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Dans 7080 
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 Topic: increasing blade speed.
Posted: 12 Apr 2017 at 11:48am |
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I want to increase the blade speed on my mower. My options are going from a 5" driven pulley to either a 4.5" or a 4". The 4" would be a 20% increase and the 4.5 would be a 10%. Any opinions on which would be better?
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Tbone95
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Posted: 12 Apr 2017 at 11:52am |
What kind of "mower" are we talking about here? 20% is a 100% better than 10%  
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Dans 7080
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Posted: 12 Apr 2017 at 12:24pm |
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Its a 21hp 48" cut lawn mower. My 12hp 38" will run circles around it because of the faster blade speed.
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Tbone95
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Posted: 12 Apr 2017 at 1:28pm |
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In that case, give 'er he!! and go with the 4" pulley. In any drive system, the only downfall to speeding up a driven component is a proportional loss of torque the component delivers. Can't hardly think that's going to be the limiting factor on a mower blade.
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JoeM(GA)
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Posted: 12 Apr 2017 at 1:40pm |
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please remember many newer mowers built in the last 15 years or so are
built with smaller, cheaper (fewer balls) bearings, or sealed bearing that are barely capable of dealing with the speed and load they run at now, increasing the speed
may push them towards premature failure. I might push the 10, at 20 I'd have to look at pretty hard at the spindles and idlers.
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BenGiBoy
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Posted: 12 Apr 2017 at 1:47pm |
 Dans 7080 wrote:
Its a 21hp 48" cut lawn mower. My 12hp 38" will run circles around it because of the faster blade speed. |
If you ever go buy a new mower you want to get a commercial one, the way it was explained to me. The mowers that they sell to you (non-commercial) are limited by the government (of some form) on how many MPH the blade tip can go, the commercial ones are not, hence why a commercial rig can mow at 15 MPH and you can only go 4 or 5 MPH on a home-owner special.
I would increase the blade speed all I could and make really sure I had the blades torqued on so they would not be in danger of falling off.
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Dans 7080
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Posted: 12 Apr 2017 at 1:54pm |
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I'm leaning toward the 4". The mower is an 03 I think. The 38" is a late 80s before the blade tip restriction. All the spindles have grease fittings on them so I may up my maintenance on them with the faster speed. I did find new pulleys are available so if the 4" doesn't work I can always make another size and I'm just out a couple bucks.
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DiyDave
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Posted: 12 Apr 2017 at 6:05pm |
Might have to get a shorter belt, too, if the idler won't take up enough slack...
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Dakota Dave
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Posted: 12 Apr 2017 at 6:05pm |
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All commercial mowers are limited to 19,000 FPM blade tip speed. Most noncomercial mowers run around 17,000 FPM.Exceeding tip speed leads to greater projectile damage and greatly increases the possability of the blade disenegrating. A flying blade piece will go through the mower deck and the RV that your mowing near. the blade tip went through the inner and outer skin of the RV. I corrected Fps to Feet Per Minute. I made a pretty big error.
Edited by Dakota Dave - 12 Apr 2017 at 7:58pm
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CrestonM
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Posted: 12 Apr 2017 at 9:18pm |
BenGiBoy wrote:
Dans 7080 wrote:
Its a 21hp 48" cut lawn mower. My 12hp 38" will run circles around it because of the faster blade speed. |
If you ever go buy a new mower you want to get a commercial one, the way it was explained to me. The mowers that they sell to you (non-commercial) are limited by the government (of some form) on how many MPH the blade tip can go, the commercial ones are not, hence why a commercial rig can mow at 15 MPH and you can only go 4 or 5 MPH on a home-owner special.
I would increase the blade speed all I could and make really sure I had the blades torqued on so they would not be in danger of falling off. |
x2! Go commercial or go home. Residential stuff of today isn't the residential stuff of the 50s.
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steve(ill)
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Posted: 12 Apr 2017 at 9:39pm |
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You better listen to Dave and figure out the speed of the blades.. Calculate the speed of your small mower, then calculate the speed of the present mower. I doubt they are more than 10% different.. Changing the speed of the blades can cause bearing or blade failure. Bigger diameter blades will have slower rotation speed than smaller blades... Look at why a rear chopper mower has a 540 RPM input and slower output.. DIAMETER.
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Like them all, but love the "B"s.
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Brian F(IL)
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Posted: 13 Apr 2017 at 7:52am |
Not trying to rain on your parade, but I did what you're suggesting many years ago on a mower deck of mine. I thought it was a great idea. Unfortunately, the resulting improvement didn't really make much, if any, difference at all.
Good luck to you.
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Tbone95
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Posted: 13 Apr 2017 at 8:25am |
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Yeah, all that bad stuff could happen......I'd still giver 'er and see what happens.
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Dakota Dave
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Posted: 13 Apr 2017 at 8:28am |
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I wouldn't worry about the bearings or the blade falling off. I've seen an aircraft propeller that has been over speed . The propeller tips were shredded. Aircraft propellers are made to much higher standards than lawn mower blades. On an 84" propeller it only takes a couple hundered extra RPM to push the tip speed past the speed of sound and than they come apart. 19k FPM is about 1/3 the speed of sound.
Edited by Dakota Dave - 13 Apr 2017 at 8:31am
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DougS
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Posted: 13 Apr 2017 at 8:39am |
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The faster blade speed might help in normal grass. The loss of torque will hurt you when you're in heavier grass or when you hit a tough spot. I don't try to out-think the designer. I'd think a smaller pulley would cost a few cents less and they'd go that way in the first place if they could.
