7018 Vs. 7018AC, Need Layman's translation please:

[Mark @ Everlast]

I think there is equal reason to run at a full 540 so you can cut cleanly through instead of bouncing off and the blades swinging back under or bending.

The problem with newer import tractors is that they reach 540 at near maximum operating speed of the engine. It puts more stress on drive line components in the tractor.

However, high engine speed (up to the maximum pto speed) and low ground speed has always been considered the safest way to 'hog.

 

[Shield Arc]

I have a question about heat and tempering and possible blade failure: If, one were to heat the offending blade up to straighten it out, is this some kind of major no-no that would change the metallurgy and make it brittle and therefore turn it into a flesh seeking missile of some sort?[/SIZE

I certainly don't recommend hardening a lawn mower blade, (just to cheap to worry with) but a brush hog blade may be different. I have a brush hog but haven't used it in 8 or 9 years. But here is the procedure I use to harden carbon steel, first preheat the area too 1100-degrees (dull red) and let free air cool slowly. This removes any stress in the material.
Next reheat the same areas to a dull red then quench in oil until cool. Used motor oil works well as it adds a small amount of acids at the same time. This method also surface hardens the steel. Reheat the steel area again too 500-degrees then quench in oil until cool. This quench adds temper (return flex) to the steel.

 

[Soundguy]

and remember.. if he simply heats it to bend it, then lets it slow cool.. it will actually soften the metal up a tad.. ie.. annealing to improve ductility..

soundguy

 

[rjmack]

Reheat the steel area again too 500-degrees then quench in oil until cool. This quench adds temper (return flex) to the steel.

that's called 'chasing the colours' and 500* is way too hot.

you only chase it to straw, and light straw better than dark.:thumbsup:

i have done quite a bit of that building custom chisels, but i would think that a hog blade would be fairly tough to do without an automated process of some sort.

edit: i just checked a colour chart and i guess 500* isn't really all that much hotter in terms of degrees (it's been a long time) but it really does make a big difference in terms of colours, and that is a big deal. even at dark straw you are risking stess fractures just from the quench, never mind the first time you hit it with a hammer.

 

[Chilly807]

I don't ever mow at the 540 pto speed rpm on my tractor. Most of the time, I'm hardly above 1.400 RPM's. I can't crunch the numbers for knowing the exact engery contained in the blades, but I always feel better not having them spinning at max rpm. It has to lower the danger by a decent margin, at least that's what my gut tells me.

My tip speed at 540 is 16,363 feet per minute, which translates to over 186 miles per hour. I often mow slightly under or slightly over 540, but it's usually pretty close. A 10 pound chunk of sharpened 3/8 or 1/2 inch high carbon steel at that speed is a nasty customer.

Sean

 

[Soundguy]

mowing at rated speed is most efficient. I'd not practical to mow at a speed so low that your blades are no longer dangerous. you get diminishing returns.. slower blade.. less energy.. less cut quality, less suction, and the machine is out of it's power band.. AND the blade is still spinnig fast enough to be a deadly missle IF it broke.

you want near 100% safe.. get a weed whacker...

soundguy

 

[Big Wave D]

I usually run around the 1800 RPM mark. When I get into thicker grass, or, especially, shrub-sapling-briers I wick up the RPM's to very near 540 speed.

Most of what I'm dealing with is fallow fields with more woody weeds than anything. I'm more wanting to knock the weeds down. I don't really mow much during growing months so it's not particularly thick or heavy mowing.

Running at slower speed allows me to stretch my 'diesel dollar' and time afield a bit. Every little bit counts.

 

[Mark @ Everlast]

"lugging" a high rpm diesel to save fuel is hard on bearings...

 

[Big Wave D]

Thanks Mark,

I'll remember that one. What I've been bush hogging over the past month or so is hardly lugging anything at the moment. The middle of July, it'd be another story I imagine.

 

[Gary Fowler]

I think someone is confused about 500 degrees. If you are talking F then you aren't doing anything useful or harmful. Until you get above 800F there is absolutely no risk. 500c is 930F and you are approaching the point where rapid cooling could make some slight changes. In the annealing process, you want to rapidly cool the specimen by immersion in water or spraying with water to quickly cool from 1300 F or higher to below 800. This softens the steel which you would want to do to a cutting blade. Surface hardening will occur when you surface cool (dipping quickly in and out in oil or water) and then allowing the internal residual heat to heat the surface again then repeat the dipping process till the specimen is cooled below 800F. The constant heating (expanding) and contraction (cooling) of the molecules make them shrink closer together thereby making the iron more dense at the surface and much harder. Add that to the transformation of the carbon in the iron to carbides when it is cooling and you get hardened steel. Higher carbon steel will make for harder and more brittle product. If you want to straighten carbon steel, you can heat it to 1000F which should be a dull cherry red without damaging the temper if you allow it to air cool or even bury it in sand so it cools slowly. 1000F is low enough that you wont significantly alter the mechanical properties.