Dual Blades on Finish Mower

[TractorGuy]

I've been wanting the try out the dual blade idea on my finish mower for some time. My current blades didn't really need sharpening but I touched them up while they were off. I sharpened an old set and put them on top of the better blades. I tack welded them together to keep them square if I hit something.

The tractor is a 3 range hydro and the mower is a 6' Agmate. I usually have to run about half speed in the mid range for a decent cut. With the double blades I was able to get about the same quality cut at full speed in mid range. I can only run full speed over about 2/3 of my property but that should cut mowing time down a bit.

I eventually plan to make some blades using disc mower blades. Not sure if I will do it using bars or a round plate. I would prefer a round plate. You can get a dozen replacement blades for about $20 with those. Not sure how long they last compared to regular blades so that will be another experiment.

 

[airbiscuit]

I'm looking forward to seeing your disc blade approach.

 

[Doughknob]

Did you bend those dual blades so they both cut at the same height?

 

[TractorGuy]

Did you bend those dual blades so they both cut at the same height?

No I just stacked and tacked. I didn't bother to check for evenness. Identical blades would only be a 1/4" different but these are two different brands so they may have a different offset.

The top blades are original and the bottom blades are Caroni.

After mowing the whole field I wasn't as impressed as I was to begin with. I did push it as fast as possible to test results. I am finding some areas with uncut strips so I will probably have to slow tractor speed next time if I let the grass get that tall.

 

[Randy Gann]

Check out Meg-mo blades. They are too expensive for me. If too expensive for you — this might be a good design for you to copy and fabricate your own.

 

[Doughknob]

You tried running the tractor at higher than the rated pto rpm right?

 

[TractorGuy]

You tried running the tractor at higher than the rated pto rpm right?

A little but not by much. I usually try to run it at the next mark beyond the PTO mark on the tach.

It tends to drop RPMs as I mow and I have to keep bumping the throttle up. I need to look into adjusting the tension on the throttle.

Ideally you want to increase blade speed but most PTO finish mower boxes don't spin the blades like a quality zero turn would. I installed a larger pulley to increase blade speed a little but I would like to spin them much higher.

The double blades DO put more stress on the engine in high grass. You can feel and hear the engine working harder. This grass was overgrown and thick. I was laying a pretty good mat of hay down.

 

[Doughknob]

I cannot see the belt configuration in your pics. But if you mean you put larger pulleys above each blade, that would decrease blade speed.

 

[LD1]

I cannot see the belt configuration in your pics. But if you mean you put larger pulleys above each blade, that would decrease blade speed.

Since he said a larger pulley and NOT larger pulleys....I took that to mean the drive pulley and not the spindle pulleys. So that WOULD increase blade speed.

I too have messed around with double blades.....and different deck designs react differently. I see yours is a rear discharge.

There is no doubt in my mind that double the number of cutting edges HAS to be an improvement to cut quality. But....I am not sure about double the air flow with twice as many high lift blades. If you try to "lift" more air than can easily be discharged out the back....the air has to go somewhere. And what I have found is that it tends to bounce off the deck and blow down and out all sides. Which is the opposite effect of what you want to"suck" the grass up. Too much airflow/turbulence isnt a good thing.I have found that dropping one or both sets of blades to a low-lift type does alot better. Currently I have a set of standard lifts paired with gators (muching blades done have nearly as much lift) and it is about as good as it can get in terms of cut quality, clipping distribution, etc

What do the clippings look like? Do they look chopped up finer than before?

 

[mark in portugal]

I did the same thing; went from 2 single bars to 2 crossed bars, doubling the blades.
My deck is bigger though. 1/2 ton and 9-1/2' wide.

I figured the only reason not to was if there wasn't enough torque available to do the extra work.
The deck maker says 20hp, and my tractor has 85Hp, so there's some spare.
Anyway, I now mow about 1-1/2x higher wheel speed. it saves about an hour each time I mow the grass.
I also use the deck as a brush cutter for the outer fields and it does bog down a little in high pto gear but it works ok.

 

[CADplans]

Gravely sells an accessory to "X" two mower blades on this type deck,,

I have two blades on this machine, even though it is mostly used for rough cutting.
The machine mulches the stuff I cut much finer,,
and it makes this "bushhog" mow grass much finer.

The Meg-Mo is exactly what you envision creating.
Buy the Meg-Mo,, then you do not have to worry about your design failing, and throwing disc mower blades out of the mower.

 

[WranglerX]

No I just stacked and tacked. I didn't bother to check for evenness. Identical blades would only be a 1/4" different but these are two different brands so they may have a different offset.

The top blades are original and the bottom blades are Caroni.

After mowing the whole field I wasn't as impressed as I was to begin with. I did push it as fast as possible to test results. I am finding some areas with uncut strips so I will probably have to slow tractor speed next time if I let the grass get that tall.

When I had issues of quality of cut I always need to slow down and actually let blades do the work....

 

[CADplans]

After mowing the whole field I wasn't as impressed as I was to begin with. I did push it as fast as possible to test results.

I am finding some areas with uncut strips so I will probably have to slow tractor speed next time if I let the grass get that tall.

The issue may not be your blades, single, or double,, the problem may be your V-BELT,,
The horsepower rating of the v-belt is probably WAY below 27HP,, so your belt is probably slipping.

