Draft buffers

[mstacy]

Tim,

This is a great thread you’ve started. How are you measuring draft loads (spring scale, load cell, etc)?

I would love to see draft versus time graphs if you are using some sort of electronic data aquisition. The load buffering you refer suggest that periodic variation in ground speed is the root cause. I had not considered this but it makes perfect sense. You don’t happen to have any ground speed (vs time) data do you?

Load buffering could potentially benefit the team (and the teamster getting more work out of them) by reducing shock loads. Also consider than an elastic tug allows the team to use momentum to start a load without hurting themselves (akin to pulling a stuck car out of the ditch with a nylon strap instead of a steel chain).

Thanks for the insight on wheel compliance too. You make a great point about the importance of substrate hardness as a factor in the overall rolling resistance.

Great work. Keep it up.

Regard,

Matt

[Tim Harrigan]

mstacy;17471 wrote:

Tim,

I would love to see draft versus time graphs if you are using some sort of electronic data aquisition. The load buffering you refer suggest that periodic variation in ground speed is the root cause. I had not considered this but it makes perfect sense. You don’t happen to have any ground speed (vs time) data do you?

Here is a graph showing speed variations with the associated variations in draft. This is with oxen pulling a 1200 lb sled. There are other sources of variation in draft and pulsing, tillage is a good example with changes in soil, depth, moisture etc.

[near horse]

Is there anything we can say about the relationship on your graph between draft and speed? This might be “the chicken or the egg” type of question but does one seem dependent on the other? Were the animals allowed to move out at the speed of their choice or were they held to a certain range (like only walking)? I know the speed and draft measurements were taken very close together in time but were they actually simultaneous? It just seems that ground speed should end up being dependent on draft – higher draft we get a lower speed. That looks like the case in some instances while in others – not so. Or is it a constantly shifting relationship in which higher draft results in a momentary slow down which in turn causes the animals to increase pull which then increases ground speed momentarily …..

Oh the challenges of applying physical concepts to biological systems! Good work Tim.

[Andy Carson]

I think Carl’s comments give not only a good way to think about this but also good way to mathematically model draft power loss and Tim’s force and velocity charts provide some data to do this with. Forgive the switching back and forth to metric and english measurements, I have an easier time doing physics in metric and I’ll dual list any important units… I see that in Tim’s example, the draft forces for the 1200 lb sled fluctuates between about 400 and 600 lbf, that is a differance of 200 lbs (890 Newtons). Force=Mass*Acceleration, so an 890 newton force should result in an acceleration of 1.63 m/s/s (F=MA, 890=545*A). Applied over 0.2 seconds, this would yield a velocity increase of 0.327 m/s (1.63*0.2) and a final velocity of 0.886 m/s (0.559 initial velocity + 0.327). 0.886 m/s is 1.98 mph, and the sled clearly does not reach this speed. This represents at least one source of power loss, but is it significant? The sled does fluctuate in velocity, with several fluctuations between about 1.25 and 1.5 mph (0.559 to 0.671 m/s) occuring in the 0.2 second range. This is a difference in velocity of 0.112 m/s (0.559-.671), and is a result of an applied 305 newton force (F=MA, F=545*0.112/0.2). So, as little as 37% of the peak force (305/809) in these high draft peaks is applied to the sled, and fully 63% might be wasted! As even the highest differences in draft force yield nearly the same speed, very high peaks in force are even more wasteful. These are the areas I am targetting. This analysis assumes alot (including that the sled/ground friction forces are constant and are overcome by the low draft forces) and probably represents a “best case” scenario. But still, it is nice to see that are there are large amounts of power that can be recovered from the high draft peaks.

[Carl Russell]

This is really helping me to visualize why the logging arch is such an effective tool, regardless of the high draft. Hitching the log so that the chain is tight and vertical from log to hitch point, allows the horses to advance with relatively light resistance. In fact the resistance increases as the line from hitch point to the far end of the log straightens out.

As the horses move forward, the front of the log comes off of the ground before the log even begins to move, which loads the hitch with the stored energy of the forward motion in the form of the weight of the log suspended off the ground. Now the weight of the log is positioned to fall back to the ground in a forward motion, so it is applied directly to the effort that the animals are exerting.

