Rail Expansion & Rail Joint Design
Printed in Modeltec, September 1989
Use with permission of author
See also Rail expansion.
For any miniature outdoor railroad trackage to be properly engineered, the effects of seasonal weather variations upon the right-of-way must be carefully taken into account. One of the most important considerations is the natural linear expansion and contraction of the rails due to seasonal temperature changes as well as direct solar heating.
In regions where the rail temperature differences are extreme, and where little or no allowance for the linear expansion and contraction of the rails has been designed into the trackage, lateral and vertical track misalignments called "sun-kinks" will likely occur. At their worst, these sun-kink conditions will be noticed where tangent trackage has pushed itself into wild "S" shapes or humps, or where curved trackage has thrown itself out of a smooth radius or curve transition, and where sharp kinks may have also been produced at the rail joints. Many miniature railroad hobbyists have discovered these problems only after having built an otherwise precise and well-designed track layout, and later found their trackage heaved and kinked into absurd new alignments after the first season of operation.
All common metals each expand and contract at different but uniform rates per degree of temperature change. These uniform rates are called Coefficients of Expansion. The Coefficient of Expansion for aluminum is approximately twice the Coefficient of Expansion for steel. Trackage built with steel rail is usually a little more stable than trackage built with aluminum rail, but in either case the effects of temperature changes on the trackage must still be dealt with in track design. The popular use of extruded aluminum rail on most typical (up to 7-1/2" gauge) miniature railroads has presented some special problems, which this article will attempt to explain.
The Coefficient of Expansion for aluminum varies only slightly between the different common aluminum alloys, and here we will use the coefficient 13.5 x 10-6 inches, per inch material length, per degree Fahrenheit temperature difference. To put it simply: if the difference in degrees Fahrenheit (TD) is known, and the rail length in inches is known, then the amount of linear Expansion and Contraction (EC) can be calculated for the TD range. What the EC works out to for a 12 inch length of aluminum rail is 0.000162 inches for every degree TD, or:
- EC in Inches = 0.000162 x TD x Rail Length in Feet
It can be seen from this, that a TD of only 20°F results in just over 1/32 inch linear change in length in a 10 foot aluminum rail.
The seasonal difference (summer/winter) in temperature of a particular region is important, however, aluminum rails can also pick up a large quantity of heat from direct sun exposure, which can affect the overall amount of EC. Aluminum rails also respond quickly to sudden fluctuations of temperature, which adds a little to trackage problems. For most regions of the United States the seasonal TD can be averaged to approximately a 120°F range (summer = @100°F maximum, winter = @-20°F minimum), and aluminum rails can easily pick up another 35°F of heat from direct sun exposure. Therefore, if we can agree that a total of 155°F of TD range is a typical figure to work with, a 10 foot length of aluminum rail can vary 1/4 inch or more in length, annually. If allowances for these changes in rail length are not incorporated into the design of the rail joints, it is easy to see how extreme "sun-kink" problems result in a section of trackage. In a 100 foot track section, 2.5 inch EC can develop - and this pressure must go somewhere — it moves the track out of alignment.
The design and construction of rail joints to allow for the total EC of each rail is usually accomplished by jig-drilling two oval-shaped (or oversize) bolt holes in the rail web at the ends of each rail (or sometimes oval-shaped bolt holes can be found in the rail-joiners). (Ed: An oversized hole could also be drilled to achieve the same effect as an oval hole.) The length of each oval-shaped bolt hole is determined by the total amount of EC divided by 2 (two rail ends), plus the drill diameter for the bolt size. Then, when the rail joints are assembled with the rail-joiners, and the bolts are not snugged excessively tight, rail "slippage" in the joint can occur. A maximum EC gap will form between the rail ends when the rails are at their coolest contracted state, and the EC gap becomes less as the rails warm and expand, up to the point of zero EC gap in the rail joint. Some experimentation is necessary to get a feel for the proper bolt tightness — a firm rail joint is desired — but it must be permitted to rather easily slip as rail expansion and contraction occurs. All holes in the rails and rail-joiners, as well as the cut ends of each rail should be carefully deburred, to better allow for smooth slippage.
