Blade Tip Speed...Whats yours?

[TLS]

I am seeing a lot of questions pertaining to blade tip speed. Obviously the faster the better, right?....to a point. What I am wondering is, at what speed is the best cut possible at? It would have lots of variables. Blade design is one, blade to baffle distance would be another. Deck design is also important. I'm sure that there are alot of different variables.

Also, another variable is engine RPM and what RPM the factory "mower" rates the fpm at. As an example, my '97 Dixie with a 25 Kohler "yeah!" was delivered to me with a free load speed of 3450 RPM. That seemed a little slow, so I looked in the owners manual and found that they should be at 3600 RPM. So I adjusted it to that. Much better cut, like night and day!. Then after reading Lawnsite, and how many ran as high as 3850 RPM, I then adjusted them to 3750 just to be safe. I noticed another improvement in cut. At what speed does Dixie attain the 19000 + fpm speed? 3450, 3600, 3750, or 3850? These jumps in RPM have a measureable affect on blade tip speed as well as quality of cut.

Next would be...If the drive pulley on the engine is the same diameter as that on the spindle (are they?) then the spindles turn at the same RPM as your engine. Are some "geared" up or down with pulleys to adjust this spindle RPM? How about the shaft driven decks, are they spinning faster or slower. The length of the blade determines tip speed, so a 72" deck needs to turn slower than a 60" deck.

All right, all you engineering students, what is the formula for determining the blade tip speed when spindle RPM is known? How can we learn spindle RPM when it is shaft driven?


BOY, do I need to get out there and start cutting or what!!!

Sorry for the long post!

Carry ON!!!

[Eric ELM]

There was a thread on this topic not long ago about figuring out the fpm of the blade by the rpm's. I worked the last 2 days for the first time in ages, I'm to tired to find it. :(

[MikeGA]

try thisn

http://www.lawnsite.com/showthread.php?threadid=10790

[TLS]

Thanks Mike, must have been plowin' that day! Professor Martin explained the formula's perfectley!

[mowingmachine]

That Martin guy beat me to it. I was hopen I could be the hero to explain that. His formulas are right and he did a great job of explaining it. As far as the shaft driven decks it is the same thing and it comes down to gear ratios. If the gears inside each of the gear boxes are the same size all the way through then the spindle speed will be the same as the engine. So if the input gear on the gear box is say 4" and the output gear is 2" then the spindle shaft speed would be twice as fast but you would cut the torque in half. Most gear boxes are a 1:1 ratio as well as pulley sizes from the engine to the spindle. Here is a good rule of thumb for a 60" deck. Most 60" decks have 20" blades and at about 3600 RPM the blade tip speed is 19000 feet per minute. The 19000 f/m is the industry safety standard and I would stay below it for liability reasons. If something goes wrong on one of your jobs and you are above that speed it is no longer the manufacturers fault and you will be the one getting sued. Hope this helps.
Mowingmachine

[mowingmachine]

Sorry I wasn't very clear about the rule of thumb for a 60" deck. This only holds true if the engine pulley is the same size as the spindle pulley.
Mowingmachine

[ncmow]

Faster is not always better. ANSI regulates this speed because of the thrown objects test. Anything above 18,500 ft/min and the pattern dispersed fails ANSI standards.

As for the formula, I've got one easier that the previous posts. Use a photo tachometer and a piece of reflective tape. Put a piece of tape on a spindle sheave and measure rpms. Then plug into below:

Blade Spindle RPMs X 3.14 X Blade Length in Inches / 12

This will give you blade tip speed in ft/min

[Richard Martin]

Let me see now, where did I leave that photo tach? What the heck is a photo tach and where do you buy it?

[Eric ELM]

Richard, your mower should be a 1 to 1 ratio. If the pulley on the motor and the pulley on the deck are both the same size your spindles should be turning the same speed as your motor. You have a tach which is fairly close, so if you are running 3600 rpm's and your blades are 20.5" long, you should have 19,000 fpm blade speed.

[mowingmachine]

ncmow,
kinda like to know where you got the ANSI standard to be 18,500 f/m. The industry goes by 19000 f/m.
mowingmachine

[OldLawnMowerMan81]

I am new to this forum and I would like some help here in regards to blade tip speeds of my lawn equipment listed below.

