Thicker blades = slower tip speed?

[JDTank]

I replaced the blades on the in-laws old Craftsman some months ago, but when I purchased new blades, they only had blades that were twice the thickness of the ones before. While these seem heavy duty and likely better to have, I have noticed when I engage the mower deck now, it comes extremely close to stalling the machine. It used to slow the engine down when we engaged the old thin light blades, but nothing like this. Obviously I blame this on heavier blades = more mass to get moving then there was before.

It is only an old 19HP B&S Turbo cool engine. I am wondering, once it get's up to speed, will I still have the same blade tip speed as I did before? As I said, these blades are likely twice the weight. I am concerned that given the situation, and the old weak engine, that it will spin the blades significantly slower to make a difference. I want nothing more then to see this machine go to the bone yard, but I don't want spindles or anything to start going out, only to find out it's my fault for running heavy steel blades.

Not sure if it matters but these are 21 inch blades on a 42 inch deck.

 

[beenthere]

If they "get up to speed" then the blade tip speed will be the same.

Only if it does not "get up to speed" will the blade tip speed be less.

With twice the thickness, I'd worry more about the other parts holding up to the extra weight and the need for a longer length bolt (if held on with a bolt).

 

[JDTank]

If they "get up to speed" then the blade tip speed will be the same.

Only if it does not "get up to speed" will the blade tip speed be less.

With twice the thickness, I'd worry more about the other parts holding up to the extra weight and the need for a longer length bolt (if held on with a bolt).

The bolt was more then fine, actually it was about 2 inches long, maybe even a little more. There is some kind of funky design in the middle of the blade, the spindle it mounts to has the same design, like a key system.

I am not overly concerned about spindles or bearings because this machine is about 15 years old, and actually has holes rusted through the deck. The owners refuse to replace it, but I have a feeling this year will be it's last season. The steering is so stripped you can turn the steering wheel half a turn before the actual wheels begin to move, and it isn't worth it to spend $300 in steering gear for a mower that is just plain worn out, especially when the entire deck is junk too.

The front spindles are so badly worn down I am waiting for them to just snap one day.

 

[Deere Dude]

The bolt was more then fine, actually it was about 2 inches long, maybe even a little more. There is some kind of funky design in the middle of the blade, the spindle it mounts to has the same design, like a key system.

I am not overly concerned about spindles or bearings because this machine is about 15 years old, and actually has holes rusted through the deck. The owners refuse to replace it, but I have a feeling this year will be it's last season. The steering is so stripped you can turn the steering wheel half a turn before the actual wheels begin to move, and it isn't worth it to spend $300 in steering gear for a mower that is just plain worn out, especially when the entire deck is junk too.

The front spindles are so badly worn down I am waiting for them to just snap one day.

It sounds like the blades are the best part of the tractor.:laughing::laughing:

 

[beenthere]

Aha!
Now I remember you (the OP) are the one trying to push the in-laws to get rid of their old mower.
Good luck.
Maybe it will be a happy new year for you.

 

[JDTank]

It sounds like the blades are the best part of the tractor.:laughing::laughing:

They are. The only value it holds is in the fresh oil, air filter, fuel filter, spark plugs, and blades!

They never clean out there deck. It had been 5 years since the underside was cleaned. Needless to say, all that grass composting under there has rotted the deck to almost nothing. Believe it or not, the deck has not been removed on this machine in 5 years. I was the first one to take the deck off in 5 years time. They go 3 years before oil changes, or maintenance of any kind.

 

[SensibleNick]

As "beenthere" said: same speed, just more effort needed to accelerate them. If they weigh twice as much, they're going to have twice the inertia.

Theoretically speaking, the greater mass means they have a greater stability too... Assuming the blades are correctly balanced, you need not worry about the bearings/spindles that hold them: They'll be fine

 

[J_J]

You are turning more weight, and therefore the blades will probably turn slower. If the engine has enough HP, using the extra HP designed into a cutting machine, it may have enough to spin at the original speed.

You could check the rpm using a photo-tachometer.

You paint a white mark on the spinning item and aim the photo-tachometer at the spinning item and it will show the rpm.

First aim it at the pulley on the engine with blades not turning

Then aim it at the lighter/thin blades while spinning.

Change to thicker blades and check the rpm again.

I think you will observe three different rpm's

Engine with no load.

Engine with a light load.

Engine with heaver load.

 

[SensibleNick]

You are turning more weight, and therefore the blades will probably turn slower.

There is not a single law of physics that supports the statement above...

