1、Best Practices Entry: Best Practice Info:a71 Committee Approval Date: 2000-03-16a71 Center Point of Contact: MSFCa71 Submitted by: Wil HarkinsSubject: Vehicle Integration/Tolerance Buildup Practices Practice: Use master gauges, tooling, jigs, and fixtures to transfer precise dimensions to ensure acc
2、urate mating of interfacing aerospace hardware. Calculate overall worst-case tolerances using the root sum square method of element tolerances when integrating multiple elements of aerospace hardware.Programs that Certify Usage: This practice has been used on Saturn I and Saturn V, Space Shuttle Ext
3、ernal Tank (ET), and Space Shuttle Solid Rocket Booster (SRB) programs.Center to Contact for Information: MSFCImplementation Method: This Lessons Learned is based on Reliability Practice No. PD-ED-1219; from NASA Technical Memorandum 4322A, NASA Reliability Preferred Practices for Design and Test.Be
4、nefit:Using prudent and carefully planned methods for specifying tolerances and for designing, manufacturing and mating major elements of aerospace hardware, will result in a cost-effective program with minimal rejects and waivers, and will avoid costly schedule delays due to potential mismatching o
5、r misfitting of major components and assemblies.Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-Implementation Method:Introduction:Elements of large aerospace hardware, such as those encountered in the Space Shuttle program, are often (1) manufacture
6、d in diverse locations; (2) manufactured and assembled by different centers, prime contractors, and subcontractors; and (3) manufactured and assembled in varying climates and environments. Several additional factors must be considered in establishing design tolerances and in providing jigs and fixtu
7、res to assure that the major elements can be mated successfully prior to launch. Specifically, the size and weight of these major components and assemblies (such as the ET and SRB) are so great that special consideration must be given to hardware deflection and deformation due to vehicle mass; wind
8、loads; and environmental factors, such as temperature, humidity, and atmospheric contamination. A variety of methods of calculating and allowing for tolerance buildup and for ensuring matching components at the assembly site have been developed to meet the specific needs of these large hardware elem
9、ents of the Space Shuttle program. No one method suits all needs. In some instances (in the ET project, for example), the overall tolerance between major critical dimensions is the sum of all of the “worst-case“ tolerances of the subassemblies. In the SRB project, for example, the root sum square of
10、 the tolerances of segments is used to arrive at the tolerance on major critical dimensions. In addition, adjustable supports are used at critical attach points to permit minor variations in matching and assembling these two major Space Shuttle hardware elements.This practice provides selected metho
11、ds that have proven successful in ensuring that major elements of aerospace hardware will be successfully and accurately assembled both in the factory and in the field; and it provides methods and definitions of dimension and tolerance buildup practices that have proven successful in designing, buil
12、ding, and flying large aerospace vehicles.Master Tooling/Jigs And Fixtures:Master tooling should be used when machining a number of interchangeable parts to ensure that each part will fit and function properly. Another method of assuring interchangeability of parts during manufacturing is through th
13、e use of jigs or fixtures. This method is used primarily when an operation such as welding, drilling, or reaming is performed by hand on interchangeable parts.Example:Thiokol, Inc. is under contract to NASA MSFC to fabricate the motor segments of the SRB. These segments must fit together precisely w
14、hen they are assembled at KSC; therefore, each segment must be indexed when it is manufactured by drilling the indexing holes in the tang and clevis joints of the Solid Rocket Motor (SRM) segment using a master tool. All of the master tools are made from transfer gauges, and the transfer gauges are
15、made from a master gauge, resulting in the same indexing regardless of where the segments are manufactured (see Figure 1).Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-refer to D descriptionD refer to D descriptionDMaster Gauge:Provided by IHSNot f
16、or ResaleNo reproduction or networking permitted without license from IHS-,-,-The master gauge is a stable, heavy cast iron fixture into which the master interface hole pattern has been precision bored. The bored holes are lined with pressed-in, hardened bushings (see Figure 2).The exact location of
