Can someone explain "Blade Tip Speed" please
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Bowhunter
Gold Member
Re: Can someone explain \"Blade Tip Speed\" please
A faster tip speed gives grass less time to react to the blade, in other words it has less time to bend before being cut.
A faster tip speed gives grass less time to react to the blade, in other words it has less time to bend before being cut.
thcri
Elite Member
Re: Can someone explain \"Blade Tip Speed\" please
I believe the speed is actually measured right at the tip of the blade. And yes a good finish mower will go faster than others.
murph
I believe the speed is actually measured right at the tip of the blade. And yes a good finish mower will go faster than others.
murph
Bird
Rest in Peace
Re: Can someone explain \"Blade Tip Speed\" please
The most common speed of the rear PTO on tractors, at least compact tractors, is 540 RPM (revolutions per minute). Then the mower gearbox can have different gearing to turn the output shaft at different speeds. Then if the blades are belt driven, the speed can be changed by using different sized pulleys. On most walk behind mowers, the output shaft is the end of the crankshaft, so the blade is turning at whatever speed the engine is designed for. Instead of revolutions per minute, blade tip speed is usually stated in feet per minute; i.e., how many feet the tip of the blade moves, in a circle of course, each minute. So even at the same RPM (revolutions per minute), the blade tip speed (feet per minute) will be higher on a longer blade (bigger circle means the tip of the blade has to travel farther to go all the way around on one revolution). Did I understand your question right?
The most common speed of the rear PTO on tractors, at least compact tractors, is 540 RPM (revolutions per minute). Then the mower gearbox can have different gearing to turn the output shaft at different speeds. Then if the blades are belt driven, the speed can be changed by using different sized pulleys. On most walk behind mowers, the output shaft is the end of the crankshaft, so the blade is turning at whatever speed the engine is designed for. Instead of revolutions per minute, blade tip speed is usually stated in feet per minute; i.e., how many feet the tip of the blade moves, in a circle of course, each minute. So even at the same RPM (revolutions per minute), the blade tip speed (feet per minute) will be higher on a longer blade (bigger circle means the tip of the blade has to travel farther to go all the way around on one revolution). Did I understand your question right?
thcri
Elite Member
Re: Can someone explain \"Blade Tip Speed\" please
Wow Bird,
I never had it explained to me like that. I never realise it was feet per minute. I should be able to go home now? I learned something new today.
murph
Wow Bird,
I never had it explained to me like that. I never realise it was feet per minute. I should be able to go home now? I learned something new today.
murph
Re: Can someone explain \"Blade Tip Speed\" please
The King Kutter Rear Discharge 72" shows a "Tip Speed" of 18,463 FPM. This seems higher than any other mower that I have researched including the more expensive brands/models. This is why I was asking, becaise according to the Tip Speed being greater, this would make the KK mower the best cut of anyone. Is this correct?
The King Kutter Rear Discharge 72" shows a "Tip Speed" of 18,463 FPM. This seems higher than any other mower that I have researched including the more expensive brands/models. This is why I was asking, becaise according to the Tip Speed being greater, this would make the KK mower the best cut of anyone. Is this correct?
Bird
Rest in Peace
Re: Can someone explain \"Blade Tip Speed\" please
Yep, that is a high blade tip speed. So, in theory at least, it should cut better if all other factors are equal. But I think some other factors can affect the speed and quality of the cut; i.e., the length of the blades, how much of the leading edge of the blade is sharpened, how sharp, what is the shape of the blades, are they shaped to create a vacuum to pull the grass up into the blades, what's the shape of the housing and/or any baffles to direct the clippings to whatever kind of discharge it has, etc.
Now those are just things that I think affect the final outcome and I don't know enough to be able to look at a mower and say it would be better or worse than another mower without actually trying them.
Yep, that is a high blade tip speed. So, in theory at least, it should cut better if all other factors are equal. But I think some other factors can affect the speed and quality of the cut; i.e., the length of the blades, how much of the leading edge of the blade is sharpened, how sharp, what is the shape of the blades, are they shaped to create a vacuum to pull the grass up into the blades, what's the shape of the housing and/or any baffles to direct the clippings to whatever kind of discharge it has, etc.
Now those are just things that I think affect the final outcome and I don't know enough to be able to look at a mower and say it would be better or worse than another mower without actually trying them.
