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Author Topic: external force/weight application  (Read 778 times)

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Offline charles mann

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external force/weight application
« on: September 06, 2018, 12:34:48 AM »
not sure if i titled the subject properly. but after stacking and stickering the slabs, are ratchet straps a wise choice to add weight/downward force on a bundle of slabs? or even nylon banding, similar to what they use when items are banded to a pallet for shipping? 

iv got several hue pecans, a black walnut around 48" at base and 38" at the top of the 12' log. I'm not sure if the will be enough weight from the slabs being stacked on top of each other. yes, the bottom will have enough, but the last 2-3 top slabs, including the top cut, I'm afraid there wont be enough weight to prevent warping. the slabs will be cut 2.5" thick.

also, how long should I air dry the pecans? 1 is over 60" at the small end of the log and the other is just over 48" at the butt, but due to a limb, is close the same at the top.behind the limb, i estimate around 42-44". 
1 was struck by lightening and has been down for 6-8 months, but still had plenty of moisture seeping from it when i separated the limb/canopy structure from the log. the other has been down only a month, maybe 2. 

would the lightening struck pecan be worth my time to mill it?
Temple, Tx
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Offline kelLOGg

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Re: external force/weight application
« Reply #1 on: September 06, 2018, 07:19:12 AM »
I use ratchet straps in my kiln but, then, I dry MUCH smaller stuff than you. I put them about 2' apart and retighten as drying progresses.
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Offline WDH

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Re: external force/weight application
« Reply #2 on: September 06, 2018, 07:59:43 AM »
Time to air dry the pecan depends on how thick that you cut it.  4/4 about 6 months.  9/4 at least one year.  It will gray stain on you if there is not good air flow. 
Woodmizer LT40HDD35, John Deere 2155, Kubota M5640SU, Nyle L53 Dehumidification Kiln, and a passion for all things with leafs, twigs, and bark.  hamsleyhardwood.com

Offline GeneWengert-WoodDoc

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Re: external force/weight application
« Reply #3 on: September 06, 2018, 08:51:46 AM »
The straps put a lot of downward pressure on the pieces at the edge of the topmost layer, but not much downward pressure on the pieces in the center of the layer.  There can be some sideways pressure on all the pieces in the top, but only if all the pieces are tight, edge to edge.  We did a test on a strapped pack by pulling lengthwise on pieces and found it was fairly easy to pull pieces out from central locations in the top five layers.

We do know that sun and rain effects are big factors in warping of the top several layers, so a roof is really important.

Any top weight is effective, but in general, about 75 pounds per square foot seems to be the most practical target.  This is around 6 of concrete uniformly across the top.  

Note that green wood has a density of about 4000 to 6000 pounds per 1000 BF, so for 4/4 that is about 4 to 6 pounds per BF or 4 to 6 pounds per square foot.  So the effect of 6" of concrete is roughly the same as 12 to 18 layers of 4/4 lumber.

A key concept is "uniformly" rather than a lot more weight in a small location.  Because the weight is transferred from one layer to the one below through the stickers, consider a load that is 4 feet wide and 12 feet long with 1-1/2"wide stickers 24" apart.  This means 7 stickers per layer.   The total weight of the concrete with 75 pounds per square foot is 75 x 4 x 12 = 3500 pounds.  So, this is about 500 pounds total per sticker, uniformly along the 4 length of the sticker.  Each layer of 4/4 would add 35 pounds to the weight on the stickers.

Sometimes people will add extra stickers to a load to get flatter lumber.  More stickers means the same amount of weight is more uniformly distributed.

Another key is that the weight needs to be applied at any time that the lumber is changing moisture content, not just in air drying, or not just in kiln drying if the moisture will change after kiln drying.  We did some testing in Arkansas with southern pine dimension lumber using weights attached to the roof of the kiln.  The lumber had no weight at the end of drying when it was pulled out of the kiln at varying MCs and then allowed to spend a few days in an outside shed where the MC equalized.  During this final MC change (equalization), we got a lot of warp on the topmost layers.u
Gene - Author of articles in Sawmill & Woodlot and books: Drying Hardwood Lumber; VA Tech Solar Kiln; Sawing Edging & Trimming Hardwood Lumber. And more

Offline charles mann

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Re: external force/weight application
« Reply #4 on: September 06, 2018, 10:29:36 AM »
Time to air dry the pecan depends on how thick that you cut it.  4/4 about 6 months.  9/4 at least one year.  It will gray stain on you if there is not good air flow.
Not sure what the 4 stuff is in terms of inches. From what iv gathered, 8/4 is 2, so a 2.5 slab is going to take a yr to air dry. Wow, not bad then. I thought the rule of thumb was 1 yr per 1 in of thickness. 
Now is that yr of air drying a time before mc is to a low enough level to put in a kiln, or is that year for ready to use material? 
Temple, Tx
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Offline YellowHammer

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Re: external force/weight application
« Reply #5 on: September 06, 2018, 12:26:44 PM »
4 in the denomominator means quarters of an inch so 1/4, 3/4, 2/4 and 4/4 etc.  its a general, approximate term but generally defines the gross dimensions of the boards.

The drying rate of wood in air is dependent on species, location, season, wind, and other factors.  There is published data defining the minimum air drying EMC that wood will reach based geographic location.  Its a general guideline.  

The one year per inch is an easy number to remember, but lots  of factors are in play.  In the summer with fans I can air dry 8\4 Poplar in a couple,  maybe 3 months.  

Easiest way to track the local drying rate is your moisture meter.  It will give you feel for your locale.  


