[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.

[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.  

 

 

[Andy Carson]

Adding traction is a great way to go and calculations show this is critical.  You reach a point of deminishing returns at some point though.  Friction coefficients reach a practical maximum at around 1, which makes good sense to me (this menas the force to drag something is rarely greater than the force to lift it straight up).  Tractor style tires get close to this a 0.7 or so.  Crawler tracks can exceed this slightly, but might add a great deal of complexity and reduced manuverability.  Either are tons better than an auto tire.  calcium in the tires would increase the weight substantially, which I know was not a goal, but if you have that much weight spinning, it would provide somewhat of a flywheel effect, which I would think would be important.

[Andy Carson]

“What is the added drag of the transmission and is it worth”

I did some reading about this, because I find it interesting as well.  I am not going to be able to give a definative answer, but was able to find enough information to give ideas of the general range of power loss.  The rule of thumb for automobile applications where all gears are lubricated (IE transmission/rear differential) is a 15% power loss from flywheel to the rear wheels, but this varies a lot.  Interestingly, one of the biggest single source of power loss is in the rear differential itself, as the direction of the rotational power takes a 90 degree turn.  Modern hypoid gearsets (the kind that are curved as well as bevelled) increase the total gear contact area and the strength of the differential.  These are common on heavy truck applications, some tractors, and other rear wheel drive applications where strength is important  because of high torque.  The increased contact area in this design also causes more friction and power loss.  Power loss in the range of 6-10% are normal for these types.  Simple bevel gears results in less power loss, assuming they are also appropriately lubricated, and torque is matched to the application.  Power losses from inside a manual transmission range from 1-2% for high gears where input and output rotational speed are identical to over 5% if there are large differences between input and output rotational speed.  Overall, this isn’t alot of power loss, as the direction of the rotation remains the same, and I do agree you get more speed options, which might be important.  In my view, the addition of a transmission does not mean the resulting machine will be 6-20% less efficient than a PTO cart wihtout a transmission.  The PTO cart still has to have a differential, or something like a differential, and this is half or more of the power loss.  I my mind, it might be 1-5% less efficient, depending the transmission.  Not a big deal. 

The concept of speed itself is a very interesting one.  Power (from a physics standpoint ) is force x velocity.  Force in a PTO cart is fixed by 1) the weight of the the PTO cart and 2) the friction coefficient of the traction system.  Gearing can change rotational speeds of PTO shafts, and change torque radiacally, but it CANNOT make more power.  People get that idea from internal combustion engines where when they shift down, the engine runs at a higher RPM, and the engine does indeed make more power at this higher RPM.  Animals do not work this same way.  

This is not to say that there are not “tricks” you can play with reguard to speed and power generation with respect to ground drive systems.  Here’s an example:  Lets say a 800 lb cart moves at 4 MPH spins the PTO shaft at 400 rpm and generates 1 horsepower (This is just for easy numbers and doesn’t come froma real calculation).  If the horses slow to 3 MPH, the PTO will spin at only 300 rpm (which is obvious), but the power also goes down to 0.75 horsepower (power=force x velocity).  Gearing in a transmission can make the PTO shaft spin at 400 rpm, but it will still only have 0.75 horsepower.  In this case, a greater load would just spin the wheels.  An alternate way to overcome this limitation in power at low velocities is to increase the force to match the lower velocity.  If the traction system (IE wheels) is fixed, the only way to do this is by adding weight.  To get back to the example, if an 800 pound cart moves at 4 MPH and generates 1 hp, it needs to weigh 1066 lbs (4/3 x800) to generate 1 HP at 3 MPH.  Big addition in weight…  Adding additional weight sovles many problems and is the driving force behind a lot of these applications.  No wonder one of the main differences between a “heavy duty” Iand J cart and there regular cart is weight.  Twice the weight, in fact. 

Still adding weight (especially the weights we are talking about) is not always desireable, especially in hilly country.  Because power is force x velocity, and the only way to increase force is with weight (assuming you don’t want to play with traction systems), it seems critical that all ground driven work be done at speeds that are as fast as the animals can efficiently walk.  This is a big deal.  Much much bigger for ground drive applications than in other work!  This argues that gears might more useful for changing the PTO shaft speed to accomplish different jobs.  This might be a major attractive feature of gearing.  Similarly, this analysis demonstrates that the gears will not be useful in allowing the same task to be a accompished at a lower forward speed (unless more weight is added to the cart).

I am not sure how much of this was obvious already, but I hope this helps thinking these things through.

