Draft buffers

[Andy Carson]

John,
I think that for a buffer to be effective, it must not only absorb the peaks in force, but also release that power in a way that be converted into useful work. I think it is likely that a pneumatic collar pad could be designed such that it could capture the peak force, but I am more doubtful of it’s ability to release that captured energy in a slow, long push. It is interesting that I was just talking to an engineer friend on mine who had been thinking about the use of a pneumatic air bag (made from load levelers). One abvantage to this system is that the pressure could be adjusted over a pretty wide range for different application. That essentially adjusts the initial spring constant of the system. The downsides are that while a spring can be adjusted in the field with a wrench, an airbag would need an air pump, and probably a high pressure one. Also, force versus volume in an air bag is an exponential fuction (pressure doubles every two fold reduction in volume) which means that there might be a limited range of travel where the response rate would properly act as a buffer in this system. That range can be extended (as in the long compression spring) by increasing the length of the apparatus. This might be great if it weren’t that I am trying to minimize the size (and especially the length) of the buffer to preserve the line of draft.

PS. Some of my experimental observations can speak the importance of response rate. I was able to adjust the initial spring rate in the first buffer by adjusting both the preload on the spring and the load on the sled. Although this study is very limited, I did not see any evidence that spring response rate can be too slow to be effective (as in a light load with light preload). I did, on the other hand, see that response rates can be too fast for efficient buffering. This was most evident during harmonic oscillations of the sprign with a light preload pulling a medium load. When the natural peak due to horse stride occurred at the same time as the most compressed oscillation of the spring, the spring nearly bottomed out. From my observations of the horse, this looks like this process results in a hard “pop” in draft, followed by a return to a normal pull. In theory, there ought to be a “push” of energy return there, but I can not see one and it is likely too fast to be useful.

[jac]

Thanks for your answers guys.. After I posted it I started thinking that an air collar wouldnt be a great thing anyway.. not because of the buffer aspect, tho now you point out the comparisons with springs I see how it might not work so well.. but because the manufacture of them would take collar making into the realms of big industry and away from small family run business’s.. Andy that idea for the leaf sprung singletree is interesting. Sometime in Europe early last century they had something similar but as yet I cant find any pictures..
John

[jac]

I found this on the http://www.freepatentsonline.com page.. I know there not draft buffers as such but equalisers… still interesting to see how our forefathers tried to ease the strain on their teams.. check out US number 1546002 and 1667790…
John

[Andy Carson]

Here’s the new draft buffer, this design incorporates a small leaf spring into a singletree. It’s very simple, easy to build and adjust, and doesn’t add any appreciable length to the setup. the leaf sprign seems a little less perdicatble with it’s response, and I am going to have to calibrate the spring with known weights, but am pretty happy so far. I am most concerned about the sliding of the spring loops over the angle iron. I greased the contact points, but if it takes a beating, i might put some plastic over it to keep down the friction.

[Andy Carson]

Here’s the new draft buffer, this design incorporates a small leaf spring into a singletree. It’s very simple, easy to build and adjust, and doesn’t add any appreciable length to the setup. The leaf springs seems a little less linear in it’s response, and I am going to have to calibrate the spring with known weights, but am pretty happy so far. I am most concerned about the sliding of the spring loops over the angle iron. I greased the contact points, but if it takes a beating, i might put some plastic over it to keep down the friction.

[Tim Harrigan]

Looks good, don’t worry about the initial design too much, you will probably end up changing it after you work with it for a while anyway. It is the action that is important at this point. Nice idea.

[Andy Carson]

Report from the leaf spring draft buffer…
One of the important differences between this buffer and the coil spring buffer is that friction must be overcome to compress the leaf spring buffer. Personally, I attribute most of the differences I noticed between the coil and leaf spring buffers to this attribute. In all honesty, the spring rate is also different as well as the distance of spring travel, but these seem like subtle differences to me and do not account for the phenomenon I observed…

1. Easing the starting of a load
This seems just as efficient with the leaf spring as with the coil spring. As the forces are rather high here, it is not surprising that the additional friction from the leaf spring has no effect here.

2. Smoothing of the forward speed/prevention of stopping
The leaf spring was not as efficient at smoothing forward speed as the coil spring. It is definitely smoother than without the buffer, but doesn’t achieve the “boat-like” feel of the coil spring. I attribute the small “bumps” to transmitted draft forces that were not great enough to overcome friction in the leaf spring.

3. Reducing maximum draft forces
Unknown…

4. Easing the strain on the front quarters
Still a definite decrease in strain on the front quarters, but not quite as efficient as the coil spring.

Even though the leaf spring does not seem as efficient of a buffer as the coil spring in some ways, it has one HUGE advantage. It can operate over a very wide range of loads with no need to adjust the preload on the spring. in fact there is really no need to preload the spring at all. I found this out by pulling a standard weight sled with a “light” preload. When I noticed the buffer was acting as it should (I had expected oscillations), I adjusted the sled weight and preloads up and down and the spring acted as it should at all weights and preloads I tried. Probably because of the friction, I do not see any harmonic spring oscillations at light preloads, and there was no threat of the spring bottoming out even with a heavy sled. The design also maintains a line of draft that is efficient and doesn’t take up any more space than a standard single tree. I am pretty happy with this! I might try something to decrease the friction a little, but I don’t think I want to get rid of it altogether anymore. I think it’s almost done!

[near horse]

Andy, you are a busy guy! Nice work. Please don’t take my questions as criticism but just thinking aloud and inquiry. So ….

In the leaf spring design is the leaf only held in place on the angle iron with the middle compression bolt and the ends are completely free? If they are free is that intentional?(I ask because in other applications the ends of the springs are anchored to mounted shackles….).

