Mowing Brush Mower Vibrates

jesseb3@att.net · Aug 2, 2009

This is a question regarding static versus dynamic balance. I'm not a physics expert - hopefully there are some on this forum.

I just bought a 4' brush mower in very nice condition, for only $350. I've run it a few times, and cut a small tract with it. It does do a good job, however, it does tend to wobble.

So, I dropped the stump jumper, and checked the balance. It balances very well on the corner of a piece of angle iron.

The, I checked the shaft... if it's bent, I can't measure the run out with my crude methods - I don't think that's it.

Then, I took the blades off of the pan, and checked it, and it's bent. Laying angle iron on the hub, you can see that the pan is bent about 1/2 up on one side. This isn't the part of the pan made of just sheet steel, rather I'm measuring across the hub to the bolt holes for the blades. Whatever did the damage bent the 1/2 steel that the brace is made of.

So, my question is whether or not that's enough warp to cause the wobble that I'm feeling. It does surprise me that it would - again, it's minor enough that it remains statically balanced, but is it enough to cause dynamic imbalance?

Thanks for any input!
Jesse

whistlepig · Aug 2, 2009

jesseb3@att.net said:
This is a question regarding static versus dynamic balance. I'm not a physics expert - hopefully there are some on this forum.

I just bought a 4' brush mower in very nice condition, for only $350. I've run it a few times, and cut a small tract with it. It does do a good job, however, it does tend to wobble.

So, I dropped the stump jumper, and checked the balance. It balances very well on the corner of a piece of angle iron.

The, I checked the shaft... if it's bent, I can't measure the run out with my crude methods - I don't think that's it.

Then, I took the blades off of the pan, and checked it, and it's bent. Laying angle iron on the hub, you can see that the pan is bent about 1/2 up on one side. This isn't the part of the pan made of just sheet steel, rather I'm measuring across the hub to the bolt holes for the blades. Whatever did the damage bent the 1/2 steel that the brace is made of.

So, my question is whether or not that's enough warp to cause the wobble that I'm feeling. It does surprise me that it would - again, it's minor enough that it remains statically balanced, but is it enough to cause dynamic imbalance?

Thanks for any input!
Jesse

I think the wobble is because of the size of the tractor. The size of the implement so to speak. I ran a 5' brush hog behind a 3,000# MF135 for many years with no vibration at all from the brush hog. I now run a 5' brush hog behind a 1,700# Kubota. The vibration is much more noticeable. I think the tail is wagging the dog. Brush hogs are not precision instruments.

AMR · Aug 3, 2009

I have an el-cheapo TSC rottary cutter that I use on my B3030, the tractor definately takes on a wobble. I run it around 2,000 RPMs and it cuts fine and reduces the wobble. I did all the inspection I could on it and attribute it to a fairly light tractor and probably less than high quality implement. I must admit that I have hit some pretty nast stuff with the cutter, mostly rocks, but a stump or two, curb by accident, metal fence, surveyors monument and who knows what else. I like to cut pretty close to the ground so when the tractor wheels dip ever so slightly ahead of the mower it is trouble.

My total brush hogging time per year is less than 20 hours so I don't pay much attention.

art · Aug 3, 2009

The blade tip speed on a bush hog is probably 12,000 ft per min, for a minimum. Think about it, it's going in a circle!!!! Being bent or having a poor build would for sure give a vibration.

EE_Bota · Aug 3, 2009

I am sure this is not your problem, because you would have noticed.
But I have had the problem, so others may have as well.

The blades are meant to swing freely so if you hit an object, the blade will swing out of the way. Once, mine was wobbling pretty bad, and upon checking, the blade was no longer free on the mount stud, and had seized at a slight angle. That set up one heck of a wobble. I struck it with a hammer a few times, no luck. So I put penetrating oil on it, and took off mowing. It was easy to tell the instant it broke loose...no more wobbling.

I can see how the condition you describe can cause a wobble. Having the mount stud of one blade higher is like applying a downward leverage force on the shaft. The blade "wants" to sling down to normal height again.

DavidAubke · Aug 3, 2009

I've run a 5' and a cheapo 4' off my little BX2350 and the vibration isn't bad enough that I'd use the term "wobble". Occasionally something will jam up under there and get it dancing but normally it's what any reasonable person would expect. I'll bet what Jesse is feeling is more than just standard operating vibrations.

I don't have a solution but I wouldn't be so quick to dismiss it as normal.

jesseb3@att.net · Aug 4, 2009

I forgot to mention that I previously borrowed a buddy's King Kutter 4' brush mower, and didn't seem to notice the wobble/vibration. Mine is used, and I may have to live with it, but it sure seems like there's room for improvement.

I've had a few thoughts since my original post...

The blades do indeed swing freely, but, since the one blade is mounted via a bolt that goes through a hole that is canted due to the bend, it may be binding the blade. After all, the weight of that blade when travelling at that speed has to cause some pressure on the pan. This may be keeping the blade from aligning properly while in use.

Also, that 1/2" deviation is just at the blade connection point. If you extend that deviation over the length of the blade, it may end up causing a difference of two inches or more at the blade's tip.

I've done some research on balancing rotating objects (OK, I asked a bright individual at work.) He explained, with the help of Google, that a bent pan would cause dynamic imbalance, through what's specifically referred to as "couple imbalance", i.e. the axis of inertia is not aligned with the axis of rotation. (Imagine running a shaft directly through the center of a rotating wheel, but do it at an angle. Sure, it's still statically balanced on the shaft, but get it spinning, and it will wobble.)

The pan is at a machine shop now. He's going to try to bend it back to position in a press. I hope to have an update shortly.

Jesse