[Andy Carson]

My dog is still guarding well, but hasn’t demonstrated much enthusiam for pulling.  I think he’s a little too gentle for this kind of thing.  His gentleness is a great benefit in other areas, though.  Making a young persons first experience with animals positive is an important good job, too, right?

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[Andy Carson]

jeesh, for that price I might by it and I dont even know what I would do with it!

I and J shows an 8 foot sickle bar pulled by a team.  That is an improved designed though, and might be 50% improved over traditional designs.  I dont know much about your 9 ft bar, but I might guess 3 horses might be needed to run it.  This is a major guess on my part, others would know better.

I googled the weight of the teamster 2000 as 1400 pounds.  I am guessing there might be 300 lbs on the dolly wheel, which leaves 1100 lbs on the drive wheels.  On dry ground with good tires, I would guess this cart would convert the power of 3 horses. Good tires and ry ground are big factors. If your land is flat, you can always add more weight too, and increase traction.  You might not need it, but weight is an easy mod.  You could shorten the bar too.  Theres lots of posibilies if it does work, and it might without mods. At this price, I see no downsides. I have had that response to some I and J products as well, nice, but not always the nice price.

[Andy Carson]

George, I wasn’t doing these calculations to show that these carts don’t work.  I was meaning to demonstrate the critical points of the design.  The I&J cart actually has almost all the features that seem to be important from these calculations.  Pneumatic tractor-style tires maximize traction.  They yield substantially more power than road-type tires.  On some terraign, this might double the power obtained by car/truck style tires.  The adjustable tongue keeps the weight over those wheels.  Notice no power-robbing dolly wheel on this one.  The weight on this heavy duty cart is substantial at 1500 pounds.  This is very close to the 1280 lbs I calculated to be needed to start a “4 horse load with tractor tires.”  You might notice that the weight of thier “regular” PTO cart, which is demonstrated with a team of horses, is 750 lbs.  Very close to the 640 lbs I calculated for a 2 horse load…  This seems like a well engineered machine that has taken into account the factors that the analysis would predict are critical.  In my opinion, to equal the I&J machine, the teamster 2000 would need to 1) remove the dolly wheel 2) come up with a balancing system to deal with the tongue weigh of a ~1500 lb two wheeled machine and 3) switch to tractor tires. 

Alternatively, a heavy flywheel with a couple adustable one-way dog clutches to let the flywheel spin up without robbing a lot of power, but deliver that power when the wheels start to slip (IE prevent the wheels from locking) might let one half the weight of the whole rig, but that is another topic.

Here’s the link to the I&J site, and no, I don’t work for them! 🙂

[Andy Carson]

I did an analysis of the power generated from ground drive systems.  I think this analysis is important and interesting because it may reveal where the greatest improvements can be made.

Obviously, having more traction is always better for generating power.  Increasing traction can be done by increasing weight (which lowers overall efficiency), of by changing tires or traction systems (which may increase complexity).  But how much traction is enough? 

If we assume that we are using pneumatic tires, the coefficient of friction will be from about 0.35 to 0.75.  Tractor type tires on dry surfaces are at the higher end of this range, road type tires on wet surfaces are at the lower end.  Note also that even for a higher efficiency tires, this coefficient will vary wildly from point to point in the field, especially at wet spots.  Still these numbers provide somethins to work with.  This means that if the cart weighed only 100 lbs, it might take only 35 lbf to drag it without turning the wheels if it has road style tires and 75 lbf to drag high efficiency tractor style tires.  I will also compute crawler tread at a friction coefficient of 1.  Some references report crawler tracks as 50% increased over tractor tread (which would give a friction coefficient of 1.1), but I think a friction coefficient of 1 is a better upper limit (even though things like dragster tires go past 1).

A rule of thumb is that animals can pull 15% of their body weight at a steady rate, so 2 1600 lb animals exert a pull of 480 lbf, 3 animals pull 720 lbf, and 4 animals pull 960 lbf.  If the work is ground driving, the wheels need to start spinning and not skid over the ground.  This is the first force I will calculate.

With road tires of wet turf (friction coefficient of 0.35), it takes a cart that weighs more than 1370 lbs to start a “two horse load” with no slippage (480/0.35), a 2060 lb cart is needed for a “3 horse load” (720/0.35), and a whopping 2740 lbs is needed for a “4 horse load” (960/0.35). 

Change to a tractor-style tire and this picture gets more promising.  A 640 lb cart would start a “2 horse load” (480/0.75), a 960 lb cart will start a “3 horse load” (720/0.75), and 1280 will start a “4 horse load” (960/0.75).

