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Changing deck pulley to turn blades faster ?


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#1 4tom ONLINE  

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Posted March 14, 2017 - 10:06 AM

Sears tells me to run at full throttle when cutting with the deck for best results. I usually run at about 25-30% throttle. Would it make sense to do a pully swap to spin the blades faster? Has anyone else thought about this? I figure the time to act would be before the deck gets back under the tractor, probably april sometime.

Edited by 4tom, March 14, 2017 - 10:06 AM.

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#2 boyscout862 ONLINE  

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Posted March 14, 2017 - 10:15 AM

I wouldn't change any hardware. I don't like to run any engine at full throttle for any length of time but, 3/4 is fine for working. Do not lug an engine down. It is hard on the crank and bearings. When mowing I use the highest gear that gives a good finsh. That is usually 4th(out of 6) but if the grass is sparse it can go up to 5th. Sometimes when I'm days late I have to mow in 3rd. Good Luck, Rick


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#3 tater195 ONLINE  

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Posted March 14, 2017 - 10:15 AM

why not run it at full throttle?


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#4 MiCarl OFFLINE  

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Posted March 14, 2017 - 11:30 AM

Remember, you're not actually moving the throttle.  You're adjusting the rpm target and the governor manages the throttle to achieve that.  If you were actually working the throttle the engine would disintegrate the first time you depressed the clutch.

 

The engine is designed to produce its maximum output at about 3,600 RPM.  If you "throttle" it back it might not produce enough power and you could actually end up overworking the engine.  Add to that the cooling fan is only running at part speed.......  Increasing the deck speed by changing pulleys will make the problem worse.  If you run the engine at 1,800 rpm and change pulleys to get the deck speed to normal the engine will need to produce twice the torque required in the original configuration.  Producing that torque will add a lot of heat and wear.

 

In reality, with an appropriately sized mower, the governor will typically only need part throttle to maintain the rpm.  It'll open the throttle if the load increases when you hit a thick patch of lawn (and you've heard that happen).

 

Now if the deck is sized so the engine needs full throttle to maintain 3,600 rpm you're going to have a problem.  This is supposedly what happened to the Strongland tractor company when they put 60" decks on 18HP tractors.


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#5 4tom ONLINE  

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Posted March 14, 2017 - 11:31 AM

It has the 21hp briggs, never seems to work hard in any gear. Cuts OK as is. Sears says the mulching blades should be spun as fast as possible. I see no reason to run full throttle in a lower gear to spin the blades faster, burn more fuel and work the motor harder. Just a thought before I prep and mount the deck for cutting. I thought a larger drive pully, or a pair of smaller deck pulleys would be a simple job and might accomplish the same thing.

#6 EricFromPa ONLINE  

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Posted March 14, 2017 - 11:32 AM

Increasing the Pulley size will make the engine work ALOT harder.I usually mow at 3/4 throttle unless I want to mulch leaves or piles of grass then it's Full throttle with the deck Up 2 notches.


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#7 4tom ONLINE  

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Posted March 14, 2017 - 11:41 AM

Thanks for the replies. I ask so I can learn. I will let it be for this year.

#8 secondtry OFFLINE  

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Posted March 14, 2017 - 12:32 PM

  I see no mention in this discussion of the government mandated blade tip speed. Many of the older mowers had the blades running at much higher speed. The government decided that us uneducated peons needed protection and required the manufacturers to cut the blade speed back for safety. So the actual blade speed has little to do with engineering but is determined by law. In the fine print somewhere it is probably deemed illegal to change the blade speed. I am sure that others here know much more about this than I. Don  


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#9 larrybl ONLINE  

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Posted March 14, 2017 - 03:53 PM

I might be mis understanding here.

Air cooled engines need air flow = 3600 rpm

Splash lube = 3600 rpm

And if you have a electric PTO, you need 3600 rpm to keep the battery from going flat.

 

Changing the deck pulleys for faster with less rpm is asking for trouble,

 

My thoughts.


Edited by larrybl, March 14, 2017 - 03:54 PM.

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#10 twofitu OFFLINE  

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Posted March 14, 2017 - 04:10 PM

Most of the time the manufacturer recommends operation at WOT for cooling purposes; cooling the engine and/or the hydrostatic transaxle. You have to figure the manufacturer designed the engine to operate at rated speed with the rated load for the rated lifespan.
I think the reason blade speed was limited was so the blade tips couldn't exceed the speed of sound. Mower blades are not dynamically balanced or engineered for the stresses involved in trans-sonic speeds.
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#11 KennyP ONLINE  

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Posted March 14, 2017 - 04:23 PM

Most of the time the manufacturer recommends operation at WOT for cooling purposes; cooling the engine and/or the hydrostatic transaxle. You have to figure the manufacturer designed the engine to operate at rated speed with the rated load for the rated lifespan.
I think the reason blade speed was limited was so the blade tips couldn't exceed the speed of sound. Mower blades are not dynamically balanced or engineered for the stresses involved in trans-sonic speeds.

I don't know! I have a neighbor who just bought a new JD riding mower and that things sounds like a copter taking off when he turns the blades on! Gotta be spinning pretty fast!


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#12 twofitu OFFLINE  

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Posted March 14, 2017 - 04:58 PM

I don't know! I have a neighbor who just bought a new JD riding mower and that things sounds like a copter taking off when he turns the blades on! Gotta be spinning pretty fast!


It is Pi day and if I remember correctly Pi x Dia. = Circumference. You could ask Mr. Google what the speed of sound is in fps and then use a tachometer to find RPM on the spindle at no load. The tip will be traveling at the highest speed. My JD sounds much different with the high-lift bagged blades compared with the triple-duty blades (the ones it came with) or the gator mulchers. Tip speed is one of the limiting factors on airplane propellers/engines and that was discovered the hard way. I'd just take the manufacturer's word for it in this case. Just sayin'
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#13 James Bosma ONLINE  

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Posted March 14, 2017 - 05:22 PM

I don't know! I have a neighbor who just bought a new JD riding mower and that things sounds like a copter taking off when he turns the blades on! Gotta be spinning pretty fast!

 

Ahh  the turbo decks


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#14 drbish ONLINE  

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Posted March 14, 2017 - 05:27 PM

My thinking is if you needed a pulley swap for better results they would have been made that way,Manufacturer's do not just guess what size of pulleys should go on a tractor
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#15 stiemmy ONLINE  

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Posted March 14, 2017 - 06:28 PM

Either way you decide to go pulley swap, or not, I never heard of anyone smokin an engine because of a lower throttle setting in all the years I been putzin with small engines. Lack of maintenance is likely the culprit here.
If the manufacturer wanted continuous operation, your throttle would be...off, idle, and WO with detents to prohibit any other operation. Just my .02 Cheers;)

Edited by stiemmy, March 14, 2017 - 06:33 PM.

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