The Measurement of the Temperature of Sliding Surfaces, with Particular Reference to Railway Brake Blocks

Abstract

A pyrometer, which covers the temperature range 200 to 900 deg. C. (390 to 1,650 deg. F.) with a response time of 10⁻³ seconds, has been developed for studying the temperature of stationary and moving surfaces. The values of the emissivities of bright and oxidized steel surfaces, determined by this method, agreed with other published work. The pyrometer has been used to determine the temperature of the surface of a railway tyre, during brake applications under various conditions, just as it emerged from beneath the brake block. Evidence is adduced to prove that the temperatures measured are not very different from those actually under the block. It is shown that the possible change in emissivity of the tyre steel is insufficient to invalidate the results. The variation and extent of the area of contact between brake block and tyre were investigated simultaneously with the temperature measurement. It was shown that high tyre surface temperatures (over 800 deg. C.; 1,470 deg. F.) are an inevitable result of “strip braking”. The preferred formation of heat spots between the spokes of the wheel, and the unequal growth of heat spots across and along the tread due to tyre distortion, were examined. An attempt to eliminate strip braking by reducing the length of the standard brake block to three-quarters, one-half, and one-quarter of its original length was made. It was found that reducing the brake block to one-quarter of its original length completely inhibited heat spot formation, reduced by a factor of 2 the standard deviation of the stopping times under standard conditions, and reduced the maximum temperature attained during brake application from over 800 deg. C. (1,470 deg. F.) to under 400 deg. C. (750 deg. F.). The deleterious formation of martensite was thus eliminated. It was found that the wear of the half-length brake blocks, expressed on a thickness basis, was not twice but maybe equal to that of a full-length block. Since the cost of a brake block is somewhat proportional to its size it follows that a shorter block may be more economical than a full-length block. It is concluded that the optimum size of a railway brake block must be determined separately for each material and set of operating conditions.