Hydraulic hose assembly cleanliness

Published:  13 August, 2015

This is the first part of a short series of articles based on the British Fluid Power Association’s recommendations and best practice for hydraulic hose assembly cleanliness.

The cleanliness level of manufactured hydraulic hose assemblies (i.e. high production volume as opposed to low volume service hoses) will, in most cases, be specified by the customer reflecting their requirements for the total system cleanliness, where the hose assemblies will be used.

This level is referred to as the ‘Required Cleanliness Level’ (RCL) and is an important parameter in the management of the system cleanliness throughout its life.

The RCL specified is dependent upon the contaminant sensitivity of the systems components, the operating pressure levels, and the life and reliability of the machine where the hydraulic system is installed.

As the RCL is a specific customer’s requirement it is unreasonable to impose this upon hose manufacturers who work on bulk or batch production principles, but may rarely know of the customers’ requirements. It is sensible, therefore to achieve a lower minimum standard for ‘normal’ production throughput, and then put into place additional processes towards the end of the production process that will achieve the higher levels of cleanliness, if specified by the customer.

Production cleanliness levels for hose assemblies

For finished hose assemblies the required cleanliness is defined by three levels of system sensitivity as shown in Table 1.

Specifying RCLs in terms of ISO 4406 codes presents problems to OEMs and

manufacturers, as both require some knowledge/guidance on the number of particles much

larger than the >14 μm(c)/15 μm limit within the standard ISO 4406. The only other

‘international’ source for guidance is SAE AS 4059 which goes up to 70 μm (c)/ >100 μm. See ISO/TR 16386 for an explanation of the dual sizes. Even then data on larger sizes

may be required and there is no published guidance.

Suggested maximum particle counts for various levels of contaminant sensitivity

In an attempt to overcome limitation stated above, members of the BFPA Technical

Committee - Contamination Control (TC 6) have used current particle count data and

particle size distributions to generate representative maximum particle numbers allowed for

the three sensitivity levels stated within Table 1. The following comments should also be


• Like all particle size distributions, the numbers of particles reduce greatly as the size of the particle gets larger. This means that the concept of ‘zero particles’ at these sizes does not exist in a statistical sense.

• The preferred volumetric unit at ISO is 1 mL. The data at these sizes are very small and result in decimal numbers at the larger sizes and to lessen this impact, the particle counts have been referred to a volume of 100 mL.

• Microscope particle counts at > 1 μm are as stated within the standard, but this is considered not to be practical.

• The equivalent AS 4059 classes do not duplicate ISO 4406 distribution. AS 4059

• Table 1 convention is used.

The above information has been edited from the British Fluid Power Association’s guidance document BFPA/P111 ‘Recommendations and best practice for hydraulic hose assembly cleanliness’. The full document contains further information and clarification on a number of the points referenced above. For more information about the document and how you can obtain a copy, please contact the Association at info@bfpa.co.uk, or Tel: 01608 647900. More guidance from BFPA/P111 will be featured in the BFPA Top Tips page in the next edition of Plant & Works Engineering.

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