Bush Hog blades hitting deck

[jb1390]

I bought a 6' bushhog off of craigslist a couple months ago. It is a small-name brand manufactured in pennsylvania. In general it seems to be pretty well built, with channel steel for side structure, and 2.5" box tubing for frame rails above the deck. I've run a few styles of blades with it - the original blades had a slight bend in the corner, which provided a slight updraft. I used these blades for some heavy brush cutting the first day I had it, and I noticed a ring of dents starting to appear around the deck - which I figured was due to the blades throwing debris upward into the sheet metal. I changed the blades shortly afterward to a flat style, that I use for heavy brush cutting. The ridge did not seem to get worse, I probably put about 20-30 hours on it through briar patches and other brush with the flat blades.

I recently mowed 13 acres of pasture - which is 99% grass, with the remaining 1% being weeds and rocks. I installed a set of high lift blades, which worked very nicely for lifting the grass up where weather/tire tracks had laid it over. The cut was great - but I noticed the ridge getting worse- to the point that there are a few small holes all the way through the deck. I did hit a fair number of rocks while mowing. I was also running at pto speed or slightly higher - which would make the blades less prone to swing when hitting an obstacle due to high centripetal force.

From what I've read, the blades should never hit the deck, unless something is wrong - but I can't find anything obviously wrong. There is probably 3" or so of clearance between the high lift blade and the deck, and very little slop <.010" in the spindle pin. The blades are not bent, and the stump jumper is in great shape - verified to be good using a dial indicator. The blade bolts are also in good shape - pretty tight, with just enough clearance to allow the blades to turn. When I push hard on the blade, I can make it bend - everything else (including gear box mount) is tight.

My only theory is that when I hit low, angled rocks, it deflects the blades enough to push them up into the deck. Close examination indicates that the blades are actually touching, and it's not just debris being thrown into the deck. This particular mower has lower sides than some (like a woods) - which would likely make it more susceptible to the blades hitting due to lower clearance. The high lift blades also sit closer to the deck than flat ones. It seems like the easiest fix is to add some thicker material to the bottom of the deck - so that when the blade hits it doesn't matter. Also, pull the rocks out where possible - though hitting rocks is par for the course with a bush hog. I could also move the gearbox and extend the sides - though that seems like significantly more effort than adding some wear plate to the underside.

Anybody else run into this kind of issue?

 

[kebo]

Obviously something is moving. Everything may feel "tight" when you check it by hand, but at full speed things can move around. I suspect that there is enough play in the bolts that hold the blades on, and enough wear in the pivot holes on the blades, that they can move up just enough to hit the bottom-side of the deck. You might need to pull the blades off for a closer inspection. Maybe just go ahead and replace them, and at least eliminate that potential cause. Good luck.

 

[John_Mc]

If you are in fact bending your blades by pressing on them by hand, I have to wonder if you are using the correct blade. No way I can bend my blades with just hand pressure. I might get a bit of flex in where they bolt on to the stump jumper, but not bend the blade itself.

 

[4570Man]

There's no way you should be able to bend bush hog blades by hand. Mine are about 3/4 thick.

 

[jb1390]

Obviously something is moving. Everything may feel "tight" when you check it by hand, but at full speed things can move around. I suspect that there is enough play in the bolts that hold the blades on, and enough wear in the pivot holes on the blades, that they can move up just enough to hit the bottom-side of the deck. You might need to pull the blades off for a closer inspection. Maybe just go ahead and replace them, and at least eliminate that potential cause. Good luck.

They are brand new blades. The bolts are very tight on the blades, with no apparent wear.

 

[jb1390]

If you are in fact bending your blades by pressing on them by hand, I have to wonder if you are using the correct blade. No way I can bend my blades with just hand pressure. I might get a bit of flex in where they bolt on to the stump jumper, but not bend the blade itself.

There's no way you should be able to bend bush hog blades by hand. Mine are about 3/4 thick.

The blades are 1/2" by 3" blades - pretty standard size. If the blades are not held in place, no way I could bend them - but when bolted to the stump jumper and the deck upside down, I can deflect them with some force. Nothing else seems to move, just the blade. A quick FEA analysis suggests a 1/2" x 3" steel bar that is 2.5 feet long (very similar to blades)will deflect 4.5 inches with 500 pounds of force. The clearance to the deck with high lift blades is less than that - and I think hitting a low rock could easily apply hundreds of pounds of upward force on the blade.

I should also mention that I did remove the blades, and they are not permanently bent. Also, rotating them around and checking side clearance indicates that everything is correct.

 

[4570Man]

How wide is your cutter? I'm guessing the blades are more likely about 18" long. They are also very likely harder than the steel bar in the test.