17、 each hole is determined by independent inspection and is entered on the master gauge drawing as a basic no-tolerance dimension. This drawing, and the master gauge it depicts, describe and establish the mastered hole pattern. The master gauge is used as a template when bushings are potted into the t
18、ransfer gauge.Transfer Gauge:Transfer gauges are stable, rigid fixtures into which hardened bushings are potted with an epoxy compound. During potting, the bushings for the transfer gauge are held in the correct position by potting pins located in the master gauge. (As shown in Figure 2), transfer g
19、auge is fitted over the master gauge before potting, and the transfer gauge bushings are located precisely before potting using the potting pins. After the potting material has cured, check pins are inserted first through the master tool and then through the new bushing location.refer to D descripti
20、onMaster Tool/Drill Jig: DMaster tools are fixtures into which hardened bushings are potted with an epoxy compound. The transfer gauge is used as a template when bushing are potted in the master tool (see Figure 3). The bushings are held in position by the transfer gauge and potting pins. After the
21、potting material has cured, check pins are inserted through the transfer gauge into the master tool to verify the location of the potted bushings.Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-Master tools are used as drill jigs to assure that the a
22、ssembly holes and pins and the indexing holes and pins in the tangs and clevis joints of the SRM segments are precisely the same (see Figure 4). Master tools are made from the same material as the SRB segment casings. Therefore, the coefficient of expansion is the same for both. It is critical that
23、the master tools and the SRB segments be subjected to the same environment until they stabilize with the area temperature before attempting to mate them or initiate drilling.refer to D descriptionInspection pins are used to verify hole locations in the SRM segment relative to the master tool.Mobile
24、Launch Platform (MLP) Preparation for SRB Stack: D1. The support post #2 under the MLP is adjusted to a certain height in reference to a benchmark in the Vehicle Assembly Building (VAB). There is a corresponding support post #2 and benchmark at the launch pad.2. Once support post #2 has been adjuste
25、d to the correct height, a triangular reference plane is established using two other points under the MLP.3. This reference plane is then transferred to the top of the MLP.4. The eight support posts (four per SRB) are then adjusted by the following means: A. Shims are added between each support post
26、 and the haunch which is permanently attached to the MLP to raise the support post to a reasonable height.B. Shims may be added under each spherical bearing for height adjustment up to a maximum of 0.5“ (see Figure 5).Provided by IHSNot for ResaleNo reproduction or networking permitted without licen
27、se from IHS-,-,-C. Eccentric bushings and the eccentric spherical bearings are rotated to bring them into alignment with a bias to give the SRBs a very slight inward pitch (towards ET). The bearings are then locked so they cannot move when the SRB aft skirt is installed.refer to D descriptionSRB Sta
28、cking: DThe SRBs are stacked on the spherical bearings on the MLP in five separate sections, one section at a time. The aft skirt, kick ring, and aft motor segment are preassembled in another area and stacked on the MLP as the aft booster assembly. The aft center segment is then stacked on top of th
29、e aft booster assembly. The forward center segment is stacked on top of the aft center segment. The forward motor segment is stacked on top of the forward center segment. The forward skirt, separation ring, frustum and nose cap are also preassembled and stacked on top of the forward motor segment as
30、 one unit. All of these sections make up one SRB. The fit of one section with the next is ensured because their mating parts (tangs and clevis joints) were all drilled using a master tool. There is no alignment adjustment between the sections and the deviation from vertical in the “y“ plane is +0.82
31、99“ per stack.Tolerance Buildup Practices: External Tank:Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-In the External Tank project, manufacturing drawings have tighter tolerances than the interface control documents (ICD) to eliminate the need for
32、 waivers if tolerances are exceeded slightly. The ICD tolerance for the overall length of the external tank (ET) is +0.74“, while the manufacture drawing tolerance is +0.62“.The ET assembly drawing overall length tolerance represents the worst-case stack-up of the major ET assemblies, and the lower
33、level assembly drawings use the same philosophy. For instance, the aft dome roundness is +0.50“, the barrel attach points are +0.02“, the machined and fabricated (formed) parts are +0.03“, and thin sections are +0.10“.ET design engineers strive for tolerances of fabrication tools up to 10 times bett
