Investigations into the ageing of hydraulic flexible hose assemblies used in mobile longwall supports in the coal mining industry

Status:

completed 10/1994

Aims:

In the past, malfunction of hydraulic hose assemblies has repeatedly lead to accidents in the coal mining industry (mobile longwall supports). In approximately two thirds of the cases, hose-specific deficiencies were the reason for malfunction. There was the suspicion that the performance of hose assemblies suffers more from the particular operating conditions and environmental factors in the mine than it does with industrial uses. The project, initiated and supported by the institution for statutory accident insurance and prevention in the mining industry, was aimed to investigate the application- dependent reduction of the operating efficiency of rubber- steel hose assemblies to be able to formulate recommendations as to the admissible service life of these products in mobile longwall supports.

Activities/Methods:

Tests were conducted in accordance with DIN 20022 Part 2 "Fluid-systems; rubber covered hoses with wire braid reinforcement; type 2ST" on two new 2ST-hose assemblies from four different manufacturers. In the same way, used 2ST-hose assemblies from six manufacturers were tested on the basis of DIN 20022 Part 2. The used hoses were made available by two mines, and subjected to testing after different periods of use in coal mining, viz. six months, one year, one and a half year and two years. The results achieved in the impulse tests (number of load alterations until breakdown) on the basis of DIN EN ISO 6803 "Rubber or plastics hoses and hose assemblies - Hydraulic pressure impulse test without flexing" served to assess the performance properties of the hoses in the new state and - after a defined period of time - in the used state

Results:

Impulse testing of new hoses revealed considerable differences in terms of quality, the quality range going from 2% to more than 300% of the required minimum pulse number specified in DIN 20022 Part 2. Quality control, especially in the manufacturing plants, must be clearly improved. The hoses' performance characteristics suffered considerably from use in the coal mines. After six months of use only about 14% of the minimum pulse number were attained on average; at the end of two years this value had further decreased to approx. 2%. The maximum service life of hose assemblies in the mining industry should be shorter than indicated in ZH 1/74 (four to six years). As a first orientation, a service life of one to two years is suggested. A definite recommendation, however, will depend on the practical experiences gathered by the mining industry and the competent German Berufsgenossenschaft.

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