Improving the Service Life of the 100-meter Green Bank Telescope Azimuth Track

Arthur Symmes *, Robert Anderson, Dennis Egan National Radio Astronomy Observatory (NRAO)

P.O. Box 2, Green Bank, WV USA 24944

ABSTRACT

The NRAO Green Bank Telescope (GBT), located in Green Bank, West Virginia, is supported by 16 steel wheels which rest upon a composite steel and concrete Azimuth Track, 210 feet (64 meters) in diameter. From the start of observing in February 2001, the Azimuth Track design presented an operational problem for NRAO. By the spring of 2001, slippage of the top plate on the base plate was causing hold-down bolt failures. In July 2002, wear between the top and base plates (fretting) had become evident around the entire track circumference. NRAO engineers took immediate action to reduce both the track slippage and wear problems. But in January 2003, cracks were discovered in two adjacent top plates; by 2006 the top plates were cracking at a rate of almost one a month – an alarming rate given the design service life of 20 years. This paper will summarize the engineering analysis efforts that were subsequently conducted to assess the root cause of the GBT track degradation problem. We will also discuss a trial modification section that was installed in June 2004. Finally, we will discuss the design solution that was developed to remedy the track performance problem.

Keywords: Green Bank Telescope, azimuth track replacement, structural analysis, finite element analysis

1. INTRODUCTION In vertical profile, as illustrated in Figure 1, the original 210-foot diameter azimuth track consists of a wear plate (2.25 in thick by 19 inches wide), a base plate (9 inches thick by 27 inches wide), a 5-inch thick splice plate below the joints between each base plate, 4 inches of cementious grout, and a reinforced concrete ringwall some 22 feet deep to bedrock. The wear plate is affixed to the base plate by high tensile bolts. There are 48 sections of wear-base plate combinations around the circumference. There is over 1 million pounds of load on each of the GBT 16 steel wheels, which are 52 inches in diameter. NRAO is not aware of a wheel and track assembly that supports a heavier loading.

The GBT Azimuth Track difficulties were initially recognized in late 2000 shortly before the GBT began commissioning observations. A chronology of GBT azimuth track problems is provided in Table 1.

Table 1. GBT Azimuth Track Deficiencies Chronology.

Autumn 2000 Tangential slippage (“walking”) of base plates and wear plates detected

February 2001 NRAO commissioning observations begin

Spring 2001 Numerous wear plate hold down bolts fatiguing and breaking just below the head

December 2001 First evidence of significant amounts of grout water seepage

March 2002 Excessive tilting of azimuth wheels at track joints detected

July 2002 Significant surface deterioration between wear plate and base plate evident

November 2002 First fatigue failures of new bolts

January 2003 Cracking of first wear plate identified

* asymmes@nrao.edu; phone 1-434-296-0338; fax 1-434-296-0255; www.nrao.edu

Figure 1. GBT Azimuth Original Track Configuration.

2. INITIAL REMEDIATION EFFORTS Beginning in the spring of 2001 continuing until 2007, various remediation efforts were undertaken to resolve the azimuth track deficiencies as they were identified. While many of the initial track remediation efforts were conducted by the GBT Contractor during the telescope commissioning phase, several remediation tasks were conducted by NRAO personnel. In conjunction with these remediation efforts an expert Review Panel was convened by NRAO to discuss the track problems and develop potential long-term corrective actions.

2.1 Reduction of base plate tangential slippage

In an effort to stop the base tangential motion, or slippage, with respect to the ground, the GBT Contractor welded the base plates to the splice plates. This appears to have been somewhat successful, as the base plate motion was reduced from ±0.060 inches to ±0.002 inches. Eight of the original 48 splices were not welded, ostensibly to allow for thermal expansion and contraction. This splice plate welding effort was completed in the early part of 2001.