34、er than the flight hardware tolerances. For example, if the target tolerance on a part is +0.1“, engineers strive to make the tool tolerance +0.01“. If the worst-case tolerance on a part is +0.1“, they strive to make the tool worst case tolerance +0.05“, or no greater than one-half of the part toler
35、ances.ET design engineers use the ANSI Standard for Dimensioning and Tolerancing for block tolerances; i.e., x.x=+0.10“, x.xx=+0.03“, x.xxx=+0.010“, and x.xxxx=+0.000x“.In some instances, match drilling is used during ET fabrication to ensure a perfect fit rather than using machine-to-drawing holes
36、which would require a very tight tolerance on the machining process.Methods of Calculating Tolerance Buildup:In aerospace hardware, two methods are normally used to calculate tolerance buildup. They are the root sum square (RSS) method and the root mean square (RMS) method. Each seems to serve best
37、for particular applications.The RSS method is generally used for calculating the tolerance buildup of large pieces of hardware like the SRBs when they are assembled at KSC. This method of tolerance buildup assumes a random ordering of the various combinations of the interface theoretical tolerances
38、and resulting misalignments.The following formula is used for RSS calculations:refer to D descriptionD Where: A, B, C, and D are the tolerances of mating segments of an assembly.The RMS method is generally used in calculating the tolerances of piece parts for small assemblies Provided by IHSNot for
39、ResaleNo reproduction or networking permitted without license from IHS-,-,-such as pumps and valves. The RMS method is also used in connection with surface roughness. The roughness value is assumed to be approximately equal to the square root of the mean value of the squares of the heights and depth
40、s of the surface roughness irregularities measured from the nominal surface in micro-inches. This value is considered to be representative of the surface condition because it is assumed to give appropriate emphasis to the peaks and valleys comprising the surface. The following formula is used for RM
41、S calculations:refer to D descriptionD Where: A, B, C, and D are the tolerances of mating segments of an assembly, and N is the number of tolerances.Environmental and Physical Factors:When designing a part and establishing tolerances, it is important to consider both the environment where the part i
42、s initially manufactured and assembled, and the environment where the part may have to be replaced or reassembled. Factors to be considered include assumptions as to whether the part has to be disassembled and reassembled at a later time and/or different location from the initial assembly. Tolerance
43、s of the part will be determined by expected variations in temperature, material, coefficient of expansion, humidity, wind load, or cleanliness.Example:Several precision components such as the LO2and LH2pumps and valves in the Space Shuttle Main Engine may be assembled initially in a clean environme
44、nt of 68 degrees and low humidity. If it is anticipated that one of these parts may have to be replaced on the launch pad at KSC, the fit of its parts could be affected if the temperature was 98 degrees and the humidity 95 percent, if the wind was blowing at 45 mph, or if sand and dirt were present
45、in the local atmosphere.Potential Cost/Schedule Impact Avoidance:Cost and schedule are two of the most important factors to consider when establishing tolerances. Designers should never specify a tighter tolerance on any part or component than is absolutely necessary for that part to fit and functio
46、n properly. Tighter tolerances require more precise machining, and result in potential scrappage of parts or components at inspection. Excessively tight tolerances also require more time for machine setup and machining as well as extra time for inspection. The loss of material, the extra time for ma
47、chining and inspection, and the potential overtime required to meet a schedule can result in higher overall costs.Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-Example:The critical mating surfaces for interface mounting flanges designed for a pump
48、or valve are required to be flat within .002“. If the designer were to unnecessarily require the same tight tolerance on the size and location of the mounting holes, flanges could be unnecessarily rejected if the holes did not meet the specifications. This could increase the cost of the flanges beca
49、use of the wasted material and machining time.Technical Rationale:The rationale for tolerancing is to assure that the majority of small and large parts will fit and function as they were designed to when they finally come together as an overall assembly. It is also essential that these parts can be disassembled and reassembled if ne
