Technical Corner

IT IS FALL, AND YOUR LAYOUT IS MOVING!

It is fall in the Rockies.  The Aspen trees have gone through their annual colorful event and have dropped their leaves.  The elk have done their best for those who traveled to the high meadows to listen to their bugling.  Lawns have been mowed and trimmed for hopefully the last time this year.  Evaporative coolers have been drained and covered for the winter.  Sprinkler systems have been blown out so that the pipes will not burst about the time The National Western Stock Show comes to Denver.  There is snow in the high country along the Front Range, a lot of it.  Good!  All is well and it is time to head to the layout room – it is train time in the Rockies!

Our layouts have seasonal changes also, and they are somewhat coincident with the seasonal weather changes.  The layout probably gets worked on and operated more in the cooler time of the year.  The change of seasons is typically marked by changes in relative humidity.  The outdoor humidity increases in the cooler months but heating dries the air resulting in low indoor humidity.  In the Rocky Mountain Region we have very low winter time humidity, and indoor humidity can be really low.  

Seasonal changes seem to be the time of the year when we hear of modelers having electrical and operation problems as gaps close and maybe even the track buckles.  It may take a few weeks after the heating season starts for the gremlins to show themselves.  The frustrated operator may claim that everything operated perfectly all spring & summer and now “I have a short I cannot find” or “My favorite stretch of track, the stuff I cannot reach, has gone wonky.”

Model railroad layouts move.  Some time is required for the structure to conform to the new temperature and humidity conditions as the seasons change. Temperature changes in the layout room, maybe 10-15 even 20 degrees F. Humidity changes a lot, perhaps more than one might realize

Green wood may have a moisture content of more than 60%.  Kiln drying is generally required to lower the moisture content to the 6 to 8% range appropriate for wood used inside a heated building.  Subsequent changes in indoor relative humidity (RH) will result in moisture content that varies from near zero at 0% RH to near 25% at 90% RH and a temperature of 70 degrees F.

A book, THE SHRINKING and SWELLING of WOOD and ITS EFFECT on FURNITURE, published by Perdue University and quoted again below, gives an example of wood movement with actual calculated dimensional change.

“Moisture content changes of only a few percent are sufficient to cause significant shrinking and swelling of wood. In the case of the 32-inch wide sugar maple table top, for example, a change of 6 percent in moisture content results in a change in width, DW, of DW = 32· (9.9/100)· 6/30 = 0.63 in.  Failure to appreciate that changes of this magnitude occur in service may lead to serious problems if the furniture is not designed to accommodate them.”  

Longitudinal shrinkage is about 2 to 4% of radial shrinkage and that would amount to about 0.013 to 0.025 inch change in the length of a 32 inch long sugar maple table top.  Extrapolating this example for a 36 inch long table top yields a change of about 0.016 to 0.028 inches.  This will be compared to the change in length of a 36 inch length of Nickel-Silver rail caused by some temperature change.

Machinery's Handbook says that the thermal coefficient of expansion of Nickel-Silver is 9 micro-inches per inch per degree F.

Suppose that Nickel-Silver rail is laid when the layout room temperature is 75 degrees F.  Extreme operating temperature variation might be plus or minus 20 degrees F.  Layout operation at another time, when the temperature is 55 or 95 degrees F will be affected by the change in length of the rail.  

A 36 inch length of Nickel-Silver rail will shrink or expand (9x10^-6)*(36)*(20) = 0.0065 inch with a temperature change of plus or minus 20 degrees F. 

Wood, including plywood and flakeboard, moves significantly as the relative humidity and subsequent moisture content of the wood changes.  The longitudinal movement is about 0.016 to 0.028 inches over a 36 inch length. The radial or tangential movement is much greater.

Rail will move a small amount versus changes in temperature but nothing at all versus relative humidity.

The ratio of wood movement versus rail movement is about (0.016 to 0.028)/0.0065 or 2.46 to 4.31 times.  Roughly speaking, the wood under the rail on a layout will move about 2.5 to 4 times as much as the rail as temperature and humidity change.  And note that is considering the movement of the wood in its best direction.

If your layout suffers from a closed gap this winter, the likelihood that it was caused by an expanding rail is essentially zero.  

If you are laying track, leave a 0.02 inch gap at every rail joiner.  The clickety clack will sound good – unless you are modeling continuous welded rail. And, do not solder the rail joiners.  And, still further, wire a dropper wire to every piece of rail.

Oh, just one more thing, condition new lumber by letting it become acclimatized in the layout room for a couple weeks before building benchwork.

Well, another thing, the thermal coefficient of expansion of wood is about 0.5*10^-6 per inch per degree F.  This is 18 times LESS than Nickel-Silver and is hardly worth mentioning.

REFERENCES:

THE SHRINKING and SWELLING of WOOD and ITS EFFECT on FURNITURE

FNR 163 Published by Perdue University, Cooperative Extension Service, West Lafayette IN.