2.2 Reduction of wear plate tangential slippage and bolt breakage

In the fall of 2000, in addition to base plate movement, it was recognized that there was substantial relative tangential slippage (“walking”) between the azimuth track wear plates and the base plates. In this same time frame, several hold-down bolt breakages were experienced around the azimuth track circumference. In order to alleviate both problems, the GBT Contractor adopted three methods to improve the wear plate–base plate interface. First, the number of hold-down bolts for each wear plate was increased from 12 to 36. Second, the diameter of the hold-down bolts was increased to 1.5 inches. Third, the length of the hold-down bolts was increased to 5 inches. These efforts appeared to be successful in eliminating the excessive tangential movement of the wear plate with respect to the base plate; however, bolts began failing again in November 2002. The relative motion between the wear plates and base plates was reduced from ±0.130 inches to ±0.003 inches.

2.3 Reduction of water seepage

In the winter of 2001–2002, continuous track covers were installed by NRAO to help reduce the amount of water seeping into the track's grout. It was expected that reducing the amount of water in the grout would reduce the overall track system wear rate. The covers have greatly reduced the amount of water falling on the track and flowing into and out of the grout, although some water still reaches the grout. Water seepage into the grout is of concern because: (1) the water flowing through the track/grout system carries grout particles away with it especially given the high pressures generated by the telescope motion; and (2) the highly reactive silica fume (caused by incomplete mixing of the dry-pack grout when it was installed) found in the grout during an inspection by Modjeski and Masters [1] can be activated on contact with water, causing uneven expansion of the grout and possibly damaging the foundation.

2.4 Reduction of excessive wheel tilts and wear/base plate surface deterioration

Excessive wheel tilts were first recognized in March 2002 when it was discovered that the azimuth wheel assemblies, shown in Figure 2, were exceeding the design specifications for tilt while running over the joints in the track. Because of the static load distribution when a wheel crosses the 45-degree splice joint, a certain amount of wheel tilt is to be expected. However, after visual inspections identified what appeared to be excessive tilts, instruments were installed to monitor the actual wheel tilts which lead to the conclusion that tilts at several splice joints were exceeding specified limits.

Figure 2. One of the four GBT “Truck Units;” each Truck Unit is outfitted with 4 Wheel Assemblies.

Initial speculation was that failure of the grout under the splice joints was responsible for this new degradation of track performance. In order to test this speculation, a section of cementious grout was replaced with epoxy grout. It was quickly determined that replacing this section of grout made no difference on the measured tilt magnitudes. It was during subsequent track inspections following this grout replacement that the excessive deterioration to the mating (or faying) surfaces between the war and base plates was identified. This surface deterioration, which closely resembles the “fretting” wear found on gear bearing surfaces, was generally found around the entire circumference of the azimuth track. In addition, the deterioration was evenly distributed on the surfaces of both the wear plates and the base plates. In the areas adjacent to the track splice joint, the resulting gap between the wear and base plates was found to be on the order of 0.060 inches. A representative sample of the track fret gaps is shown in Figure 3.

The assessment at this point indicated that a strong correlation existed between the excessive wheel tilts and the established fret gaps. The decision was therefore made to install shimming material to close the fret gaps, especially at the splice joints. During the summer of 2002, several shim materials were tested; following several months of testing, a molybdenum-bronze impregnated Teflon material was determined to have the best performance. However, even this Teflon material had a limited life span of between 8 to 12 months which required continuous monitoring of the wheel tilts to identify when the shims had degraded to the point that wheel tilts were approaching excessive levels.

3. AZIMUTH TRACK EXPERT PANEL REVIEW Following the appearance of the fret gaps and the associated excessive wheel tilts concern increased within NRAO over the structural adequacy of the azimuth track because of the growing scope and extent of the identified problems. NRAO determined that there was sufficient internal concern regarding the engineering of the azimuth track design to warrant convening a panel of engineering experts to review and discuss possible options to ensure satisfactory long-term performance of the GBT azimuth track. A special meeting of this Review Panel, composed of academic and professional engineering experts, was convened in October 2002 to evaluate the cause and remediation of the wear in the GBT azimuth track at which time data supporting the various track problems was presented [2]. It was the intent of NRAO to pursue analysis and modification of the track, up to an outright replacement based on a new design. The Panel considered the risk of new, unforeseen failure mechanisms inherent in a new design, and recommended that selected trial

measures and further testing be done in parallel with detailed finite element analysis (FEA) and investigations into a new track design.

