ML20087N531

Emergency Diesel Generator Rocker Arm Capscrew Stress Analysis

ML20087N531
Person / Time
Site:	05000000, Shoreham
Issue date:	03/31/1984
From:	STONE & WEBSTER ENGINEERING CORP.
To:
Shared Package
ML19270B897	List: ML19270B896 ML20087N531
References
NUDOCS 8404030556
Download: ML20087N531 (28)

Text

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March 1984 EMERGENCY DIESEL GENERATOR ROCKER ARM CAPSCREW STRESS ANALYSIS Prepared for:

TDI OWNERS GROUP l

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EMERGENCY DIESEL GENERATOR ROCKER ARM CAPSCREW STRESS ANALYSIS Prepared for TDI EMERGENCY DIESEL GENERATOR OWNERS GROUP MARCH, 1984 Prepared By STONE & WEBSTER ENGINEERING CORPORATION 1'

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TABLE OF CONTENTS Section Title Page 1

APPLICABILITY l

2 EXECUTIVE

SUMMARY

2 3

OBJECTIVES 3

4

SUMMARY

OF SERVICE CONDITIONS 4

METHODS OF ANALYSIS 5

5 5.1 Determination of Applied Stresses 5.2 Determination of Endurance Limits 5.3 Thread Distortion Analysis 5.4 Thermal Stress Evaluation I

6 DISCUSSION OF RESULTS 9

7 CONCLUSIONS 11 APPENDIX A STRESS

SUMMARY

I APPENDIX B FATIGUE / LOADING DIAGRAMS i

APPENDIX C COMPONENT DRAWING.

APPENDIX D TASK DESCRIPTION APPENDIX E-REFERENCES L

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..n SECTION 1 APPLICABILITY This report is applicable to the TDI Nuclear Stand-by Service Diesel Generators utilized at the Shoreham Nuclear Power Station.

Other TDI Nuclear Stand-by Service Diesel Engine Rocker Arm Capscrews, as part of the TDI owners Group Design Review / Quality Revalidation effort, will be evaluated seperately.

Reviews will include identification of the specific capscrew design installed and a comparison to the Shoreham's analysis as to differences in materials and loading conditions, if any.

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SECTION 2 EXECUTIVE

SUMMARY

An analysis was conducted for two rocker arm capscrew designs utilized in the TDI Nuclear Stand-by Service Diesel Generators.

This analysis served to evaluate the various rocker arm capscrew product improvements incorporated by the 4,

manufacturer.

The designs considered were the original " straight section" type and the modified " necked down" design as detailed in Reference 1.

The purpose of this analysis was to evaluate these capscrew designs based on the criteria referenced in the Component Design

(

Review Task Description, (see Appendix D).

This analysis concluded that both original and modified rocker arm capscrews are adequately designed for the given service conditions.

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SECTION 3 OBJECTIVES The objectives of this analysis are to evaluate the functional attributes of the TDI rocker arm capscrews as detailed in the Component Design Review Task Description (see Appendix D).

The capscrews under consideration include the original and modified TDI designs as detailed in Reference 1.

Task Description details include:

1.

Evaluate the stresses at the minimum cross-sectional area (A(min)) resulting from capscrew preloads, cam follower acceleratien lor'3, valve spring loads and residual cylinder i

pressure forces.

2.

Compare the total resultant stress to yield and endurance limits for the capscrew.

3.

Evaluate the thread specification for resistance to distortion and creep.

4.

Compare the material utilized in the rocker arm capscrew to ASTM A-193.

The details of this analysis are provided in SWEC calculation number ll600.60-245.1-M1 (Reference 2).

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SECTION 4

SUMMARY

OF SERVICE CONDITIONS The rocker arm capscrews serve to transmit camshaft follower acceleration loads, valve spring loads and residual cylinder pressure forces from the rocker arm shaft to the cylinder heads.

To achieve these design objectives, the rocker arm capscrews are preloaded with a specified torque.

This produces a tensile load p) on the capscrew which is and subsequent preload stress (S y converted to a clamping force on the rocker arm shaft.

In addition to the preload stress, a cyclic fatigue stress (S ) is induced on the capscrews due to the above referenced b

loads.

This cyclic stress alternates about the total applied mean stress (S ) as shown in Appendix B, Figure 1.

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SECTION 5 METHODS OF ANAYLSIS

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Determination of Applied Stresses 5.1 In determining the stresses applied to the orignal and modified rocker arm capscrews, the tensile preload resulting from the applied torque is first calculated by the relation g= T/.2d Eq. 1 (Reference 3)

F where T= applied torque, LB -in f

d= nominal stud diameter, in due to torque T F y= stud preload, LBf The resulting tensile load is taken to act uniformly over the minimum cross sectional area (A(min)) of the capscrew so that i

the resulting preload stress is maximized.

For the original design, A(min) is located at the capscrew thread root whereas for the modified design A(min) is at a smooth, polished " necked down" section between the threads and the capscrew head (see Appendix C L

Figure 1).

