Draft buffers

[Tim Harrigan]

near horse;17428 wrote:

One other question regarding one of your graphs Tim. Why did using nylon harness on the steel wheeled units result in higher average horizontal draft for both oxen and horses? Didn’t the nylon harness give lower values for the rubber-tired wagons?

The nylon did give lower values for the rubber-tired wagon when pulled by the horses. There was not much difference with the ox drawn rubber tired wagon, perhaps because all the pull was through a single rope rather than 4 traces and the applied pull did not match the elastic coefficient of the nylon rope.

I think with the higher draft of the steel-tired wagons it seems like the elastic connections pre-loaded a certain level of pull in the traces and the recovery speed of the nylon was too slow to return many of the pull forces and allow an accumulation of low draft pulls.

[Tim Harrigan]

Carl Russell;17433 wrote:

……… but it still brings me back to where does the energy go that is required to do the work?

There is energy wasted in every mechanical system as friction, heat, etc. It seems like the spiked forces that are in excess of what is needed to move the wagon is wasted energy. The goal of a buffer is to capture that excess energy and return it as useful work. So it is an attempt to improve the mechanical efficiency of the system and on the high end minimize the soft tissue buffering. The forces that the animals buffer could be seen as wasted energy as well.

[Tim Harrigan]

Carl Russell;17433 wrote:

The unbuffered must surge forward and reduces the draft more than the buffered one. Somehow it seems there must be a balancing there somewhere.

It seems like loads that are surging forward are going to be light loads so maybe it is not quite so important. With heavier loads it would be most helpful to buffer the high end loads which is what the nylon did with the rubber tires and horse harness.

I think the balance is the real challenge when considering a draft buffer. Generally they are going to be effective over a small range of pulls and you work your team over a wide range of pulls. That is why I come back to shock load protection for most situations.

[Carl Russell]

Tim Harrigan;17438 wrote:

There is energy wasted in every mechanical system as friction, heat, etc. It seems like the spiked forces that are in excess of what is needed to move the wagon is wasted energy.

What I guess I mean, is even though the buffering reduces the average draft, it can’t reduce the amount of energy that is required to do the work, with or without the buffering system.

The buffered system must recover the load over a longer distance than the un-buffered system. It would seem that even the heavy loads would have a slingshot affect, so although the unbuffered system will have spikes that measure higher, it should have dips in draft that measure lower.

If you use the stationary load as zero, then all draft measurements associated with forward movement will be cumulative, and the system with the highest highs will have the higher average. However if you took all draft measurements and graphed them around the median values then, I presume, you would see that the unbuffered system also has it’s energy advantages.

All-in-all an interesting concept. I’m glad to know how draft changes can affect the animals. I also know that I can address that in how I harness and condition my animals. For me it is like so many other things about draft animals, it is good food for thought.

Thanks, Carl

[Andy Carson]

@Tim Harrigan 17440 wrote:

Generally they are going to be effective over a small range of pulls and you work your range over a wide range of pulls.

This became abundantly clear when I started to do some of the math to figure out the rate of the spring, the length of the spring, the reduction in forward speed, and the preload. The one I am attempting to design may be useful in buffering average loads of around 300-500 pounds, and the user would need to adjust the preload on the spring within that 300-500 range. It would be a simple thing, but it would need to be done. The spring won’t kick in until the force exceeds 25% of the average, and will lose it’s ability to buffer (bottoms out) if the load exceeds 1000 lbs. It is designed for one horse, and geared around back calculations of force from my stoneboat work. If someone wanted to use the system for two horses, I suppose they might want to make two of these things, or they could make one bigger one. The bigger one would require a different spring though, not just a different preload on the “one horse” spring. A spring that can store this amount of energy and release it gently is a pretty big spring, but the way, the one I will use is a coil spring of about 4 inches in diameter and 12 inches long. Because there is an adjustment in preload that would need to be performed for each different job, and the system fails if the maximum loads are very high, it might be less useful for a logger than a farmer.

[Tim Harrigan]

Carl Russell;17433 wrote:

By comparing the two hitches to the zero of the stationary load, the spikes are not off-set by the reaction forces… or are they, and I can’t see that either?

Can we see these numbers showing high and low spikes above and below the average as zero? (like the wagon on the hay ground chart). That would show me more accurately how the buffer softens the line relating to draft.

Carl

Can you elaborate on this? I am not sure exactly what you are asking.

[Tim Harrigan]

Countymouse;17447 wrote:

A spring that can store this amount of energy and release it gently is a pretty big spring, but the way, the one I will use is a coil spring of about 4 inches in diameter and 12 inches long.

Is this a compression spring or extension spring? I think the Pinney springs mentioned earlier by JAC were extension springs (4) that were attached to the single trees. It will be interesting to see what you come up with and how well they work.

[Andy Carson]

Compression spring. I order it from McMasters and it’s not here yet, so I still have to play with how to rig it up. It’s probably going to resemble a beefed up drawbar spring (like a porch swing spring) and will probably use a long threaded rod or bolt through the middle to preload the spring. Turning the screw in or out to adjust preload will be a little annoying in this mock-up, but I think investing time in a better mechanism is probably a little premature right now. Any ideas on how I might set up an objective test of it’s efficacy? I don’t have Tim’s measuring equipment, but if I was thinking if I made a close up video of the spring in action, I could possibly determine the max draft force by the length of the movement of the spring… Maybe I could then preload the spring to a much greater degree (such that only very high draft could compress it) and take another video over the same course. If the spring is sometimes compressed at the high preload, but never obtains that same level of compression with moderate preload, I think this demonstrates a reduction in maximum draft forces… Not as good as Tim’s system, but it could be a decent “first pass” using equipment I have and might help me tweek things if needed. Any thoughts?