However if the words "turning" and "turn" are replaced with "accelerating" and "Accelerate" then it becomes entirely accurate

I hate to disagree with helpful fellas such as yourself JJ, but there's a little bit of me that can't stand by and watch the laws of physics get trampled

 

[J_J]

I don't mind being corrected. But,

Here is the reason that I say that. If I run my Dixie chopper at a certain throttle, that gives me a certain rpm with standard blades, and if I add a heaver blade, the blades do not turn as fast, and the engine slows down. I must have lost some energy somewhere.

Does the rpms not have anything to do with mass.

I have done it and didn't like it , and went back to the blades that I always used.

If I want the heaver blades to turn faster, I need to give it more gas to increase rpm.

I may not know the exact laws of physics, but I can tell that the blades are not turning as fast, with the same amount of gas flowing into the carb.

I have done the same thing with weed eater line.

If the line is extended to a larger diameter, the engine can not run as fast. Cut the line to the correct size, and you regain the rpms.

I am assuming that it takes a certain amount of HP to move a certain mass.


I know you are going to give me a lesson on certain things, so go easy, and don't beat me up to much

 

[JDTank]

I may try and find thinner blades for it.

The store I bought them from was "very adamant" about selling thicker blades, telling me how they last longer and what not. I told them I already knew that, but this is a weak old machine that doesn't have a lot of time left anyways. According to them, they don't sell any thinner blades, just thick heavy steel ones.

Like I said, if it breaks it would be a good thing, at the same time, you hope no one get's injured when a spindle breaks at high speed and metal goes flying everywhere. Most of it would stay under the deck, but you never know, some pieces may get under the deck sides or come out the discharge chute.

 

[SSdoxie]

It sounds like that is same mower I have. I put the heavier blades on several years ago after bending 3 thin ones accidentally hitting gopher mounds. Only thing I found is the belt strecth is increased while trying to bring the blades up to speed. Replaced original belt with another form Sears, that lasted a year. Then I replaced with a HD belt from Tractor Supply, one of the green ones. That was about 5 years ago and still going strong. last year I did fail a spindle bearing but figuring mower is 14 years old, no complaints and local auto parts had the bearings @ $6.00 a pair versus Sears $80 for the spindle/bearing assembly - they don't sell just the bearings.

 

[SensibleNick]

I have done the same thing with weed eater line.

If the line is extended to a larger diameter, the engine can not run as fast. Cut the line to the correct size, and you regain the rpms.

I am assuming that it takes a certain amount of HP to move a certain mass.


I know you are going to give me a lesson on certain things, so go easy, and don't beat me up to much

I'm not in the habit of beating people up JJ - only when they seem to be reveling in willful ignorance do I take the gloves off.... and that Sir, is not you

With the weedwhacker line prescribing a larger diameter, there's definitely a lot more work for the machine to do - the increased wind resistance is noteworthy for a machine that spins it's wire so fast with so little torque. Plus, with such a light weight wire to spin, they need the RPMs to actually get the whacking done . Deck mowers also have an optimum speed, but due to the flywheel properties of the blade, it's far lower, and they have far more torque

In short... when the mass you're moving is revolving around the same point - like a wheel, flywheel or propeller, The mass is only relevant when it comes to getting it up to speed.... the thing that requires more power is the resistance that the air offers.

Wind resistance becomes more relevant at higher speeds: such as weed whackers.
Swept area is also directly linked to the power needed, and given that the calculation of the area involves the radius squared the power needed raises exponentially as you increase the diameter.

Anyway... we're getting seriously off track here I hope my attempt at explaining my "argument" is understandable

 

[aczlan]

I don't mind being corrected. But,
Here is the reason that I say that. If I run my Dixie chopper at a certain throttle, that gives me a certain rpm with standard blades, and if I add a heaver blade, the blades do not turn as fast, and the engine slows down. I must have lost some energy somewhere.
Does the rpms not have anything to do with mass.
<snip/>
I am assuming that it takes a certain amount of HP to move a certain mass.

Do the two sets of blades have the same profile? (ie: high lift vs high lift, mulching vs mulching)
How are you measuring the speed of the blades?
I have swapped blades for heavier ones (on a Cub Cadet LT1050) and while it took longer to spin up, it held the engine RPM better and cut better with the heavier blades.
I have a hard time seeing where two blades of the same type (ie: high lift vs high lift, mulching vs mulching) would cause a difference in blade speed even if they were the same weight.
I have seen that the heavier blades will spin up slower, but they also slowed down less when you hit a patch of heavy grass.