Following two days of meetings, discussion, and inspection of the azimuth track, the Review Panel provided a number of recommendations for investigating possible long-term improvements to the existing track system. The most significant Review Panel recommendations are summarized in Table 2.

Table 2. GBT Review Panel Primary Recommendations for Investigation of Improved Track Long-Term Performance

• Refinement of the existing azimuth track finite element analysis (FEA) of the track splice joint to include:

o Assessment of the interfaces between the wear and base plate o Assessment of shear forces at the base plate-wear plate interface and on the

bolts connecting them o Assessment of the impact of the voids between the plates caused by the fretting o Assessment of the effects of increasing the bolt preload or increasing the

effective length of the bolts to prevent fatigue failure

• Develop and implement a splice joint stiffening of the base plate to provide improved continuity

• Concurrently with base plate stiffening test, remove existing wear plates and replace with two 7-foot sections and one 14-foot section in order to bridge the stiffened base plate splice joint; the intermediate joints should be symmetrical

Figure 3. View of a representative fret gap found on the GBT Azimuth Track; shown in the picture is a rubberized casting

of the fret gap existing between base plate and the wear plate.

A secondary recommendation proposed by the Review Panel was a re-evaluation of the GBT weight to ensure that realistic wheel loads are used in all track evaluations. Because of the uncertainty of the telescope weight as derived from design documentation, the Review Panel suggested that measurement of the actual wheel loads may be required. In response to this recommendation, weighing of the GBT was performed by NRAO in April 2003. Weighing of the GBT determined the maximum wheel load to be 1,099,674 pounds; this maximum wheel load is approximately 5 percent greater than the design wheel load of 1,045,440 pounds used by the antenna Contractor during the design phase.

4. ASSESSMENT OF LONG-TERM AZIMUTH TRACK SOLUTIONS 4.1 Dynamic/non-linear analysis – FEA of azimuth track using rolling wheel

In response to the analysis recommendations made by the Review Panel, NRAO contracted Simpson, Gumpertz, and Heger (SG&H) of Boston, MA, to conduct extensive high-level, non-linear finite element analysis (FEA) studies of the track splice joint. These FEA studies were intended to quantify the interaction of the wear and base plates including plate slipping in way of the fretting areas; the detailed FEA also modeled the effects of hold-down bolt embedment length and pre-tensioning. FEA efforts were conducted by SG&H from 2003 through 2005; during this period SG&H conducted detailed FEA of both the existing track splice joint as well as the track configuration that was ultimately selected for the track replacement. A representative example of one of the SG&H azimuth track FEA models is shown in Figure 4.

Using the analyses conducted by SG&H, NRAO was able to confirm that the appearance of the fretting degradation (gaps) found on the wear and base plates could be attributed to the large relative motion that occurred between those track components. This degradation primarily occurred before the addition of the increase number of hold-down bolts; however, some degree of fretting wear continued after the addition of the bolts. The susceptibility to fretting wear was determined to be largely a function of the high contact pressure under the wheels and the relatively large relative motion in this area.

The SG&H analysis results also confirmed that fatigue related to stress reversals generated by the passing wheels were responsible for the failure of hold-down bolts. The information gained from this part of the analysis was used for the track redesign effort. For the new track configuration, based on the SG&H results, the hold-down bolts were moved away from the wheel path (which required a wider wear plate), the effective hold-down bolt length was increased, and the hold-down bolt preloading was increased. These two configuration changes combined to reduce the bolt stress levels sufficiently to provide a minimum bolt fatigue life of 20 years [3].

Figure 4. Illustration of detailed GBT azimuth track FEA model prepared by SG&H to study rolling wheel effects.

