This Railway Tech article gives an overview of Wheelset friction and rolling resistance in railway wheelsets and how it relates to other sources of resistance for a train. It is assumed that the reader has basic knowledge about terminologies of mechanical engineering like bearings, axle load, friction, wheelset, etc.
What is a Railway Wheelset
A wheelset is usually defined as the assembly of an axle, wheel, and bearings (see picture below).
Resistance of Train Wheelset –
The traveling resistance of a train is often expressed as how much resistance force is there per unit of axle load. A practical unit is often [N/kN], i.e., how many Newtons’ force of resistance is acting on the train per kilo-Newton axle load.
Total Resistance Force = Friction at wheel & track interface + Friction in wheel bearing + Air resistance
Friction at wheel & track interface and Friction in wheel bearing (mechanical resistance)
Off course, the resistance force (being a combination of frictional and air residence force) is thought to act in the opposite direction of the motion of the train. For low-speed trains (say below 50 km/h) most of the resistance of a train comes from the wheelset. More specifically from the contact between the wheel and rail. The wheelset resistance is more or less independent of speed and is usually estimated to be 2 to 2.5 [N/kN]. This includes the friction of the wheelset bearings, which normally accounts for 10% of the total wheelset rolling resistance in modern trains. Earlier trains had journal-type bearings in which the friction was much higher compared to modern rolling element bearings.
There can be miscellaneous factors affecting the resistance such as the wheel & rail tracks manufactured as per applicable standards for material (generally rolled or forged steel) and quality parameters. For example, defects in wheel & track material, wheel surface wear, imbalance, track geometry, and other factors may further affect friction (resistance).
Also, the resistance will change (increase or decrease) when the train is moving on a gradient track or a curved track compared to a straight and leveled track. The gradient resistance is the extra force required to lift a train up a gradient (slope) or conversely is the extra force “pushing” the train down a hill. Obviously, It is a positive quantity when going up the grade and a negative quantity for going down the grade. Similarly, when a train travels around a curve, due to the track resisting the direction of travel (ie the train wants to continue in a straight line), it experiences increased resistance as it is “pushed” around the curve.
Curve radius impact – the tighter the curve radius the higher the resistance to the train
Rolling Stock Rigid Wheelbase impact – the longer the rigid wheelbase of the vehicle, the higher the resistance to the train. Modern bogie stock tends to have shorter rigid wheelbase values and is not as bad as the older style 4-wheel wagons.
Air Resistance on Train
For higher speeds, the wind resistance will start to dominate. One reason for this is that wind resistance is proportional to the speed at the power of two. The graph below (taken from UIC data) shows the development of the resistance of a passenger train as a function of speed.
Further, there can be various conditions such as still air, air moving in the opposite direction, air moving in the same direction, air coming from a lateral (perpendicular) side, or even air moving at an angle to the train’s direction of movement. As wind drag is heavily dependent upon the velocity of the train so When the wind is present, then the resultant velocity of the train and the wind is used for the calculation of the wind resistance.
Total wind resistance = Wind Drag Resistance + Lateral Force Resistance
Additionally, air drag further increases if the train is moving through a tunnel which is known as tunnel resistance. A train entering the tunnel generates a compression wave (imagine a piston moving in a hollow cylinder). The friction of the displaced air with the tunnel wall produces a pressure gradient and, as a consequence, a rise in pressure in front of the train. Off course, the tunnel resistance will depend on Train Profile, Train cross-section, Tunnel Profile, and Tunnel cross-section. If the tunnel cross section is small then there will be less space for air and hence more resistance. Further, If 2 trains move through a tunnel then compression waves due to the movement of one train will generate additional resistance or drag for the other train.
Conclusion
The conclusion is that the friction of wheel bearings does not contribute significantly to the resistance of a train. It is however important to reduce the friction of railway wheel bearings to a minimum since this reduces the operating temperature of the grease in the bearing. A lower operating temperature will significantly increase grease life and consequently the maintenance interval of the bearing. Conversely, if the bearing runs at high operating temperature continuously (often called Hotbox) it may result in damaged wheelsets and in extreme circumstances fires, and derailments.
End of topic.
Key Words: The aerodynamic drag of high-speed trains; Wheelset friction and rolling resistance, Aerodynamic Drag; Boundary Layer Transition; Computational Fluid Dynamics; Computerized Simulation; Rail Transportation; Rapid Transit Systems; journal type bearings, rolling element bearing