# Presentation: 590956

## ALI Aperture SelectorFailure Analysis

**S.E.  Forman**
**V.J. Cerrati**
**J. A. Mendenhall**
**L. M. Mendenhall**
**4 November  2002**

## ALI Aperture Cover

**Aperture Selector: mechanism on ALI aperture cover used during **
**solar calibration; blocking plate moves to uncover array of 7 slits**
**and returns to original position. **

**Blocking**
**Plate**

**Slit array under**
**blocking plate**

**Motor**
**Thermal**
**Shield**

## Aperture Selector Details - 1

**  ****Motor-Resolver Vendor: Astro Instrument Corporation, Deerfield, Florida; **
**     ****Part number: CV5-A**
**  ****Ball screw stall torque: 9.73 inch ounces**
**  ****Ball screw pinned to motor shaft with 0.125 inch steel pin**
**  ****Plate linear speed: 0.19 inches/second; screw rotational speed: 91.2 rpm**

**Photographs of Structural Thermal Model (STM) Aperture Selector**

## Aperture Selector Details - 2

**Aperture Selector Details - 2**

**Fully closed**

**Fully open**

**Motor- resolver-ball screw-ball nut**

**3 Dimensional Computer Aided Design Model**

## Ball Nut – Jam Nut Assembly Process

**1**

**2**

**3**

**4**

**5**

**Hand tighten brass spacer**

**Slide Al plate over brass spacer**

**Ball nut assembly**

**Add brass washer**

**Hand tighten jam nut;**
**Add 2 set screws & staking epoxy**

## Ball Nut – Jam Nut Details

**Ball Nut – Jam Nut Details**

**Blocker plate**

**Ball nut**

**Stainless steel**
**jam nut**

**Ball screw**

**Ball nut**

**Jam nut**

**Blocker plate**

**Brass spacer**

**Brass washer**

**Epoxy **
**staking**

**Epoxy **
**staking**

**Staked**
**set screws (2)**

## Pre-Flight Selector Testing

**Functional tests during development (mechanism and instrument levels)**
**Structural thermal model (STM)**
**Flight unit (~25 open and close cycles in air and vacuum)**
**Qualification tests (Instrument level)**
**STM vibration and thermal balance testing**
**No issues during either test **
**Selector temperature varied between -10° and -30 °C in vacuum**
**Flight vibration and thermal vacuum testing**
**Unpowered selector opened slightly during vibration test; problem traced to loose set screws in jam nut; epoxy staking added **
**Fix evaluated in STM selector vibration test with no problems**
**Thermal soaks with spacecraft simulator at 50° C and -30° C and 4 thermal cycles (-10 to +40° C) in vacuum (coldest selector temperature -40° to -50°C)**
**Life Tests (at STM mechanism level with epoxy staking)**
**240 open and close cycles in vacuum; half at 0°C and half at 40°C**
**Thermal soaks in vacuum at -10° C and 50° C**
**Spacecraft Level Tests**
**Vibration, thermal cycling and acoustic testing (no selector problems) **

## Aperture Selector Failure

**On 5 July, 2002 (Day 186), aperture selector opened and partially closed; no further controlled movement possible**
**Uncontrolled “opening” movements observed during later solar calibrations (Days 186,193,207 & 227) **
**Resolver telemetry indicates motor shaft rotation after failure**

**On-Orbit Focal Plane Response Data**

**Normal**
**Operation**

**5 July 2002**
**Operation**

**Superposition of data**

**Horizontal lines represent increased focal**
**plane response during solar cals after**
**initial failure (more light is entering slits)**

## Possible Failure Modes and Comments

**Possible Failure Mode****	****	****            Comments**

**   ****Power supply failure**
**   ****Switch failure**
**   ****On-board software error**
**   ****Command error**
**   ****Telemetry error**
**   ****Resolver circuit failure**
**   ****Resolver mechanical failure**
**   ****Ball screw retainer bearing frozen**
**   ****Selector guide bearing(s) jammed**
**   ****Motor-ball screw coupling failure**
**   ****Blocker plate clamping relaxed**
**   ****Ball nut jammed**

**Not likely; command and telemetry similar to previous selector cycles**

**Not likely; requires double shear**
**failure of steel pin; motor incapable**
**of developing sufficient torque **
**to shear pin (10 in-oz      ~100 psi pin**
**stress)**

