Mowing Help with rotary mower vibration?

[CraigM]

My rotary mower has developed a vibration. If I am mowing along and hit something substantial, the mower will vibrate badly. It shakes the whole tractor. If I look back, I can see the back edge of the mower oscillating in the roll axis. When this first happened, I climbed off and checked everything I could think of. The blades are free to pivot, there was nothing caught in it, the driveline is intact and turns freely with no telltale noises or bumps. Trial and error has taught me that when it starts to shake, I can stop it by stopping the mower, and raising and lowering it through its full travel several times. The mower blade has to be completely stopped, or this procedure won’t work. This will correct the vibration about 90% of the time, but the next time I whack a chunk of heavy branch or other object hidden in the grass, it starts to shake again.

I have looked at it several times and can find no hint of where the problem is. Given the symptoms, does anyone have any good ideas where to start looking for the problem?

For what it’s worth, I’ve found that you can use the hydro as a brake by stepping on the clutch and leaving your foot on the hydro pedal. The mower backdrives the system and the tractor and mower come to a stop in about 10 seconds as the mower runs out of momentum.

 

[KiotiJohn]

Some of the things I've notice cause this are:

Sway chains too loose, should be fairly tight with a rotary mower. (tighten both sides making sure mower is evenly set between the tractor wheels)

Blades have something wrapped up in them.

Mower is not correctly level vertically and horizontally (adjust the right lift arm up or down till deck is level, adjust the sway chains until mower is evenly set between the tractor tires)

The obvious ones you've already checked for.

I think it's most likely the sway chains being too loose. Once you hit something, loose sway chains allow the momentum of the blades to get the vibration going, but if chains are good and tight, this can't happen. With the unlevel situation, I've found that it throws off the balance at the PTO shaft and allows vibration at times.

I hope some of this helps. John

 

[RoyJackson]

Check the gear box mounting bolts. Use a wrench to ensure they are tight.

Give the PTO shaft a good pull (tractor off, of course) to make sure there's minimal lateral movement and it is fully seated on the gear box end. May as well do the same to the tractor end of the shaft. While you're doing that, check the "U" joints of the shaft to ensure imperceptable play in them.

Check the blade spindles both above and below the deck, as well as any attachment bolts. In fact, while you're there, check all the bolts used to assemble the mower.

Make sure you've no more the 3/8th's to 1/2" movement when pushing on the belt at it's longest free running length (or, where your manual says to check tightness).

Run your tractor at low RPM's (say between 1000 and 1500) and see if there is any wobbling motion.


Let us know what the problem is when you figure it out...

 

[Rowski]

I think what maybe happening is the blades are getting caught up with each other. When you hit something solid of the blades are supposed to break away. When they break away they may contact each other and stay stuck untill the mower stops (centrifigual force keeps blades stuck together).

It would be wise and safe to check the basics like the others have mentioned. But by the sounds you have a major balance problem more so than loose bolt or u-joint.

 

[RoyJackson]

"When you hit something solid of the blades are supposed to break away."

Maybe I'm confused...are we discussing a finishing mower or a rotary cutter?

 

[Rowski]

</font><font color="blue" class="small">( My rotary mower has developed.... )</font>

</font><font color="blue" class="small">( The blades are free to pivot... )</font>

Duno...Sounds like maybe a hybrid mower /forums/images/graemlins/wink.gif

 

[RedDog]

How big is this mower?

On my 12 footer when I had a gear box going out, (there are 3 gearboxes on it) it started shaking.

I think you may have something going out in the gearbox, or possibly the gearbox is a little loose, as somebody else mentioned.

Second possible thing would be a bent blade causing this.

Good luck, and let us know what the problem was,,,,, RedDog

 

[normde2001]

"The mower backdrives the system and the tractor and mower come to a stop in about 10 seconds as the mower runs out of momentum."

I can't troubleshoot your vibration problem from here, but you definitely need to start by buying an "overrunning" clutch. That should not happen. Do you have any wobble/play in the "U-joint" driveshaft connections?

 

[CraigM]

John: I’ll admit to being lazy and leaving the sway chains loose after hooking up. Had to swap the rotary for the belly mower, but in a week or so I should be bush-hoggin again. I’ll tighten up the chains and see if it helps.

Roy, RedDog: Before I put the rotary on again, I’ll go over the whole thing again for loose bolts. Did look it over already, but this time I’ll do it carefully in the shed, not quickly in the field.


Rowski, Roy, it’s a conventional bush hog type 4’ rotary mower. I’m sure that the blades are not folding back and getting stuck. I’ve looked under several times when it started shaking. I let it coast down before I got under it and it shook the whole time as it spun down. The blades were unbent, extended and swinging freely. I get the feeling that there is a hint of the problem in the procedure that I have found fixes it. Why in the world would simply raising and lowering the thing make it stop? Why would this only work with the mower not turning? I’m hoping that someone smarter than I will catch the hint and solve the puzzle. If not, then I guess I’ll go over the whole thing this winter.

Norm: The tractor is hydrostatic. The hydro makes the backdriving issue irrelevant in normal operation. What I am doing is an abnormal operation that bleeds off the momentum in the spinning mower quickly by deliberately using the backdriving. I thought it was a cool twist on an otherwise bad thing.

Thanks all, for the thoughts.

 

[slowrev]

Blade carrier loose or wobbling on shaft slightly ? Just a thought.