Knowledge about rails(铁路轨道小识)
The old way of laying steel rails was to leave gaps between the lengths of rail so that there is room for expansion. Nowadays the railway lines are continuous, and there might be many kilometres of rail joined end to end with no gaps. Rails of length 18m are produced from a lump of steel. They are thoroughly welded together to give a length of 360m by passing an electric current of 4000A through two rails in contact end to end. When the rails are carried to the site, the engineers measure the temperature and calculate how much the rail must be stretched so that it has the same extension as the expansion through heating up to 43°C. The far end is clipped down, leaving a gap between two rails. An "hydraulic stressing machine" clamps to the ends, both sides of the gap and stretches the rail being laid. As the ends are brought together, they are thoroughly welded through a chemical solution called "thermit process".[ 本帖最后由 DF11香港青年 于 2008-12-7 09:40 编辑 ]
Illustrations a 插图a
http://www.sertomanwfl.com/Laying%20Rails.JPGlaying steel rail
铺设铁轨
http://www.girr.org/girr/tips/tips1/rail_gap.jpggaps between the length of rails铁轨之间的空隙
[ 本帖最后由 DF11香港青年 于 2008-12-8 17:43 编辑 ]
Illustrations b 插图b
http://www.nt.gov.au/aarc/news/photolib/Adrail35_med.jpgPreparation for Thermit Weld
準備用热剂焊接轨条
Photograph by Steve Strike, Outback Photographics - © AustralAsia Railway Corporation
由Steve Strike摄影 内地摄影 - 澳洲铁路公司
http://www.nt.gov.au/aarc/news/photolib/Imgp0796.jpg
SA Premier Mike Rann and NT Chief Minister Clare Martin ignite the Thermit Weld
公司领导層Mike Rann 和 Clare Martin點燃热剂焊缝
Photograph by AustralAsia Railway Corporation
由澳洲铁路公司摄影
http://www.nt.gov.au/aarc/news/photolib/Imgp0798.jpg
Thermit Welding process in action
进行热剂焊缝过程
Photograph by AustralAsia Railway Corporation
由澳洲铁路公司摄影
http://www.nt.gov.au/aarc/news/photolib/Adrail75.jpg
Rail showing Thermit Weld prior to completion
完成热剂焊缝过程前的轨条
Photograph by Steve Strike, Outback Photographics - © AustralAsia Railway Corporation
由Steve Strike摄影 内地摄影 - 澳洲铁路公司
[ 本帖最后由 DF11香港青年 于 2008-12-8 18:13 编辑 ]
Question & Answer Time-slice
Question 1In the passage above, two welding processes are mentioned. Why these two welding processes are necessary?
Rails are made from molten metal. When they have just emerged from the mill, their temperature is below the melting point. To ensure that the rails are straight, the rails are placed over a series of rollers and awaits for complete solidification. These rails are arranged in parallel and at the same level. Usually, these rollers are limited in size. Thus, the rails emerging from the mill would not be too long.
After solidified, the rails are welded together in the factory. Again, because of transportation, the rails should not be welded too long in the factory.
After the rails are brought to the site, another welding process will be carried out to form one single rail between two terminus.
[ 本帖最后由 DF11香港青年 于 2008-12-9 20:04 编辑 ] 看看海外的:lol 领导亲自上阵，焊接无缝钢轨！
呵呵，做领导真是好！~
Question and Answer time-slice
Question 2Why a current of 4000A helps to weld the rails together?
By P = I^2 * R , a large current can produce sufficient heat to raise the temperature of the metal to its melting point. In this way, welding and joining of rails can be carried out.
P is the electrical power
R is the resistance of the metals at the junction
[ 本帖最后由 DF11香港青年 于 2008-12-12 22:52 编辑 ]
Addtional knowledge related to laying rails
To testify the welding materials, the rails are laid in cold days so that the joints would not be broken because of over stretching due to temperature fall. However, laying rails in hot days has the advantage of reducing the stress required during rail laying.More about hydraulic stresing machine
This machine cannot fulfill the job if1. the cross-sectional area of rails is larger, or
2. the temperature difference between hot and cold days is more than 23°C in other countries.
If this machine cannot fulfill the job, the rails should be laid on hot days. However, the welding material must be well-tested for their strength against stretching before being applied.
Pandrol clip
http://www.scalefour.org/resources/images/Pandrol1.jpgThis photo shows a Pandrol clip found between rails and sleepers.
The clips are to fix the railson the sleep against stress due to
1. bending of rail round corner
2. pre-stress
3. over-stretch at low temperature
4. compressive stress at too high temperature.
中文翻译
以前铺设铁轨的方法是在相邻的铁轨之间预留空隙，使它们有空间膨胀。现在的铁轨线是连接在一起的，会有很长的公里数的轨条头尾相连，没有空隙。18米长度的轨条由一堆钢制成。4000A的电流通过两根轨条末端的接触面，使它们彻底地焊接在一起成360米长的新轨条。当轨条运到工地，工程师会测量温度，再计算需要延伸的长度，确保结果跟在摄氏43度延伸时的情况一样。轨条的末端被夹住，使它们之间留下空隙。液压机器钳紧轨条的两端，固定空隙，再延伸轨条。两条轨条的末端早开始接触後，它们透过铝热剂彻底地焊接在一起。 支持一下科普贴如有下文我稍后把我的回帖删除
问答时段
问题1在以上文章，提及了两个焊缝程序。为什麼需要这两个程序?
轨条由熔化的金属制成。当它们刚出厂时，温度比熔点低。为了保证轨条挺直，轨条放在在一连串滚轴上等待完全固体化。这些轨条都平行地和在同一高度排列。通常滚轴尺寸有限，因此出厂的轨条不会太长。
固体化后，轨条在工厂裏焊接在一起。为了运输，轨条不宜焊接得太长。
轨条运到工地后，另一焊缝程序会展开，为的是使两个火车终点站之间用单根轨条连接。 不错，哈哈。。。。。
问答时段
问题2为什麼4000A的电流有助把轨条焊接在一起?
按照公式「电功率 = 电流^2 * 轨条连接处电阻」，大的电流能够产生足够热使金属的温度上升至其熔点，然后就可以进行焊接和连接轨条的工作。
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