO, run the revs up a bit and disengage once it begins to shake. Doing this a few times seems to correct the problem. The unit re-balances and works vibration free.

 

[4x4moses]

If you have other PTO-mounted attachments, you may want to see if the wobble/shaking persists. If not, you'll know the problem is in the cutter. If the problem persists with the other implement, you'll know something is off with the tractor.

 

[Torvy]

Not sure what parts you have handy. I'd probably use a different PTO shaft to rule that out completely.

If the mower was raised, it's less likely to problem with blades unless they got stuck together. I think you would have noticed that when you inspected there.

Pictures can help. Someone may notice something y'all missed.

 

[dmheil]

I just got the seal on my gearbox replaced & brand new blades. Woods 10’ cutter with 3 point hitch, took it out today and worked so well, until I crossed a ditch that had washed out during the winter and was deeper than I thought. My husband had to get the shovel to dig me out. Them when I turned on the PTO to start cutting that field, the cutter shook VIOLENTLY. What happened and how do we fix it? Please!!!

 

[dmheil]

My Bush Hog is not as large (BH-16, 6 feet) but I somehow bent the PTO shaft, no idea how, and the vibration from the shaft being bent about shook me out of the cabin. Replacing the PTO shaft solved the issue. Just looking at the shaft you could not tell it was bent, but looking at it when running at speed....it was out of round by about three inches.

 

[Barker806]

I have a Brush bull rated for a 4" tree and snap the blades from time to time. I never seem to be able to find the missing chunks and that is a bit troublesome.
Best of luck !

 

[4x4moses]

Also try disconnecting the PTO shaft from the cutter. Then, using a tape measure, check the distance from the deck to the front of the shaft. Then rotate the cutter's gearbox, and measure again. Do that at least three or four revolutions. If the measurements are all the same, your spline shaft is not bent. While you've got the PTO shaft off, check to see of there to see if there's any in/out movement to the spline/shaft on the gearbox.

 

[ras5309]

Question: how long did you let it run when it began shaking violently? Did you shut the pto off immediately upon feeling the vibration? A rotary cutter (brush/bush hog) has to spin up some rpms to let centrifugal force "throw" the blades out straight, as a few members have said already. If nothing looks bent/stuck/damaged, engage the pto and run up the throttle some (to maybe 1000-1500 rpms & see if it stops vibrating. It should balance out in about 5-10 seconds. What your experiencing may be normal.

 

[SteveD55]

I unhooked the cutter, and pulled the PTO shaft in and out and it slid perfectly. Oh! The stub on the mower,,, that’s hooked to the middle gearbox,,, how can I tell if it’s bent?

Maybe a universal joint on the PTO shaft is damaged?

 

[bgr]

The heavy shaking you describe is typical when one of the blades is not swinging freely around its pivot pin in the on the end of the "T" which comes out of the gearbox. Usually it is not a major problem. Lift the mower up(be sure to put a prop under it) and check each blade to see if it swings freely. If not, usually the problem is a bit of bark or a vine or old fence wire or something else which is causing the blade to bind up. If you find anything wedging the blade get it out. The blade can also swing back around far enough to jam it against the "T" piece. If that is the case tap it loose with a hammer.
Be sure to check the pivot pin to make sure it is OK once the blade is free.
Almost all the time a stuck blade is what is causing that heavy shaking and it is worth checking first, but be sure to look at all of the mower hardware after you fix a stuck blade.
Going backwards down into the ditch probably swung one of the blades back around and jammed it against the "T" bar.

 

[lwarner901]

The plastic shielding may slide easily, but the steel prop shaft inside might be slightly tweaked.

What you might want to inspect is the input shaft to the mower.

Also inspect the prop shaft U-joints. When you put the mower on an extreme angle, it may have tweaked a U-joint.

Start with the simple/easy things like bent blades, or blades that are not swinging properly, then move to prop shaft, u joints, then look at the splined input shaft on the mower

Update, the connection at the gear box and PTO shaft was off, somehow the bolts were loose and the connection was not tight. Tightened

The plastic shielding may slide easily, but the steel prop shaft inside might be slightly tweaked.

What you might want to inspect is the input shaft to the mower.

Also inspect the prop shaft U-joints. When you put the mower on an extreme angle, it may have tweaked a U-joint.

Start with the simple/easy things like bent blades, or blades that are not swinging properly, then move to prop shaft, u joints, then look at the splined input shaft on the mower

problem solved, husband found that the connection from the PTO shaft to the cutter’s gearbox was wobbly, he tightened up some bolts and it’s purring now. Thank ya’ll for your help!!!

 

[scootr]

Good to hear it, thanks for letting us know.

 

[Williy]

It's always GREAT to hear PROBLEM SOLVED!

willy

 

[JLinOK]

I just got the seal on my gearbox replaced & brand new blades. Woods 10’ cutter with 3 point hitch, took it out today and worked so well, until I crossed a ditch that had washed out during the winter and was deeper than I thought. My husband had to get the shovel to dig me out. Them when I turned on the PTO to start cutting that field, the cutter shook VIOLENTLY. What happened and how do we fix it? Please!!!

I thought the first posts about dirt would be right. I had that problem once from a screw that worked itself clear. Also, listen to the gearbox and see if it is some problem in there. You might disconnect the PTO shaft and see if it is coming from the tractor or the implement. Is the PTO shaft bent?

 

[California]

Related - now that all the knowledgeable people are here - how important is it that the blades swing freely?

I have one that blade that takes some force to swing by hand. I've assumed that centrifugal force when running means that this is irrelevant. I occasionally experience out-of-balance vibration upon starting out which I assume is caused by this stiff swivel. Should spraying penetrating oil help? I'm reluctant to disassemble it.

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