I have four lawn mowers (and one lawn tractor):

1 - a 1987 Sears Craftsman 38321 20" rear bagger
engine - Craftsman, model 143.374012 (Tecumseh TVS90-43352E), engine speed 3200 rpm (high) and 2100 rpm (low)
Blade tip speed (from ncmow's post) - 16,755 fpm (high) and 10,996 fpm (low)

2 - a 2002 Sears Craftsman 38761 22" side discharge mower/mulcher
engine - Craftsman Eager-1, model 143.024200 (Tecumseh LEV105-347001A), engine speed 3050 rpm
Blade tip speed (from ncmow's post) - 17,567 fpm

3 - a 1973 Sears Craftsman 9026 20" side discharge
engine - Craftsman Eager-1, model 143.234062 (Tecumseh ECV100-145035A), engine speed 3000 rpm
Blade tip speed (from ncmow's post) - 15,708 fpm

4 - a 1993 MTD 123-478C084 21" rear discharge, self-propelled
engine - Briggs & Stratton Quantum XTE, model 124802-3110-01, engine speed 3200 rpm
Blade tip speed (from ncmow's post) - 17,593 fpm

5 - a 2006 Sears Craftsman 27681 LT3000 42" deck lawn tractor
engine - Briggs & Stratton Intek OHV (with AVS), model 31P977-0635-E1, engine speed 3300 rpm
What is the blade tip speed for this tractor? What formula do I use to determine blade tip speeds for tractors?

Thank you,



Ben (OldLawnMowerMan81)

[mowtech]

I am new to this forum and I would like some help here in regards to blade tip speeds of my lawn equipment listed below.

I have four lawn mowers (and one lawn tractor):

5 - a 2006 Sears Craftsman 27681 LT3000 42" deck lawn tractor
engine - Briggs & Stratton Intek OHV (with AVS), model 31P977-0635-E1, engine speed 3300 rpm
What is the blade tip speed for this tractor? What formula do I use to determine blade tip speeds for tractors?



The easiest way is to measure the rotational speed of the spindles with a tach and use that speed for calculating the tip speed. If you actually want to calculate it from the engine speed you can do this by calculating the ratio of the drive pulleys--engine to spindles, and using that ratio to calculate the spindle speed. This is easier said than done because the actual pitch diameter of the pulleys is hard to measure without the proper tools. A close approximation would be to use the pulley diameters. The drive can either speed up the spindles or slow them down. The device with the smallest pulley will be spinning the fastest.

[OldLawnMowerMan81]

So the formula for blade tip speed is as follows:

For walk-behind push-type mowers:
Measure blade circumference (tip to tip) (ex. 20" blade = 5.236' circumference)
Find physical engine speed (ex. 3200 rpm)
BTS: 3200 x 5.236' = 16,755.2 fpm

For walk-behind self-propelled mowers:
Measure circumference of drive pulley (ex. 5.06" pulley = 1.325' circumference)
Measure circumference of engine pulley (ex. 5.5" pulley = 1.44' circumference)
Find physical engine speed (ex. 3200 rpm)
Measure blade length (tip to tip) (ex. 21" blade = 5.498' circumference)
BTS: 1.325' / 1.44' x 3200 x 5.498' = 16,189 fpm
(This formula could be different for those mowers whose engines have PTOs. Could anyone please give me an alternate formula for self-propelled mowers whose engines have their own auxiliary driveshafts?)

[mowtech]

So the formula for blade tip speed is as follows:

For walk-behind push-type mowers:
Measure blade circumference (tip to tip) (ex. 20" blade = 5.236' circumference)
Find physical engine speed (ex. 3200 rpm)
BTS: 3200 x 5.236' = 16,755.2 fpm

For walk-behind self-propelled mowers:
Measure circumference of drive pulley (ex. 5.06" pulley = 1.325' circumference)
Measure circumference of engine pulley (ex. 5.5" pulley = 1.44' circumference)
Find physical engine speed (ex. 3200 rpm)
Measure blade length (tip to tip) (ex. 21" blade = 5.498' circumference)
BTS: 1.325' / 1.44' x 3200 x 5.498' = 16,189 fpm
(This formula could be different for those mowers whose engines have PTOs. Could anyone please give me an alternate formula for self-propelled mowers whose engines have their own auxiliary driveshafts?)

Not quite. You don't actually need the circumference of the pulley because the circumference is proportional to the diameter. (Pi x Dia)

For a belt driven mower:

Tip Speed (feet/minute) = Engine Speed (RPM) x (Pitch Dia Engine Pulley / Pitch Dia Spindle Pulley) x 3.1415 x Blade Length (inches) / 12

For a shaft driven mower:

Tip Speed (feet/minute) = Engine Speed (RPM) x Gearbox Ratio x (pitch Dia Gearbox Pulley / Pitch Dia Spindle Pulley) x 3.1415 x Blade Length (inches) / 12

This assumes gearbox is driven directly off the engine.

[Ooomwizard]

My blade speed is about 2 mph.. then I have to shave again the next morning.

[Lawn-Sharks]

My blade speed on my snapper is 1,000,000.00 Fpm..not really but is sure would feel like that if i stuck my hand under the deck!