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Four blade brush cutter

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STEVEM
Join Date: Jul 2003
Posts: 26 VIRGINIA
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2005-02-15          106201


Has anyone had experance with a four blade brush cutter? The one I'm looking at is a 5 foot Condor brand. It's used and I can't find any information about them, such as recomended HP ratings
I have an L2900 Kubota 33 HP. I'm using a 5 foot two blade cutter now and it works fine with this tractor.
Does a four blade cutter work better than a two blade cutter?
Is there such thing as a cutter with a faster blade tip speed? If so would that be a better way to go?
I'm looking for a better cut and moore ground speed.




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Murf
Join Date: Dec 1999
Posts: 7249 Toronto Area, Ontario, Canada
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2005-02-16          106218


While I'm not familiar with the condor brand, per se, 4 blade bush hogs.

They were an attempt to make a single cutter that would do finish cutting and rough cutting all in one. It failed.

The physical design of a bush hog is such that on anything less than pool table flat ground they will tear up the turf if you lower them to 'finish cutting' heights. They simply do not have the anti-scalp design or wheels necessary.

As for cutting faster, or better, that is good question. As for better, probably they would leave a slightly nicer looking cut, but only if the blades were kept very sharp and well balanced.

As for faster, not likely in any significant manner, it takes a certain amount of horsepower to cut a given volume of standing grass, period. They only way to practically cut more grass is more horsepower.

Best of luck. ....


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denwood
Join Date: Jul 2004
Posts: 542 Quarryville PA
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2005-02-16          106240


I would tend to agree with murf in your instance. Your HP will be the limiting factor. As far as nicer, I doubt it. If 2 blades are sharp they will be nice too. You don't see many high end finish mowers with 4 blades and their job is to look good. Faster yes, but maybe only on paper. Having 4 blades will mean another blade is cutting in half the time it would take for 2 blades(given equal tip speed), meaning you could go twice as fast and still have the same amount of grass being cut per blade. ....


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denwood
Join Date: Jul 2004
Posts: 542 Quarryville PA
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2005-02-16          106241


The question is how fast do you have to go to overwhelm a 2 blade cutter(not considering HP). I think that speed would be too fast to be truly usefull and not bouncy and hard on equipment. If you had the HP to mow x square feet of grass per minute, it would be better to go wider and slower than narrower and faster in my opinion. I would however look at tip speed of same sized mowers since faster tip will result in more cuts/given time and allow slightly faster ground speed with the same amount of grass cut per rotation, and a higher quality cut. ....


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hardwood
Join Date: Dec 2002
Posts: 3583 iowa
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2005-02-16          106243


I agree that going faster with the same cutter will result in more power needed in tall stuff or a poorer cut in short grass. I doubt that four blades would do much better than two sharp ones. Just a question I've allways had, but never taken the time to do the math. As blades become dull the outermost part of the blade wears off first, so at a given forward travel speed and a known rotor speed how far forward does the unit advance before the next blade makes it's cut? Half an inch, three quarters, or an inch? Food for thought. Frank. ....


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Murf
Join Date: Dec 1999
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2005-02-16          106246


It's a relatively (he says tongue planted well in cheek) calculation Frank.

At say 5 mph, the tractor, and we hope, the cutter too, are moving forward at the rate of 5,280' an hour or divided by 60 minutes, 88' per minute.

Using a 5' cutter in our example, one blade has to travel 188.5" to make one full rotation. The 5' cutter would, at 540rpm drive speed, have a tip speed of about 12,725' per minute. That means it would travel 152,700" per minute. If we divide by the circumference of 188.5" we get 810rpm.

So, know we know that in one minute, a) the cutter moves 88', b) the pair of blades turns 810 times, and therefore, c) a blade makes a pass, 1620 times in 88', or 1056". So to get a 'per cut' figure, we divide 1620 times by 1056" and get 1.534.

Now, after all that, we know that each blade cuts about 1.534" per cut.

Don't you feel better knowing that now? LOL.

