PTO cart

[j.l.holt]

If you used a steel wheel and put several feet of steel cable under the rim, you would add a lot of weight for traction and not have it add to the over all weight much. It is not suspended  as we think a heavy  would be.

[Eli]

If it was set up that 540 is hi gear on the transmission that is usually direct drive not as much drag from the trans mission. When you shift down you would have a little more drag from the transmission but it would be more than offset by the mechanical advantage of slowing down the out put shaft. If you end up with an over drive auto trans mission just find a gear that is direct drive 1 turn input = 1 turn out put most likely fourth on a fine speed.    Eli

[Andy Carson]

This is a tricky thing about gearing on ground drive stuff.  If you are used to tractors, etc, you are used to the idea that you can use a lower gear and go slower and you have mechanical advantage.  This is not always the case with ground drive stuff.  Gears alter torque/force, but they cannot change power.  Here’s an example of what I mean.

A 2 MPH tractor produces the same power (force x velocity) as a 4 MPH tractor because the velocity that is refered to here is the velocity of the power element -the crankshaft.  The speed of the cranshaft has no relation to the ground speed, and this is key.  The slow tractor has the same power as the fast tractor, but has twice the time to accomplish a task (IE, it takes 10 minutes to get around the field rather than 5).  You could also think or in terms of applied torque and/or force.  The torque/force of the slow tractor is twice that of the fast one due to gearing.  Either way, this is great example of mechanical advantage that we can all relate to.

Doing the same math with a ground drive system is different.  a 2 MPH ground drive cart produces half the power (force x velocity) as a 4 MPH cart because power is directly related to ground speed (assuming the cart is making maximum use of its weight in both cases).  The slow cart has half the power of the fast cart, but has twice the time to accompish the work.  Half the power, twice the time = no net mechanical advantage.  It would, no doubt, be easier for the animals, because they would have to produce less total power.  This is not due to mechanical advantage, though, it is simply moving slower and getting less done.  Thinking about torque in this example might be easier, even with different gearing.  The instantaneous torque for either high or low gearing is determined by the weight of the cart and the traction system and is not effected by ground speed.  Speed only enters into this if you ask how much power (torque x velocity) you can generate at whatever RPM you are interested in.  For a cart of x pounds, power will always increase linearly with ground speed (4 MPH is twice the power of 2 MPH), but you would have to bale hay faster too (you are covering and baling twice as many bales at 4 MPH).  There just isn’t a free lunch here.  I am beginning to see why ground drive mowers do not have different gears…  I know it is a different application, but it is something to think about.  

 

 

[Eli]

I was not really trying to gain power but loose rpm and saying the transmission really wouldn’t  cause much drag in a direct drive gear. And if you down shift to slow down the pto  but maintain ground speed changing gears would not make it harder to pull. When we raked with a tractor we ran the pto at 1/2 speed and drove faster Because it was gentler on the hay.  If I build a ground drive pto cart I will have more than one speed on the pto. But I would probably use a chain set up like on a 6000 series John deer corn planter rather than a transmission. Eli

[j.l.holt]

I understand what you are saying,,just a little hard for me to put into words,,This is how I got a mind set on the simple  horse power in put and the speed/tork/power  out come.    A 20mi. bike ride in hilly country on a old 10 speed bike gets it set well in your mind just how power in, and  tork /speed out work together. Or apart.  I seen the relation ship real clear by the time I got home.

[Andy Carson]

Eli,
I agree that a cart that was running a PTO at half speed would pull easier, but if the cart could run the PTO at full speed it has substantial weight. Running the PTO at half speed means it weighs twice as much as it needs to. This represents a substantial loss of efficiency. You see what I mean? I do like your concept of just swapping out gears to change gear ratios. It sounds simple and effective. I think if I was going to make adjustable gear ratios, I might also put a place to hold weight (like suitcase weights) as it seems like it is just as important to adjust weight as it is to adjust gear ratios. Perhaps more… I was Googling some info about ground drive combines because in some ways these represent the ultimate in ground drive technology. These can be over 15 tons. Granted these bohemoths were pulled by 30-36 horses, but this represents a weight of roughly half the weight of the animals pulling it. Massive.