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The tensile loads produced in the capscrews are converted to l

l a clamping force on the rocker arm shaft which serves to resist the residual exhaust pressure, valve spring and cam-follower acceleration loads.-

The residual exhaust pressure loads are caletlated by multiplying the cylinder pressure at the exhaust valve opening point (130*ATDC) by the valve face area.

The valve s

spring forces are calculated by multiplying the full spring l

deflection by the applicable spring constant.

These forces are transmitted to the intermediate rocker arm via the intermediate I

pushrod.

In' determining the camshaft follower acceleration p

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t-1, loads, the follower velocity and acceleration are calculated by taking first and second derivatives of the follower displacement as a function of cam rotation.

To obtain the maximum load condition, the mass of followers is multiplied by the maximum value of the acceleration.

The cam follower loads along with the pressure and spring loads are resolved into a single vector acting on the intermediate rocker arm as detailed in Appendix B Figure 4, As detailed in Reference 2, the total preload on the rocker arm shaft exceeds the resultant forces by a factor of nine (9).

For components preloaded in this manner, the cyclic stress due to an alternating load is given by:

S

=+F

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q.

(Reference 3) 3 rb A(min)

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Where S

= alternating stress b

= alternating load Frb = Fr b

K +K, 3

and K3 = spring rate for the stud K, = spring rate for the section joined by the capscrews F

= maximum resultant force 5

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As per reference 3, the spring rates Kb and K,are determined by:

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where d = nominal capscrew dameter, in E = modulus of elasticity, LB /in g

L = length of joined sections, in p) and the-alternating stress once the preload stress (S y

(+S ) are determined, the total applied mean stress (S ) is b

defined as:

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CA 5.2 Determination of Endurance Limits The endurance limits (S,) for the original and modified designs are determined by:

K K

q.

( eference 3)

S, = K, K3 c

d e

f e

where S '= the endurance limit of a rotating beam specimen K"" = component surface factor K

= component size factor K

= reliability factor c

K temperature factor K*d== modifying factor for stress concentration K

= miscellaneous effects factor The endurance limit of the two designs is compartJ as well as the total applied mean stress (S ) and the yield strength (S )

t y

of the material in order to determine the suitability of the component design for the given service.

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minimum area is calculated by:

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where S

= torsional stress due to tightening s

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=

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frdr where T = 75% of the applied torque, in-LBf dr = root or minimum diameter I

Based on the maximum normal stress theory, thread distortion and subsequent failure during preload application will not occur providing Sn4Sy.

5.4 Thermal Stress Evaluation Thermal stresses in the rocker arm capscrews are qualatatively evaluated by considering the physical constraints of the joined ccmponents and the coefficients of thermal expans' ion for the materials utilized.

As detailed in Reference 2, the thermal stress (Sgy) in the capscrew is defined as:

St1 = X (

Tra -

Tes) Ecs where X = coefficient of thermal expansion 'F"

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change in temperature of the rocker arm J;

assembly, 'F Tes =

change in temperature of the

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p SECTION 6 DISCUSSICN OF RESULTS Per Reference 5, fatigue failure in bolts and studs is avoided if the total applied mean stress (S ) is below the yield t

point for the component material and the endurance strength exceeds the cyclic stress by an acceptable margin.

As per Appendix A Tables 1 and 2, the total applied mean stres's (S I t

taken at the minimum cross sectional areas A(min) is 41.0 ksi and 50.8 ksi for the original and modified stud designs respectively.

The higher value of S f r the modified design is primarily due t

to the " necked down" nature of the minimum area.

Per Reference 4, the yield strength for the stud material is approximately 105 ksi.

As a result, the criteria for a fatigue resistant design, namely S 8

and S A S are satisfied for both t

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e designs.

This is graphically depicted by Appendix B, Figures 2 and 3.

As shown in Appendix A, Table 1, the modified design utilizes an endurance stress of 37.6 ksi with a cyclic stress of

+0.6 ksi (see Appendix B, Figure 3) and therefore, satisfies the second criteria for a fatigue resistant design.

This increase in endurance strength is primarily attributed to the location of the minimum area.

As detailed in Reference 2, A(min) is located in a polished section of the capscrew, free of any significant stress concentration factors.

This allows for K,and K, to be set equal r

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to 1 in Eq. 3, thereby increasing the endurance limit (Ref. 2,3).

Note K,.and K, are less than 1 for the original design since the minimum area is located at an unpolished, high stress l

concentration section (thread root) of the capscrew.

During engine stand-by and operation, minimal thermal and creep effects are present in the rocker arm capscrews.

Due to the relatively uniform temperature distribution within the rocker arm assemblies, (i.e.

Tra = Tes) effects due to thermal stresses are negligible.

Additionally, since the capscrew is confined to the rocker arm assembly and experiences relatively low temperatures, effects due to creep and stress relaxation are also negligible.

Since the yield strength of the material is not exceeded, thread distortion will not occur for the given service conditions and installation procedures.