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skipwelte
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Posted: 13 Apr 2017 at 8:40am |
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i did that once on a b210, i found that the increased blade speed loaded down the engine a lot more, it wasnt sucessfull. It must of been to large of an increase, I went back to about the same size pulley and got along ok. the tip speed increase alot as it made a lot more noise.
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Stan R
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Posted: 13 Apr 2017 at 10:59am |
Power required will go up by the cube of the rpm. https://en.wikipedia.org/wiki/Affinity_lawsThus, 1.2 x 1.2 x 1.2 = 1.7 or 70% hp increase. Not sure what engine hp increase it will be as some HP is serving the movement of the mower.
Edited by Stan R - 13 Apr 2017 at 11:00am
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HudCo
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Posted: 13 Apr 2017 at 11:04am |
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the blade tip speed is just as fast as the 38 with shorter blades more than likley there is somthing ealse that needs to be adjusted engine rpm needs to 3400 deck needs to be slighty higher in the front than rear , i have a blade tip fps speed chart acording lenght of blade but i cannot find it now. more than likely the blades need to be replaced we have all ready had three mowers come to the shop his year that a piece of blade has went throught the deck and all push mowers . a few years ago a customer brought a push mower and when we checked it in we done the usual things while the customer was there like check the oil and air filter condition he said it did nt sound right and started it and the blade came out and punctured a riding mower tire about fifteen feet away then just spun on the ground for a few seconds sure glad no ones feet where involved
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Brian Jasper co. Ia
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Posted: 13 Apr 2017 at 5:31pm |
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You want the front lower than the back. Having the back lower increases the power needed to do the same job.
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FloydKS
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Posted: 13 Apr 2017 at 9:26pm |
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I have been waiting to 'chime in'... you also want the blade to be sharp, nothing slows a blade like trying to push a butter knife thru grass. Of course the ends get dull first cause they do all the cutting.
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DanD
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Posted: 13 Apr 2017 at 10:04pm |
Brian Jasper co. Ia wrote:
You want the front lower than the back. Having the back lower increases the power needed to do the same job. |
Actually the manual for Simplicity mowers says to have the front of the mower a little higher than the rear. With the blades pointing straight front to rear, it says to have the front of the center blade 1/8 to 1/4 inch higher than the rear of the two side blades. This is for a 48" three blade deck such as found on a Sovereign tractor.
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DougS
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Posted: 14 Apr 2017 at 5:40am |
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I would think if the front were too much higher than the rear each blade of grass would be cut twice, requiring more horsepower. 1/8 inch wouldn't be noticeable.
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Lonn
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Posted: 14 Apr 2017 at 6:43am |
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Well yeterday I just bought a new Simplicity ?????? the size bigger than the Broadmoor. I wish they'd use numbers, names just don't stick in my head. Anyhow I am going to read the manual tonight. I used to set up new AGCO Allis mowers when I worked at the dealership in the early and mid 1990's and back then the front of the deck had to be lower than the rear or it would take too much power to cut the grass.
One other thought, and I'm not calling anyone an idiot because I've seen this before, but are the blades on right or are they on upside down? We had a guy trade in a brand new Homelite because he had no power and was cutting terrible. When I was going over the Homelite for resale I noticed whoever set it up had put the blades on upside down. I think he bought a 1600 series AGCO Allis from us and of course was happy with it.
Edited by Lonn - 14 Apr 2017 at 6:43am
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Stan IL&TN
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Posted: 14 Apr 2017 at 7:17am |
Well you are all wrong. Now that that is out of the way, have a good Friday. 
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Tbone95
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Posted: 14 Apr 2017 at 7:25am |
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Lonn
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Posted: 14 Apr 2017 at 7:30am |
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I've been all wrong for a very long time, just ask my wife
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Gerald J.
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Posted: 14 Apr 2017 at 8:37am |
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Another problem with reducing the diameter of the driven pulleys on the blade shafts is that the smaller pulleys will lose torque capability and so be more prone to slipping if loaded hard. That slipping will wear the belt rapidly and the pulleys too. The reduced torque capacity comes from a shorter length of belt meeting the pulley. This will happen while the higher speed will need more torque at the blade shaft.
Gerald J.
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Stan R
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Posted: 14 Apr 2017 at 8:45am |
Gerald J. wrote:
Another problem with reducing the diameter of the driven pulleys on the blade shafts is that the smaller pulleys will lose torque capability and so be more prone to slipping if loaded hard. That slipping will wear the belt rapidly and the pulleys too. The reduced torque capacity comes from a shorter length of belt meeting the pulley. This will happen while the higher speed will need more torque at the blade shaft.
Gerald J.
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Assuming the current belt contact area is at design with NO over design contact area....
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WNYBill
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Posted: 14 Apr 2017 at 8:57am |
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Be careful increasing speed, unit was not designed for it. On a brush hog type mower, the front should be at least 1/2 inch lower or you will be cutting the grass twice . I believe our Dixie Chopper zero turn is level, to make a nicer looking finish, but those blades are only 20 inches long. When all else failes READ THE OWNERS MANUAL
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Dans 7080
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Posted: 14 Apr 2017 at 1:42pm |
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Update. I got the 4" pulley on and it cuts way better that I ever thought it would. The blades are new and everything else is up to snuff. I will replace the belt because it was wore out before I changed the pulley, but the same size belt will work. Power is not an issue as the faster speed gets the grass out a the deck faster. Its a whole different machine. The blade tip restrictions are a joke if you ask me. I am very satisfied with the results and thank you all for your input!
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