sweettractors
Advertiser
Re: Can someone explain \"Blade Tip Speed\" please
<font color="blue"> Tip Speed being greater, this would make the KK mower the best cut of anyone. Is this correct? </font>
If you base your assumption completely on BTS, That would be correct, However, as Bird said, there are many oter determining factors that contribute in total, to a overall high performance machine. We find a higher quill bearing/housing failure rate in mowers with the highest BTS. --Ken Sweet
sweet@scrtc.com
<font color="blue"> Tip Speed being greater, this would make the KK mower the best cut of anyone. Is this correct? </font>
If you base your assumption completely on BTS, That would be correct, However, as Bird said, there are many oter determining factors that contribute in total, to a overall high performance machine. We find a higher quill bearing/housing failure rate in mowers with the highest BTS. --Ken Sweet
sweet@scrtc.com
Re: Can someone explain \"Blade Tip Speed\" please
Ken,
What tip speed(at the highest FPM) do you recommend for optimum cuts without too much speed to cause bearing failure? I was thinking between 16,000 & 17,000 FPM but not sure. I had really not ever thought of having too much tip speed until you mentioned this. I guess it's like a vehicle engine running at ultra high RPM's and then it breaks quickly due to higher stresses. Thanks,AL /forums/images/graemlins/shocked.gif
Ken,
What tip speed(at the highest FPM) do you recommend for optimum cuts without too much speed to cause bearing failure? I was thinking between 16,000 & 17,000 FPM but not sure. I had really not ever thought of having too much tip speed until you mentioned this. I guess it's like a vehicle engine running at ultra high RPM's and then it breaks quickly due to higher stresses. Thanks,AL /forums/images/graemlins/shocked.gif
Pooh_Bear
Platinum Member
- Joined
- Jan 2, 2005
- Messages
- 763
- Tractor
- Early 1949 Ford 8N
Re: Can someone explain \"Blade Tip Speed\" please
17000 ft/min is approx mach .25 where mach 1 = speed of sound.
You have seen that trick where a guy yanks a table cloth off a table covered in dishes. The trick is to yank it so fast the dishes don't have time to react so they stay on the table.
Ever use a sling blade, swing it slowly and it doesn't work as well.
Swing it fast and it slices rite thru the toughest grass.
A fast mower blade works like that.
A fast mower blade also generates more lift of the grass blades.
This pulls them up straighter to get a more even cut.
There are two ways to increase blade tip speed.
Increase the RPM or increase the blade lenght.
Tip speed is calculated by multiplying RPM x Pi x blade length.
blade lenght in inches will give you inches/minute.
Blade lenght in feet will give you feet/minute.
The blade rotates in a circle. Draw a circle around the blade.
Imagine the circle as a wheel. if the wheel were rotating
then how far would it go in one minute. That is tip speed.
17000 ft/min is approx mach .25 where mach 1 = speed of sound.
You have seen that trick where a guy yanks a table cloth off a table covered in dishes. The trick is to yank it so fast the dishes don't have time to react so they stay on the table.
Ever use a sling blade, swing it slowly and it doesn't work as well.
Swing it fast and it slices rite thru the toughest grass.
A fast mower blade works like that.
A fast mower blade also generates more lift of the grass blades.
This pulls them up straighter to get a more even cut.
There are two ways to increase blade tip speed.
Increase the RPM or increase the blade lenght.
Tip speed is calculated by multiplying RPM x Pi x blade length.
blade lenght in inches will give you inches/minute.
Blade lenght in feet will give you feet/minute.
The blade rotates in a circle. Draw a circle around the blade.
Imagine the circle as a wheel. if the wheel were rotating
then how far would it go in one minute. That is tip speed.
have_blue
Veteran Member
Re: Can someone explain \"Blade Tip Speed\" please
</font><font color="blue" class="small">( <font color="blue"> Tip Speed being greater, this would make the KK mower the best cut of anyone. Is this correct? </font>
We find a higher quill bearing/housing failure rate in mowers with the highest BTS. --Ken Sweet )</font>
Blade wear is another consideration. Running at full RPM, the blades on my 72" Woods really get dull fast. After a 7 acre job, they have long since quit cutting and rely on pure velocity to explode the grass like a Weedeater string. Backing the 540 RPM down to ~450, my blades stay very sharp for 1-2 cuts of my 7 acres of grass and light weeds, and leave a great finish. (It runs much quieter too!) It takes 5 minutes to do a light touch-up sharpening on the machine.
If you're cutting 1-3" brush, I guess you need all the tip speed you can get.
</font><font color="blue" class="small">( <font color="blue"> Tip Speed being greater, this would make the KK mower the best cut of anyone. Is this correct? </font>
We find a higher quill bearing/housing failure rate in mowers with the highest BTS. --Ken Sweet )</font>
Blade wear is another consideration. Running at full RPM, the blades on my 72" Woods really get dull fast. After a 7 acre job, they have long since quit cutting and rely on pure velocity to explode the grass like a Weedeater string. Backing the 540 RPM down to ~450, my blades stay very sharp for 1-2 cuts of my 7 acres of grass and light weeds, and leave a great finish. (It runs much quieter too!) It takes 5 minutes to do a light touch-up sharpening on the machine.
If you're cutting 1-3" brush, I guess you need all the tip speed you can get.
have_blue
Veteran Member
Re: Can someone explain \"Blade Tip Speed\" please
PB,
That formula only works if you know the blade RPM. You would have to know it, or you would have to know the gearbox ratio to calculate it. My cutter's manual lists the gearbox ratio, (but not the tip speed) so others may as well.