[j.l.holt]

If just weight and traction are your biggest factors. What would adding duel wheels do for you. and how about if the added ones were filled with concrete?  Both weight and traction when you needed it,cheep and quick.

[Andy Carson]

JL, I used to rides bikes alot too, so know what you are talking about. Part of the difference is conceptual and part of it is just language. The conceptual part is that a bicycle, just like an engine, produces power independant of ground speed. You can tire yourself going slow up a steep hill or going really fast on the flat. Very different speeds, but the same exhurtion/power output. Conceptually, this is because you are pushing on the pedals at a given force producing a certain torque. This torque is translated through gears to the wheels producing a forward force that you feel. When you use a lower gear, you feel the increased force pushing you forward because the torque is multiplied. In common language, alot of people call this an increase in power, but it is not power in the physics sense. It is an increase in torque at the cost of forward speed. Because power (in the physics sense) is force x velocity, you can slow velocity and increase force without changing total power.  When you go up a hill on a bike, you feel the need to shift down because getting up the hill requires more force to maintain a given speed than on the flat. One might either increase power (power proper) by increasing force while keeping the same rpm and forward speed, but we all know this is tiring. It is usually better to shift down, keeping power (power proper) roughly the same while increasing torque and instantaneous force so the hill can still be climbed at a slower speed. This is intensy confusing because people use the word “power” all the time when they really mean “force”. It is also confusing because there are so many examples (bikes, cars, tractors) where going slower due to gearing means you have a mechanical advantage. It makes one think that slow is always more forceful (some would say powerful, but this is not power in the physic sense) and fast is always weaker. When it is the speed itself that produced the power, like is a ground drive system, this relationship doesn’t always follow.

[j.l.holt]

Like you write up and under stand most of it, but is a little deep for a old country boy.   I’m going to have to stick with the thinks I know.  Shoeing horses and  tinkering with junk… My wife says I have tinker-ides.  Hates it when I start a project.   I get consumed with trial and ere ideas.

[bendube]

Andy,

Thanks for helping me to understand the physics behind this.

Let me know when you build the ultra-heavy-duty forecart for 8 horses with crawler tracks and a power-regulating flywheel!

It seems like having a way to add/subtract water for weight might be a good way to change weights relatively conveniently.

 

[Eli]

I think I’m going to set up a still and run a tractor on some type of home brew. It just sounds simpler. lol.     Eli

[j.l.holt]

Eli,,,,as in wood gasification?

[Andy Carson]

“Let me know when you build the ultra-heavy-duty forecart for 8 horses with crawler tracks and a power-regulating flywheel!”

I find all this stuff interesting, but really don’t have enough land to justify such a thing.  I realize that some of this seems impractical, and it might actually be.  They are ideas that would need some tinkering to get to work.  I like to do the mathmatic modelling to see how much might be gained by the tinkering, and because I feel it is a meaningful thing I can contribute.  My rough guess is that the crawler tracks aren’t worth the complication compared to tractor tires.  They might yield a 30% reduction in weight, but this is might only result in a 3-12% improvement in efficiency (depending on hills and wheel design).  The flywheel, though, still seems like it would be as it would translate more directly to driveshaft power.  Still, either of these somewhat complicated design aspects is not near as important as the simple addition of raw weight over the drive wheels.  You are right, Bendube, that the bulk of the work is probably in getting these or other designs to work in the real world.  Still, I think it is meaningful to make sure the concept makes sense on paper before going to the trouble of building and troubleshooting a piece of equipment.

[j.l.holt]

You have good points there for sure. I don’t use math that much so it easier to build then ”tweak”’ as I go. More weight then another horse would be the easiest  for most folks to do, or at least me.  I have seen tracks that were kinda home built for small trucks and jeep type.  Built from snowmobile track and boogie wheels. they now sell them so they should be durable.  Seen them set up for full size pick ups.

[Roscoe]

If you need the ultra-heavy-duty just for one implement, like the baler, it’s probably easyer to convert the implement in ground drive. You will save a lot horse power with avoiding draging a lot of dead weight around, especially in hilly country.

Tracks have a lot of friction, I doubt that there is some advantage over wheels on level ground. To figure out, how the relations are between weight reduction, efficiency and resistance, could be a nice future project for Andy 😉

But if you need tracks that you not sink, it’s definitly the wrong time for field work…

[Author]

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