Aaron Z

 

[jenkinsph]

Do the two sets of blades have the same profile? (ie: high lift vs high lift, mulching vs mulching)
How are you measuring the speed of the blades?
I have swapped blades for heavier ones (on a Cub Cadet LT1050) and while it took longer to spin up, it held the engine RPM better and cut better with the heavier blades.
I have a hard time seeing where two blades of the same type (ie: high lift vs high lift, mulching vs mulching) would cause a difference in blade speed even if they were the same weight.
I have seen that the heavier blades will spin up slower, but they also slowed down less when you hit a patch of heavy grass.

Aaron Z

That is my take on this too. I wouldn't be too concerned about the effect of blade thickness. I would wear them out before buying new ones.

 

[JDTank]

That is my take on this too. I wouldn't be too concerned about the effect of blade thickness. I would wear them out before buying new ones.

I concur, was thinking this myself, but I thought it doesn't hurt to ask. The only stupid questions are the ones that go unanswered.

 

[beenthere]

JD
Do you have the X530 all picked out for your in-laws to buy when this Crapsman disintegrates? :laughing:

 

[JDTank]

JD
Do you have the X530 all picked out for your in-laws to buy when this Crapsman disintegrates? :laughing:

It's either the X530 or X360 for them.

The biggest frustration I have is the fact that they buy cheap junk. They flat out refuse to entertain the idea of spending money on anything of quality, when there's a "perfectly good craftsman at sears for $1,500" Whenever the subject comes up, immediately they start talking about buying used, which infuriates me. John Deere equipment very rarely sells for cheap, even used. The difference in price is almost always so slight, your better off getting a brand new machine, with full warranty.

As I have said before, I know I shouldn't take it so personal, but when I am the one doing the work and cutting the grass, I want something nice to ride around on. I am giving my time for free, and my mechanical expertise, as I am the one who maintains it now. Sure, I could pull the plug on it any time I want and say take your cheap machine and cut your own grass. I actually enjoy doing it though, it's a time you can forget about life, and just simply...cut some grass.

More and more I think I am going to buy my own machine, the X740 I have been talking about for a long time, and make a deal with them. I will cut all the grass, I will pay for the machine, it will be mine, but you pay all of my fuel expenses, and oil change expenses. Any other maintenance and payments will fall upon me. This way I can use the X740 for a bit of a "side business" as well. Several of the wife's friends around the area are already interested in having me power sweep there very long driveways (one is about 1,000 feet long) and cut there grass. I know I could make $60 an hour with this machine on weekends, just a few blocks from home.

If they agree to pay for my fuel and oil changes, why not? It saves them from having to buy there own machine, and I get my own machine!

There's one golden rule that I will not falter on though...when I DO finally get my diesel deere, no one's *** will warm that seat except for mine.

 

[Deere Dude]

My BIL had a mowing business and always used others mowers and just charged for his time. Now he bought a big JD front mount and is done with mowing with other peoples junk, but has to charge more obviously.

He had a good friend who also mowed for money and was convinced by him to get a John Deere because he was mowing a bunch or other people lawns with a small "Brand X" mower. He was having trouble keeping it going and my BIL had to fix it for him all the time because he is handy with that.

So what he did, he went and bought a small JD, almost the smallest one he could find, but he was happy with it, surprisingly. Finally after a year of commercial mowing with it it started to fall apart on him and he began to complain and rant and rave to my BIL about how crappy John Deeres are, "It's a John Deere, it's supposed to last a long, long time." My BIL told him, not being used that way.
If you have a lot of lawn and rough lawn you have to get a bigger mower, like what you have in mind. Then it will last longer than a 1oo series or whatever the cheapies are.

Good Luck

 

[mfreund]

Do the two sets of blades have the same profile? (ie: high lift vs high lift, mulching vs mulching)
How are you measuring the speed of the blades?
I have swapped blades for heavier ones (on a Cub Cadet LT1050) and while it took longer to spin up, it held the engine RPM better and cut better with the heavier blades.
I have a hard time seeing where two blades of the same type (ie: high lift vs high lift, mulching vs mulching) would cause a difference in blade speed even if they were the same weight.
I have seen that the heavier blades will spin up slower, but they also slowed down less when you hit a patch of heavy grass.

Aaron Z

x2 I have a push mower for the really steep ditch and noticed that with a heavier blade it took more to slow the mower. I did not notice any slower getting up to speed.