 

[jb1390]

How wide is your cutter? I'm guessing the blades are more like about 18" long. They are also very likely harder than the steel bar in the test.

Just went and measured - they are a tad over 30". The cutter is 6'.

Modulus of elasticity doesn't change much among steel types. Yield strength varies significantly. Yield strength is what determines when a part will stay permanently bent - modulus of elasticity determines the "spring rate". Modulus of elasticity is what is used for FEA - I'm very confident that the analysis is correct - though the amount of force that could be applied to the blade while mowing is a guess. I'm thinking that around 300+ pounds isn't unrealistic though - which would be enough to get the blades to touch.

Standard flat blades have a couple more inches of clearance, which is why I think they did not cause dents. It seems like the landpride, woods, etc mowers that I've looked at have much deeper sides than mine, and hence more tolerance to blade bend etc with no contact to the structure. Also on the woods, the structure is underneath the deck - so a little bit of contact would hardly be noticed, and would not be apparent from above.

 

[LD1]

Measure your exact clearance between. The blades and deck.

3" don't sound like it's anywhere close to enough. The cutter is probably not built for....or spec'd to run lift blades.

If they are a common size, you night be able to find some lift blades with more offset.

 

[rd_macgregor]

Reinforcing the deck by welding plate on the underside could conceivably make the problem worse, since it would reduce clearance. Like LD1, I suspect the unit isn't intended to be used with the hi-lift blades.

 

[kthompson]

First I would do is track down the manufacturer recommended blades. On rough cut I have not known anyone who uses a blade with lift in it. That really sounds like a light (compared to true rough cut blade) weight after market blade especially if it is the one you can get to flex by hand.

I am assuming the blades bolt directly to the stump jump pan so must be bolted on the bottom. I have seen rough cut there is a bar across the stump jumper the blades bolt to and it would be possible to bolt the blades on the top of those bars which would not give the blades the clearance needed.

As to how common hitting rocks are blessed rocks are not normal here but depending on their size would say not good. A broken blade, heavy piece of wood can travel a lot farther than many realize and do damage even then. Know of a heavy piece of wood traveling at least 60 feet and hitting a car door hard enough to cause a large dent. Have known of broken blade to fly across a house.

If you have the proper blades and they hit then somewhere something was changed such as the gear box, stump jumper to affect the clearance of the blades.

Oh, verify if there is suppose to be a spacer to lower the blades on the bolts. Very possible to give easier rotation of the blades in case of hitting something solid and they have been loss over time. kt

 

[LD1]

I dont have any numbers or measurments handy, but I am pretty sure that all of the bushhogs I have owned had the blades ALOT more than 3" from the deck at rest.

Heck, an average stump jumper pan is at atleast 3" in depth. Then there is the 1/2" or so clearance between it and the deck. Then throw in the 2-2.5" offset in the mower blade and we are about 6" from the deck. And I have never seen a high lift bushhog blade that had more than ~1-1.5" wing. So without measuring....I'd say all my mowers have at least 4.5-5" of clearance with a high lift blade. And those mowers to include my woods DS96, Bushhog 105 and 306, And international IM600, and a standard duty KK5'.

Perhaps you could share some pictures of this mower, the blades, and a good shot of the underside showing the deck, blades and stump jumper / blade carrier?

 

[jb1390]

Measure your exact clearance between. The blades and deck.

3" don't sound like it's anywhere close to enough. The cutter is probably not built for....or spec'd to run lift blades.

If they are a common size, you night be able to find some lift blades with more offset.

I will measure later today. You very well may be correct - that the cutter was not designed correctly for lift blades. The sides are fairly low compared to many cutters I've seen - finding blades with more offset might cause the blades to rotate below the sides, which would not be a safe way to go IMO.

I thought about moving the gearbox and extending the sides as a solution, but that seems like a lot more effort and money than adding some 1/4" Hardox wear plate or even mild steel to the bottom.

 

[jb1390]

Reinforcing the deck by welding plate on the underside could conceivably make the problem worse, since it would reduce clearance. Like LD1, I suspect the unit isn't intended to be used with the hi-lift blades.

Agreed, adding material will reduce clearance, and if not done properly could cause further issues. Though I am thinking 1/4" material, and planning on using high strength material that is designed for impact and wear - so a hit from the blade wouldn't cause damage like it is now.

 

[jb1390]

First I would do is track down the manufacturer recommended blades. On rough cut I have not known anyone who uses a blade with lift in it. That really sounds like a light (compared to true rough cut blade) weight after market blade especially if it is the one you can get to flex by hand.

I am assuming the blades bolt directly to the stump jump pan so must be bolted on the bottom. I have seen rough cut there is a bar across the stump jumper the blades bolt to and it would be possible to bolt the blades on the top of those bars which would not give the blades the clearance needed.

