Draft buffers

[Carl Russell]

Countymouse;18700 wrote:

Carl,
I’m curious if you have had a chance to try out the buffer yet. Do you have any thoughts you might share? Don’t worry about hurting my feelings if you don’t see an effect. I feel like I do, but I wanted to hand it off to someone else to see if they see the same thing…

I will be putting it to use today with some pine logs. Been pretty busy in the garden lately, and working the team in the woods.

I’ll report back, Carl

[Andy Carson]

I am planning on presenting the theory and practical applications of draft buffers at the 2010 NEAPFD. In thinking about how to best explain the concept, it has become clear to me that any amount of power stored in the spring (as represented by spring compression beyond the average draft) MUST be released back into the system. It seems obvious to me now, but really the power has no where else to go… Steel springs are incredibly resilient and the power will not simply be dissipated. Because releasing the stored energy produces positive work (pulls the load forward), then a properly designed draft buffer HAS to be producing a positive effect. That is, unless it is either 1. Not compressing over the average draft 2. Not decompressing 3. Produces harmonic oscillations resulting in slow fluctuations in velocity or 4. Dissipates stored energy. How much power is recovered and how much pracical impact this has on work are still open questions.

[Carl Russell]

Andy I had a chance to use the single tree in the woods the other day. Things move to fast and variable for me to notice much difference, but I did see it compress the spring a few times. I was a little worried about the ruggedness as it is quite a bit lighter than what I use in the woods.

I want to try it a few more times, just skidding logs on a main trail befoe I send it back to you…I was twitching with it, and with stops, and turns, and variations in the terrain it was hard to see what was happening.

I think you are on the right track in terms of describing how it works, and I think the theory is sound. I can’t wait to hear/see your presentation.

Carl

[dlskidmore]

@Countymouse 18950 wrote:

any amount of power stored in the spring (as represented by spring compression beyond the average draft) MUST be released back into the system.

Not entirely true, some energy can leave the system in the form of heat. Anything that stores or transmits energy has an efficiency rating, of power output/power input. If there was no power loss, a spring that is pushed will bounce back and forth forever and never stop, but it does, because it takes energy to deflect the shape of the spring, and this turns into heat.

That said, I use a shock line for most all dog drafting. It makes it easier on the animal, taking the shock/vibration out of the starts and sudden stops of the load. The energy loss is not considerable in comparison to the comfort it gives the animal, allowing it to work harder/longer.

[dlskidmore]

@Tim Harrigan 18251 wrote:

Here is a picture of the nylon rope. The dense rubber insert is compressed when the rope is under tension.

This is exactly what we do for the dogs, only heavy duty. I’ve not found an entirely satisfactory method of affixing the bungie inside the tugs. My most successful was French whipping, but that’s starting to give out on my current set of lines. Thankfully poly and waxed twine are both cheap, so I can just re-thread the bungee through a new rope and try again.

Some of my previous attempts:
[IMG]http://lh4.ggpht.com/_5XMXouoXO_c/SOlYMsPov6I/AAAAAAAAB08/B_ohJ2OytDA/s144/IMG_0610.JPG[/IMG][IMG]http://lh6.ggpht.com/_5XMXouoXO_c/SOlYQ_PlkPI/AAAAAAAAB1E/aaqky95XVgg/s144/IMG_0601.JPG[/IMG]

[dlskidmore]

@dlskidmore 18973 wrote:

This is exactly what we do for the dogs, only heavy duty.

Correction, the dog lines are not compressing the rubber insert, the nylon is squished up over it, and the ends are fixed to the rope, allowing for it to stretch. Only in case of bungee failure or extreme load does the load transfer to the nylon. This compression method has less shock reduction, but would be less prone to failure than the tension method.

[Andy Carson]

Carl, are you saying it compressed the spring entirely, as in the spring bottomed out? I was a little worried about logging with it because of the heavy and variable loads involved, but thought I would see what you could notice anyway. I almost threw in the third leaf spring in case your loads bottomed it out, and am kicking myself for not doing that now. Longer term impovements in efficiency are probably only going to be noticable on longer duration work, but I also noticed an improvement in starting loads. I guess it was hard for you to say one way or the other with your twitching? I suppose it would be, as a fair comparison would really require twitching the same piece of woode over the same path twice, and is not really a very useful activity…

[Carl Russell]

Andy, it did not bottom out, and seemed to be pretty strong. I did notice it compressing a few times, not bottomed out, but it was hard to watch it, and also hard to tell how it could be making the enterprise easier, or more efficient in terms of effort by the horse.

My difficulties are not related to the device, more related to the activity, and the related conditions.

The “lightness” I referred to had to do with the steel and the design of the single-tree. First of all, having the leaf spring on the front of the evener required that I hitch the heal chains longer that I typically would, so that the mare could get adequate extension on the variable terrain.

Then, being just a straight piece of angle stock with no support to prevent folding, I was nervous that I would fold it. You know, skidding over humps and through hollows the varying angle of draft can put strange forces against the single-tree, and I wanted to preserve the piece.