Funny you mentioned the mach number. I have always believed that the grass breaking mach 1 from being whipped violently is partly responsible for fast blade wear, and certainly for much of the noise and bedlam going on under the deck. /forums/images/graemlins/smile.gif
PB,
That formula only works if you know the blade RPM. You would have to know it, or you would have to know the gearbox ratio to calculate it. My cutter's manual lists the gearbox ratio, (but not the tip speed) so others may as well.
Funny you mentioned the mach number. I have always believed that the grass breaking mach 1 from being whipped violently is partly responsible for fast blade wear, and certainly for much of the noise and bedlam going on under the deck. /forums/images/graemlins/smile.gif
Pooh_Bear
Platinum Member
- Joined
- Jan 2, 2005
- Messages
- 763
- Tractor
- Early 1949 Ford 8N
Re: Can someone explain \"Blade Tip Speed\" please
</font><font color="blue" class="small">( That formula only works if you know the blade RPM. You would have to know it, or you would have to know the gearbox ratio to calculate it. My cutter's manual lists the gearbox ratio, (but not the tip speed) so others may as well.
)</font>
That is what I meant. Blade RPM. Not PTO RPM.
I should have made that clear.
I watched a show on the History channel once about engines.
A guy talked about jet engines on commercial airlines are
getting so big and powerfull that the turbine blades are
approaching a tip speed of mach 1. And they weren't sure
what would happen if the tip speed exceeded mach 1.
Tip speed for lawn mowers for home owners are limited.
I have a question into another board to find the limits.
And commercial lawn mowers have higher allowed limits.
I'll post when I find what the limits are.
Pooh Bear
</font><font color="blue" class="small">( That formula only works if you know the blade RPM. You would have to know it, or you would have to know the gearbox ratio to calculate it. My cutter's manual lists the gearbox ratio, (but not the tip speed) so others may as well.
)</font>
That is what I meant. Blade RPM. Not PTO RPM.
I should have made that clear.
I watched a show on the History channel once about engines.
A guy talked about jet engines on commercial airlines are
getting so big and powerfull that the turbine blades are
approaching a tip speed of mach 1. And they weren't sure
what would happen if the tip speed exceeded mach 1.
Tip speed for lawn mowers for home owners are limited.
I have a question into another board to find the limits.
And commercial lawn mowers have higher allowed limits.
I'll post when I find what the limits are.
Pooh Bear
sweettractors
Advertiser
Re: Can someone explain \"Blade Tip Speed\" please
First Choice Groming mowers are: FPM is 13700 on the 4ft, 14100 on the 5 ft, 14200 on the 6 ft and 15300 on the 7 ft. At 540 PTO, Blade RPM is 3130 for the 4 ft, 2650 for the 5 ft, 2240 for the 6 ft and 2030 for the 7 ft. We have been selling this brand since the company started and have not had 1 failure on the quill assembly.--Ken Sweet
Email sweet@scrtc.com
First Choice Groming mowers are: FPM is 13700 on the 4ft, 14100 on the 5 ft, 14200 on the 6 ft and 15300 on the 7 ft. At 540 PTO, Blade RPM is 3130 for the 4 ft, 2650 for the 5 ft, 2240 for the 6 ft and 2030 for the 7 ft. We have been selling this brand since the company started and have not had 1 failure on the quill assembly.--Ken Sweet
Email sweet@scrtc.com
Highbeam
Super Member
Re: Can someone explain \"Blade Tip Speed\" please
To get the higher blade tip speed, the gear box has to have taller gears. This would reduce the ability of the mower to cut big thick stuff, right. Kinda like driving around in high gear makes you less able to climb a hill. My thought is that it would be easier to stall a blade geared this way when the going gets tough.
If a guy wants to buy a rotary mower for use as a finish mower then go for max tip speed. If you want to bust some brush, gear it down.
To get the higher blade tip speed, the gear box has to have taller gears. This would reduce the ability of the mower to cut big thick stuff, right. Kinda like driving around in high gear makes you less able to climb a hill. My thought is that it would be easier to stall a blade geared this way when the going gets tough.
If a guy wants to buy a rotary mower for use as a finish mower then go for max tip speed. If you want to bust some brush, gear it down.
ThistleKop
Silver Member
Re: Can someone explain \"Blade Tip Speed\" please
<font color="blue">Tip speed for lawn mowers for home owners are limited.
I have a question into another board to find the limits.
And commercial lawn mowers have higher allowed limits.
</font>
Pooh bear,
The maximum blade tip speed for homeowner mowers is 19,000 FPM. This has been the OPEI standards for many yrs. When these products came under the stds of the CPSC a few yrs ago, they used the same std. I remember when the engine rpms were lowered to comply with this std. Quality of cut has suffered since.
TK
<font color="blue">Tip speed for lawn mowers for home owners are limited.
I have a question into another board to find the limits.
And commercial lawn mowers have higher allowed limits.
</font>
Pooh bear,
The maximum blade tip speed for homeowner mowers is 19,000 FPM. This has been the OPEI standards for many yrs. When these products came under the stds of the CPSC a few yrs ago, they used the same std. I remember when the engine rpms were lowered to comply with this std. Quality of cut has suffered since.
TK