In this way the animals can move more weight from less effort, because the energy is captured and reused, or added back into the continued effort. Any situation in the progression of work where the log drags harder provides another opportunity to capture the extra effort, as apposed to the exhaustive effort from increased draft.

This is why although the angle of draft is not ideal for the bio-mechanics of the horse, the equipment is giving more than a direct advantage of lift. It is reducing friction, but perhaps more importantly it seems to also buffer and capture energy, that when released, compounds the constant effort already being applied.

The other point about the log and chain as buffer is that it is unlimited in its effectiveness, and it is automatically adjustable to whatever the load or draft demands, as apposed to measuring, and pre-loading a coil spring.

Hmmmmm????:rolleyes:

Carl

[Andy Carson]

Carl, I couldn’t agree with your logic more. On the arch, you are probably mostly storing the energy by elevating the log. I have no doubts that the swinging chain could store all the excess energy produced during the “spikes” and think it’s likely that it would be released at a correct rate to be useful. Maybe if the chain is nearly horizonal, you might get a “pop” in energy release (as opposed to a “push”) but I don’t know how often that happens. Also, if the log is suspended from a nearly verticle angle (like if it was winched up), the rate of fall is close to zero and the system might fail to effectively store energy until the chain and log swings back to a “ready” angle, which might take a little time. Logs being hauled (as least from what I’ve seen) usually reside in between these two extremes and are probably in a position to immediately act as an effective buffer. Doing a more detailed analysis of the rate of fall for a log on a swinging chain requires calculus (I’m pretty sure) and I’m a bit too rusty to do that in public… The real beauty of the system, at least to me, is that the buffering capacity AND the draft load are both proportional to the weight of the log. In fact, the example is so similar that is might provide an effective demonstration of the usefulness of the buffering principle. Has anyone had to hitch to a log in such as way that half the log is elevated but the log was chained so as not to allow a swing??? I would guess that this would be more tiring to the horses and illustrates how effective buffers can be… I don’t have a logging arch, but someone out there might want to do a little experiment and compare the effort to pull a suspended and swinging log to the effort required to pull a suspended but not swinging log… It would just take a chain, a couple chains, a come-along, and a little “conditioning” time. The weight of farming implements is not proportional to the draft they produce, so some other energy storage system (like a spring) is needed. Preloading the spring is analogous to swinging the logging chain back to a “ready” position where it is ready to store and release energy in a timely manor. The maximum load is analogous to the load that would make a the chain on a logging arch nearly horizonal, which also represents the maximum that the logging arch can buffer. Not really that dissimilar when you think about it…

[Tim Harrigan]

near horse;17482 wrote:

Is there anything we can say about the relationship on your graph between draft and speed? …. Were the animals allowed to move out at the speed of their choice or were they held to a certain range (like only walking)? ……I know the speed and draft measurements were taken very close together in time but were they actually simultaneous?….. It just seems that ground speed should end up being dependent on draft – higher draft we get a lower speed.

The animals were advanced at a normal ground speed for that work. I am not sure about the graph, generally higher speed means somewhat greater draft. Work is force times distance and greater speed gives greater distance. That would be more so with tillage tools than wagons. Tillage draft generally increases fairly rapidly with speed. The draft and speed measurements were simultaneous. I have not tried to line them up exactly to see how they lay out, there is probably a very slight lag in the draft measurement relative to the time stamp. I do not think the animals are that sensitive to draft regarding their speed in the normal range of drafts. They seem to have a comfort zone that they move in but they will get out of it with really big loads.

[Tim Harrigan]

Now I wish I had gathered more data when we were working with the arch. There is always another day.

I think we often underestimate or fail to recognize the subtle insight and application of mechanical advantage that so many of these farm tools and implements provide. The harness and ox yoke, eveners and single trees, there are so many examples of brilliance from those humble craftsmen.

[Carl Russell]

Countymouse;17496 wrote:

…… Has anyone had to hitch to a log in such as way that half the log is elevated but the log was chained so as not to allow a swing??? I would guess that this would be more tiring to the horses and illustrates how effective buffers can be… I don’t have a logging arch, but someone out there might want to do a little experiment and compare the effort to pull a suspended and swinging log to the effort required to pull a suspended but not swinging log…….

