PTO 'string cutter' ?

[PILOON]

While what I really want is a bush hawg, I am toying with the idea of making a glorified 'PTO string cutter' as I think I have the necessary components.

I have a gravely 90 deg gear box that came from a bush hog type tool attachement as well as an assortement of PTO shafts to drive the gear head.

My idea is to weld up a suitable 3 point framework to support this 'box' and fashion a head to attach a cable holder (might be a couple of big washers that would 'clamp' a cable (maybe HD nylon cord or even wire rope).

What I need to 'cut' is ditch growth alongside of my driveways that consist mainly of reeds, berry bushes and generally less than 1 inch new growth.

The gear box is close to a one to one ratio but my tractor has 3 PTO speeds, 550, 750 and I seem to recall the other is a little over 1000 rpm.

What speeds do bush hogs generally spin?

What chances do I have of success?

Anyone ever do a similar project?

What suggests to me that this should be feasable is the fact that there exists self powered walk-behind (8-12hp range) units of this type often demonstrated on TV.

 

[schmism]

What speeds do bush hogs generally spin?

What chances do I have of success?

Anyone ever do a similar project?

What suggests to me that this should be feasable is the fact that there exists self powered walk-behind (8-12hp range) units of this type often demonstrated on TV.

Brush hogs generally get there effective cutting force from the mass of the rotating heavy arse blades.

You have to spin a 'string" so much quicker in order to impart the same energy into your grass/weeds/brush etc.

in short i dont think you could spin your PTO version fast enough with parts on hand.

 

[mjncad]

I like the idea, and I suggest you find out what the walk behind string trimmer spins the trimmer head at, and find a gear box with the proper gear ratio to approximate the same head speed using your PTO.

My only concern is that even with the heaviest nylon string is that you'll spend more time changing broken string than whacking weeds. I quit using string on my gas trimmer, and went with a mini brush cutter head that has three reversible plastic and steel cutting edge blades. It cuts the weeds and I only have to stop for three reasons: 1. The trimmer ran out of gas, 2. I ran out of gas, 3. The job is complete.

Or you could do what one of the hosts on the British car TV show Top Gear did when they went gardening for charity. He used a shotgun for blasting weeds and trimming tree branches. What a hoot!

 

[excelagator]

There is one I saw in an old paper issue in Farm Show mag. It was mounted off to the side of the tractor for wackin' weeds down a long fenceline.

 

[Bird]

What chances do I have of success?

Do keep us informed. And I hope you succeed, but right now, I believe your chances are nil. Many years ago when I first heard of the first string trimmers hitting the market, I wondered why you couldn't just use an electric drill and fishing line. Turned out the answer was real simple; electric drills just don't turn fast enough. I don't know where you could get a gear box that would convert your PTO speed of 540, or even 1000, to the kind of speed necessary for a string trimmer.

 

[gemini5362]

First of all the disclaimer. I have never done this and do not know how well it works. Hopefully someone that has done it will add some experience and practical information to this. Several of the old timers used to tell me that they would use a piece of chain in place of blades on their brush hog. They said it worked better and was a lot cheaper to replace than blades. If that is an accurate description then you obviously dont need the mass of the blades and the rotational speed of the standard brush hog will work. If you are going to build a rotary cutter I would surely try some 5/16 inch chain that will hold up a lot better than the plastic line that is used on string trimmers. I have the push string trimmer with a 6.5 horse engine and when I hit anything big or solid the line does not last long. I think your idea trying to use the string will not be practicable. The chain however might be a workable idea.

 

[Villengineer]

use a belt system to get the rpms up just like they do on rfm's. You could even build a 2 head unit which would cut a wider swath, but keep the trim line shorter.

 

[toivo1037]

Instead of string, or chain, how about some very heavy steel cable...

 

[Treemonkey1000]

Steel cable flexing will break fairly quickly. Plus strands of that breaking off and flying around could puncture people and tires. Chain might work. To get more rpm's couldn't you use a care rearend to change the speed up? If you PTO the axle hub instead of the driveline input you would get more speed I think. A 4:10/1 rearend for example.

 

[LBrown59]

What I need to 'cut' is ditch growth alongside of my driveways that consist mainly of reeds, berry bushes and generally less than 1 inch new growth.

I have what I feel might be a better similar idea
I'm itching to try out if only I had a welder and could weld.
I need it specifically for mowing/trimming out over ditches and banks.

 

[Pooh_Bear]

Here is a link to get ideas from:

3 point mounted weed eater for tractor

Weed Chopper

Pooh Bear

 

[LBrown59]

Here is a link to get ideas from:

3 point mounted weed eater for tractor

Weed Chopper

Pooh Bear

I need to get 4 to 6 feet out from the rear tire.
This thing and others i've seen similar to it won't do that.

