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