Additionally, bending stresses on the capscrews are negligible due to the relatively low cyclic forces acting on the intermediate rocker arm (see Appendix B, Figure 4).

The material specified for the modified rocker arm capscrew is AISI 4140 hardened to 25-30 (Rockwell).

This material meets or exceeds the requirements of ASTM A193 Grade B7.

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SECTION 7 CONCLUSIONS Reference 2 provides an analysis of the total applied mean stress (S ) and the endurance limit for two TDI rocker arm t

capscrew designs.

Based on this analysis, both original and modified designs (Reference 1) are adequate for the intended service.

However, the modified design is more fatigue resistant due to the relatively high component endurance limit.

The threads utilized for both original and modified rocker arm capscrew designs adequately resist distortion.

Additionally, the material utilized for the modified rocker arm capscrews meets or exceeds the requirements of ASTM-A193.

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APPENDIX A - STRESS

SUMMARY

TABLE 1: STRESS

SUMMARY

- ORIGINAL DESIGN TABLE 2: STRESS

SUMMARY

- MODIFIED DESIGN i

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STRESS

SUMMARY

ORIGINAL DESIGN

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S B - ALTERNATING STRESS f

S E - ENDURANCE LIMIT S T - TOTAL APPLIED MEAN STRESS i

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TABLE NS 2 STRESS

SUMMARY

i MODIFIED DESIGN SPL SB SE ST KSi KSI KSI KSI i

50.2 t 0.6 37.6 50.8 i.

l SPL-PRELOAD STRESS SB - ALTERNATING STRESS SE - ENDURANCE LIMIT ST - TOTAL APPLIED MEAN STRESS I

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APPENDIX B - FATIGUE / LOADING DIAGRAMS e

FIGURE 1:

CYCLIC FATIGUE LOAD DIAGRAM FIGURE 2:

FATIGUE DIAGRAM - ORIGINAL DESIGN FIGURE 3:

FATIGUE DIAGRAM - MODIFIED DESIGN FIGURE 4:

INTERMEDIATE ROCKER ARM-FORCE DIAGRAM A

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FIGURE N24 INTERMEDIATE ROCKER ARM FORCE DI AGRAM

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APPENDIX C - COMPONENT DRAWING f

FIGURE 1:

MODIFIED ROCKER ARM CAPSCREW r

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NOTES

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2. REF TDI DWG O2-390-04-OJ W

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APPENDIX D - TASK DESCRIPTION COMPONENT DESIGN REVIEW: DR-03-390G-1 n

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RCCKER ARM CAPSCREWS Classification B PART NO. 02-290G Comoletion 3/1/84 PRIMARY FUNCTION:

The rocker arm capscrews transmit resultant loads from the valve springs, valve opening pressure cushrods, and rocker arm assemblies to the sub cover and cylinder heads.

FUNCTIONAL ATTRIBUTES:

1.

The rocker arm capscrews must have sufficient strength to withstand the necessary preload and oscillation loads witnout fatigue cracking, unacceptable preload relaxation or thread distortion.

SPECIFIED STANDARDS:

None E:ttmated G.mpieltgn EVALUATION:

1.

Determine.thu stud d imens t or.s frorn existing design 2/17/84 drawings and evaluate the st ress at the minimum cross-sectional area resulting from the apolled preloads assuming un2 form thread lubrication and load distribution.

Stress concentration fa: tors ce inc!uced for the thread root area will in the analysis for the previous TDT dow a g n.

2.

Determine the stresses expertenced et the :. n :."

• O/10/84

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area resulting from push rod motion, selve spring deflection and valve opening cressure.

3.

Determine the total resultant bolt stress e.nc 1/20/84 comoare to yield and endurance limits.

4 Compare capscrew design and mateiial specification to ASTM A-193.

5.

Evaluate the thread specification for recistance to distortton and creep.

6.

Perform similar analysis on the previous un: form cross section capscrew design.

REVIEW TDI ANALYSES:

1.

Review any TDI stress analyses associated with cesign/

material changes.

INFORMATION REQUIRED:

Est imat ed l

ClODie512D 1.

Caoscrew preload (hole-down force)

/ 9 / 84 2.

Capscrew lubrication 2/9/84 1

3.

Capscrew design drawings and material specif teat ions 2/9/84 4.

Rocker arm geometry and drawings 2/9/84 p]?.

5.

Valve spring constants, free l e n g t *., compressed C / ?.' 84 g./

length 6.

Operating loads on the capscrews d

7.

Valve pop-open pressure in cylinder 2/25/84 y

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L-APPENDIX E - REFERENCES Reference 1:

TDI Drawing 02-390-04-0G - Original Design TDI Drawing 02-390-04-0J - Modified Design Reference 2:

SWEC Calculation 11600.60-245.1-M2

-Reference 3:

Mechanical Engineering Design, J.E. Shigley, 3rd edition I

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Reference 4: ' Engineering Properties of Steel, ASM 1982 Reference 5:

Simple Diagrams Aid in Analyzing Forces in Bolted Joints, Assembly Engineering, G. Meyer 1972 6

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