SECTOR FOCUS: RAILWAY LUBRICANTS
Maintaining safe and quiet railways
Lubricants are used on the railway for multiple reasons including: ensuring safety, reducing wear and suppressing noise. To reduce noise and increase wheel and rail life, “top of rail materials” (TOR) are used at the wheel/rail interface. This conditions the rail to provide a consistent friction/ traction value, which in turn leads to less micro slip and slides. Currently, in Europe, the use of TOR materials is predominantly used to reduce the squeal emitted by the train wheels in curves.
The squeal, which is an unpleasant high pitch noise can be generated from the wheel/rail interface as the train proceeds around curves. As the train moves through a curve the wheel pairs (which are fixed) travel different distances, and thus some sliding between the wheels and rails is inevitable. When the friction characteristics of the wheel/rail interface allow it, the wheels can enter an unsteady dynamic. This unsteady dynamic is due to the wheels alternating between two sliding speeds, generating vibration and noise.
Noise reduction is achieved by controlling the friction characteristics at the wheel rail interface. To reduce noise the friction should increase with the percentage creep. Creep is defined here as the percentage of sliding relative to rolling at the rail/wheel interface:
where Us
is the sliding speed
between the rail and wheel and UE
is the mean speed of
the wheel and rail as they pass through the contact.
Historically it has been difficult to test and rate TOR materials, without the use of full scale field trials. In some recent work a new test has been developed and may be included in the relevant standard.- TS15427 - Railway applications –Wheel/ rail friction management – Part 2-2: Properties and characteristics – Top of Rail materials”.
Dr Marc IngramDr Marc Ingram, Ingram Tribology
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The new test uses a PCS Instruments MTM, which consists of a ball and disc loaded against each other and driven independently. The friction force acting between the ball and disc is measured by a load cell. The linear speeds at the point of contact is varied from being equal to a difference of 10%. This difference in speed introduces sliding into the contact and produces the creep curves. In a similar condition to what is experienced in the field. A very small amount of TOR material is added to the disc prior to testing, mimicking the small concentration used in the field. A specially developed mask is used to ensure the rig operator can apply the correct quantity of material accurately. A photograph of the test rig, assembled and ready for testing is shown in Figure 1.
10 LUBE MAGAZINE NO.150 APRIL 2019
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