As to how common hitting rocks are blessed rocks are not normal here but depending on their size would say not good. A broken blade, heavy piece of wood can travel a lot farther than many realize and do damage even then. Know of a heavy piece of wood traveling at least 60 feet and hitting a car door hard enough to cause a large dent. Have known of broken blade to fly across a house.

If you have the proper blades and they hit then somewhere something was changed such as the gear box, stump jumper to affect the clearance of the blades.

Oh, verify if there is suppose to be a spacer to lower the blades on the bolts. Very possible to give easier rotation of the blades in case of hitting something solid and they have been loss over time. kt

The blades bolt directly to the bottom of the stump jumper pan. I do not think there should be a spacer, since the bolts have a keyway that aligns with the stump jumper to prevent rotation while tightening the bolts.

The high lift blades I believe are land pride blades, and are a standard 1/2" by 3" blade, with a nice flare in it to help stand the grass up.

I agree that high lift blades do not seem to be particularly common - though I also read of many people who have trouble getting grass to cut well when laid down by tires. The cut was amazing, even in areas that were laid down by rain, and in the tire tracks.

I also would like to reduce the number of rocks in that field - but in Connecticut, I'm going to hit many more rocks unless I quit using the mower. Just comes with the territory. It seems that worn out blades or worn out bolts are the most common causes for a blade coming off - and I inspect them regularly - I understand the energy involved and the potential consequences of a failure.

 

[jb1390]

I dont have any numbers or measurments handy, but I am pretty sure that all of the bushhogs I have owned had the blades ALOT more than 3" from the deck at rest.

Heck, an average stump jumper pan is at atleast 3" in depth. Then there is the 1/2" or so clearance between it and the deck. Then throw in the 2-2.5" offset in the mower blade and we are about 6" from the deck. And I have never seen a high lift bushhog blade that had more than ~1-1.5" wing. So without measuring....I'd say all my mowers have at least 4.5-5" of clearance with a high lift blade. And those mowers to include my woods DS96, Bushhog 105 and 306, And international IM600, and a standard duty KK5'.

Perhaps you could share some pictures of this mower, the blades, and a good shot of the underside showing the deck, blades and stump jumper / blade carrier?

Yes I think pictures would be helpful. The 3" is an estimate from memory - I'll pull out the camera and tape measure when I have a chance.

 

[Notforhire]

Agreed, adding material will reduce clearance, and if not done properly could cause further issues. Though I am thinking 1/4" material, and planning on using high strength material that is designed for impact and wear - so a hit from the blade wouldn't cause damage like it is now.

Twenty years ago I knew of a rental business that had a BH 285. Being a rental unit, it got a lot of rough use. On the underside of the deck they had welded a ring about four feet in diameter, made from (I think) 5/8 X 1 steel stock. This limited the blade deflection and prevented the blades from hitting and slicing through the deck.

That said, I can't imagine that an arrangement like that would be necessary under normal use.

 

[jb1390]

Here are a couple pictures of the underside of the deck. The blades are actually only 2.5" from touching. I was checking out the brand new Landpride mowers at a local dealership the other day, comparing the design to mine. I asked for some pricing on them - seems to be around 2500 plus tax for an equivalent (but better built) mower. The blades on the new one seemed to have about the same amount of slop as mine, which indicates that nothing is worn out on my mower. There was, however, a tremendous difference in the clearance of the blades - for a couple reasons. One, it had much higher sides to begin with, and the stump jumper is installed a little lower. The other is that they install the subframe underneath the deck, instead of above it. The sales guy told me that many manufacturers are moving that way, because if a blade hits a rounded rock, it gets pushed right up into the deck if it's too close. If the subframe is underneath, the thicker steel works to protect the deck. He told me this without me talking about any of my issues - but it sounds like exactly what's going on with my mower.

I also got some pricing on steel to reinforce mine - it would be around 220 for 1/4" mild steel, and around 450 for 1/4" hardox 450 for a 4x8 sheet sheared into 16" strips. I'm not sure yet which material I'd prefer, but I'm inclined to just reinforce the deck of mine anywhere the blades touch, and buy a better cutter in a few years.

 

[LD1]

I believe the single biggest reason for the push to go with the supports on the underside is ease of cleaning off the top of the deck.

As to your situation....yea thats too close with them high-lifts. You might be able to source some generic blades that are the correct length and have a little more offset?

 

[Gary Fowler]

For a lot less money, I would take those blades to a machine shop and have them bend them to give more offset in the blades. Cold bending should not affect the tempering although I doubt that hot bending in the existing bends would hurt the tempering either. It would take 2 bends per blade to do it right and keep the cutting edge level.
Of course this option would only be used if you cant find replacement blades with enough offset in them.