Despite the excellent analyses conducted by SG&H, two problem areas were not resolved by their efforts:

• The root cause of the wear plate cracking was not identified by the SG&H FEA results. In fact, the SG&H results suggested just the opposite; that is, the stress levels developed by SG&H showed that the wear plates should not be experiencing any failures at all. It is noted, however, that all the analyses conducted by SG&H assumed that all track components maintained their original dimensions; no consideration of the fretting effects on plate geometries were considered in the SG&H analyses. Subsequent static FEA conducted by NRAO (as presented in following sections) indicated that inclusion of fretting effects had a dramatic influence on the stress results.

• A walking track phenomena, or creepage, was also anticipated but not observed in the results of the SG&H numerical analyses. This phenomena is well documented qualitatively [4], however, it is lacking in applicable quantitative stress analysis. It was expected that SG&H analysis, using a rail structure much simpler that those

encountered in the rail industry, would be able to shed light on creepage and potential associated internal stresses. However, in spite of state of the art hardware and software, it was not demonstrated numerically that creepage occurred and therefore it is feasible that additional local stresses associated with the dynamic behavior of the track/rolling wheel interaction remain unidentified.

4.2 Static analysis – FEA analysis of wheel tilt at splice joints

One of the primary reasons for conducting a static FEA of the azimuth track was to gain a better understanding of the structural mechanism that causes the wheel tilting action at each track splice joint; an example of measured wheel tilts is provided in Figure 5. In Figure 5, “Tilt-In” is defined as tilting or leaning of the Azimuth Wheel inward toward the antenna pintle bearing; “Tilt-out” is defined as tilting or leaning of the Azimuth Wheel outward away from the antenna pintle bearing. It should be understood that excessive tilting of a wheel assembly could lead to structural instability and potentially collapse of that wheel assembly. Controlling, reducing, or eliminating this wheel tilting is one of the major objectives associated with the track modification or replacement. It is also apparent from past experience that the wheel tilting at the joints exacerbates the fretting problem between the wear plate and the base plate. Therefore, without a complete understanding of what occurs as a wheel approaches and passes over each joint, it will be difficult to satisfactorily determine a suitable corrective action.

Figure 5. Measured azimuth wheel tilts as the Wheel crosses a track splice joint (shown is a plot for Track Splice 13).

In order to study the static interaction of the wheel with the azimuth track, a NASTRAN FEA model was developed for a representative section of the azimuth track incorporating three splice joints. The rolling wheel load is modeled by applying a patch load, representing a Hertzian contact area [5], at various wheel locations along the track circumference relative to the joint. The FEA model prepared for this analysis consists of nominally 133,676 nodes and 116,049 eight-noded solid elements. It must be emphasized that the NASTRAN FEA models used for this analysis study do not precisely simulate the interaction of the wheel and track components. Specifically, the slip surfaces between the wear and base plates and between the base plate and the cementious grout are accurately not represented. The FEA model used in this static analysis is shown in Figure 6.

A sample of the FEA deflection results for the Azimuth Wheel patch load positioned at the center of the splice joint (that is, Wheel = 00.00 inches) is presented in Figure 7, which shows the wheel at -6.67 inches (counter-clockwise from the splice joint center). The peak deflection of the wear plate top surface at this location is -0.0206 inches; by comparison the peak deflection with the wheel at the splice joint center location (Wheel = 00.00”) is -0.0214 inches. Figure 8 provides “snapshots” of the wear plate top surface deflections as the Azimuth Wheel patch load is moved from the -19.00 inch position clockwise to the +19.00 inch position (relative to the splice joint center). In regard to wheel tilts, it is noted that when moving from the -19.00 inch wheel position clockwise towards the center of the splice, the wheel begins to tilt out (to the right) as it approaches the center – this shown by the increasingly “red” regions to the right as the wheel nears the splice joint. As the patch load crosses the splice joint, the tilt returns to “zero” at the splice joint center point and the then the wheel begins to tilt in as the wheel continues clockwise away from the splice joint center. The wheel tilts return back to “zero” at roughly 20 inches from the joint center.

Figure 6. NASTRAN FEA Model used for static analysis of azimuth wheel rolling over track splice joints.

