Product category:
Seals
News Release from: Freudenberg Simrit LP | Subject: Hydraulic seals
Edited by the Engineeringtalk Editorial
Team on 18 October 2004
Hydraulic seals work safely with
zinc-free media
Tests have shown that zinc-free hydraulic media have been wrongly blamed for damage to hydraulic seals.
Zinc-free hydraulic media have been in existence for almost 30 years During this period the additives have been continuously modernised and modified by the manufacturers
This article was originally published on Engineeringtalk on 18 Feb 2003 at 8.00am (UK)
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The direct contact between the hydraulic medium and hydraulic seals results in interaction that is incalculable here.
In this context it was repeatedly claimed in the last two years that there is a direct association between zinc-free hydraulic media, hydraulic seals from PTFE bronze compound and the very fine striae that have occurred in certain cases.
This is completely unfounded, as the latest studies have shown.
The reason for the formation of very fine striae is due to quite different reasons, and a direct dependence between cause and effect cannot be shown for zinc-free hydraulic media in general terms.
As increasing numbers of operators of hydraulic systems are switching over to zinc-free media, no time should be wasted in clearing up the confusion that has arisen in this regard.
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The problem here is not the zinc-free media: the essence of this problem is the formation of the striae.
In order to identify the damage mechanism producing these very fine striae testing was carried out on a test rig with the aim of reproducing such striae.
The following parameters were varied for this purpose: material of sealing components; material of contact area; hydraulic media - in particular different zinc-free media; and operating parameters.
At no time during these tests, which solely aimed at producing defined striae, was it possible to reproduce a typical striae pattern.
Nor was it possible to produce a striae pattern in combination with all zinc-free hydraulic media tested.
As it is general knowledge that the formation of striae is directly associated with insufficient lubrication, the next test series was carried out under extreme dry running conditions through the absence of hydraulic media.
Analyses showed that these conditions result in abrasion products.
This involves the known polishing agents iron oxide and chromium oxide with a particle size of approximately 0.2um.
Examination under the scanning electron microscope have revealed that iron oxide and chromium oxide particles become attached in particular to the bronze particles of the PTFE bronze compound.
Here it is especially interesting that the quantity of these abrasion products is 10 times greater with PTFE bronze than for PTFE glass and PTFE carbon fibre compound.
As it is at the bronze particles that these abrasion products collect in an entirely relevant quantity, a direct association to the formation of very fine striation can be concluded from this.
Very fine striae are also a direct result of insufficient lubrication.
This can be summarised in three core statements.
In particular the use of PTFE bronze may cause the typical pattern of damage to form with insufficient lubrication and depending on the operating parameters.
No direct association to individual oil types - ie including zinc-free hydraulic media - or surfaces was found.
However, certain zinc-free media react with bronze so that it is difficult to evaluate any effect occurring here.
The pattern of damage known as "very fine striation" was already clearly defined by Simrit in 1992.
The abrasive accumulation now established during these extensive series of tests is an unmistakable indication for the causes of the damage mechanism resulting in very fine striae.
To avoid the formation of striae it is first of all necessary to analyse the operating parameters which might result in insufficient lubrication.
This may occur for example in closing cylinders when the cylinder makes a relatively long stroke without pressure and the full operating pressure does not act on the sealing system until the final millimetres.
Here no lubricating film can form, and in such cases the use of PTFE glass compound is recommended.
In many cases the use of PTFE bronze seals in combination with zinc-free hydraulic media is noncritical.
This of course depends on the special additives in the specific hydraulic medium used.
In all cases Simrit specialists are on hand with help and advice.
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