Dynamic pressure testing of hydraulic flexible hose assemblies - Ageing effects of storage conditions

Status:

completed 10/1999

Aims:

Hydraulic flexible hose assemblies are generally made of rubber hoses with a wire reinforcement and corresponding fittings. Compared to metals, rubber materials start ageing much earlier, not only under operating conditions but also during storage. For this reason, first orientation values for the storage time and service life of flexible hoses and hose assemblies were defined in the eighties already, e.g. in DIN 20066, Part 5 "Hydraulic fluid power; Hose assemblies; Assessment of service performance". It was recommended that flexible hoses should not be stored for more than four years, and that the maximum service life of hose assemblies should be six years, including two years of storage. Little experience was available when these values were fixed. The project was aimed to determine the effect conditions and times of storage have on the service performance of rubber-steel-hose assemblies. The project was launched by the institution for statutory accident insurance and prevention in the mining industry, and the BG expert committees "Chemistry", "Iron and Metal II and III", "Materials handling and load-lifting equipment", "Civil engineering", and "Transport".

Activities/Methods:

New hose assemblies and new hoses of pressure stages 2ST and 4SP, sold by the meter, were stored under four different storage conditions by a manufacturer. Storage times were two to three years, four to five years and seven years. Comparative tests were conducted on new samples which had not been stored at all. The criterion by which the service performance was assessed before and after storage of the hoses was the result of the impulse test with and without flexing according to DIN EN 6802 "Rubber and plastics hoses and hose assemblies with wire reinforcements; hydraulic impulse test with flexing" and DIN EN ISO 6803 "Rubber or plastics hoses and hose assemblies; hydraulic pressure impulse test without flexing".

Results:

2ST-hoses and hose assemblies turned out to suffer clearly more from storage than did the 4SP-samples. The absence of visually identifiable deficiencies is no guarantee for a sufficient service performance. Even if the visual check does not reveal any change in the outer appearance of a hose, its functional performance might be impaired. Storage conditions play an important role. Unless (optimum) storage in accordance with DIN 7716 "Rubber products; requirements for storage, cleaning and maintenance" is possible, hose assemblies should be stored in a closed room under usual workshop conditions. Open-air storage should be avoided, as it leads to the biggest loss in performance. The orientation values for maximum storage times (four years for hoses and two years for hose assemblies) as referred to in ZH 1/74 "Safety provision for hydraulic hose assemblies" and DIN 20066, Part 5 do not need to be revised; they were confirmed by the test results. Slightly extended storage times can be accepted in special, well-founded cases.

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