Expansion joints
Summary: Expansion Joints allow the track to accommodate expansion due to temperature and humidity changes. This is particularly true for large scale layouts outdoors.
Outdoor Railways
Garden railways experience great temperature fluctuations every day. In some areas, temperatures can swing as much as 50-60º in a single day. Throughout the year as we go through different seasons, we get even wider temperature ranges. These temperature fluctuations cause our railroads to expand and contract - the primary reason why we never attach our tracks to the ground and why they are floating on ballast like their bigger brothers.
The use of expansion track is vital only for the long sections of track. Track on turns and curves are fine as the track should float in and out as the rail expands and contracts. A portion of track that has 20 feet (6 meters) or more should use an expansion section. This prevents track from buckling in the heat or large gaps that the trains cannot cross without derailing, loosing power, or causing extra wear in the cold (winter/night).
Here are some guidelines that DCCWiki recommends:
- Less than 20 feet (6m), you may not need an expansion section, but it can't hurt. Not need until past 10' (3m).
- If you have 20 feet, put an expansion in the middle so you only have 10 feet at most between areas of track that can relieve pressure.
- Every 20 feet of track for runs longer then 40 feet.
Examples
- 10 feet / 3m: None needed.
- 20 feet /6m: One in the middle of the run.
- 30 feet / 9m: one somewhere in the middle. If one end has a tighter turn into the straight section than the other, place the expansion section closer to the wider turn.
- 40 feet / 12m: One 10 feet in from both ends, leaving 20 feet in the middle.
- More than 40' /12m: one at 10'/ 3m, then every 20' or 6m.
Indoor Railways
While indoor empires do not experience the same degree of temperature and humidity swings, expansion and contraction is an issue. Trying to maintain a stable environment can be challenging in some situations.
Very little of the expansion will come from the actual track. Brass and Nickel Silver are both copper alloys and will similar expansion characteristics. The big factor is the layout construction itself. Most modellers use wood, which will change with the ambient conditions.
If using wood for the benchwork, always allow it a few days to acclimate to the conditions before using it. Well-seasoned dry lumber is important! Plywood is another material, which while being made of wood, is more stable.
Over time, wood will move. It will cup and twist. Experienced woodworkers know this, and will construct things to minimize the effects of cupping over time.
Choosing Lumber
A few simple rules for wood construction
- Make sure you are buying dried wood, preferably kiln dried. If possible, deal with a lumber dealer, not a big box retailer.
- Purchase wood with straight grain. That stuff with the wild grain can warp in strange ways.
- With dimensional lumber, such as a 1×4, expansion occurs mostly in long dimension. Expansion of an almost undetectable amount in width, and nil in thickness. If you follow normal benchwork practice, that means your layout might be a mite taller or shorter depending on the weather – not a big deal.
- Seal the wood. Either dewaxed shellac or polyurethane minimizes moisture exchange and thus wood movement and/or warping. Shellac (such as Zinsser Sealcoat) dries almost instantly, but polyurethane will smell less during application.
- Finally, if this is all too much, buy a sheet of hardwood plywood and cut it up into 1x4s or 1x3s. Best bet is maple or birch. Avoid buying the cheap stuff. But it's still a good idea to seal it.
- Pile your lumber neatly to prevent it from warping or twisting.
Trackwork
Do not solder every joint. At intervals leave a loose joint so the rail can move with the benchwork. If needed for power districts or detection purposes, cut gaps in the rail and fill with styrene or another material, then shape to match the profile. This eliminates the rail expanding to fill the gap and making an electrical connection in the process.
Expansion of Nickel Silver
Nickel Silver has an expansion coefficient of about 16 ppm per ºC, or 0.002% (0.0016%). This means a 10m length of rail would expand as follows:
10m × 0.002/100 = 10 × 0.00002 = 0.0002m or 0.2mm
OR: 10 × 16 ×10–6 = 10 × 0.000016 = 0.00016
Multiply by 1000 to convert that number into millimetres: 0.00016m = 0.16mm (160 microns) per change of 1ºC.
- Therefore a 10ºC change in temperature would result in the rail changing length by 1.6mm, or ≈ 63 thou. That 10m length of rail would now be 10.0016m.
Further Reading
External Links
- Thermal Expansion Calculator - Remember to use Custom and the CTE (ɑ) is 16 for Nickel Silver. There are also presets for various wood species.