Changing mower pulleys for more power

[cjpenny89]

Has anyone ever thought of changing the pulley diameters on the pto and the main pulley of mower deck to help out with the low Horse Power Problem?

I am over looking this here? I mean I have saw some mowers with little pto pulleys and larger main mower pulleys.

I have about a 8" pulley on my pto and about a 4" pulley on my mower.

so I am guessing that it should be 1 to 2 every one turn of the pto is 2 turns of the main mower pulley?

What would happen if a guy could put a 10" or 12" pulley on it would I then possibly get 3 turns of the mower per one turn of the pto?

Or the bigger the pto pulley the more power it takes to turn it?

Just some ideas any body else care to chim in?

[JBall8019]

chad,
you wont have the torque to mow if you change the pulley configuration. all you need is around 2300 rpm for mowing.
john

[ljw]

Chad, I would imagine that the manufacturer has already maximized the sheave configuration for the existing HP. Any increase in mower speed, i.e., using a larger drive sheave would overload the motor. But I don't really know of anyone that has tried it. Something that I have always wondered about is the possibility of using larger pulleys using the same drive to driven pulley ratio. This would give you more "belt wrap" which would possibly allow you to operate with a lower belt tension. Lower belt tension would increase the life expectancy of the bearings.

Larry

[evielboweviel]

smaller pulley on the Pto helps with a tired motor. However the blade tip speed is less and you should mow in first gear
Ron

[cjpenny89]

Thanks guys was just thinking and sometimes that gets you into trouble

[Matt Kirsch]

You want to do the exact opposite, which is, run a SMALLER pulley on the PTO. 8" is TOO BIG. Mine is 6" in diameter, maximum. That pulley must be for something else.

If you've got an 8" drive and a 4" driven, and the motor's doing 1800 RPM, you're running the mower blades at 3600 RPM. Cripes-a-mighty, you must be generating lift with that thing if you can get 'er spooled up! Do the wheels even touch the ground?

[Rick Prentice]

Hi Chad. I know of four Cubs with mowing decks in my neighborhood. Mine has an 8" along with two others. My friend George has a newer setup with a 6" pulley. The Woods manual that I have suggests the blade speed of 3600. Looks like there's pro's and con's. I've been commented on how well my Cub mows grass(by John Deere owners), now that I acquired evielboweviel's old o.d. green Lo-boy . That's one sweet running machine. Rick

[John *.?-!.* cub owner]

If my memory serves me corerctl (happened once), the 59 inch Woods originally used a 6 1.2 inch pto pulley, and the 42 inch woods used a 8 inch pulley.

[awander]

Power is Torque multiplied by rotational speed.

You can't get "more power" out of the Cub by changing how you use the attachments. It has a maximum power that it can put out(unless you change the engine or the fuel).

If you change the speed of the mower blades(by changing pulleys) then you will also be getting adifferent amount of torque on the blades. the fiaster you spin them, the "less-hard" you will be spinning them(less torque) The slower you spin the blades, the harder(more torque) they will spin.

The use of the mower requires enough torque on the blades to actually curt what you are cutting, and also enough speed to make the cut stuff fly out of the mower, etc.

[Rick Prentice]

I'm scratching my head again . I know what the engineering figures say on paper, but if you install a smaller pulley, versus a larger pulley, on the pto shaft, the engine will work easier, that's just simple mathmatics, at least in my simple brain . As soon as I find more time, one day, I'm going to prove this theory of mine

Rick

[awander]

Yes, the engine will work easier, but to do the SAME AMOUNT of work, it will have to work faster.

What most people think of when they say "power" is actually "how hard" something pushes.

A lot of torque(or a large force) seems more "powerful", but technically speaking it is not.

Power is work/unit time. So when you go to a smaller pulley, the engine "pushes harder" but the belt moves slower. The "power" is the same in either case.

A useful way of looking at it is the use of a lever(which is what a pulley really is, it's just a lever that keeps going around and around)

If you use a longer lever to lift a load, you push "less hard", but you have to push for a longer a longer distance.

Power is work(force x distance traveled) per unit time. In the "lever" example, going to a smaller pulley gives us a larger force. But the disatnce travelled(how fast it is spinning) drops proportionally, so the distance times the force is the same.

So the only way to get the engine to do more work with the smaller pulley is to speed it up!

Hope this makes it clearer instead of more muddy.

The key, I think, is that when we say "power", we really mean "force" or "torque", but that is NOT what it means in the science.

[Matt Kirsch]

Okay, let's not make this too complicated!

What it all boils down to is, he's asking the Cub do more than it is capable of. It simply lacks the torque/power/whatever to run the mower at that speed AND move AND mow grass at the same time.

The Cub can't do that much work. It needs to be configured to do LESS work. Reducing the size of the PTO pulley will reduce the torque/power/whatever needed to turn the mower deck, leaving more to move the tractor and actually cut grass.

Both my Cubs have ~6" pulleys for driving their respective 59" decks.

According to the online parts list at http://www.cngco.com, it appears the PTO drive pulley for the Woods 59C2 is 6.1" in diameter.

[awander]

Well, you are not wrong, but it IS kind of complictaed.

Using a smaller pulley on the PTO means the mower blades will turn more slowly. It also means that MORE torque will be applied to the mower blades to make them turn.

If the slowly turning blades NEED that extra torque to cut the grass they are cutting, the net effect on the tractor's engine will be the same. the same amount of power will be used to turn the blades as they cut.

It is very possible that the slower-moving blade will NOT cut as well. This may or may not cause more torque to be needed, depending on what is being cut and a myriad of other factors.

On the other hand, if you have done it, and it wiorks for you, then we can say that for the type of mowing you do, a smaller pulley works. If you have noticed that more power is available to the tractor to move itself in this situation, then we can again say that for the stuff you mow, at the speeds you mow it, it works well to use a smaller pulley.

But I bet that if you mowed a heavier growth of grass, you might be able to reverse this, so that it didn't work as well. in that case, you might be better off with a larger PTO pulley, or with the stock size.

[Matt Kirsch]

You're missing the point, which is that the mower pulley on his Cub is the WRONG ONE! Torque be danged, it's the WRONG FlIpPiN PULLEY

*pant* *gasp* *cough* *sigh*

Okay, back to the physics lesson: No, it does not mean that more torque will be applied to the blades to make them turn. The same amount of torque will be applied to turn the blades at a lower RPM. Horsepower is a function of torque and RPM (I believe the formula is HP = Torque * RPM / 5252). Same torque, less RPM means less horsepower required to turn the blades.

Game. Set. Match. Don't mess with a mechanical engineer with a major in math and a minor in physics

I just double-checked my C-3 Danco deck, and the pulley does appear to be 8" in diameter. The Woods pulley is definitely closer to 6". Maybe that's why the Farmall seems so much "gutsier" than the LoBoy... Anyway, both decks cut just fine, and the Woods is cutting some pretty heavy stuff at times because Dad doesn't always have time to keep up with the lawn mowing.

[awander]

Matt:

You're right that at the PTO, torque is not going to change. I was incorrect in my statements. The torque, and the RPM, are the same, so the power will be the same.

What changes, of course, is the linear force on the belt, and the linear speed of the belt, when you change pulley size.

This linear speed and linear force difference, when applied to the pulley on the mower deck, willl result in a different torque and different RPM on the mower blades.

What I was trying to say, is that slowing down the blade ROPM may REQUIRE more torque at the blades to actually do the cut.

I think I am right about that.

BTW, I am also an engineer, though I specialize in electronics. I learned a long time ago that a degree doesn't make me right....