A Small-Scale Experimental Study of the Damage Due to Intermittent Shoe Braking on the Tread of High-Speed Train Wheels

Abstract

Recent interest in emergency shoe braking in high-speed trains stimulated research on the damage due to sliding contact between brake blocks and wheel treads. Bi-disc tests, with rolling–sliding contact between specimens extracted from wheels and brake blocks, were previously performed in constant working conditions to reproduce the temperature of the wheel during braking. However, braking is often a discontinuous operation, obtained by intermittent contact between brakes and tread, to prevent excessive heating. In this article, braking is simulated by discontinuous tests, periodically interrupted to cool down the specimens. Three wheel steels (HYPERLOS, SANDLOS, and CLASS B) were tested against the same cast iron brake material. Measurements of the friction coefficient, contact surface temperature, and weight variation were performed. Moreover, optical microscopy observations and hardness tests were done on the cross section of the wheel specimens. Material transfer from the brake to the wheel disc with the formation of a discontinuous third-body layer was observed. This layer, when worn away, likely contributes to surface crack nucleation. In addition, the debris of both materials enhances abrasive wear. If compared with the continuous tests, the intermittent tests showed higher friction coefficient fluctuations and in some cases significantly different weight variation. Among the steels, the weight loss and subsurface hardening of HYPERLOS was higher in the discontinuous tests than in the longest continuous tests. CLASS B showed similar behavior in both tests. An effect of the temperature cycles on the friction coefficient and the wear behavior was hypothesized.

Keywords:

• Shoe brake wheels
• intermittent braking
• material transfer
• wear
• surface cracks