SURFACE FINISH GUIDELINES

1 FINDING THE OPTIMUM SURFACE FINISHTo achieve the optimum seal performance it is critical for themating hardware SURFACE FINISH to have defined characteristicsthat address the roughness, directional orientation or lay andhardness for reliable seal efficiency and life. As the Aerospaceindustry transitions from the traditional hard chrome platingtoward the use of HVOF (High Velocity Oxygen Fuel) coatings, thedefinition of an optimum SURFACE FINISH has also recently hard chrome plating had been effectively utilizedfor dynamic surfaces as the traditional coating method.

2 To thisend, Aerospace seal gland specifications have been publishedwhich include SURFACE FINISH requirements that are proven to becomplimentary to efficient seal performance, primarily when usedwith established elastomeric and PTFE contact is important to recognize the distinct differences between hardchrome plating and HVOF coating. Most notable is the manner in which chrome and HVOF interact with seals and bearings in dynamic applications. Experience with chrome has shown that with increasing time in service, the chrome SURFACE FINISH will polish and become smoother.

3 Conversely, HVOF surfaceshave been found to remain relatively constant regardless of theaccumulation of dynamic cycles with seals and bearings. As aresult, Greene, Tweed has established distinct recommendationsfor chrome and HVOF-coated OF SURFACE FINISH PARAMETERSG reene, Tweed s recommendation for SURFACE FINISH utilizes thedefinitions of the parameters as found in ISO 4287 for chromeplated or coated or finished information below references the Rp, Rz and Rmr surfaceparameters. This information is intended for reference only, andthe specific ISO standard should be consulted for more This parameter is a measure of the largest peak and can be used to gauge the potential aggression of the HVOF surfaces are most generally comprised of tungsten and carbide particles, a low Rp is recommended toassure that no irregularly high peaks are present that could act to score or slice the seals and This is a measure of the average peak to valley height of thesurface.

4 Since boundary layer lubrication present on the surfacecan be beneficial to reduce the friction and corresponding wearon the seal, the Rz characteristic provides an indication of thesurface s potential to maintain the desired lubrication to the originally known as the 10 Point Height, this characteristic123 SURFACE FINISH GUIDELINESP arameter Definition of the Parameter ISO 4287: 1997 RaThe arithmetic mean of a SURFACE FINISH profile within asampling lengthRpThe largest peak height of a SURFACE FINISH profile within asampling length measured from the mean lineRzThe average of the sums of the largest valley depth to thehighest peak of a SURFACE FINISH profile within a samplinglengthRmr (Tp)

5 The ratio of the material length at a specified depth for asurface FINISH profile 2007, Greene, Tweed all rights reserved All trademarks are property of their respective FINISH \ product datahas recently been re-defined by the ISO specification. Simply, it isnow the average of the highest peak to the deepest valley of fivesampling lengths taken across an evaluation length of a surfaceprofile (1997) = 1/5 { i},where Zi= (Pi+Vi) for each sampling lengthRmr This parameter provides information on the relative materialratio at a specific evaluation depth.

6 In many applications this infor-mation is used to evaluate the load bearing capacity of the surfaceprofile. Along with the other SURFACE FINISH parameters, Greene, Tweeduses Rmr (tp) to determine the consistency of the SURFACE profile. Evaluation of Rmr (tp) is somewhat misunderstood. To clarify, thesurface FINISH profile is evaluated by a generated Abbott-Firestonecurve. This curve plots the percentage of material present in asurface profile based on the depth from the highest peak. Using the Abbott-Firestone curve, the parameters COand C1determine the evaluation depth for determining the Rmr (tp)DYNAMIC CHROME SURFACE FINISH RECOMMENDATIONSE lastomeric Contact SealsGreene, Tweed s recommendation for elastomeric contact seals coincides with the specified SURFACE FINISH definitions of the governing Aerospace specifications MIL-G-5514, AS4716 and AS4832.

7 Ra = 16 inches ( meters) MaximumNote: In accordance with the previous revision of Aerospace Specification AS4716, Ra = 8 to 12 inches ( to meters) is considered optimum for elastomeric contact recommendation balances the two important aspects of desirablesurface characteristics for elastomeric contact seals the promotionof boundary layer lubrication between the seal and the hardwaresurface and minimal aggressive wear between the hardware andthe elastomer seal is important to note that there has been some consideration in the Aerospace industry to expand upon the recommendationscurrently found in the Aerospace specifications by controlling othersurface FINISH parameters beyond Ra.

8 This is allowable with theadditional parameters defined based on finishing and processingcontrols and historical performance data. Note, however, thatconsideration regarding the possible impact that the control ofthese additional parameters may have on other system productionlead time. Economics should be balanced against the net benefitachieved by specifying parameters other than Ra. Acknowledgingthat the chrome SURFACE FINISH is likely to polish/change after ashort time in service, the long-term, net positive effect of controllingadditional parameters for chrome, may be PEAKDEEPEST VALLEYABBOTT-FIRESTONECURVE COis a set percentage belowthe highest peak to eliminate possible measurement error C1is a defined distance beneathCOand defines where the surfaceprofile is evaluated Where C1intersects the X axis is the stated Rmr (tp)

9 For a surfaceprofile 2007, Greene, Tweed all rights reserved All trademarks are property of their respective DS-US-AS-032surface FINISH \ product PTFE Cap/Contact SealsDue to the fact that PTFE materials are inherently self-lubricating,the need for boundary layer lubrication between the seal and thehardware is less critical than with elastomeric contact seals. As aresult, hard chrome plating should be finished in such a manneras to reduce the boundary layer lubrication while still promotinga less aggressive SURFACE FINISH topography.

10 As a result, Greene,Tweed recommends:Ra = 4 to 8 inches ( to meters)With the use of both elastomeric and PTFE contact seals it isimportant to obtain a neutral grain orientation to the machinedsurface. Many finishing techniques such as single point turning,grinding, honing, roller burnishing and polishing are used in the manufacturing process. A process that induces a directionalorientation to the structure of the SURFACE can negatively influencethe leakage and wear behavior of the selected seal and should be OF THE MATING SEALING SURFACE A dynamic hardware SURFACE hardness that is generally below 42 HRCwill promote seal-induced polishing and wear to the mating metalsurface and also inhibit the use of higher performance.