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Offline Ianab

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Re: external force/weight application
« Reply #6 on: September 06, 2018, 03:41:46 PM »
1" per year is pretty much a worst case scenario. If you leave wood that long you can be pretty sure it will be dry. You may also have been looking at a stack of dry wood for the last 10 months. Wood can be dried from green in most kilns, although some species need slow drying, and that ties up your kiln for a long time. So some folks air dry a bit first, then finish in the kiln. Takes longer, but means more wood through the kiln per month if you can finish 4 loads, vs 1 load from green (for example). Takes longer, and you have more inventory sitting in the drying stacks, but you are getting 4X the wood dry.
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Offline charles mann

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Re: external force/weight application
« Reply #7 on: September 06, 2018, 07:51:47 PM »
Im gonna try and search through the topics in this section for drying times for certain species. I just got a line on 12 black walnuts between 25 and 36. Intially, i only wanted to mess with my 2 pecan logs and branch sections and my 40+ walnut log and branch section. But now with these new walnut trees, ill have enough for a while. Not sure if my 20 conex will hold all the pecans and first walnut log. 

Still have to build my mill AND insulate and get a fan and heating source for my conex. A LOT of hustling to do with 6 days a month at hm to do it. 
Temple, Tx
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Offline WDH

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Re: external force/weight application
« Reply #8 on: September 06, 2018, 10:22:51 PM »
Now is that yr of air drying a time before mc is to a low enough level to put in a kiln, or is that year for ready to use material?
Low enough to put in a kiln.  Still not dry enough for furniture or inside use.  I try to air dry to get below 20% moisture content before putting any wood in the kiln. 
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Offline GeneWengert-WoodDoc

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Re: external force/weight application
« Reply #9 on: September 07, 2018, 08:08:43 PM »
You can get air drying times for various locations, various species and 4/4 and 8/4 on page 41 Table 4.2 in Drying Hardwood Lumber. This table clearly shows one year per inch is grossly incorrect.
Gene - Author of articles in Sawmill & Woodlot and books: Drying Hardwood Lumber; VA Tech Solar Kiln; Sawing Edging & Trimming Hardwood Lumber. And more

Offline charles mann

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Re: external force/weight application
« Reply #10 on: September 07, 2018, 08:28:40 PM »
You can get air drying times for various locations, various species and 4/4 and 8/4 on page 41 Table 4.2 in Drying Hardwood Lumber. This table clearly shows one year per inch is grossly incorrect.
Thank you sir. 
Ill see if i can find a pdf version online and download it. 
Temple, Tx
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Offline charles mann

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Re: external force/weight application
« Reply #11 on: September 07, 2018, 11:05:57 PM »
You can get air drying times for various locations, various species and 4/4 and 8/4 on page 41 Table 4.2 in Drying Hardwood Lumber. This table clearly shows one year per inch is grossly incorrect.
Mr. Gene, 
found copy online in pdf and found 1 on the USFS site with the help of my fire manager. 
again, thank you sir for you input and insight.
Temple, Tx
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Offline GeneWengert-WoodDoc

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Re: external force/weight application
« Reply #12 on: September 08, 2018, 09:31:22 AM »
When to stop air drying and go into the kiln is an interesting issue.  All species can be kiln dried green from the saw.  The issue is that air drying, even with some quality loss due to poor drying weather (too wet or too dry), is usually less expensive than kiln drying green from the saw.

Air drying is attractive also because it allows more loads through the kiln in a year's time.

Once air drying starts, it is best air dry down to about 25% to 30% average MC because going into the kiln at higher MCs can lead to color differences, shell to core.  That is, the outside of the lumber will develop one color due to the humidity, temperature and drying rate in air drying, while the core will get another color due to the heat and faster drying.  Bi-colored lumber, shell to core, is often a defect.

For the whiter colored woods, oftentimes air drying is too slow to get the whitest color, so these woods are almost dried green from the saw even though it is expensive.

If air drying is done too long, there will be more warp, more risk of interior checking, more risk of fungal and insect damage, more risk of staining, more risk of warp, and so on.  A large roof minimizes some of these risks by keeping rain and sun off the lumber.  Long air drying can result in final MCs of 14-15% MC. 

Kiln start-up...if the lumber in air drying has beeen exposed to an average of 65% RH or 12% EMC, then the kiln should be started no higher than 12% EMC, and it is usually better to be started 1% or 2 % drier than the average outside EMC.
Gene - Author of articles in Sawmill & Woodlot and books: Drying Hardwood Lumber; VA Tech Solar Kiln; Sawing Edging & Trimming Hardwood Lumber. And more

Offline rjwoelk

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Re: external force/weight application
« Reply #13 on: September 08, 2018, 11:23:19 AM »
I see gene answered my question before i asked. 
I always thought of the straps too the only thing i was going to add to them was a coil spring from a cultivator to keep the tension.
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Offline GeneWengert-WoodDoc

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Re: external force/weight application
« Reply #14 on: September 08, 2018, 12:53:00 PM »
A very stiff metal bar held at each end by springs and placed over each sticker crossing, with the other end of the spring fastened to the lumber in a lower layer, can apply force.  With 2 sticker spacing and 4 wide pile, each bar and two springs would have to apply about 560 pounds to get the same effect as 70 pounds per square foot.  Note that this is the same force required for a strap at each sticker crossing.
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Offline Don P

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Re: external force/weight application
« Reply #15 on: September 08, 2018, 09:35:44 PM »
In some glueup operations a deliberately bowed caul is used to deliver pressure evenly to the center of a panel as clamps are applied at the ends. I think a little math with the elastic modulus and size of the caul would give the curvature needed to flatten out at a pressure which should give fairly uniform pressure across the stack.

Although with the ratchet on one side, I'm not sure that is true as you tighten.


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