 

[RonMar]

To cut grass cleanly, It is not so much RPM, as tip velocity of the cutting string that is important. The one that Pooh bear linked to has a 3130 RPM spindle with 540 PTO RPM. That triangular deck shape houses a large pully under the gearbox, and a small pully out at the spindle to achieve this 5.8:1 ratio. It has a 24" cut diameter, so the string tips are traveling roughly 328 feet per second. A typical 14" handheld string trimmer has to turn nearly 5400 RPM to get this same string tip velocity which is why their 2 stroke engines really sing... As the string wears/breaks, they don't cut as well. This velocity also applies to blades. That is why most push mowers that are direct driven off an engine shaft don't go much bigger than about 21". A 3600 RPM gas engine driving a 21" blade yields around a 329 FPS at the blade tips...

Anything you build will need to have that same string tip velocity to have a reasonable chance of being successfull.

Good luck

 

[Brad_Blazer]

They used to sell weedeater heads with chains instead of string. I remember a story about a man who killed his young granddaughter using that attachment when it broke, flew some 50', and struck her head. Any type of cutter that behaves like a whip will be consumed in the process and occasionally shed chunks. It is much safer it the material is lightweight so it does not shed bullets.
Here is the recall of the "weed wizard"
Recall: Weed Trimmer Heads With Metal Chains Recalled by Weed Wizard -- Nolo

 

[gemini5362]

That is an interesting weed trimmer for a three point hitch. Based on my experience using a weed trimmer with a 6.5 horse engine that trimmer will havae you spending more time replacing string than you do trimming anything other than grass. Basically it looks like the same head that I have on my string trimmer only it has places for 4 pieces of plastic cutting string and mine that I push like a lawn mower only has 2 but same mounting design on the string.


I imagine there is some difference between the chain on that weed wizard and a piece of 5/16 inch chain. Also if it breaks better chance of it not going flying into space when it is underneath the deck and aprons of a brush hog attachment.

 

[frank_f15]

i think you will just waste time and material for something that wont work, IMHO just bite the bullet and get a real rotary cutter.

 

[PILOON]

Hey Guys, thanks!

My design is now to be adapted from PoohBear's link (maybe more reach) and I can derive the required speed (thanks RonMar) with a pully arangement easily.

I did , a few years ago, attach steel cable to my Huskvarna weed wacker and stopped fast when I realized that I was hurling bits of cable hither and yonder.
I now use the square nylon (biggest available) lengths in the gripper head with excellent durability.
At first I would fill my pocket with precut lengths expecting to use up a good handfull per drive side, but no they last quite long. In fact I believe I merely used 4 or 5 lengths.

My plan will now be to make a 3 point mount for the gear box and a side arm as per PoohBear's link (essentually an offset channel box) with a quill arrangement to mount the holder head.
In fact I might even purchase the conversion adaptor as sold for my Husky and attach it to the quill. (the quill being a shaft and 2 bearings, unless I simply buy one)

It can't work any worse than my big husky weed wacker does, only I can sit and drive rather than walking! (Getting old--er and lazier)
Also I can raise or lower at will with the 3 point.
Also for ditch management the offset arrangement is COOL!

But the info I needed was RPM's, so thanks guys!

Oh, yes I'll post pic's, probably as a finished project with stages.

Heck I could also adapt a 16" saw blade for bigger stuff! Why not? I do that now on my Husky wacker. (12" blade)
Using an offset belt drive I like as I feel that this is somewhat a safety feature as with 'just right' tension (OK, now I need to have a tensioner=easy enough) I can introduce controlled slippage to prevent sudden stoppage in the event I hit a stray piece of rebar for example.

Sure, I'll have safety shields etc as I know what it is like to have pebbles etc hitting the legs (even dry twigs hurt).
I found that something like HD inner tube rubber or carpet does an excellent job in absorbing/deflecting flying bits and pieces.

 

[gemini5362]

I will be eagerly awaiting to see your results.

I also use the largest multiside plastic pieces that you can get for a weedeater on mine. If I am cutting in anything that has any kind of large brush in it or big weeds even. I have gone through a dozen strings in a couple of hours. I usually buy a row of the string then cut it to lengths to thread in my unit. Like I said I am eagerly awaiting to see your unit and find out how well it works

 

[dqdave1]

Maybe chainsaw chain instead of nylon.

 

[PILOON]

Maybe chainsaw chain instead of nylon.

That would scare me as one link at the required rpm would result in about the same velocity as a .22 cal bullet, remember, I tried using wire rope on my string cutter and was finding bits of wire imbedded in my trousers and exposed skin.

If anything I'd prefer a saw blade as blades are designed for reasonably high RPM usage plus there are thousands of weed wackers running them.
Also a blade will cut weeds no problem with advantage of also handling the weeds reeds etc.(might make for more slimy gooey juice than a string however)

In the last hour I was gathering the components that I would use to fab up my 'tractor wacker' and it should not be a very long project to assemble.

The most difficuly will be fitting a suitable pully to my gearbox, but worst come to worst I might simply weld one to the gearbox output shaft as afterall it will become dedicated to the project anyway.