Post by stanmorek on Jan 5, 2006 19:00:36 GMT
For those of you who are interested, I've some notes which I made during a visit to a PWI presentation on welded rail.
The Permanent Way Institute Seminar 31st March 2004
Southbank University
Rail Stress & Rail Stress Management
1.0 Introduction
The development of continuously welded rail was driven by the need to eliminate joints in the track for an improved ride quality and to reduce maintenance. Continuously welded rail (CWR) is also five times less likely to experience track buckles compared to jointed track. Despite these benefits, there is also a need to manage stress levels in CWR to control performance and risk.
2.0 History of Continuously Welded Rail
The first Thermit welded track appeared in 1936 after improvements in metallurgy and experimentally enhanced mechanical joints were developed in Britain by London Transport. Further tests were performed up to and after the War where the results of these tests were reported in an ICE paper in 1948 which discussed the concepts and principles of CWR. The tests had confirmed that rails could be theoretically welded to an unlimited length provided that adequate measures were taken to relieve stresses within the rail (the balancing of tension and compression forces). Trials undertaken in the 1950’s showed that in practice, the total length of welded rail achievable was only half of a mile. At the time, the length of welded rail was limited by the only available method of achieving stress free temperature (see section 2.0) in CWR. This was through solar heating. Further developments were carried out with the emphasis on rail fastenings and the prevention of rail buckling.
Modern CWR consists of Pandrol type rail fastenings and improvements in achieving stress free temperature through mechanical means of hydraulic tensors. The current Network Rail guide to CWR is the RT/CE/S/011 Handbook.
3.0 Stress Free Temperature
Thermal forces due to changes of temperature within rails induce stresses. Compression in rails is due to rises in temperature and tension is due to falls in temperature. These stresses are managed by mechanical joints in rails such as expansion and adjustment switches. Stress free temperature (SFT) is achieved when the internal rail forces are in equilibrium. There are also resisting forces due to rail fastenings, sleepers and ballast as is lateral movement which is also restrained by the same. Disturbance of ballast, sleepers or rails will upset the state of equilibrium in CWR and will subsequently require re-stressing as will the effects of creep and braking and accelerating of trains over a period of time.
Track in the open sections experiences daily and seasonal temperature changes and the value of SFT is determined as a mid-range temperature where there are no overall compressive or tensile forces in the rail. Normally SFT is 27ºC in Britain and the degree of pre-stressing of CWR required during installation is through natural or artificial means (see section 4.0).
The force in the rail, P = EAαT
Where: Α, Rail cross-sectional area; α, coefficient of expansion of steel; T, temperature variable; E, elastic modulus of steel;
4.0 Methods of Stressing Continuously Welded Rail
The challenges faced by P-Way engineers in correcting or achieving SFT is the time available (possessions), the environment (air temperature) and available resources. The types methods of achieving SFT in CWR are classed as either thermal or mechanical.
Natural – The principle of “Sun Locking” is to allow the sun to heat the rails. The rails are mounted on rollers to ensure an even distribution of stress during expansion. This operation can only be carried out when the rail temperature is 21-27ºC. Due to the narrow band of allowable working temperature, there is the risk of having to complete work when the rail temperature is out of range.
Heating – In the past, rails were artificially heated by gas heaters. However, this method would often lead an uneven distribution of rail temperature damaging rail fastenings. The operation must also be carried when actual rail temperature is below SFT.
Stretching – This method uses hydraulic tensors to mechanically apply the correct amount of stress to the rail whilst sections of rail are anchored down to allow stretching. This is a complex procedure and is highly competence dependant where uneven rail stress distribution due to incorrect calculations is a risk. Significant forces are also need to be applied during cold weather working.
The stressing of CWR in a typical three hour procedure using 12-15 men is as follows.