[jac]

Pioneer do a neat spring hitch set up for their sulky plows ..dono if that would be any use for what you need ??
John

[Tim Harrigan]

CM, sounds like a good plan. Do you have a set of tractor weights or something hang off the buffer to calibrate the pre-load? Seems like you could measure the threads and get a pretty good idea of where you are in short order.

[Carl Russell]

Tim Harrigan;17448 wrote:

Can you elaborate on this? I am not sure exactly what you are asking.

Tim, I have been having a hard time wrapping my head around these findings, and thought that if I could see a graph that showed the changes in draft force as work progressed then I could see the peaks and valleys. I have been stuck on how the load responds to the spikes in force, ie. the slingshot effect.

But just trying to figure out how to explain that better, and with your comment about lost energy, I have come to a better understanding.

The unbuffered system creates a spikey “wave” of changing force, which have energy dumps at each peak and valley. With the buffer, as soon as the force begins to increase the energy is stored in the “spring”, and then released, beginning immediately, and not only prevents the spike, but allows the animals to “re-use” the pre-exerted energy. This creates that Sine-wave that is much more efficient.

It isn’t that the load takes less power to move it, it is just that the energy exerted from the animals is used more efficiently.

I would think that having a buffer on each trace, like in the experiment would give more range of effectiveness, as a pposed to the draw-bar spring analogy.

If we look at the logging arch again, then the ability to load the swinging chain with forward energy probably has as much effect on the efficiency as getting the log off of the ground. Of course the log must come off of the ground to load the chain that way, but the motion of the chain allows the team to store their forward energy and apply it to further advancment.

Sometimes this brain of mine….. Thanks for providing all this information, and allowing me to delve into this in my way….perhaps if my desktop wasn’t crammed full of tax forms….

Carl

[Tim Harrigan]

Yes, there is the potential for more efficient energy use if the buffer response matches the demands of the load, but based on some of the trials it looks like they could be a drag on the system as well. If they can give the energy back in a productive way. It would be good to be able to view a graph of the pulling forces in progress as work progressed but it happens too fast to visualize it. It is a series of pulsing that is hard to put in context. It looks like electronic noise. The advantage of the frequency graphs that I showed is that they accumulate the forces of interest so you can see where they are over time.

The singletree springs would have the advantage of dividing up the load into smaller segments so they could be smaller. I would like to know a little more about their design parameters to be able to predict when they might be of benefit.

The logging arch example is interesting, of course the log would have to be chained up short to get it off the ground. In addition to the lift, and the potential forward movement of the log, there is additional buffering in the start as the arch begins to move an instant before the log potentially creating an easier and smoother start.

[Andy Carson]

When I had been thinking about this at the beginning, I had been thinking of two smaller extention springs, mostly based on the Pinney springs and as in Tim’s experiments. As I started to put pen to paper and see what spring constants would be useful, it became clear that no one spring would be useful for heavy jobs without some sort of preload. That lead me to compression springs as the system to compress would possibly be more simple. Another advantage of a compression spring is that when they are overloaded, they simply bottom out. An extension spring can be stretched beyond the point of return by a rock or other “immovable” object, and would require another mechanism to make sure this didn’t occure. I doubt this is a problem for Pinney-type overload springs, as thier spring constants are probably very very high, but for these types of drafts buffers it might be. When looking into the range of preloads that the spring must experience, and the speeds they must respond to, it came out that a longer spring was needed. The short springs can store just as much energy, but it would probably be released in one big “pop” as opposed to a long steady “push.” If we want to target 1/5-2/5 of a second response and release time, a spring of at least 8 inches in length is needed and a 10-12 inch spring is probably better. Dividing up the load between the traces does mean the diameter of the wire (and the resulting weight) used to make each spring could be smaller, but the time of response (and the length of the spring) would need to remain the same. Also, if a 12 inch spring had a diameter of much less than 4 inches, I start to worry about the potential of the spring bending sideways or “buckling” instead of compressing evenly. So, you still end up with a spring of approximately the same size, but just two instead of one. Also, one would have to make sure the springs are adjusted to the same preload, which seemed like more fuss than required… At first pass, it seemed like the spring forces would be distruted efficiently through the singletree to a drawbar spring. Was the only benefit of the two spring set up to divide the load and possibly reduce the size of the spring?

[Tim Harrigan]

Countymouse;17464 wrote:

At first pass, it seemed like the spring forces would be distruted efficiently through the singletree to a drawbar spring. Was the only benefit of the two spring set up to divide the load and possibly reduce the size of the spring?

I think so, but I am not sure. It seems like you are thinking it through pretty clearly. Overextension is my concern with the extension springs as well unless like you say the spring constants are very high. In that case it begins taking you back to shock load protection rather than task buffering.

[jac]

I seem to remember an old advert that was reproduced in a book for sprung loaded equalisers. This thread has got me thinking about the draft buffer theory. To my mind a team doing heavy discing for example will have a far heavier draft on ploughed land but have an extra buffer effect with each foot print in the form of compression beneath the hoof and a slight slipage..as opposed to the team doing harrowing on pasture with heavy draft but not as much.. if any.. slip or compression of foot.. so lose out on a small amount of buffer effect.. Just my way of seeing it.. The Pioneer advert does state the spring will help in sudden load situations…
John

[Author]

Posts