Also, do you think that the original arc of the spring before compression would impact how the leaf setup performs? I recall that some of the offroad enthusiasts used to have their leaf springs re-arced to make the curvature more severe.

[Andy Carson]

Thanks Geoff, and don’t worry about criticism. Without constructive criticism, I wouldn’t have gotten this far…

Yes, the spring floats on the angle iron and is only attached through the center bolt. The spring is prevented from twisting by the angle iron, and the center bolt holds the spring in place really well even without shackles. I had initially been thinking about incorporating a shackle-like system if this showed potential but am not so sure now. The shackle would add minimal length and largely get rid of the friction, but it seems the friction is helping somewhat in this system. I would like it to be a little slicker though. I had sprayed the ends of the spring with WD-40, but think I will try some more advanced lubricants before I make a shackle. I have also been thinking of attaching some teflon to the contact surface between the angle iron and the spring ends. That might be as friction-less as a shackle anyway…

The leaf spring arch is a funny thing and very counter intuitive to me. It seems stiff at the beginning, then gets more responsive, and might stiffen again when the spring is almost bent the wrong way. I will know more when I do a complete calibration curve. I started one, then didn’t believe the numbers and decided to do it again with the same set-up I used for the coil spring. The spring I bought was from tractor supply (part number #1750195) although I am using only two of the three leaves it came with. I took out the middle leaf as it seemed too stiff with all three leaves. My horse rarely got through that stiff initial spring constant into the “responsive area” with all three leaves, but with two it works great. Drilling springs, by the way, is tough work. I got a tip off some other message board that you can use a masonry bit for it. True, but it ruins the bit. At least I didn’t ruin one of my good bits!

[near horse]

Even the 2 spring leaf is stiff? I think of those “springs” everyone puts on wagon seats (old school) – those things seem pretty soft to me – or maybe I’m just a big old lard bucket!

[PhilG]

here is one we welded up for logs

[Andy Carson]

Looks great Phil. Does it seem to help with the loads? Does the spring rate seem appropriate? Any oscillations or threats of bottoming out?

[Andy Carson]

Another report from the leaf spring buffer…
I tried a variety of different lubricants for the spring ends where they rub the angle iron. I found that the spring ends really don’t need much lubrication to increase efficiency quite a bit. The biggest problem is figuring out a lubricant that doesn’t attact and hold alot of dirt. Heavy grease, for example, works great but grabs dirt really fast and the efficiency starts to fall fast once it’s dirty. Lighter lubes (WD-40 and similar) tend to get dried up by dust and quit working pretty fast as well. The best by far (at least from what I’ve tested) is the silver anti-sieze lubricant. It’s plenty slick, stays slick when dirty, and though some miracle of chemistry tends to move the dirt out from between the contact surfaces. When efficiently lubed, the leaf spring buffer appears to be just as efficient as the coil spring at starting loads, smoothing forward speed (the boatlike feeling is back), and easing the strain on the front quarters. It still doesn’t need a preload to prevent harmonic oscillations. Perhaps there is still enough friction with lube to prevent these… Or, perhaps harmonic oscillations are not possible (or more difficult) on a variable rate spring. At any rate, I tried a variety of loads on the sled with and without the buffer and the buffer seems to help decrease the total effort by about 12%. This is an EXTREMELY subjective measurement based on how comfortable and tired my horse was pulling these different loads (815 lbs on sled without buffer required about the same effort as 920 lbs with buffer). It’s definetly not something I want to hang my hat on, but it gives me some sort of idea on if this is even worth measuring. I think it is. Tim has forwarded some directions on how to put together a simple pull meter, so that’s the next step. I’m not sure if you all are still interested in the “play-by-play” but this still seems like a good place to report my findings and hear about others…

[Tim Harrigan]

Good stuff, Andy. It will be interesting to find out how close 12% is to what you measure. Keep up the good work.

[Tim Harrigan]

I was curious to see if the nylon rope would provide more buffering of sled draft under less resistance than a sled on bare ground so I did some tests in January when the ground was frozen and 3 inches or so of snow. The attached graph shows the pulling forces for six starts and a 5 second pull with a 1000 lb sled (scoot). The starting forces spiked to an average peak of 614 lbf for the nylon rope and 664 lbf for the steel chain. The average continuous pulling force once the sled was moving was 77 lbf with the steel chain and 80 lbf with the nylon rope. Even though the starting chain draft is higher I do not think the difference is large enough with these 6 comparisons to conclude there is a real and predictable difference between the chain and the nylon rope.

There are some interesting things about this. One is the surprisingly high starting spike that lasted about one second. I do not think it actually took this much force to start the load, but neither was I holding them back to ease slowly into the load to miminize the starting spike. It shows me that a if yoke fits well the team will not hesitate to step into and accelerate the load. If a team is reluctant to start a load there may be an equipment problem, or the load may be too big for the team to handle confidently.

Also, once the load on the sled began moving and developed momentum it required only a small effort, a pulling force equal to about 8% of the total weight of the load, to keep it moving. This is about the same effort needed to pull a steel-tired wagon of the same weight. So a sled or scoot is a very efficient method of conveyance on frozen and snow covered ground. The frozen ground was important, the sled runners did not cut in the ground which would have increased draft. Also, the snow was only a few inches deep so the sled was not pushing snow. That also would have increased draft.

It reminds me of the work we did measuring logging draft with tongs. The draft when skidding a log on bare ground was actually lower than when skidding on snow covered ground when the ground was not frozen. The log not only cut in similar to the bare ground, it plowed snow in doing it.

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