Flail Mower Let's talk flail mowers

[DieselBound]

Relevance of rotor diameter?

I'm [back to] looking at flails [now concentrating on one for my NX5510 rather than the B7800]. Wondering if there's any significant quality and performance issues related to different rotor diameters. I realize that knife/cutter tip speed is, given same rotor RPMs, greater with a larger diameter rotor. I can understand rotor tube thickness being an issue as well (thicker being better). Just don't know that if there were two candidates of flail with their only difference being that of rotor diameter which would be preferable (why?).

 

[Jchawk]

Man this sucks. Where did you purchase this? I was looking at similar ditch mowers but now I think i will stick with hydraulic side shift. Weight for those heavy duty ditch mowers would have been an issue anyway.

I'd rather not say, yet, depends on how they handle this.
It was shipped across country, so whatever happens I'm sure will be a headache.

 

[Ricn]

Shipping makes warrantee a pain for us west coast folks; hate that everything comes from the east, even if made in china! I had a bearing fail in my new flail and although the seller would warrantee the thing, he would not pay shipping so changed it out here. No biggie but had to dismantle the drum and such to change it out.. Not sure why it failed, bad bearing or, more likely because I over greased it. Really like this unit, no regrets.

 

[Fallon]

Re: Relevance of rotor diameter?

I'm [back to] looking at flails [now concentrating on one for my NX5510 rather than the B7800]. Wondering if there's any significant quality and performance issues related to different rotor diameters. I realize that knife/cutter tip speed is, given same rotor RPMs, greater with a larger diameter rotor. I can understand rotor tube thickness being an issue as well (thicker being better). Just don't know that if there were two candidates of flail with their only difference being that of rotor diameter which would be preferable (why?).

Bigger diameter will be stronger, not that it is a relevant measure in this situation. Driveshafts are hollow rather than solid because most (over 90% I think) of the torque transmitting strength of a shaft is the outside. So they make them hollow to safe weight & increase performance. For a flail though, you are more concerned with running into stuff & denying it than torque transmission. So all things being equal, thicker walls will be better than a bigger diameter.

Mower (not just flail) tip speed is reasonably optimized already. Much faster & the centrifugal force on the blades makes self destruct even without hitting the occasional rock. You can make longer knives to match a smaller rotor & maintain the proper tip speed.

 

[DavesTractor]

Re: Relevance of rotor diameter?

Bigger diameter will be stronger, not that it is a relevant measure in this situation. Driveshafts are hollow rather than solid because most (over 90% I think) of the torque transmitting strength of a shaft is the outside. So they make them hollow to safe weight & increase performance. For a flail though, you are more concerned with running into stuff & denying it than torque transmission. So all things being equal, thicker walls will be better than a bigger diameter.

Mower (not just flail) tip speed is reasonably optimized already. Much faster & the centrifugal force on the blades makes self destruct even without hitting the occasional rock. You can make longer knives to match a smaller rotor & maintain the proper tip speed.

Great input Fallon. I know on rotary mowers blade tip speed is set by ANSI standards to 19,000 fpm. That might also be the flail standard, I don't know. If so, and as you suggested, the circumference of the actual cutting circle can be made the same despite differences in the rotor diameter buy using longer knives or hangers. It would seem that a larger cutting circle, by whatever method, would allow slower spindle and gearbox output speeds. That can't hurt the longevity of bearings and may reduce heat. A couple of the more expensive brands built on the west coast are Rears and Vrisimo. Both are mowers meant for every day, even all day, mowing in orchards. These mowers get hundreds of hours a year on them. Rears uses a much larger rotor, larger housing etc. Vrisimo uses a more conventional size. I've always wondered about that. Would there be more of a beneficial flywheel kinetic energy effect when hitting tall grass with the larger rotor? Would there be more space "under the hood" for chewing up thick or tall grass? I think these are other considerations that often come along with a larger rotor and bigger housing overall. I generally run a Vrisimo since we are a dealer for them, but in really tall or thick grass it can really consume the horsepower trying to cut up and eject the clippings and I have wondered if a larger rotor with more room under the hood would help in that regard.

 

[DieselBound]

Fallon & Dave,

Yes, great input!

I was aware that tip speed can be altered based on knife/cutter length. It's hard to find info on actual knife/cutter length.

One flail that I'm closely scrutinizing has a 5" diameter rotor (on the lower end of diameter for the size I'm looking at) which is 8mm in thickness (a little on the thin side compared to other flails, though it's hard to get such specs from others).