I have used a bunk cart where the logs are elevated on one end and chained to a bunk. The load moves very easily.

I would say that the comparisons should be one of dragging log versus log swinging on a high hitch, as I think the buffer affect is adding advantage over the dragging log, not a suspended log.

I just think the discussion is most useful in helping to understand where energy is wasted, or where it can be captured to make a limited living power unit more efficient.

Suspending the logs completely provides significant advantage, but if the buffer theory is strong, then in any case where draft spikes do to changes in landscape etc. it could be beneficial to consider how to ad a buffering system.It may not be so beneficial as to out way the practical application, but knowing that the concept is pertinent, is a good place tp start.

Carl

[Andy Carson]

Sorry, when I wrote about the “suspended and swinging” log, I meant suspended and swinging on just one end. The other end would be dragging. I didn’t explain that very well… I agree that there would be little buffering advantage in a completely suspended log. I think a bunk cart would be a good comparison, provided the attachment point on the log was roughly the same as with a chain type logging arch. If the log was moved up onto the bunk cart much, there would be less dragging weight than with the arch and it wouldn’t be a fair comparison anymore. If the dragging weight is similar, how does the effort compare between a bunk cart and a logging arch using a chain? Is there a noticable difference in effort?

[Carl Russell]

Countymouse;17510 wrote:

Sorry, when I wrote about the “suspended and swinging” log, I meant suspended and swinging on just one end. The other end would be dragging. I didn’t explain that very well… I agree that there would be little buffering advantage in a completely suspended log. I think a bunk cart would be a good comparison, provided the attachment point on the log was roughly the same as with a chain type logging arch. If the log was moved up onto the bunk cart much, there would be less dragging weight than with the arch and it wouldn’t be a fair comparison anymore. If the dragging weight is similar, how does the effort compare between a bunk cart and a logging arch using a chain? Is there a noticable difference in effort?

I understood what you meant, and I was making reference to the same thing you describe above. If the log is on the bunk at 18″ above the ground then the draft angle is similar for the horses, but there is virtually no friction.

When using a typical arch-cart, without a winch, the logs are usually hanging vertically below the hitch point, and often are either on the ground, or only a few inches above, when resting. It is when the cart moves forward that the upward lift occurs, and the “suspension” is a result of the difference between power and the drag on the log.

If a log is suspended 2-4″ above the ground, say on a Go-devil, then the hitch point is much lower, providing the animal with a much more suitable angle of draft. This way the animal can provide lift on the front of the log, and utilize their bio-mechanical advantage more efficiently. The load is moved more easily, and because of this the comparison would not be fair.

People who have used a cart like a pioneer cart to skid logs, where the draw-bar is only 8″ off the ground can attest to the fact that the cart gives virtually no advantage, and possibly draws harder than just ground skidding with loose rigging, precisely because the hitch point is not high enough to provide any lift.

This is why the arch-cart is so innovative. Getting the log off of the ground has been demonstrated to provide advantage, but when combined with a mechanism that can capture the spikes in draft it goes outside the box.

Carl

p.s. Oh, and I wanted to say then you add to this the maneuverability, safety, and functionality of the design, and there is a very important tool.

[Andy Carson]

A picture of the much debated spring type draft buffer… I will test it tomorrow. It was pretty easy to put together, and only took about an hour once the spring was here. I have to admit, I am having doubts about the math to come up with this spring rate now, because the spring “feels” kinda light without a preload. With a good preload, though, I can’t move it… I’ll know if I was way off tomorrow.

[Tim Harrigan]

Looks good. For sure it will take some trial and error but with a little work it might work quite well. If your idea to trim the top 10% of draft forces can be achieved with careful observation it could be a useful task buffer. It would be a good learning tool also if you can calibrate the breakover forces for a range of compression settings. It will be interesting to see how well it works.

[Andy Carson]

That was a very interesting experiment and I learned a lot! To test the draft buffer, I made three 20 minute runs with a 1000 lb sled with different preloads on the spring.

