Locomotive Wheels - Stress and Fatigue

Generally, when a wheel slips it is the rail that suffers, not the wheel. The contact point is at a varying point on the circumference of the wheel, whereas it is it usually confined to a small area on the rail. So the rail wears out first.

Rails flex underneath the wheel as it passes along.

Does that help?

Just want to clarify, are you talking about the wheel or the wheel's tyre or both together? Like Cars, Trains have replacable tyres. They are made of different steel hot drawn from a single nut. They are shrink fitted onto the wheel. The actual running dynamics are quite complex as both rail and tyre have special curves and angles to facilitate cornering and load bearing requirements.

I think they do develop cracks and there have been derailments due to broken or loose tyres. In the old days they had wheel tappers to inspect train wheels, they now use ultrasonic inspection. Cracked tyres are probably less of a problem today due to improved metallurgy.

First, I have absolutely no experience with trains other than the bridges they cross, but I will take a stab.

The principal stress on a locomotive wheel is compressive, think of it as an ever changing column of steel below the axle. Some bending occurs on corners, however trains generally do not take very sharp corners so the bending stresses are probably pretty low.

Fatigue is primarily related to tensile stresses (or bending stresses which include tensile stressess), and if you remember your materials class, cycles to fatigue are related to the magnitude of the stress. Fatigue is governed by:

1. The number of cycles (probably lots of cycles on a train wheel)

2. The range of the stress (primarily tensile) (I bet the range of tensile stress is pretty low for a train wheel)

3. The existence of a flaw. (as a failure could be relatively catastrophic, I am guessing that train wheels are made with very tight quality control.

So, because the 2nd and third variables are very small, the number of cycles required is likely incredible high.

Just a stab!

-Y/E

Train wheels wear out look for the video on you tube about the guys that change them all the time.

Also the Steel of the wheels work hardens with use. The softer matrix supports the self harding surface. As new material is exposed it is hardened by use. This makes them very tough. The same concept is used for tunnel boring cutters.

Brad

Hi DA. Ansari, Locomotive wheel - one of most SAFETY CRITICAL COMPONENTS of a Rolling Stock. They are subjected to Static & Dynamic Loading, in addition to Thermal stresses during braking. Fatigue life is quite long due to good design, better material selection, good manufacturing practices, better quality control & maintenance. In order to improve the Fatigue life of wheels, the following steps are taken care of by the manufactures: 1. use of High Carbon Steel material with Carbon content ranging from 0.57 to 0.67 % 2. Optimization of Heat Treatment process is carried out to improve fracture toughness. 3. Rim Hardness range is maintained at 300 to 341 BHN to ensure life & reliability. 4.Beneficial compressive Hoop Stresses in Rim developed by RIM Quenching. 5. Wheels in Austenitic state are impinged with water in a rig fixture under controlled condition on Tread. These stresses resist initiation & propagation of fatigue cracks. In spite of all these precautions, Wheel failures do occur due to the following reasons: 1. Wheel flanges & tread wear since tractive & braking forces are transferred through them. 2. Breakages from punch marks. 3.The formation of flat & martensitic heat affected zones on the running surfaces which causes cavities & crack networks to develop on the tread. 4. Thermally & mechanically induced fatigue. 5. Operational defects due to poor brake block alignment, improper axle box clearance. 6. use of poor quality Brake Blocks. Even the best wheels made by reputed manufactures suggest periodical check ups at regular intervals. Hope this helps you. Rangasamy