The Permanent Way Institute Seminar 31st March 2004
Southbank University
Rail Stress & Rail Stress Management
1.0 Introduction
The development of continuously welded rail was driven by the need to eliminate joints in the track for an improved ride quality and to reduce maintenance. Continuously welded rail (CWR) is also five times less likely to experience track buckles compared to jointed track. Despite these benefits, there is also a need to manage stress levels in CWR to control performance and risk.
2.0 History of Continuously Welded Rail
The first Thermit welded track appeared in 1936 after improvements in metallurgy and experimentally enhanced mechanical joints were developed in Britain by London Transport. Further tests were performed up to and after the War where the results of these tests were reported in an ICE paper in 1948 which discussed the concepts and principles of CWR. The tests had confirmed that rails could be theoretically welded to an unlimited length provided that adequate measures were taken to relieve stresses within the rail (the balancing of tension and compression forces). Trials undertaken in the 1950’s showed that in practice, the total length of welded rail achievable was only half of a mile. At the time, the length of welded rail was limited by the only available method of achieving stress free temperature (see section 2.0) in CWR. This was through solar heating. Further developments were carried out with the emphasis on rail fastenings and the prevention of rail buckling.
Modern CWR consists of Pandrol type rail fastenings and improvements in achieving stress free temperature through mechanical means of hydraulic tensors. The current Network Rail guide to CWR is the RT/CE/S/011 Handbook.
3.0 Stress Free Temperature
Thermal forces due to changes of temperature within rails induce stresses. Compression in rails is due to rises in temperature and tension is due to falls in temperature. These stresses are managed by mechanical joints in rails such as expansion and adjustment switches. Stress free temperature (SFT) is achieved when the internal rail forces are in equilibrium. There are also resisting forces due to rail fastenings, sleepers and ballast as is lateral movement which is also restrained by the same. Disturbance of ballast, sleepers or rails will upset the state of equilibrium in CWR and will subsequently require re-stressing as will the effects of creep and braking and accelerating of trains over a period of time.
Track in the open sections experiences daily and seasonal temperature changes and the value of SFT is determined as a mid-range temperature where there are no overall compressive or tensile forces in the rail. Normally SFT is 27ºC in Britain and the degree of pre-stressing of CWR required during installation is through natural or artificial means (see section 4.0).
The force in the rail, P = EAαT
Where: Α, Rail cross-sectional area; α, coefficient of expansion of steel; T, temperature variable; E, elastic modulus of steel;
4.0 Methods of Stressing Continuously Welded Rail
The challenges faced by P-Way engineers in correcting or achieving SFT is the time available (possessions), the environment (air temperature) and available resources. The types methods of achieving SFT in CWR are classed as either thermal or mechanical.
Natural – The principle of “Sun Locking” is to allow the sun to heat the rails. The rails are mounted on rollers to ensure an even distribution of stress during expansion. This operation can only be carried out when the rail temperature is 21-27ºC. Due to the narrow band of allowable working temperature, there is the risk of having to complete work when the rail temperature is out of range.
Heating – In the past, rails were artificially heated by gas heaters. However, this method would often lead an uneven distribution of rail temperature damaging rail fastenings. The operation must also be carried when actual rail temperature is below SFT.
Stretching – This method uses hydraulic tensors to mechanically apply the correct amount of stress to the rail whilst sections of rail are anchored down to allow stretching. This is a complex procedure and is highly competence dependant where uneven rail stress distribution due to incorrect calculations is a risk. Significant forces are also need to be applied during cold weather working.
The stressing of CWR in a typical three hour procedure using 12-15 men is as follows.
- Site survey to identify rail fastenings and curves in track;
- Mark out anchor length tell-tales and reference points in stressing length;
- Check rail temperature;
- Cut rail gap to allow stretching;
- Unclip the rail (release rail fastenings);
- Calculate the required rail extension;
- Mark up required extension gap in rail;
- Trim the rail;
- Fit tensors to cut ends of the rail;
- Apply typically 50 tonnes hydraulic pressure through tensors to stretch the rail and close the gap;
- Clip up the rail;
- Weld up rail ends at the cut;
- Fill track beds with ballast and pack sleepers.