Larger rotor might provide more energy, though I'd figure that heavy (and perhaps longer) flails on a smaller diameter rotor could be comparable. The unit I'm looking at has hammers that are about 3.3 lbs each (18 flails).

Interestingly, the unit I'm looking at has a 7 3/4" dia roller. Large compared to a lot of comparable flails. Not sure if larger is better in all conditions, but for me I'd think it would be preferable because I have a lot of uneven ground (some ruts and such). Backing up would also likely be easier, not that backing up with a flail is encouraged.

I don't believe that there's any "optimal" as it really depends on one's use/environment. I'm just trying to figure out what is likely to work best for me: hard to part with a big chunk of money with fingers crossed! (I like to try and match up as best as possible so there's no surprises/disappointments). I don't plan on ever parting with my 6' rotary: it gets the job done no matter what I'm facing; just looking for something bigger that is more biased to cutting grass (as I open up to more grass areas).

 

[DavesTractor]

All things being equal, I'd rather have a larger rear roller to keep it from reacting as much to little dips and such. You can create a washboard effect if you mow a lot, mow real low and follow the same direction each time. The roller will drop into a dip which also lowers the cutters, then come out of a dip which raises them. The distance between your roller and cutter circle will be the distance between your original dip and the one you just created by scalping into the dirt. And it happens over and over again. I used to flail mow an area of our place down to about 1/2" and over the years created some areas of washboard. There are obvious ways to avoid that by mowing higher or going at a different angle. We recently switched to a Jacobsen HR5111 11' mower for this area (6'ish acres of irrigated "lawn") and find that I can mow a little quicker and get a little better finish, but obviously not as versatile as a flail.

 

[jthibodeau89]

So how do you get the Deere triangular clevis's loose from the knives? Are they made as one unit and you have to replace the two knives and the clevis all at the same time OR is the Deere clevis made with a gap (hidden by the strap in your picture) ??

The d-ring (clevis) has a gap hidden under the strap. The gap is just wide enough for 1 blade.

The d-ring swings in the strap, thus the strap can wear and drop the d-ring and blade assembly. There were many missing scoop knives and d-rings on the 25A when i bought it. I changed to side slice knives for cleaning up my weedy hill pastures.

On the 390 I lose 1 or 2 blade stations/year mowing about 25 acres 6 times/year. I personally prefer the bolts over a cotter pinned pivot. Judging by the dings and nicks on the bolts and rotor rib, i can see the cotter pin getting broken fairly easily.

 

[DieselBound]

[in response to Dave's last post]

I have a fair amount of trees. I'll be pushing it with a 7'+ flail; having hydraulic side shift is the mitigation.

I do mow in different patterns from time to time. Just a lot of uneven ground no matter how I look at it. I have done leveling on some really bad ruts (from skidders): those were dealt with earlier on with my B7800 and a 5' box blade - A LOT OF WORK! I'm pretty much making pasture as I go/can.

 

[Fallon]

All things being equal, I'd rather have a larger rear roller to keep it from reacting as much to little dips and such. You can create a washboard effect if you mow a lot, mow real low and follow the same direction each time. The roller will drop into a dip which also lowers the cutters, then come out of a dip which raises them. The distance between your roller and cutter circle will be the distance between your original dip and the one you just created by scalping into the dirt. And it happens over and over again. I used to flail mow an area of our place down to about 1/2" and over the years created some areas of washboard. There are obvious ways to avoid that by mowing higher or going at a different angle. We recently switched to a Jacobsen HR5111 11' mower for this area (6'ish acres of irrigated "lawn") and find that I can mow a little quicker and get a little better finish, but obviously not as versatile as a flail.

I definitely concur. Bigger tires make for a smoother ride. A roller will be the same. It will ride over smaller rough patches smoother the larger it is.

No matter what, standard rotation flails wont cut for beans in reverse. Not sure if it's the roller pushing stuff down or the flails going the other way compared to the material feed. But on both my flails they only cut 30% or so in reverse.

I think worrying about rotor diameter & knife length might not be the most useful metric. They tend to go towards the top of the permissive range. If you have a smaller rotor you can get clevis mounts or longer knives to compensate. I'd assume the more room between the rotor & the hood the less HP it would take. But it would also decrease the mulching action a bit. The same goes for more knives. Even if it's only the vertical part not the cutter tip, the more there are the more mulching that would go on & the more HP required.

Hammers & duckfoots have a lot more surface area so should have more drag & eat more HP. Not sure how the relatively flat surfaces mulch compared to many smaller side slicers though. My Peruzzo with hammers eats noticeably more HP than my ancient 917 with side slicers, but it's an apples to oranges comparison. I dont notice that much of a difference in mulching though.