Joseph Epperson, Materials Group ChairmanJoseph Epperson, Materials Group Chairman

Metallurgical FindingsMetallurgical Findings

Recovered Jackscrew Assembly

Recovered Jackscrew Assembly

• Screw attached to horizontal stabilizer

• Separated from acme nut

• Nut thread remnants on screw

• Screw attached to horizontal stabilizer

• Separated from acme nut

• Nut thread remnants on screw

Closeup of Thread RemnantsCloseup of Thread Remnants

Jackscrew with Lower Stop

Jackscrew with Lower Stop• No grease in working area of

screw• Sandy grease on lower end

of screw• Oil sheen on upper end of

screw• Grease on lower stop with

nut contact marks• Fractured torque tube

• No grease in working area of screw

• Sandy grease on lower end of screw

• Oil sheen on upper end of screw

• Grease on lower stop with nut contact marks

• Fractured torque tube

Lower Mechanical StopLower Mechanical Stop• Lower stop recovered

separately• Grease on stop• Marks on upper surface

from contact with acme nut after nut thread separation

• Created offset loading of the torque tube

• Spline damage from rotation of screw

• Lower stop recovered separately

• Grease on stop• Marks on upper surface

from contact with acme nut after nut thread separation

• Created offset loading of the torque tube

• Spline damage from rotation of screw

As Recovered Acme Nut

As Recovered Acme Nut

• Threads completely stripped

• No grease on interior or adjacent surfaces

• Grease fitting passage blocked

• Red and green grease on surrounding structure

• Threads completely stripped

• No grease on interior or adjacent surfaces

• Grease fitting passage blocked

• Red and green grease on surrounding structure

Used Nut with Worn ThreadsUsed Nut with Worn Threads

Accident Nut with Stripped ThreadsAccident Nut with Stripped Threads

Torque Tube FractureTorque Tube Fracture• Fractured in

threads at lower end of the acme screw

• Low cycle fatigue cracking

• Similar fractures could be produced by high stress in a few cycles

• Fractured in threads at lower end of the acme screw

• Low cycle fatigue cracking

• Similar fractures could be produced by high stress in a few cycles

Two & Three Dimensional Finite Element Analyses to Determine Thread Loading and Contact

Pressure Distributions

Two & Three Dimensional Finite Element Analyses to Determine Thread Loading and Contact

Pressure Distributions

-40 -33-25-18-11-3.6 3.6 11 18 25 33 40

22 MPa

2

1

3

u y=0

u y=0

Scr

ew

Nut

-70 MPa -60 -50 -40 -30 -20 -10 0 10 20 30 40

22

Conclusions from Finite Element Analyses

Conclusions from Finite Element Analyses

• Approximately 90% of screw load reacted in four threads

• Loads shift as thread wear progresses• The maximum value of the contact pressure does not

change as wear progresses - 3,000 to 4,000 average over contact area

• At large levels of thread wear (> 0.080) under normal operating loads, large bending deformations develop which are consistent with the thread remnant wear shape

• Approximately 90% of screw load reacted in four threads

• Loads shift as thread wear progresses• The maximum value of the contact pressure does not

change as wear progresses - 3,000 to 4,000 average over contact area

• At large levels of thread wear (> 0.080) under normal operating loads, large bending deformations develop which are consistent with the thread remnant wear shape

Thread RemnantThread RemnantSHEAR FRACTURE• At major diameter• Less than 10% of the

cross section remained

WEAR• Multiple bevel/stages • Unique wear /

deformation shape

SHEAR FRACTURE• At major diameter• Less than 10% of the

cross section remained

WEAR• Multiple bevel/stages • Unique wear /

deformation shapeWear occurring

during initial deformation

Wear occurring after additional deformation

Shear fracture surface

Thread Deformation SequenceThread Deformation Sequence

Summary of FindingsSummary of Findings• Material and structural conditions did not

contribute to the acme nut wear• No grease in working area of the screw• Grease not removed by ocean impact,

exposure, or recovery• Jackscrew grease not contaminated• The wear is caused by sliding contact and is

consistent with an unlubricated condition• Torque tube fractured by low cycle fatigue

• Material and structural conditions did not contribute to the acme nut wear

• No grease in working area of the screw• Grease not removed by ocean impact,

exposure, or recovery• Jackscrew grease not contaminated• The wear is caused by sliding contact and is

consistent with an unlubricated condition• Torque tube fractured by low cycle fatigue

Metallurgical FindingsMetallurgical Findings

Additional PhotographsAdditional Photographs

Lower stop in contact with the acme nut

Additional Photographs Additional Photographs

Interior of lower stop

Interior of lower stop

Remnant Positions vs Trim Positions

Remnant Positions vs Trim Positions