As shown in Figure 9, when no surface degradation as of result of fretting is considered, the wheel tilts associated with crossing the splice joint are relatively benign with an expected peak tilt of less than 0.025 degrees. However, when the influence of a fret gap is included, it can be seen that the tilts increase dramatically. For a nominally fret gap of 0.030 inches, the peak tilt increases to about 0.130 degrees; while a 0.060-inch fret gap is predicted to cause wheel tilts on the order of 0.285 degrees. It can be seen in Figure 9 that tilts of this magnitude are consistent with the tilts measured at the GBT azimuth track.

Figure 7. FEA deflection results for the Azimuth Wheel positioned at -6.67 inches from the center of the splice joint

4.3 Stress analysis in way of the splice joints

A second reason for conducting the static FEA of the Azimuth Track configuration was to identify a root cause for the circumferential (edge) and radial cracks that were occurring within close proximity of the track splice joints; examples of these cracks are shown in Figure 10 and Figure 11, respectively. The preliminary assessment of wear and base plate stresses derived from the wheel tilt FEA models provided similar results as those determined by SG&H using a more detailed, dynamic FEA. That is, based on the tilt analysis models, the calculated stresses were below the assumed working stresses for the wear plate and base plate, see Figure 12.

As a result, NRAO was still left with no explanation for the edge and radial cracking of which was occurring with increasing regularity on the wear plates. Subsequently, the FEA models were revised to include the observed fret gap. By incorporating a minimum fret gap of 0.03 inches, the nominal stresses in the wear plate were found to increase

significantly. For the NRAO fret gap studies, the fret gap was assumed to extend approximately 14 inches circumferentially from the center of the splice joint. With this assumed circumferential gap length, the calculated stresses increase substantially over roughly 75 percent of the assumed fret gap length. It is apparent from these results that significant plate bending occurs in way of the fret gaps along the track resulting in higher than expected stress conditions when the azimuth wheel crosses.

Assessment of the stress condition in way of this assumed fret gap supported the conclusion that the wear plates are cracking as a result of fatigue. Review of fatigue data for AISI 4140 plates [6] as well as a systematic evaluation of fatigue life indicated that the nominal fatigue limit for the track plates was somewhere in the area of 100,000 cycles. Based on the GBT operational history, azimuth wheel travel over joints is on the order of 25,000-35,000 cycles per year; this equates to the track wear plates achieving 105 cycles within three to four years, which is equivalent to the time frame for the appearance of cracks on the Azimuth Track.

Figure 8. FEA results for GBT Wheel Tilt Analysis for the azimuth wheel moving in a clockwise direction from -19.00

inches to +19.00 inches (relative to the splice joint center); maximum wear plate surface deflection is 0.0214 inches.

Figure 9. Calculated Wheel Tilt results from the static NASTRAN FEA; selected measured wheel tilts are included.

Figure 10. Circumferential cracks in the GBT wear plate extending from the face of the 45-degree splice joint surface.

Figure 11. Radial cracks that have occurred in the bottom surface of a GBT wear plate.

Figure 12. Calculated Von Mises (Effective) Stresses for the Original Track Configuration.

Figure 13. Fret Gap FEA results; indicates that fatigue failure in the range of 105 cycles could be expected.

5. TRACK SPLICE JOINT TEST SECTION In response to the 2002 Track Review Panel’s recommendation to develop and implement a splice joint stiffening design, NRAO undertook an effort to modify one splice joint along the Azimuth Track. This track modification effort was begun and implemented in 2003 and involved the installation of a 3-inch deep, partial penetration weld at the top of the base plate joint. In addition, the wear plate joint was shifted away from the base plate joint by 3.75 degrees, which results in a wear plate completely spanning the base plate welded joint. To accomplish the wear plate joint shift, two intermediate wear plate joints were installed on either side of the welded base plate joint; as shown in Figure 14, these

new wear plate joints were given a “chevron” configuration (one of 90-degrees and one of 120-degrees) in an attempt to minimize wheel tilts at the joint crossing.

