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OT optimal cutting speed and feed rate for lawn mower blades?

Freedommachine

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May 13, 2020
Ok, I cannot be the only one here who has thought about this while mowing grass, the mind wonders to some interesting places.

I have a lot of grass to mow, 5 acres at least. I recently picked up a well cared for Bolens DGT1700 from my local auction house. It is a well built little machine with a 17hp 3cyl Mitsubishi L3A diesel engine; 2 speed rear end; hydro drive and a beefy, shaft driven fabricated deck. There is only one problem, it is absolutely terrible at cutting grass!

The mower was advertised with a 72" deck; which is why I wanted it. Come to find out, the deck does measure 72" but it only has a 60" cut. They designed the discharge chute as an extension of the deck and mounted the landing gear out on the end. (Notice the "cutting line" sticker in the pic below)

P5121329 (1024x768).jpg

The blades were well past used up and replacements are no longer available. I ordered a set of 60" blades for an exmark off Amazon and machined them to fit the Bolens; that part was easy. I only mention it to provide some backstory.

I thought the new blades with "high lift" edges would generate more lift to pull the grass up and cut it all even; it was a little better but overall, not even close to good.

This got me thinking; I can look up specific cutting data for just about anything that cuts - at the very least, surely some bored engineer has made a YouTube video on this topic. However, to my surprise there isn't much available.

There does seem to be some lawn care forum folklore about 19000 SFM being the ANSI standard; although no one ever cites which standard this is listed under.
If I had to guess, it probably comes from basic direct drive push mowers. A 21" push mower spinning at a governed engine speed of 3600 RPM will have a cutting speed of 19,807 SFM.

My next theory was that because this was the only diesel model that used this deck, maybe the diesel is rated for a lower rpm and thus wasn't turning the blades fast enough. I checked the specs for the Mitsubishi L3A and it does run at a governed speed of 3600 rpm.

Nevertheless, I ordered a Non-contact tachometer from scAmazon and set out to get some hard numbers.

At full throttle, the little diesel runs at 3450 rpm. I checked the blade RPM from one of the spindles; at full throttle, the blades are spinning about 2400 rpm.
.262 * 2400 * 20.5" blade = 12,890 SFM.

As a comparison, I checked the engine speed and blade speed on my John Deere with a 48" deck - This mower cuts well.
Engine speed =3600rpm
Blade speed = 3600rpm
.262*3600*17.125" blade = 16,152 SFM.

The next question; how much difference does 3262 SFM make in performance? To test this, I dropped the JD mower down to 13,000 SFM and took a cut. The difference was night and day; I had to drop my feed rate down significantly and the cut quality was poor.

I still need a method to accurately measure wheel speed in order to find the correlation between cutting speed and feed rate. However, I am getting somewhere.

I think the next step will be to replace the spindle bearings in the Bolens and give the deck a once-over to ensure everything is in the best possible condition - then comes the next logical test... I will come up with a drive pulley combination to increase the blade speed to 4000 rpm - or 21,484 SFM and see what happens 😁.

Of course there are other factors at play here as well. Most decks are built with baffles in order to direct the flow of air underneath the deck. My JD has them, the Bolens does not.

Edit: I found a pic showing the Bolens deck does have baffles. My mistake, it has been a few months since I had the deck off. Notice this one has the "wheel kit" mine has the ski type design shown above. The front skis suck and will be replaced with wheels in the future.
Bolens deck:
PB280011 1.jpg

John Deere deck. It isn't the 48" but same design:

bm20505.jpg

Engine torque, depth of cut (grass height), wet or dry grass... like I said, plenty of other factors. If I tried to take them all into consideration at once, this would go nowhere. Being a machinist, I understand the math behind cutting speed and feed rate - based on the sample size of two mowers; this appears to be the largest factor in increasing feed rate while maintaining cut quality.

If anyone wants to participate in this curiosity driven experiment, if you could measure the cutting speed of your blades and post it here, that would be helpful. I am really interested to find out the SFM of the blades on some of the newer zero turns. IIRC, the Hustler hyperdrive can cut at something like 18 mph.