Best of luck.
....


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hardwood
Join Date: Dec 2002
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2005-02-16          106247


Murf, You don't know what a load you've taken off my mind, so I'll sleep lots better now. In real world I probably don't go more than 2.5 to 3 mph. So being you used 5 mph my wild guess wasn't too far off. Altho I've often thought of the question while doing my mowing (therapy) it really wasn't that important, just kind of a fun way to pass a cold Feb. afternoon. Enjoy your day. Frank. ....


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AnnBrush
Join Date: Mar 2004
Posts: 463 Troy OH
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2005-02-17          106279


Given a fixed HP tractor, you won't be able to travel any faster using 4 blades.

You are cutting less (half) grass with each blade but you are cutting twice as often (four cuts per rotation as opposed to two). Net advantage (assuming linear relationship between cutting half the grass twice as often vs all the grass at once) is zero to the observer on the tractor.

Musing on this, with 4 blades you may only wear the tips of the blades out while the remaining shank does less work (length of swath cut per blade is shorter when additional blades are installed). If this is the case you have replaced the middle of the cutting portion of the existing blades with new blades that are not needed in the first place.

There may be advantages to each blade doing it like this (for example blades wear half as quickly, but then you have to maintain twice as many). I see no advantage yet. ....


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s chrand
Join Date: Aug 2004
Posts: 77 Mid-Michigan
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2005-02-17          106285


Re inches per cut
I in no way wish to impugn the well deserved reputation of Murf, for I have learned a great deal from his posts, but there were two problems with the above calculation.
First, feet per minute at 5 mph is 5 x 5280 / 60, or 440. The ‘5’ was omitted.
The rest of it makes sense, except that the final division is reversed. It should be 1056 divided by 1620 for a result of .65" per cut (at 1mph). At 5mph that would be 3.26" per cut.
A simple formula is (1056 x mph) / (spindle RPM x N cutters per spindle).
So now I wonder, what is a good amount to cut, and should the RPM of the PTO be modulated to correspond to the ground speed?

PS All of the above is the engineer in me totally out of control!
....


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denwood
Join Date: Jul 2004
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2005-02-17          106287


Obviously you have a hydro. Ground speed vs. pto is directly linked for gear drive. You only get to choose by however many gears you have. Winding down the pto rpm too much results in a poorer cut and more tearing of the material, although I always run at a low rpm/pto unless I am mowing grass, then I use at or close to 540. ....


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Murf
Join Date: Dec 1999
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2005-02-17          106288


Mea culpa.

You are of course correct, I DID omit the '5' from the calculation, and invert the numbers.

That's what I get for talking on the phone while I make posts.

I stand corrected.

As for pto speed while cutting, it is an interesting topic. I have on occassion run 'low power' implements such as fertilizer spreaders using the 2 speed pto, set the pto to the 1000 rpm speed then run the engine at about half-speed.

However, as Denwood pointed out, blade speed is critical in making a good cut.

Best of luck. ....


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yooperpete
Join Date: Jan 2004
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2005-02-17          106289


Last summer we had some discussions about decks with 4 blades. Some said superior cutting performance is achieved. If 4 blades are cutting at the same speed as two blades the amount of cut per blade is cut in half (if they are all set to the same height). Likewise a second pass through, while the grass is being sucked up from the vacuum of the first should get you a better clean cut (i.e. same as double, triple and quadruple cut razors on the market).

P.S. I bought another CubCadet lawntractor this fall with 54" deck with 3 spindles that has been setup with double blades. I didn't get a chance to conduct a good test on its performance. Will give everyone feedback when the sun begins to shine again!

I'm not in agreement about the equation. 540Rpm at the tractor doesn't always mean a 1:1 ratio at the blades. In the machine tool industry the principle of cutting with a milling cutter is somewhat the same as cutting grass. We generally talk about chipload which would be the amount of cut per sweep of a blade as it advances. This varies as the blade rotates. If toward the front of the deck being 12 o'clock, with clockwise rotation the blade would initiate a cut at 9 o'clock and progressively get wider as it rotates toward 12 o'clock and again begins to thin as it reaches 3 o'clock. This assumption is good if the blade tips are just slightly lower at the 12 o'clock position. ....


