Brush hog gearing question; or maybe more blade speed?
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    Jim Timber's Avatar
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    Brush hog gearing question; or maybe more blade speed?

    I've got an idea in my head for a new brush hog design that should be extremely robust and reasonably compact while still having a large width of cut.

    What's used to determine the operating RPM of a cutter head? Is there a tip speed that ends up too fast or unsafe?
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    AJgrn78's Avatar
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    All I can tell you is that a brush hog blade doesn’t spin anywhere close to as fast as a belly mower blade, I wouldn’t be surprised if the gear box was close to 1:1 with the pto, and the blade spindle turned at 540 rpms. But it’s one big 60” blade so get enough blade tip speed to cut. Disclaimer, I am making some assumptions....
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    JD4044M's Avatar
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    Did a quick check on the net. It seems most gear boxes are 1 to 1+ with a point something like 1:1.13 ect and tip speed vary between 10K and 16K FPM. There are surplus places that sell them plus the spec up to 100 HP and down ratings. I will try to find it again and post it here. Here is a link to the site I was getting information on.https://www.surpluscenter.com/Gear-R...ter-Gearboxes/
    Last edited by JD4044M; 02-24-2018 at 03:32 PM.
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    Jim Timber's Avatar
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    I think the 84HP unit I used for my stump grinder is 1:1.4 increase if I remember right. It's a US made Omni and has held up beautifully in that application.

    My design will use two smaller diameter cutters on a skewed axis from perpendicular to the center-line of the tractor so there's no "stripes" of missed stuff. I'm just unsure how to design the operating speed at the blades.
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    offset blades

    Quote Originally Posted by Jim Timber View Post
    I think the 84HP unit I used for my stump grinder is 1:1.4 increase if I remember right. It's a US made Omni and has held up beautifully in that application.

    My design will use two smaller diameter cutters on a skewed axis from perpendicular to the center-line of the tractor so there's no "stripes" of missed stuff. I'm just unsure how to design the operating speed at the blades.
    I'm guessing that the tip speed is below the speed of sound because I don't hear snap, snap, snap from under a mower. Depending on the weight of the blades, there's significant centripetal force on the bolts that hold the blades on and that force increases with the blade speed so that's also something to keep in mind.

    At some point, I've run a cutter with slightly offset blades. It worked great in a straight line but as I recall, would streak in a turn. Right now, I don't even remember what it was or when I ran it.

    Treefarmer
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    Tip Speed

    The mfgrs try to keep the Tip Speed below the speed of sound and also keep the mower safe. I was discussing this years ago with some JD folks who related how the damage to the mowers increased when operators hooked 540 rpm mowers to 1000 rpm PTOs.
    Leo
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    Jim Timber's Avatar
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    The stump jumper (hub) looks to be the same size across many of these mowers with only the blade length changing. Having that much weight swinging around on a mid-pivot sure could induce some significant side loading on the axle bearings if you were putting high centrifugal (centripetal is the center-seeking force opposite centrifugal) loads on the blades when they encountered something they couldn't shear. Twin cutters actually reduces these loads and is inherently safer, with the drawback of having less force in the blade due to reduced inertia - so the drawback would be poorer performance on thicker stems (if that's an issue?).

    This isn't precisely to scale as I'm not sure how wide I'm going to go or how large of cutters I'll be using. Just for demonstrations sake, the large circle is 700px and the smaller ones are 350px. The bottom line showing the saved length isn't perfectly perpendicular but it does make the point of tail swing reduction.

    Click image for larger version. 

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    Another idea I'm toying with is timing the two cutting heads so the blades can cross-over each other's swaths. The Chinook I worked on in the Army is built this way and those have been in service for 60+ years. Originally, I was thinking of using automotive hubs and some kind of slip-clutch type drive system between the cutters, but it might be easier and more robust to just use off the shelf parts. Another advantage of that is serviceability down the road if I ever end up too messed up to machine my one-off's replacements.
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    jim timber-are u building this hog to run thru woods that is gonna be left over? or where they have cut at? i would think a 8 ft or 10 would be plenty big enough.

    i can't wait to see what u come up with. it is gonna be a one in a million i'm sure
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    jim

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    Jim Timber's Avatar
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    I've got a 5' brush hog now, and the one time I spun it up it shook pretty bad despite having brand new blades and a new gear box (I bought it from the shop when they discovered the Comet box wasn't worth replacing and they weren't aware that US made boxes aren't that expensive - they claimed the mower needed $700 in parts and I have $500 in the completed unit including buying it from them ). I'm not sure if one of the blade bushings was stuck or what (never investigated it as I had nowhere I was planning to mow at the time), but need to address that before I use it.

    My tractor is 6' wide, so that mower doesn't cover my full tire path as it is.

    The intended use will be maintaining my food plot, trails, and rifle range. I don't want to go too big, but I also don't want to be making extra passes I don't have to. The main thing is that I hate how freaking HUGE a 7' (and up) mower gets and they're just not driver-friendly when you're looking at a tail-swing the length of your wheelbase (driving the loader around the woods is bad enough and it's only 6' out the front).

    I've had this idea kicking around in my head (in various incarnations) for as long as I've had my tractor. Whether or not I pull the trigger on building it is yet to be seen. Money's still tight and I don't know what the proceeds of the logging is yet. I also have limited abilities to work at present due to the chewed finger. I'm just trying to work out the mental gymnastics for if/when I decide to make this sucker a reality. I still owe someone a lamp too (sorry Jen!), so I need to make that before I work on my own stuff.
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    Jim Timber's Avatar
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    I found an interesting item at my favorite PTO mower gear box supplier (Surplus Center) and have decided to make a timed interference-blade mower with 3 transmissions (as gear boxes are called on helicopters). I guess when you see the light it's hard to deny it. The parts will be rather expensive, but still way cheaper than a 7' or 8' mower.

    Still need to see what the surgeon says Tuesday before I get too excited about ordering the expensive parts, but I have a solid design in my head now and I can begin working on assembling my parts list.
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