It is probably reasonable to assume that 1/4 to 9/32 inch is the widest absolute EC gap dimension to design for rail joints in both 4-3/4 and 7-1/2 inch gauge trackage. This prevents abnormal wheel pounding and stress in the rail joints if the EC gaps may be at their widest setting (at times, a rail joint may be found to be pulled out to the widest gap for reasons other than temperature contraction). This absolute EC gap dimension automatically places a limit on the longest rail lengths which may be used in most trackage. For instance, using our total 155°F TD range example, rail lengths longer than 10 feet would require larger than a 1/4 inch maximum EC gap dimension at each rail joint to permit proportionately more expansion and contraction. To keep things simple, any rail lengths shorter than 10 feet could also use the same 1/4 inch rail joint design configuration, with no unusual problems arising. However, track switches, track "diamond" crossings, etc., should have tight internal rail joints which do not permit slippage. (Note: For track gauges smaller than 4-3/4 inch, perhaps even less of an absolute EC gap dimension would be required, and maximum rail lengths would therefore need to be proportionately shorter than 10 feet).
For track laying at moderate temperatures where the rail joint EC gap would be partially closed, the proper amount of gap to install as the rails are connected can be calculated according to the rail temperature of the moment. The actual tail temperature can be measured by using a simple pocket thermometer with it's probe taped to the rail and with the dial shaded from direct sun to obtain an accurate reading). In order to calculate the "corrected" gap, the maximum EC gap dimension designed for the rail joint at it's lowest temperature must be known, as well as the length of the rail, and the difference between the actual rail temperature above that of the lowest design temperature provides the "Gap Setting" Temperature Differential (TDgs). The resulting equation is similar to the first equation used above:
- Gap Setting ECinches = Max EC Gap - (0.000162 x TDgs x Rail Length in Feet)
For example, assuming that our rail lengths are 10 feet, that the designed maximum EC gap is 1/4 inch at -20°F, and that the actual rail temperature of the moment is 70°F, then:
- TDgs = 70°F + 20°F = 90°F
and:
- Gap Setting ECinches = 0.250 - (0.000162 x 90°F x 10 ft)
- Gap Setting ECinches = 0.1042
The figure 0.1042 inches is the amount of gap to install when laying this track example.
Rather than repeatedly having to do the calculations as the actual rail temperatures fluctuate, it's quite easy to plot on a graph all of the Gap Setting EC dimensions for various rail temperatures along the total TD range, for the rails of a particular length. A graph saves time, and permits track laying or other adjustments to be accomplished with the most accuracy and comfort. If desired, a more exact seasonal TD range, plus solar rail heating, rail length, and maximum EC gap figures may be developed to suit any local region by using the seasonal temperature averages and direct measurements in that region. (Ed: See Rail expansion for a spreadsheet that performs the calculations.)
When laying track in this manner, any gap-setting errors should favor a wider gap dimension by no more than 1/32 inch or so. The rail joint gaps then simply won't close up as tightly at a warmer temperature, which is preferable, and at the lowest TD temperature the rails will be pulled slightly taut, but not enough to do harm to the trackage. If desired, overall tolerances of say, plus and minus 1/32 inch on each side of the Gap Setting EC can be designed into both the rail joint configuration and the drilling jigs prior to drilling the rail ends (and these tolerances may also be plotted with lines on the graph on either side of the Gap Setting EC line).
As previously noted, a rail joint may be found to be pulled out to it's maximum gap for reasons other than temperature contraction. Rails may "creep" toward the bottom of track grades or along curves due to wheel friction pressures, etc. The trackage can be occasionally inspected for such occurrences, and the rails may be "slipped" back into their proper gap positions (most rails will slip across the crosstie surfaces, if the track was constructed by the usual methods). A relatively easy method found to accomplish this task is by using a duck-billed vise-grip plier tightly clamped over the rail head (with a rag in between to protect the rail from damage). Then, by carefully rapping on the side of the plier jaws with a hammer, the rail Should lip in the desired direction. If several connected rail lengths need to be "slipped," it becomes necessary to work each rail only a little at a time. As the joint gaps close on one end (and pull open on the opposite end) the next rail along in either direction can next be "slipped" as required.
Although the use of aluminum for rail in miniature railroad trackage has presented some special problems, it's peculiarities can be reasonably dealt with by various track engineering techniques. Aluminum rail has it's share of advantages, and is the material preferred by many persons in the hobby for many different reasons. This material will, no doubt, remain a popular choice in miniature railroad track construction for quite some time to come. It is hoped that the information presented above will be of interest and value to those who choose to use aluminum rail in their trackage.