**Both demonstrated with STM;**
**discussed on following slides**

## Failure Scenarios

**Clamping Force Relaxed (jam nut side)**
**Epoxy joints fail**
**On-orbit thermal cycling (0 to 20° C) loosens clamping of blocker plate ring**
**Ball-nut jam-nut assembly free spins (demonstrated with STM by loosening jam nut) ****(does not allow further opening)**
**Suggested fix: mechanically fasten ball nut, spacer and plate**
**Jammed Ball Nut**
**Epoxy joint fails**
**Contaminant (epoxy, paint or metal debris) trapped between ball-nut and ball screw (demonstrated with small piece of paper on STM)**
**Ball-nut unthreads from brass spacer and free spins ****(further opening occurred in lab by opening and closing aperture cover)**
**Suggested fix: shield ball-nut ball screw from particulates; mechanically fasten components and eliminate epoxy staking**
**Assessment**
**Most likely failure due to jammed ball nut**
**Uncontrolled blocker plate movements induced by aperture cover open-close vibrations of free-to-move plate**

## Jammed Ball-Nut Theory - 1

**Most common ball-nut failure modes: jamming due to trapped debris or insufficient lubrication**
**If ball-nut jams, it will rotate with ball-screw, brass spacer will unthread itself and ball-nut will free spin with ball-screw**

**Free-to-move blocker plate**

**Jammed ball-nut**

## Jammed Ball-Nut Theory - 2

**Blocker plate**

**Ball nut**

**Stainless steel**
**jam nut**

**Epoxy **
**staking**

**Epoxy **
**staking**

**Staked**
**set screws (2)**

**Lingering Question: Why is there**
**space between ball-nut and blocker plate, after failure?**
**  ****When brass spacer comes**
**free it will likely strike rotating ball screw one or more times. Friction between rotating helix and spacer will cause it to move away from ball-nut.**

**Normal**

**After Failure On-orbit**

**Jammed Ball-Nut Theory - 2**

## Probable Epoxy Failure Modes

**Strength of epoxy exceeded during ground test thermal soak at -30° C forming small crack(s)**
**Ground (-10 to 40°C) and orbital (0 to 20°C) thermal cycling causes cracks to grow until separation from metal occurs**
**Relevant material data**
**Epoxy coefficient of thermal expansion (CTE)   75 ppm/°C**
**Aluminum CTE   23 ppm /°C**
**Brass CTE  18 ppm/°C **
**Stainless CTE  12 ppm /°C**
**No primers or special treatments used on bonded surfaces**

**aluminum**

**brass washer**

**Peak tensile stress = 5200 psi**

**Allowable stress = 4000 psi**

**adhesive**

**adhesive**

**aluminum**

**brass**

**steel ball-nut**

**Peak tensile stress = 6000 psi**

**Allowable stress = 4000 psi**

**Jam-nut Side**

**Ball-nut Side**

## Summary

**Aperture selector abruptly failed to fully close on 5 July 2002 during solar calibration (Launch: 21 November 2000)**
**Slit blocking plate remains partly open**
**Resolver indicates motor shaft rotation**
**Small, uncontrolled plate movements occurred, allowing more light to enter during subsequent solar calibrations**
**Decoupling of motor from ball screw deemed unlikely**
**Shear strength of steel coupling pin way beyond torque capability of motor**
**Probable failure mode:**
**Epoxy staking overstressed during ground test thermal soak at -30°C and failed during subsequent ground, launch and orbital thermal and vibration environments**
**                                                      ****and **
**Ball-nut jammed due to particle entrapment and unthreaded from brass spacer**

## Aperture Selector Drawings

**A**

**A**

**Section A-A**

**Motor drives ball screw to move blocker plate; plate clamped to ball nut**
**with spacer and jam nut**

## References

**“****Mechanism Life Test Summary”, Dan Brosnan Memo, 29 Sept, 1998**
**Problem Failure Report after aperture selector failure during vibration test; 9 Sept 1998**
**Vibration Test Report for EO-1 Aperture Selector; Eugene Huang, 21 September 1998**
**Data Sheets for EA 934NA Epoxy Paste Adhesive from Loctite Aerospace, January 2001; also from Dexter Hysol, October ,1998**
**“****ALI Flight System Thermal Cycle Test Summary”, Ron Efromson and Dave Nathanson, 1 March 1999**
**Astro Instrument Document for ALI Parts CV-3A, CV-4A, and CV-5A, 1997 **

## Solar Calibration

## Selector Failure Animation