First person to tell me how dumb this is gets a gold star. 😉
 
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"...At full throttle, the little diesel runs at 3450 rpm. I checked the blade RPM from one of the spindles; at full throttle, the blades are spinning about 2400 rpm. .262 * 2400 * 21" blade = 13,204 SFM..."

Is the picture of the deck with two blades what the Bolens has? Are you saying the Bolens has 21 inch blades? I found that the Exmark 60 has three 20.5 inch blades. Then what does the picture represent? Please explain.

Larry
 
"...At full throttle, the little diesel runs at 3450 rpm. I checked the blade RPM from one of the spindles; at full throttle, the blades are spinning about 2400 rpm. .262 * 2400 * 21" blade = 13,204 SFM..."

Is the picture of the deck with two blades what the Bolens has? Are you saying the Bolens has 21 inch blades? I found that the Exmark 60 has three 20.5 inch blades. Then what does the picture represent? Please explain.

Larry
The deck represents Baffles.

The best blade speed depends on the dryness of the grass. Wet grass takes more speed.

Your comparison is without value unless the conditions are controlled.

Make sure the leading and trailing edges are at the same height. Double cut means a lot!

Tip speed is vital, but there is over three inches of active sharp blade. So the number is an average at best.

Depth of cut (feed rate) means everything.


I've gotten into the habit of running the chipper shredder at nearly double the 540 rpm design spec. Works great

Maybe try increasing mower blade speed through pulley size.
 
To cut requires force against what you are cutting. Sharp edge concentrates force to exceed ultimate strength of material. But blades of grass are flexible and light.
So there are two ways to create force against grass to cut it. One is to back it up with a stationary object like the bed-knife of a reel mower. The other is to hit it FAST, because the force the grass-blade sees is determined by how massive it is and how fast the impinging mower blade is trying to accelerate it

So, speed is of the essence.
 
"...At full throttle, the little diesel runs at 3450 rpm. I checked the blade RPM from one of the spindles; at full throttle, the blades are spinning about 2400 rpm. .262 * 2400 * 21" blade = 13,204 SFM..."

Is the picture of the deck with two blades what the Bolens has? Are you saying the Bolens has 21 inch blades? I found that the Exmark 60 has three 20.5 inch blades. Then what does the picture represent? Please explain.

Larry

Larry,
My apologies, I have added pictures for clarification. I suppose that's what I get for trying to write this up at midnight lol.

You are correct about the blades as well. I just checked my Amazon account, these were the exact blades I purchased, they are 20.5". I will go back and correct the math.



The best blade speed depends on the dryness of the grass. Wet grass takes more speed.

Your comparison is without value unless the conditions are controlled.

Technically speaking, yes the moisture content of the grass is important. However, if a mower can cut damp grass evenly across its entire deck width, it can do the same with dry grass.

It looks like I can pick up a soil moisture meter for less than $10, but if I have to take it that far, I will need specific parameters defined for what determines a high quality vs low quality cut from a rotary mower as well. How good is good enough?

Increasing pulley size is what I'm thinking too. Power is transfered from a solenoid operated clutch to a pulley, to a drive shaft, to a 90° gear box bolted to the deck. The gear box has a pulley on the bottom that drives the deck belt system - I'm thinking that is the pulley I need to change. This way I can avoid increasing the speed of the gear box and drive shaft.

So, speed is of the essence
From what I have observed so far, blade speed appears to be the #1 factor in cut quality and feed rate. I have noticed that when the blade speed is too low, the grass directly in front of the spindles tends to fold over rather being cut, regardless of mower wheel speed.

I want to see video of freedom running trochoidal toolpaths on the lawn - taking off a 6" patch but running his ass off full speed along the edge :D

You're just jealous of my mad M-Cam skills 🤣. Now you've got me thinking, if I take a drone image of my property and turn it into a surface model, I wonder what sort of tool path Mcam would come up with?

Hey, could be a new business opportunity; programming automated golf course mowers 🙃
 
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You know, Double mowing will solve all the issues, and leave a nice "professional" lay to the newly cut grass. ;-)

Here in VT, If it's green, and survives mowing, IT"s GRASS.

Look out for the Asian jumping worms, they can spoil a lawn in just one season.

Ask me how I know...
 
