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Discussion Starter #1
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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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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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-Reducers/Gearboxes/Rotary-Cutter-Gearboxes/
 

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Discussion Starter #4
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

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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Discussion Starter #7
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.

Rotary cutter perspective.jpg

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. :dunno:

i can't wait to see what u come up with. it is gonna be a one in a million i'm sure:munch:
 
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Discussion Starter #9
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 :laugh: ). 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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Discussion Starter #10
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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Just don't let the VA fix your finger. I compound fractured my pointer finger on my left hand. A small piece of wood kicked back off my table saw and busted it in haft and the bone was showing. I immediately pulled it strait and set the bone before it hurt so much I could not touch it. The emergency room said I did a good job on the bone setting and they left it that way? Now it is all crooked looking and a torn tendon on the tip keeps me from making it strait like the rest are. The VA told me your to old to fix it cause your not working and they would fix it if I was a younger vet. The doctor that told me that quit the VA because they would not let him fix some things on the older veterans! They left my in Pancreatic pain for 5 years and only gave me pain meds till I was up to 5 - 7.5 Hydrocodone's a day. I went cold turkey and stopped them after a outside doctor fixed me in 1 1/2 hours doing a $138,000.00 plus operation down thru my throat with very tiny little tools cutting open a duct in my Pancreas that was plugged up and not working right. After 5 years of not eating great tasting food I was able to now eat anything I wanted. The VA picked up the tab on that one and I will never let them fix me again. So now I'm just trying to fix the ate up stomach part. Good luck on your finger and don't worry you can still fix stuff if it dose not work right I have done it for years now. It cracks and pops all the time as it shifts when I bend it.
 

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Discussion Starter #12
If this was the only thing wrong with me, I'd be a happy camper. :lolol: Stage 4 arthritis in both knees, nerve damage in both hands, nerve impingement in my neck, arthritis in my neck and upper back, maximum rated messed up feet (50% rating!) - so my primary booger hook being jacked up is just one more item on a long, long list.

I was on the conference call when the lady was scheduling my outside provider authorization and heard her tell them I'm approved for like 180 visits and 6 months of PT. :eek:hmy: The provider I'm seeing is probably the best outfit in the area for specialty orthopedic care. All the pro sports teams use them.

That said, our Minneapolis VAMC is actually a high quality operation. We have one of the best med schools in the country doing our internships, and the staff surgeons are top notch. My regular ortho surgeon is a knee specialist, and he knows me well enough to know I'd blow up fake knees if I didn't have the pain to keep me in check, so he won't do them and just claims that I'm not "age appropriate" for replacements. Part of that is true, and part of it is that my medial and lateral compartments are still good (what happens when you hurt when you walk for 20 years and you stop doing it), but my knee caps are junked out. He knows a partial implant on me would be destroyed in short order, so he won't do it because once those are junk you go to full replacements and then I'd blow those up, and I'd be getting new knees until there was no bone left to glue them onto. Instead, he's holding out (as am I, now) for the stem cell treatments to come around and those are the only thing showing promise to fix what's actually wrong with my joints and not have any early-failure risk. Should be happening in the next year or so per my visit last week. I'm pretty excited about it and told him I'd be their guinea pig! :mocking:

I'd be a straight up junkie if I took enough drugs to mask my regular pain. I'd be so high I wouldn't be able to do anything, so being sober and aching and not doing anything is really the same outcome. My kidney function had dropped so my old GP wouldn't let me take NSAIDs, and that's when I realized that I need to listen to my body and not my stubborn head so much and just slow down or I'm gonna go toes up from secondary system failure. My kidney function is back to normal and I take the NSAIDs when I need to, and take my percs when I need a break from being miserable just to take the edge off, not to make me feel normal again.


And back to the thread topic:

Here's a JD 2084 behind what looks like a 5045e.

5045e 2084 mower.gif

I'm noticing most mowers have a bit of gap behind the tractor. I assume this is for quick hitch compatibility, but is there a reason not to tuck the mower deck in closer to the rear tires as long as everything clears?
 

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Interference blades

Looks like someone else had the same idea, only made it with lighter duty parts. :laugh:

https://www.everythingattachments.com/Fred-Cain-8-ft-HD-Agricutter-p/fc-208l-ac.htm

Mine won't cost 5K either. :gizmo:
I think interference blades will work well until something happens to one side like a stump or other obstruction. Then your run the chance of taking out two gearboxes at once. If the blades swing on a pivot, you'll want to scratch your head quite a bit so figure the overlap as the blades tend to fold somewhat as you move the rig from place to place without the pto spinning. Then when you put it in gear, they unfold but depending on the overlap there could be a chance the blades would hit while unfolding. Our mowers have about 5-10 seconds of a lot of vibration until the blades fully unfold as they are spinning up to speed, particularly if one side has fully folded into the stump jumper.

I noticed the powerline trim crew just has the bucket truck drop limbs to the ground and then they run over the limbs with a cutter. I couldn't see the cutter brand but it has to be tough. The tractor is fully armored as well with a pretty heavy cage set up to keep crap from flying through the glass, taking out hydraulics etc.

Treefarmer
 
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I'm noticing most mowers have a bit of gap behind the tractor. I assume this is for quick hitch compatibility, but is there a reason not to tuck the mower deck in closer to the rear tires as long as everything clears?
I'd say it's to give the operator room to get in and attach the 3 point (if not iMatch) and hook up the PTO. :unknown:
 

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Most two spindle, non folding, brush cutters have the blades overlapping about 4" and the gear boxes are timed 90° apart by the sub shaft between them. I would try for 15K - 16K fpm blade tip speed.
 

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Discussion Starter #19
^^^ Yep. That's what I've discovered as well.

Seems like they got more popular after I looked at them the last time in 2015. Still crazy expensive to buy, but I should be able to build one for under $2K.
 

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