What's new
What's new

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

I think maybe he meant to say FPT? We're getting crossed up with all these terms getting thrown around. I do that all the time with acronyms when too many are used in quick succession.
 
I suspect the easiest way to improve that crappie deck is to cut an opening behind each blade to make it a rear discharge. Passing the grass to one side is like recutting chips. The deck is too short for one sided discharge.
 
At one time Bolens made rear discharge mowers for many of their tractors. They worked fine when you were only cutting off an inch or two of grass. However, when you tried to cut off even as little as 3", they left windrows of cuttings rather than evenly distribute them across the field.
 
I think maybe he meant to say FPT? We're getting crossed up with all these terms getting thrown around. I do that all the time with acronyms when too many are used in quick succession.
Maybe, but measuring the speed of a rotating cutter in surface feet/min - and even calling it "cutting speed" - is counterintuitive. It is the correct terminology, it is just difficult to wrap your head around when we are so used to thinking in terms of RPM. I don't fault anyone for getting it mixed up.

Instead of SFM, we could also use MPH. If the blades were instead wheels of the same size; we want them traveling across the pavement at 210.227 MPH or 18,500 feet per minute. Let's say this number applies to ALL rotary finish mowers.

If my blade is 18" long, the center of the spindle would need to rotate at about 3925 rpm in order to achieve our 210mph. A 22" blade would need to spin roughly 3210rpm to achieve the same speed at the tip of the blade.


The gearbox of the 60" Bolens deck in question is actually mounted upside-down. They are driving the output side and outputting through the input side.

All evidence points toward this deck being designed around components already in their product inventory - or they used this gear box model because peerless had them in current production. Either way, they didn't care that it didn't run the blades fast enough; it cut grass sort of, so they went with it. The whole deck is ok overall but the engineering is wrong in all the right places to make this a terrible mower.

1.) The shell is bolted together, no welds.

2.) The 72" deck length from the integral chute makes it very heavy on one side and it the deck lift cannot pick it up evenly.

3.) The front 'ski's' instead of wheels cause the deck to tear up the sod and make the deck twist and hop over slight bumpy surfaces.

4.) Blade speed is far too slow.

5.)The right side ski is mounted out on the end of the deck chute-extention-section; resulting in far more twisting leverage being applied to the (bolted together) deck frame...so stupid. It should be mounted further in; between the "cutting line" sticker and where the gear wrench is sitting in my pic below.

I plan to fab a proper caster wheel set up for it, relocate the one on the right side and cut all that extra deck length off. I will also be welding this deck shell together.

20231006_103433.jpg
 
Last edited:
Gravely made their 60 and 72 " decks similar but used wheels front/back. Also the deck was one piece formed on press brake
The RH side wheels were "Inside" the cutting width.
With that type of formed deck baffling will make/break the performance.
Found picture on internet....easier than taking a picture of mine....lol
welding this deck shell together.
Not worth the time .......
 

Attachments

  • 60 deck.jpg
    60 deck.jpg
    188.7 KB · Views: 11
Have any of you guys ever screwed up so badly that all you can do is stand back and say; "oh hell no, I did not just do that - am I really that stupid?"

So, I spent a week searching for the "correct" gear set that I wanted for my mower deck gearbox. I then had to go through 3 distributors; wait 3 weeks and pay $240 to get them.

I finally got the gears, rebuilt the gearbox and got it all back together this week.
I fire the thing up and discover something that I should have already known... the tractor only has a 1800 rpm PTO speed! 🤢

The original gear set in the right angle gearbox was 1: 0.740. When Bolens designed the deck, they installed the gearbox upside down to achieve a near 1:1.25 gear ratio needed obtain a marginal blade tip speed.

When I ran the numbers to verify my current blade speed, I forgot about the reduced PTO speed. I used 3600 rpm engine speed, 1:0.74 gear reduction, 6.5" drive pulley and 6.25" spindle pulleys; the result was within 70 rpm or so of my measured spindle rpm, so I assumed it was right.

I ended up tearing it all back apart and putting the original gear set back in so that I could mow one more time for the season.