When I flail mow with my 90" flail mower, and 60HP,, if the speed is too high, and the grass too tall, the flail mower belt slips.

My SIL goes through two v-belts a season, on a 72" mower behind a JD 855,,
I am convinced he is simply driving faster than the v-belt can handle.

I have used the same mower behind my tractor, and never wore out a v-belt.

So, the condition of the v-belt may be worth checking,,,

 

[TractorGuy]

Belts are good 5L size. The design is very simple. It has two identical belts. Each one goes around the drive pulley, center pulley, and one side pulley. Both are tensioned by adjusting the gearbox in relation to the center double pulley. I check them pretty much every time I mow and they haven't felt hot like they are slipping.

I have installed a larger drive pulley on this mower from a smaller version of it. This winter I am going to replace all the spindle bearings and I plan to install the smaller driven pulleys from the smaller mower. As long as the belts don't begin to slip this will increase the blade speed even more.

It isn't a case of the mower not providing a quality mow. As long as I adjust my travel speed to the quality I want it will mow fine. I just like to modify to try and improve the cut at a higher rate of travel.

 

[dadohead]

Wow... I'm impressed with all of your experimentation! In my 25 years as an engineer at Deere, I spent about 8 years in the mower group developing finish mowers. That's how it's done... experimenting. It's a "black art".

I made Senior Engineer when I was a 'deck head' and immediately set out to bring the craft into the 21st century. There HAD to be a better way than build expensive prototypes and cut grass! We certainly had finite element analysis (FEA) capability at Deere but modeling compressible gasses is quite a specialty. I connected with engineering department at a top university. They were leaders back then at computer modeling and predicting helicopter rotor blade designs. I gave them the models for a 3 spindle mower that we knew a lot about... how it cut, it's weaknesses, etc. After 3 months (and $$$) the results: "wow.. this is a hard problem." Yeah... tell me about it! The phd of the department said something like this: "we model helicopter blades for the government... it's hard but we get results. YOU have THREE helicopter blades running next to each other, surrounded by a shroud, and introduce mass (grass) into the system!... impossible!!!" We tried some of their suggestions with awful results. Soooo... it was back to 'black art' method of building fiberglass decks, removeable baffles (cardboard and duct tape), many prototype blades, traveling to wherever the grass is, and testing. Mowers are tricky and expensive to design.

First off... what you made and tried is gospel. Your test results trumps any suggestions I could offer. But I have a few:

Be fair with your testing. You need to run each change at the same ground speed and height of cut (HOC) as your baseline. Working by yourself or using a hydro tractor make it really hard. Hydro's lead you to the speed they like to cut at (by sound and engine power feedback). We always worked in pairs so one guy could run a pace tractor or cut with the baseline mower itself. THEN we compared the results. Many skip this part. European dealers LOVE to experiment. EVERY time we visited a dealer over there they rant about our products then bring out the "magic" mower they have improved. GREAT! I really do want to see it! But EVERY time when we get the mowers at the same HOC and run and same speed together... the proto loses the 'magic'.

Know your numbers. Several numbers you will want to know: blade tip speed (ft/min or fpm) and blade pass frequency. These are particularly valuable when you start "X'g" the blades. Rotary cutters typically run at lower tip speeds: 12000 fpm to as high as 16000 fpm. 12000 fpm is the threshold where impact cutting starts; below that... you're just 'combing' the grass. ANSI limits consumer and commercial mowers to 19000 fpm for safety... that is screaming! Commercial mowers tend to run really high tip speeds for obvious reasons (that's why they need 30+hp to drive the mower too). Mower blades are axial fans. My GX/48C runs at about 16000 fpm tip speed @ WOT; sometimes the OEM publishes that data... sometimes you have to measure. Borrow or buy an optical tach and measure the spindle rpm... then tip speed is a pretty easy calculation. I can help if you'd like. Just know that as you speed up the mower (or add X blades) the power draw will go up exponentially. Knowing my 48C blade is 17" long and assuming a 4 mph pace... the blade pass is .59"/cut. X'g will cut that number in half but dramatically increase power consumption. So in super lush areas... I just slow down to 2 mph and get the same result.

Mowers are tricky. You'd think that going super high lift (or X'g the blades) would really increase lift. It doesn't. The air chamber speed goes up (and the discharge is spectacular) but cut quality usually suffers. Same with counter rotating blades... air speed goes up but lift goes down. Mulching is a good reason to X blades. Grass stays in the chamber and gets aggressively recut. Piranha/Gator type blades do the same thing for much less money though.

We would grade cut quality by viewing each part of the cut path separately: forward sweep (portions of the cut where blade(s) are travelling forward... can be weakness), between blade (high chamber air speed will often leave between blade striping), tire track (typically the worst part of a cut). The power unit's tires bend the grass blade forward prior to the mower. This is where ZT mowers shine. The designer can use deep decks and place the caster wheel in the strongest part of the cut... huge benefit. 3pt mowers have a big disadvantage in this regard. The mower location is pretty much determined and placed behind 2 BIG tire tracks. My guess is that you're seeing results from the 'forward sweep' portion of a mower in-line with the tire tracks of the tractor. The mower just can't lift the grass well enough to cut it.... it needs the 'back sweep' portion of the blade to mechanically lift and cut it.

Wow... I went long here. Sorry about that. I get excited when guys start talking cut quality. Hope this helps!