The claim that shaft wear problems associated with the use of hydraulic oils with zinc-free additives is completely unfounded, according to the latest work from Freudenberg Simrit. With the increasing use of oils with zinc-free additives, this claim has been of concern to both users and suppliers of hydraulic fluids.
A proposed direct association between zinc-free hydraulic media, hydraulic seals from PTFE bronze compound and the very fine striations that have occurred in certain cases was published at the 12th International Sealing Conference in Stuttgart in 2002. Simrit disagrees, and suggests that the formation of very fine striations is due to quite different reasons, and a direct general dependence between cause and effect cannot be shown for zinc-free hydraulic media.
The justification for this claim has been explained by the company. Zinc-free hydraulic media have been in existence for almost 30 years. During this period the additives have been continuously developed by the manufacturers. In order to identify the damage mechanism producing these very fine striations, testing was carried out on a test rig with the aim of reproducing the problem. The following parameters were varied for this purpose: sealing material, counter face material; hydraulic media, particularly different zinc-free oils; and operating parameters. During these tests a typical striation pattern could not be reproduced. This included all the zinc-free hydraulic media tested.
These striations are often associated with insufficient lubrication. Therefore, the next test series was carried out under dry running conditions in the absence of hydraulic media. This can result in the creation of abrasion products, such as iron oxide and chromium oxide with a particle size of approximately 0.2 ยตm. Further examination using a scanning electron microscope has shown that these oxide particles can become attached in particular to the bronze particles in the PTFE bronze compound. The quantity of these abrasion products can be 10 times greater with PTFE bronze than with glass or carbon fibre-filled material. The company is therefore concluding that, as it is the bronze particles that collect the abrasion products in greater quantity, this pro-