Following four years of service, the modified joint section was found to perform satisfactorily. There has been a clear and stable reduction in wheel tilts in the modified joint. It should be noted that the wheel tilts were not completely eliminated in the modified joint; however, based on the FEA analysis conducted by NRAO, comparatively small tilts were expected. Likewise, FEA analysis also indicated that substantially lower stress levels were to be expected for the modified joint configuration. Finally, plate surface wear in way the modified joint appears to be minor with no observable wear at either of the wear plate chevron joints or at the welded base plate joint.

Figure 14. Configuration of GBT Azimuth Track Test Joint Section.

6. CONCLUSIONS In December 2004, the Track Review Panel was reconvened to evaluate results of the on-going analysis and in-field joint testing. In the course of this review, the proposed track configuration for resolving the overall track problems was reviewed. At that time, a consensus was achieved that the proposed approach provided the best possible solution for extending the operational life of the Azimuth Track components within the limits of the overall telescope geometry. The primary characteristics of the new track installation, as shown in Figure 15, include:

• Installing new base plates having a minimum yield strength of 50,000 PSI

• Modifying the base plate splice joint to 22.6 degrees and installing a 3-inch deep, partial penetration weld at the top of each of the 48 base plate joints (joint angle was changed to maintain a consistent bolt spacing)

• Utilizing Full width (27 inches) wear plates having a depth of 3.25 inches and fabricated from AISI 4340 steel (having a minimum Brinell Hardness Number range of 360 to 390 and a minimum Charpy impact energy of 10 foot-pounds at 0 degrees F)

• Installing the wear plates with joints staggered 3.75 degrees away from the base plate welded joints and employing a 120-degree chevron joint at each of the 48 wear plate joints

• Employing 2-inch high tensile threaded studs with a minimum yield strength of 125,000 PSI on a 9.25-inch spacing (1,728 total) to hold the wear plates to the base plates; a bolt-tensioning system instead of bolt torqueing to be employed to ensure adequate hold-down force

• Installing a Teflon-bronze-molybdenum shimming material between the wear and base plates to eliminate the initiation of fretting

Analyses of this proposed configuration was conducted by NRAO as well as by SG&H. The analysis results confirmed that substantial improvements in track component performance were to be expected. In particular, the FEA results point

to a 40 percent reduction of the plate wear index developed by SG&H as well as a significant improvement in the hold-down bolt fatigue life, which is now predicted to exceed 20 years. An overall reduction of the wear plate stresses is also expected. An analysis of the thicker wear plates which included fret gap effects indicated a stress decrease of about 30 percent; while modest, this stress reduction with a thicker wear plate would be sufficient to provide a fatigue life exceeding 20 years even with the effects of fretting damage.

The proposed GBT Azimuth Track remediation design was implemented during the summer of 2007. Following a successful installation, the track is now exhibiting satisfactory performance for all aspects of telescope operation [7].

Figure 15 Configuration of the revised GBT Azimuth Track.

REFERENCES

[1] Modjeski and Masters, Inc., “Report on the Analysis of the GBT Track Grout,” GBT Archive, A0291 (2002). [2] NRAO Staff, Green Bank Telescope Azimuth Track Review, Internal NRAO Document (2002). [3] Gunjeet Juneja, Frank W. Kan, and Joseph Antebi, “Update on Slip and Wear in Multi-Layer Azimuth Track

Systems,” Proc. SPIE 6273 (2006). [4] Coenraad Esveld, [Modern Railway Track], MRT-Productions, Duisburg, Germany (2001). [5] S.P. Timoshenko and J.N. Goodier, [Theory of Elasticity], McGraw-Hill Publishing, New York (1970). [6] Howard E. Boyer (Editor), [Atlas of Fatigue Curves], American Society for Metals, Metals Park, Ohio (1999). [7] Robert Anderson, Arthur Symmes, and Dennis Egan, “Replacement of the Green Bank Telescope azimuth track,”

SPIE 7012-120 (2007).