Railroad track guides the train to move forward and bear the load at the same time. The flatness of rails is the cornerstone of safe comfort and high-speed running. The friction between the rail and train wheel brings out defects (spalling, shelling, cracking, rail corrugation) and leaves an irregular surface. All these defects on rails progress to worse condition if there is no timely grinding. Rail grinding is one effective technique of railroad track repair and maintenance. By using rail grinders, rail defect material is removed and the rail profile is restored. Thus the service life of railroad track is extended.
Rail defects
When talking about rail grinding, the rail defects are needed to be introduced first. Rails are subjected to extreme stress and compact from train wheels. Damages on the rail surface will inevitably appear. Common rail defects include vertical wear, side-wear, rail corrugation, contact fatigue wear (spalling, shelling, craking), plastic flow, etc.
These independent defects of the rail will be a shock each time the train wheel passing by, and then generate a load that is several times higher than normal. Therefore, the rails are under high pressure. Under normal circumstances, these damages will expand further, some severe conditions will cause the railway to fail.
Not only the rails are affected. The rails can’t absorb all the energy produced by the impact. These shocks will continue to pass on other track structure such as the rail pads and sleepers. Finally, the trackbed will sink locally and the railway loses its stability.
Optimal rail profile
The rail defects lead to the change of rail profile which causes the rail-wheel contact unstable. Rail grinding is actually to cut the surface of the rail, remove the defects, and restore the rail to the optimal profile. The optimal rail profile is shown below in the figure.
When the train is running on a straight line or a curve with a large radius, the wheel tread is in contact with the railhead area B. In this state, the contact stress and lateral creep force are the smallest; when the wheel flange is close to the rail gauge angle, that is in area C in the figure, when a conformal contact is formed, the contact stress is the smallest.
The profile of the rail head is similar to an arc shape, but the actual grinding (grinding wheel type) is to divide the rail head into different areas according to the angle, that is, the rail head profile is regarded as composed of line segments divided by angles, and the position that needs to be ground corresponds to is a certain angle.
The more serious the rail damage, the greater the amount of grinding and cutting, the greater the workload, the greater the difficulty, and the shorter the rail life. Therefore, preventive grinding and corrective grinding are like “curing diseases”, and the best way is to prevent the rails from being “sick”. Perform regular preventive grinding to eliminate the damage in the germination state.
Types of rail grinding
Pre-grinding
Pre-grinding the new rail can remove the decarbonized layer and eliminate surface defects in the production, welding, transportation and construction of the rail. It can also optimize the profile of the rail head, and improve the smoothness of the welded joint. Besides, it can repair the rail surface damage caused during the track laying process. Rail wear developed in a time of progress, ensuring good contact between wheels and rails from the beginning can limit the initial damage.
Preventive grinding
Preventive grinding refers to grinding rails regularly, restore the rail profile so that the surface stress cracks on the rail can be eliminated at the initiation stage, and the propagation of the rail rolling contact fatigue cracks can be well controlled.
Corrective grinding
Corrective grinding refers to the operation of grinding rail after the surface defects are generated. Corrective grinding was prevalent at the birth stage of rail grinding technology. It is mainly carried out to repair rails by removing the severe damage on rail surfaces such as fatigue cracks, corrugation, spalling, plastic flow and other defects.
The figure below shows the effect of preventive grinding and corrective grinding on the life of the rail. The yellow line represents preventive grinding. Each grinding will delay the development of damage and increase the life of the rail. Preventive grinding before the micro-cracks exceeds 0.2mm is the best time to block the trend of accelerated damage to the rail.
Rail grinding practice
How rails are ground? There are mainly three ways.
- Sliding shoe grinding
Sliding shoe grinding is to grind away the surface of the rail by pressing the grindstone on the rail to perform the reciprocating motion. This grinding method can only grind the rail head longitudinally, and cannot adjust the rail profile or grind the switch. Also, the grinding speed is low. This method has been gradually eliminated.
- Grinding wheel
Grinding wheel grinding usually uses a high-speed rotating grinding wheel driven by an electric motor or a hydraulic motor to grind the rail. This is an active grinding method and is currently the main grinding method. The rail is ground vertically and horizontally.
There is also a kind of passive grinding. The position of the grinding wheel is inclined to a certain angle with the travelling direction of the train. The high-speed operation of the power tractor drives the grinding wheel to rotate at a high speed on the rail. The relative movement between the grinding wheel and the rail reaches the required material grinding speed. Grind the entire rail head from the running edge of the rail to the running surface. This kind of grinding method is called high-speed rail grinding because of the fast grinding speed.
- Milling
Milling is a new type of rail profile repairing mode. The milling cutter is installed on the radial surface of the milling wheel. Each milling section is equipped with several milling cutters. There are several milling sections per milling wheel. In this way, all milling The knife forms a rail head profile to be completed.
The benefit of rail grinding
- Improve the wheel-rail contact interaction by modifying the rail profile, thereby reducing wheel-rail contact stress and wear.
- Correct or control rail corrugation. These defects will increase wheel and rail noise and speed up the deterioration rate of vehicle components and track components, even causing train speed limits.
- Control rolling contact fatigue defects. These defects will increase the risk of rail damage and even reduce the effect of ultrasonic rail flaw detection.
- Correcting or controlling other rail defects (such as wheel roll, crush, rail head vertical and longitudinal cracks)
- Reduce the adverse effects of wheel and bogie movement.
- Reduce noise and vibration, reduce vertical irregularities of common joints and welded joints, and control rail corrugation.
- Alleviate the adverse effects of the large axle load wheel and improve the wheel-rail contact conditions.
- Make the rails and wheels contact correctly, reduce the vehicle’s lateral instability (lateral creep).
The economic effect of rail grinding
- Preventive grinding can prolong the service life of railroad track. Rails ground regularly as planned can extend the life of 5-8 years.
- Reduce the repair and maintenance of fasteners.
- Reduce locomotive fuel consumption.
- Reduce wheel repair cost.