Anyway, I am over the experiment at this point. It still mows as good as it would have new - probably better with the new exmark blades. The gearbox needed bearings and seals anyway so it wasn't a complete waste of time.

The tractor, deck and snow blade are all in excellent shape for being 30 years old. I think my best option - as someone likely suggested in this thread already - is to buy a zero-turn and pass along the Bolens to a collector who will appreciate it for what it is. Unfortunately I was born with the gene that requires pain before I learn anything 😄🙄.
 
Cal Poly offers a four year degree in "Turf Management". I assume lawn mowing, theory and practice, is a part of that.
My nephew was not sure if he would be accepted into engineering so he transferred from JC into a major program of "turf management" at SLO . It is never impacted, so he got in and took all engineering courses. After a year he was called in and told to change his major to engineering since his grades proved he was capable.
BilL D
My wife bought me the shirt
 

Attachments

  • lw4.jpeg
    lw4.jpeg
    22 KB · Views: 7
Last edited:
I too have more time than should admitted to ruminating on this topic. Just do not start considering brazing carbide tips onto your blades. Yes, considered that myself as I really hate the blade sharpening effort.
 
I can relate to the original poster. I have a 2000 model Scag 48" Tiger Cub. It has over the 20 years that I've owned it, it has proven to be a pretty reliable, easy to service piece of equipment. Does a great job of cleaning up leaves. The only thing wrong with it is that it does a horrible job of cutting grass. I run down a row and look back and see all that it's missed and have to make another pass. My old single-blade Dixon mower did a much better job of cutting.

I've tried all kinds of blades, double blades, new spindles, new belts, new belt tensioner bearings, etc. I'm running a grass catcher, too. It's very dry and dusty where i live and half my yard is dirt. This thing throws clouds of dirt out in front of the deck, so I've put a strip of rubber around the front to try to cut down on that. It works a little bit. I've cleaned the deck thoroughly, etc. The only thing I haven't done is to check for & aft cutting height. I've seen that in the book, just haven't gotten around to it.

Freedommachine, when you said you had to machine the blades, have you check commercial suppliers, like MowMore? They have a large assortment of blades to choose from.
 
Freedommachine, when you said you had to machine the blades, have you check commercial suppliers, like MowMore? They have a large assortment of blades to choose from.

I spent some time searching the interwebs and nothing came up other than the overpriced NOS blades, I think that I made the right choice in the end though.

The blades I used were higher quality and of a better design. All I had to do was helix bore 2 holes in each blade so that they would bolt to the Bolens spindles.

I know that increasing the surface speed of the blades would fix a majority of the cut quality issues. I still feel like a dummy for ordering the wrong gears but oh well, failure is always a possibility when trying something new.
 
My big 7ft slasher broke in half a couple of weeks ago ....rust has inferior tensile strength ,and I ran the tractor over a big lump of wood that was previously in the city park next to me ...........they had pushed it into my place under cover of 4 ft high grass.................The gearbox is good ,so I will reframe and resheet it ......it has round edges on 3/4" thick folding blades ,and works perfectly on the 80hp tractor.
 
one time i had grass so wiry and tough mower blades would not cut it..........however it was a desert type grass,and since the never ending rains ,its all been choked out by jungle grass........... broad leaf paspalum ,and the damn stuff grows 3ft high in one week..........ive given up cutting ,as cutting stimulates growth,and now spraying roundup ...........unfortunately,some patches are immune to roundup .....and Ive also got jungle vines that are a class E weed ............I was leaving them because they strangle trees ,and Ive got a VPO on the place ,as well as 'special orders' for 'neighbourhood trees' and 'heritage trees.'
 
Guess I've never seen 'low lift' blades. Where can I find some?
That may be tough to find as the industry thinks high lift blades are the only way to go.

Look for a blade with the least amount of "curl" ...usually lower cost aftermarket blades have less lift.
One can always grind some of the curl away, of course blade must be balanced.
If you look at the blades there now, you should already see the curl being eroded away by the dirt.

It's a fine line between too much and not enough lift. The variables are many in deck design and grass itself is the biggest.
You may never get a clean cut.........lol
 








 
Back
Top