Crawler tracks let you start a 2 horse load with a 480 lb cart, 3 horses with a 720 lb cart, and 4 horses would need a 960 lb cart.  Better than the tractor tires, but I have doubts it is worth the complexity for starting a load alone…           

Once the wheels are started, the surface actually travels at half the rate the axles and the rest of the cart do.  Think of pushing on the top of a wheel rather than the back of a cart.  You get a 2:1 mechanical advantage.  This means that once the wheels are turning, they require only half the weight to keep them turning.  That’s only 690-1370 lbs for a cart with road tires, 320-640 lbs for a cart with tractor style tires, and a tiny 240-480 lbs for crawler tracks.  Here the crawler track system seems to be interesting for high power (high horse number) applications, where extra weight might not be needed.

After doing this analysis, I come away with 2 important areas to focus on for greatest improvement.  I think this matches with experiences I have read about using ground drive PTO carts, but those with more experience will have to tell me if these observations seem to match the real world experience. 

1)      Use the highest traction tires possible.  Small gains here make a very big difference. 

2)      Keep those wheels turning!  Locking them up means they are going to be twice (literally twice) as hard to start again.  In the meantime, some tools (like balers) clog up from going forward without running which further increases the force required to start up again.

[Andy Carson]

The part about the teamster 2000 that doesn’t make sense ot me as a PTO cart is that (at least from what I can tell) is has 3 wheels, but only rear two wheels drive the PTO.  That front dolly wheel removes a substantial amount of weight from the drive wheels.  Put a scale under that wheel and you will see how much weight is taken by that front wheel.  Weight is traction, and traction is power for a ground drive set-up.  One would definitely get more power out of a setup without a dolly wheel.  If you had to have a dolly wheel, best put it as far away from the center of gravity as possible.

[Andy Carson]

Here are some random conceptual thoughts about ground drive PTO carts. It seems to me that using car/truck rearends has potential for doing much lighter work, but if you want to do heavy work, I think a more “ground up” redesign might be more effective.

1. I agree with Donn that power of these carts is proportional to traction. Traction is, however, only proportional to weight if the wheels/drive system are identical. I believe it is essential to maximise traction in these systems because extra weight simply means more work hauling up hills (if you have them). There was much research done at the beginning of last century on how to transmit power most efficiently from engines to tractions, and this just works in reverse for transmitting power from traction to rotational work. It is clear that pneumatic tires are more efficient than steel wheels at transmiting power because of thier ability to float rather than dig in. If one uses pneumatic tires, I think that having good tires is important. Thinking outside the box, crawler tracks have even better traction per pound, about 50% more, compared to pneumatic wheels. I have never seen a tracked ground drive forecart though… It might work fantastically. One would have to think about how to turn, but I think that’s a solvable problem… Try to make those “wheels” slip! 😉

2. Because the power is generated from traction and the traction varies over terraign, I would think these carts would benefit greatly from a big flywheel. It’s going to be rotating rather slowly, though, so it would need to be big and heavy at the edges. I am thinking “combine sized”, not “baler sized”. Weight is a good thing here, though, because it also generates traction. If the flywheel is heavy enough to make up for fluctuations in the traction, it is going to be (actually it has to be!) hard to get started. A clutch would likely be needed. This could go a long way towards solving an intermitant tire slip problem.

Brainstorming…

[Andy Carson]

That’s true. I would encourage you to post your thesis on this site, it might be a very interesting read and stimulate conversation. I especially appreciate observations and reports from other countries or areas of the world. These reports provide alternative techniques and technology that are truly field tested, and might be applicable to current problems.

[Andy Carson]

Some people can’t use photos now?

Everyone can have a photo on the new site. We can also post photos on the site itself, rather than referancing photos on other sites. Does this answer your question, Erika?

[Andy Carson]

By the way, here’s a link to another thread discussing tie stalls
http://www.draftanimalpower.com/showthread.php?5742-Standing-Stalls&highlight=stalls

[Andy Carson]

I know it’s not exactly what you are asking about, but the niftiest tools I’ve seen recently is a manure trolley. There’s a photo below. I first saw this in Jay Bailey’s barn in Vt. When I first saw it, I thought it seemed little better than a wheelbarrow. It took only a few minutes to realize how vastly superior of a tool this is. No comparison, really, to a wheelbarrow. At this barn the manure was shifted by overhead trolly and rail to a single accumulation site, where it could be piled high and deep because it was dropped from above. Pretty slick. I would definately incorporate this into any design where you need to manage manure without a front end loader.

[Andy Carson]

For those of you with profile or avatar photos, remember to save these and upload them on the new forum.

Just so you all know, you will be able to read posts during the ~24 hour transition period, but you will not bea ble to post. This allows all the data to be moved over without potentially losing posts made udirng the transition.

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