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Murf
Join Date: Dec 1999
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2005-02-17          106291


Yooper, I think you misread the calculation.

The blade tip speed is not pendant on a given gear ratio in the drivetrain. The figure given was based on a 'standard' tip speed for a 5' bush hog, taken BTW, from a Rhino Bush Hog spec. I just happened to have on hand.

Best of luck. ....


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shortmagnum
Join Date: Nov 2003
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2005-02-17          106292


So is there any difference between a two blade cutter at 600 rpm vs a four blade cutter at 300 rpm? (Ignoring HP differences)
Dave ....


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earthwrks
Join Date: Dec 2003
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2005-02-17          106296


I use rotary cutters for cutting heavy brush, and I don't get into "finish" mowing", but what about a well-maintaned flail mower? I watched a guy cut his yard with one and looked like a golf course when he was done. I'm told that flails use more HP but could it be worth it if you are only taking off a few inches of grass? ....


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Murf
Join Date: Dec 1999
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2005-02-17          106297


Dave, yes there would be a difference, several actually. First, with an increase in speed so too comes a gain in inertia and a gain in centrifugal force. The inertia would help overcoming momentary increases in load, from say a sapling. The centrifugal force does the same basic thing, helping to keep the blades out-stretched in the cutting position when they load up instead of swinging back in. Then there is the difference in cut from the blade contacting the grass at half the speed, this would result in more of a 'hacking' through the grass instead of slicing as it does at the higher speed. The faster (to a point) a blade contacts the grass the better the cut.

Earthworks, I think you have it reversed, a flail takes LESS power to cut the same width not more. Not much more mind you, but more. For instance where the rule of thumb for a bush hog is about 6hp per foot of cutter, a flail takes only about 5hp per foot of cutter. Again, tip speed is one of the functions of the better-looking cut. They are also safer, since they don't tend to fire debris like a rotary does. We use them extensively in our work on goldf courses, they certainly do a nice job. They also sit closer to the tractor so they require less counter-balance and generally weigh less too, all of which makes them ideal for a CUT. They do tend to be rather spendy though.

Best of luck.

Best of luck. ....


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brokenarrow
Join Date: Jan 2004
Posts: 1288 Wisconsin
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2005-02-17          106330


All you engineers out there, I need help with an algebra calculation. If your traveling at 3 mph. your tip speed is 1395fpm. a two blade cutter and it is 72" wide.
Now you run over a ground hornets nest and given the rate of 275 hornets exiting the nest per second , how many hornets would actually die and how many would sting your ass assuming you increase your speed to 12mph after the first stinger hits your neck?
I will give you a real life answer in about 5 months but would like to be prepaired with enough medicine ahead of time. ....


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Murf
Join Date: Dec 1999
Posts: 7249 Toronto Area, Ontario, Canada
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2005-02-18          106357


That's a simple (in my case) calculation.

Zero. (Look at my picture # 11 & 12).

They just beat their little brains out on the glass of the cab trying to get to me. LOL.

Best of luck. ....


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STEVEM
Join Date: Jul 2003
Posts: 26 VIRGINIA
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2005-08-19          115122


After using the 4 blade cutter and mowing 18 acres I couldn't see a lot of differance in the cut of my old 2 blade cutter. However the 4 blade cutter wind rowed really bad. I don't think it had anything to do with the 4 blade design, but was attributed to poor deck design.
I purchased a Land Pride 1800 series brush cutter, it is suppose to have a faster blede tip speed, it cuts alot better than any brush cutter I have ever owned. That's in the quality of the cut and dispurment of the debre is much better. It's about as close as you can get to a finish mower and still cut rough stuff. I think Bush Hog uses the same gear box. The old saying, you get what you pay for is true in this case. ....


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