The deck should not be level. It should be level side to side but actually should be tilted down towards the front. Level or tilted up in front will cause extra drag in heavy or wet grass. Enough to bog the engine in some cases. I set mine about 1/4" lower in front than in the rear. That gives you clearance sort of like on the bottom of an endmill (really more like heeling, but same result). I can probably check my JD 62" deck (also diesel) today and tell you how fast it spins.
 
I do not see how you can get your blade to turn to fast to hurt quality of cut. Spinning blades have to cut faster than grass can bend.
I switched to reel mower. So much nicer. I can cut grass taller.
I well remember that my father bought a Cooper Clipper gas reel mower in 1949 to cut the grass around our new house. For years, he spent many hours with sheets of paper and a wrench adjusting the stationary blade. Even so, the mower would cut the grass and leave some kind of tall weed still standing. His solution was to buy a grass whip and turn the kid (me) loose on the weeds. I was very happy when he dumped the Clipper and switched to a rotary mower. The grass whip was never used again.

Larry
 
The deck should not be level. It should be level side to side but actually should be tilted down towards the front. Level or tilted up in front will cause extra drag in heavy or wet grass. Enough to bog the engine in some cases. I set mine about 1/4" lower in front than in the rear. That gives you clearance sort of like on the bottom of an endmill (really more like heeling, but same result).

Good point, I read the same thing in the Bolens manual.

I can probably check my JD 62" deck (also diesel) today and tell you how fast it spins.
That would be helpful, thank you!

I'm hoping I can increase the spindle speed by 1600 rpm without breaking anything, replacement parts are a bit scarce. I might try to find a way to implement a shear bolt in the driveshaft or something just in case. I know people hate shear bolts but it's better than breaking a gearbox.
 
Following this. I have nearly 20 acres to mow. 27 horse Deere zero turn with three blade 72 inch deck. I'm using all 27 horses... Spindle sheaves are slightly larger than the PTO sheave so I'd say spindle rpm is between 2500-3000. Blades are about 24 inches. I've observed a huge difference in performance depending on blade sharpness which isn't a surprise.

One comment on the above images: It looks like a lot more room under the Deere deck for swarf compared to the Bolens.
 
Sharp blades definitely make a big difference, especially at lower blade speeds. And there can be a huge difference between decks. More room *above* the blades between them and the underside of the deck seems to get the best results.
 
20 acres ! Jeeze. Why don't you put in corn, or oats, or alfalfa or something. At least get something back for all that work :(
It's my own personal park. Planted about 200 trees. I don't mow it all every time. Some of it gets mowed only once or twice a year. I do need to invest in a brush mower for those areas since the Z is really a finish mower. I plan for prairie restoration on 6-8 acres of it which then won't need mowing.
 
The picture of the deck you show in the original post may be different than the one currently on your tractor. Over the years Bolens made several changes in the baffling, discharge chutes, anti-scalp wheels, and roller arrangements. I believe the largest mower available from the factory was 60". However, there were several aftermarket companies like Brinley that made compatible decks.

Over the years I've had a couple Bolens machines. The most recent one was a 1983 GT1400 Eliminator Garden tractor. I also have the factory brochures and repair manuals for that era. The undersides of mowers looked similar to the one in your first post except for the baffling on the front side. From 1983 to about 1986 there were only baffles on the rear of the deck. The front side was left open supposedly for better air flow and grass discharge. I mowed about an acre of lawn on a weekly basis and had no problems with the quality of the cut. I do believe your blade speed is a bit low. If I recall correctly the blade speed on my 48" mower was 3,000 rpm.

There is a forum dedicated to the Bolens machines on the Garden Tractor Talk board. Some of the members are real experts on anything Bolens.


ON EDIT: I believe the 19,000-fpm speed standard is set for safety and noise concerns. The speed equates to about 215 mph. With the factory guarding in place a thrown object should not penetrate the housing.
 
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If you were to work out what your “feed rate” is, you will be shocked how low it is. Disc mower style mowers for cutting hay (same principle as your lawn mower but with a blade that can pivot if it hits something) are perfectly fine cutting at speeds into the mid teens of MPH, and I doubt they are allowed a much faster blade tip speed than lawn mowers which is generally limited to 19,000 feet per minute.
 








 
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