First run: 2 inches @ 127 lb/in = 254 lb preload
This preload is substantially lower than the predicted effective preload and should not increase efficiency. I thought I would give it a try anyway and just see what happened. The first thing I noticed is that the load was easier to start from a standstill, but I wasn’t really that surprised at that. After that, I was struck with how smooth the ride felt. Normally, I prefer to stand on the sled rather than rest on the rail, but I quickly found myself leaning into the rail. No quick changes in velocity. Watching the compression of the spring, I could easily see that there are two nearly equal factors that compress the spring. One is the natural movement of the horse, with spring compressions every time the rear legs move from nearly vertical backwards to the end of the stride. I would have expected that this would have been the “power” part of the cadence, but was suprized to see just how powerful this part of the stride is, and just how weak (comparatively) everything else is. Watching the spring, it appears that the front legs can only “hold on” until a back leg stroke gets into the “power zone.” Another factor that extends the spring is the natural variation in the terrain. Interestingly, the variation in the terrain has about an equal effect to the natural variation in stride power. I had expected the terrain to dominate the spring extension. Definitely wrong there… Sometimes, the compression of the spring due to stride occurred at the same time as a bump in the terrain. This caused a movement of several inches in the spring. When the energy was released, it caused a swinging effect that was too slow and subtle to feel, but you could see on the spring. I was not sure whether the sled was speeding up and slowing down or the horse was speeding up and slowing down, but one of the two was. Once the “swinging” started, it continued for several seconds causing wide variations in draft that nearly bottomed out the spring and annoyed my horse greatly. Over the 20 minute test run, the effort seemed to be about the same as without the buffer, except during those times when the “swinging” was going on, when it was a noticeably more difficult pull.

Second run: 4 inches @ 127 lb/in = 508 lb preload
This was what I would have predicted to actually help with this load. Watching the spring, I basically only see compression at times when the terrain is moderate to difficult AND it’s in the middle of the powerstroke of the horse. This still produced a smooth ride on the sled. The spring only failed to come back to the resting point after the “powerstroke” if the terrain was very difficult or we were going up a hill. The preloading eliminated the swinging effect. Interestingly, this system greatly reduced the “bobbing” head motion during the high draft areas. It’s a pretty dramatic difference (with respect to the head bobbing) but I would be speculating a lot to guess why… Perhaps others out there have ideas? Overall, I could see a reduction in draft compared to the first run, but I am not sure about how this compares to a non buffered system. It’s not a huge difference, that’s for sure. At maximum, I measured a 7/8 inch displacement, which corresponds to a maximum draft of 619 lbf.

Third run: 5 inches @ 127 lb/in = 635 lb preload
In this run, I compressed the spring beyond the maximum compression seen in run 2. If the spring is compressed at all in this run, than the draft is higher than seen in run 2. I counted 58 times that the spring was compressed in this run, which demonstrates that the spring settings in run 2 were able to reduce the maximum draft force compared to this setting and probably compared to an unbuffered setting as well. Every time the spring was compressed, it was very brief, and I am not sure about how tiring 58 0.2 seconds compressions can be over a 20 minute run… Maybe I’m wrong, but I don’t think that simply reducing the magnitude of the maximum draft force is going to be a game changer… The larger head bobbing in areas of high draft was back though, and this was a dramatic difference that occurred for larger periods of time. This might be a game changer in itself or indicate that a “game changing” phenomenon is occurring that I don’t understand… Overall, I though that this run was maybe a little more tiring than run 2, but I am biased. It’s not a night and day difference, and would require some sort of more careful analysis to convince myself or others.

I am open to suggestions of tests that would be more convincing…

[Tim Harrigan]

That is interesting. How did you calibrate the pre-load on the springs? I am not sure it is linear across the range from fully compressed to released. It is nice to hang some known load on is just to be sure. I have learned not to be too assuming about whether draft is increasing or decreasing, I still am not sure what is better from the animals point of view, a more even pull or lower total draft, and to what extent you can have either one or both at the same time. Perhaps just observing the signals from the horse are best. I am going to be in Pittsburgfor a few days in June, if it works out maybe I could bring some equipment along and measure pull to compare with what you have.

I am most accustomed to observing oxen at work. Maybe some of the horse teamsters can comment on the head bobbing relative to high-draft work or any other observations.

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