From the introduction:

“Shrinking and swelling occur as the wood changes moisture content in response to daily as well as seasonal changes in the relative humidity of the atmosphere, i.e., when the air is humid, wood adsorbs moisture and swells; when the air is dry, wood loses moisture and shrinks. Various finishes and treatments may be used to slow this process, but, in general, they do not stop it. Likewise, air drying and kiln drying the wood do not prevent the wood from subsequently gaining or losing moisture.”

From page 3:

“In general, the amount of shrinking and swelling which takes place is directly proportional to moisture content changes in the wood. Wood shrinks and swells the greatest amount in the tangential direction, about half as much in the radial direction, and about 0.1% to 0.2% in the longitudinal direction. “

LONGITUDINAL SHRINKAGE in SEVEN SPECIES of WOOD

U.S. Department of Agriculture, Forest Service.  Forest Products Laboratory, Madison, WS

U.S.D.A. Forest Service Research Note FPL-0203

From page 4:

“In the United States, the commonly accepted information on longitudinal shrinkage was summarized in 1931 by Koehler: “The longitudinal shrinkage of normal wood ranges from 0.1 to 0.3 percent.”

EFFECTS OF WOOD SHRINKAGE IN BUILDINGS

National Research Council Canada CBD-244. 

From page 4: Wood Truss Uplift

“An increasingly common effect of wood shrinkage is the upward bowing of wood trusses in winter.  This causes cracks between the partitions and the ceiling of up to 20 mm in severe cases. Wood truss uplift is primarily caused by the differential longitudinal movement of the upper and lower chord members.

Air in a well-ventilated attic space contains approximately the same amount of moisture as the outside air.  In winter the relative humidity of the outside air is fairly high; consequently, the top chords and web members will absorb moisture until equilibrium is reached with the surrounding air. The higher moisture content causes the top chords to lengthen.

The lower chords however experience a different phenomenon. Since in modern houses they are often covered with up to 300 mm of insulation, their average temperature in winter is closer to the indoor temperature. This causes the air spaces in the insulation adjacent to the wood to have a much lower relative humidity than the air adjacent to the top chords.  As a result, the air spaces adjacent to the bottom chords absorb moisture from the wood until an equilibrium moisture level is reached. The moisture content in the lower chords may decrease to less than 10% during the coldest winter months, and cause the chords to shorten.  As the lower chords shrink and the top chords expand, the peaks of the trusses are forced upward. This forces web members attached near the peaks to pull the lower chords upward, which, in turn, causes cracks between the ceiling and the partitions. If the chord members contain compression or juvenile wood, the amount of movement can be significantly increased.”

SOME BASIC CHARACTERISTICS OF WOOD

National Research Council Canada   CBD-85

From page 1:

“For Canadian woods the range of radial shrinkage is from 1.7 to 6.7 per cent, that of tangential shrinkage from 3.7 to 10 per cent.  The shrinkage along the length of the grain is not shown, but it is normally very much smaller, being from 0.1 to 0.3 per cent in total, except for some abnormal conditions that can develop. “

LINEAR MOVEMENT of PLYWOOD and FLAKEBOARDS as RELATED to THE LONGITUDINAL MOVEMENT of WOOD

U.S. Department of Agriculture, Forest Service.  Forest Products Laboratory Madison, WIS.

U.S. Forest Service Research Note FPL-073

From page 3:

“Veneer, plywood, and flakeboard specimens were subjected to various humidity conditions. Physical and elastic properties of the veneer were determined and the influence of these properties on the movement of plywood and flakeboards fabricated of like material was evaluated, The linear movement of the plywood and flakeboards was closely related to the longitudinal-to-grain movement of the veneer.”

This information about plywood and flakeboard should be emphasized.  Plywood has shrinkage characteristics similar to lumber in the longitudinal direction. This stability is due to the much higher modulus of elasticity of wood with the grain than across the grain. Alternating the direction of the grain in adjacent plies, therefore, stabilizes the plywood in both directions.  Waferboard benefits from a similar stabilizing effect because the individual wafers are randomly organized. If waferboard is soaked, however, the resulting increase in thickness can so weaken its internal bonds that it exhibits greater movement than would normal lumber.”

Note well that the word “Veneer” must not be taken out of context.  In this study, ‘veneer’ refers to the wood used to press the plywood.  The flakes for the flakeboard were made from the same ‘veneer.’  “Veneer” in this case does not refer to the very thin veneers used in making furniture grade plywood or furniture.

MACHINERY’S HANDBOOK 26th Edition

Industrial Press, Inc.  

200 Madison Avenue

New York, New York 10016

Rex G. Beistle

Submitted for publication by the Rocky Mountain Region of the National Model Railroad Association.

This original work is donated to the NMRA without expecting compensation of any sort.

If this is published by any other NMRA body, just give me credit.