a track machine for cleaning ballast of contaminants, such as fine debris and dust, that lower the resiliency of the ballast layer and prevent the drainage of moisture. Ballast becomes contaminated because the gravel is abraded and crushed by the dynamic loads of passing trains and compacted under railroad ties during track maintenance operations; in addition, particles of ore, coal, and the like are spilled on the ballast from passing trains. As a result of the impaired filtering capability of the gravel layer, moisture is retained by the contaminants, the ballast foundation becomes thin in places, the gravel is scattered, and frost heaves are formed in winter.
The most commonly used ballast-cleaning device in the USSR is the model ShchOM-D, which was designed by the Soviet engineer A. M. Dragavtsev; it is mounted on an electric ballasting machine. The device consists of an endless reticulated belt that moves crosswise to the direction of the railroad track around the rails and ties, which are elevated by the electric ballasting machine. The belt slides underneath along a blade (at a speed of 12 m/sec) that cuts under the gravel layer as the machine is moved. Fine particles of contaminants are ejected onto the shoulder of the roadbed through holes in the belt as the belt moves over the curved portion of its trajectory; large pieces fall into a bin and are then poured back onto the track and leveled off. The ShchOM-D can process up to 2,000 cu m of ballast per hour at an operating speed of 1 to 3 km per hour. Ballast-cleaning machines that operate without elevating the rails and ties have scraping mechanisms to separate the gravel and feed it into a cleaning device. They are used mainly to clean the ballast under railroad switch layouts and station tracks.
Ballast is cleaned on an average of once every 8–10 years; cleaning is done more often on track sections where contamination, such as by coal dust, is more severe. The use of ballast having special fillers, for example, asbestos, can extend the servicing period to as much as 15–20 years.