Grading Finish Blade test - the blades

[Charlie_Iliff]

Attached is a picture of the blade I cobbled up, compared to the stock PT rough cut blade for the 1845's 72" rough cut mower. The purpose of the exercise was intended to be development of finish blades which will do lawn quality cuts, but not self-destruct when encountering a little hump in the soil. (See broken mower blade thread.)
The plan was to put four on each stump jumper, 12 in all, instead of the two per spindle on the rough cut system. The PT finish blades are four per spindle, but rigidly welded to a ring, not free swinging. I'll attach a pic of that to the next post.

 

[Charlie_Iliff]

For further comparison, here is one of my blades and a rough cut blade lying on the factory finish blade ring. The factory finish blades are 1/8" thick, my blade is 1/4" thick, and the rough-cut blades are 3/8".
I selected a Gravely blade that would have about the same drop from the plane of the disk as the stock finish blades. In the photo, the ring is upside down. The blades are offset downward from the stump jumper something over an inch. The Gravely blades have a little less offset, but more "lift" - more bend upward of the trailing edge.

 

[Charlie_Iliff]

Re: Finish Blade test - failure # 1

So I mounted the 12 blades and attacked a horse pasture, where grass growth has been slow, some areas are grazed to dirt, and some weeds have grown a foot. One single-stem weed, particularly, was up a foot or so, thinly scattered over the field. To my dismay, more often than not I ran over the tall stem and it popped back up in my wake as if I hadn't been there. The cut was rough throughout. I did not have much fairfly smooth grass to try, but it seemed better cutting tall fescue back from 8" to 5" than it did on taller weeds, etc. I had the mower set high. Clearly, the very high lift blades were creating pressure distributions under the deck which flattened a lot of stuff below the blades, rather that picking it up for even cut. I learned something, I guess, but I'm not sure yet what.

 

[Charlie_Iliff]

Re: Finish Blade test - Failure # 2

The temperature when I started mowing was 94. With the rough cut blades, I have been able to mow with that temperature, but the machine running hot. With the factory finish blades, I had one genuine overheating incident. With my special super high lift swiveling set, The needle started up in 15 minutes, and would have gone all the way to engine meltdown if I hadn't stopped at 1/2 hour, 240 degrees, and the anemic alarm sounding, but inaudible even with Pro-Ears until I shut down the mower.
With this experience I did learn something. The higher the lift on the blades, the more the dust and grass fragments are blown out from under the mower, and the faster the oil cooler plugs up.
My first overheat incident was with the factory finish blades and the deck fairly low. A lot of stuff was blown out the front in that configuration. This test was with the deck high, and most was blown out the back, underneath the machine.
As testing progresses, if I don't lose interest and let the weeds grow, I'll try the deck tilted forward and back to see if it makes a major difference.
So, after the overheat, I drove it a mile, blew out the oil cooler, removed half the blades, and went back to mowing. [Continued next Post]

 

[Charlie_Iliff]

Re: Finish Blade test - qualified success

After removal of half of the new blades, so I had just two per spindle as the rough-cut blades are mounted, the deck cut much better, and in fact was a bit smoother cutting at reduced throttle than at full chat. More importantly, the temperature stabilized at 220 and stayed there for an hour. Outside temp was still over 90. The amout of debris blowing out the back was reduced enough that accumulation on the oil cooler was significantly less than with 12 blades. So, I now have 2 sets of super high lift swiveling finish blades.
I'll have to wait for the fall growth spurt to try the blades on the lawn. My suspicion is that PT 4xx owners and Scag, Exmark, etc operators will not be switching to PT 1845's with Iliff custom blades for smooth lawn work. It may be, however, that if I am actually cutting only a small bit off the top of already smooth grass, these blades will do better than the rough cut blades. For horse pastures, I have to declare my experiment a failure, and remount the 3/8" thick rough cut blades. They have minimal lift, and the engine will run a lot longer without overheating.

 

[Bird]

Re: Finish Blade test - qualified success

Charlie, that's an interesting experiment. Kinda makes you wonder whether more blades are better or worse; sounds like worse. It reminds me of the instructions with my DR trimmer/mower. It uses two strings, but you could just as easily put 4, or more, strings on it, but I haven't tried it with 4 because the manual says "Please note that installing more than two cords at a time does not improve trimming performance - in fact it can lessen it."

 

[wasabi]

Re: Finish Blade test - qualified success

<font color=blue> I have to declare my experiment a failure </font color=blue>

At least you (and we) now know more than before.....that makes the experiment quasi-successful in my book. Thanks for reporting it in detail.....thought provoking!

 

[MossRoad]

Re: Finish Blade test - qualified success

I'd think that even with two blades, as you increased the pitch of the blade to gain lift, the wind resistance would increase, thus loading the engine down more and increasing your temperature. Add two more blades and you are doubling the amount of air that the blades have to push, etc...

The same thing would happen with a string trimmer. More strings = more wind resistance = more load on the engine.

Am I thinking correctly here?

 

[Charlie_Iliff]

Re: Finish Blade test - qualified success

Bird:
Certainly more blades was not as good in this particular application. Over on Lawnsite, I see a lot of claims by those using "doubles", often with two blades of different lift characteristics. Most claim better finish cuts. With my deck, with no contouring for air flow, and a lot of volume taken up by stumpjumpers, my working theory is that the "lifted" air came back down where it pushed the grass out of the way of the blades. I also have a DR string trimmer. I think more strings slows it down too much. I don't think I have that problem with the mower spindles, but at some point I might put some kind of tach on the deck.
Now, if I can just figure out what it is I'm learning so the next step is an improvement .../w3tcompact/icons/smile.gif

 

[Charlie_Iliff]

Re: Finish Blade test - qualified success

David:
You're certainly right that more blades result in more drag, and more pitch (lift) results in more drag. The highest drag system I tried was with the factory finish blades with the deck low - at a height for a showoff lawn, that Bubenberg tells students never to do because it hurts the grass. Certainly that caused the most heat, until my 12-blade run yesterday. The stuff I was cutting with the stock blades was a lot thicker than yesterday, however, and I think would have been signiicant drag even with the rough cut system. Yesterday, I overheated it even at part throttle, where I have been able to cut heavy stuff wide open with no problem if the oil cooler was clear. I think that load is certainly a factor, but less of one than even a 20% blockage of the cooling air flow.