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Cylinders, seals need better texture specifications

A discussion group has been formed to work towards improved specification for hydraulic piston rod surface texture. A meeting, led by Bob Flitney, a UK consultant in seal R&D and surface texture, was held in Stuttgart, Germany in October and attended by four seal manufacturing companies and three cylinder manufacturers.

History shows errors
For years, seal catalogs specified only a height parameter, such as Ra, Rt, or Rz. During the early 1980s, many of these recommendations were not included in ISO standards for seals because no international agreement existed about the parameters that should be used nor the methods of measurement. However, research shows that the detail of the surface texture is important, and that additional parameters measuring secondary finishing, such as bearing area or skew, should be used.

Seal standards prepared more recently have included the specification of Ra, Rz, and bearing area, Rmr. Even more recent work shows that the use of additional parameters, Rp, Rk, and Rv, allow improved surface texture specification and more consistent performance. These parameters can be measured with most modern standard surface texture instruments.

Debate raises challenges
Members of the group suggested that satisfactory performance had been obtained for years because the normal industrial process for manufacturing chrome bar provided a satisfactory texture due to the grinding, polishing, and chroming techniques used. Detailed texture specification was less critical if the Ra value, or other height parameter, was satisfactory. As alternative surface treatments and machining processes have been introduced and reliability criteria become more stringent, specifications have been found to be inadequate. Some typical coating processes that are now used include ceramic, welded metal coating, metal spray, and nitriding.

Doubt also exists among some manufacturers about whether this more inclusive range of parameters should be required. However, they agree that there will be a progressive trend to replace chrome across industries, which will lead to increased requirement for improved texture specification. This trend is being created by both environmental and economic factors.

The way forward
Although some parameters have been provided, it was determined that adopting these parameters without qualification was inappropriate. Further work across a wide spectrum of applications and finishing methods will be necessary to ensure reliable optimum values. The group expressed interest in cooperative work across several research organizations to permit the use of extensive facilities to provide the large amount of data required. Liaison across Europe and with the major fluid power research work in the USA was viewed as a way forward.

The group will meet again to assess progress, most likely during Hannover Fair in April. Each of the seal manufacturers is investigating the most appropriate texture values that may be proposed for use with its seals.

Research organizations have been invited to investigate the potential for collaborative research. For more information, contact R.K. Flitney at +44 1799 501659, [email protected], or visit

TAGS: Seals