Typically my logging single-trees are tubular steel (1.5-2″Dia.) with 1/2″ steel rod bent around the ends forming loops for the heel chains, the back behind at an angle to the hitch point. This is based on the old spread chains transferring the pressure from the pull that is straight in line, to a pull that is compressed against the ends of the tube.

I realize that with the leaf spring the pull is not directed at the center of the single-tree because it is spread out by the spring, but none-the-less the it just looks like the angle stock would be too light to sustain an errant angle of draft.

Carl

[Andy Carson]

Carl, I wouldn’t worry about bending the single tree bending until the spring bottoms out. Besides, if it bends it’s easy to bend back. I would probably recommend a beefier one for logging in general, but this ought to show the concept… Actually, if you baby the buffer it is less likely to show any positive effect. The spring needs to be pulled hard enough that it compresses fairly often. That is probably a decent load even for a logger. I understand you hessitation with a new piece of equipement, but it was designed for a good working load and below that load the spring is likely to just sit there doing nothing. Besides, like I said, I can bend it back.

[Carl Russell]

So today I had a chance to use the buffered singletree in a situation where the ground is more consistent. I noticed that on a 12′ log with about 70 bf would compress the spring slightly and the drag was enough that it never really returned to the normal position until I stopped the horse.

It definitely functions as a shock-load absorber, but as far as increasing efficiency by returning stored energy it appears that this spring isn’t quite stiff enough.

I pulled a few smaller logs, and a few larger. The small logs didn’t really have any effect on the spring, but the larger ones, although not bottoming it out, created enough drag that the spring never recovered until I stopped the horse.

I didn’t have time, nor logs, enough to switch back to my standard singletree to see how the horse reacted, but he seemed to move pretty much the same as he usually does. I have to say that he must have felt the elasticity, but in this type of situation it was hard to evaluate how that affected him.

Carl

p.s. yes I was taking pictures while twitching these logs. Handy cell phone and responsive horse…..

[Andy Carson]

Great pictures Carl, I was never able to make any action photos look that good. Too bad you don’t see much of a differance though… It might be hard to notice a differance with such variable terraign and loads, but I am curious if the loads might still be too light. If the buffer was working well, it would probably reduce loads by about 15% Do you think you would notice the differance between a 70 bf log and an 80 bf log? That is one of the reasons I ended up doing some testing with heavier loads on the sled, because I really do notice the differance between a 1000 lb load (for example) and a 1150 pound load. In other words, the heavier the load the easier it is to see differances between buffer and no buffer. On a sled it is easier to adjust the laod, though and I understand you kinda have to work with what nature gives you. I am not sure what a 70 bf log weighs or what the drag would be like. When the buffer is working at maximum capacity, the spring threatens to bottom out. If the spring is barely moving the load is still too light in my mind. Thanks again for all your thoughts on this, it is really helping me think about how to improve and strengthen the design.

[mitchmaine]

russell’s question about wooden trees got me to wondering if wooden trees and eveners offered any buffer over their steel counterparts. we went to steel mostly cause when you abandon them like it usually happens, there they are rusted instead of rotten when you go looking for them.

mitch

[dlskidmore]

@mitchmaine 19178 wrote:

got me to wondering if wooden trees and eveners offered any buffer over their steel counterparts

I would think that would depend on design more than anything else. A steel tube would have very little buffering capacity, while a piece of flat steel, placed so the stresses would bend it the thin way, would have more spring than the wood.

[Carl Russell]

I figure that 70bf weighs about 6-700 lbs.

My point about the flex is that although it did compress the spring about 1-2″, there was enough consistent draft that there never was any chance for the spring to recover, thereby delivering the stored energy back to the task. It made for a squishy hitch, which may have some positive effect, but it was hard for me to see that it actually could increase efficiency. I would be more inclined to see a buffer as beneficial if it were able compress and then recover while the hitch was still moving.

Carl

[Andy Carson]

Hmmm… 600-700 pounds is in the range that I would have expected to see a positive effect. I am also disturbed by the fact that you did not see any decompression of the spring and you are correct that this means the buffer is not working as designed. It is difficult to imagine how energy will be returned without spring decompression. As I think the draft forces are likely to fluctuate just as much in your hands as in mine, I am most likely to blame friction here. I suppose it’s also possible that my horse is less smooth than yours, but that’s part of why I wanted a second opinion. So, for improvements thus far:

1. Generally strengthen the singletree with bracing and tubing similar to a standard singletree

2. Make brackets to act as pivot points one the ends of the spring (so as to reduce friction and make buffer more responsive)

Any other thoughts?

PS. When you say the spring never recovered, do you mean it never decompressed at all or just never decompressed completely? A partial decompression while moving also indicates stored energy being returned to the system. With heavier loads, I never saw the spring completely decompress, it just fluctuated between different degrees on compression. This is still a good thing from a design point of view. Compressing and not moving, though, is pretty bad.

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