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Bolt failures in the medical industry Milan van Bree March 2013
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Page 1: Presentation3 aangepast

Bolt failures in the medical industry

Milan van Bree

March 2013

Page 2: Presentation3 aangepast

Content

1. IntroductionWhat makes the medical industry specialExamples failure modes

2. Loose boltsRoot cause/ Design rules

3. Broken boltsRoot cause/Design rules

4. Stuck boltsRoot cause/Design rules

5. Electric conductivityRoot cause/Design rules

6. Human Safety!

Page 3: Presentation3 aangepast

What makes it so special

• Reliability, Human Safety• Clean, High Friction

• & to be cleaned!• Machined Female parts ( No loose parts)• Dust/ bacteria• Corrosion, due to acids

– Puke/vomit

• No (unless sealed)

Page 5: Presentation3 aangepast

Examples Loose & broken bolt

M1,2!

Page 6: Presentation3 aangepast

Loose bolts II

Root cause• High Friction due to

• Material combinations and surface deposition technologies

• GD&T and smooth surfaces Ra<1.2 • Clean surfaces• Over constrained parts.

• Material CTE differences• During bake out

Page 7: Presentation3 aangepast

Design rules preventing loose bolts

• Awareness of the friction, MEASURING

Page 8: Presentation3 aangepast

Design rules preventing loose bolts • Calculate the bolted connection with acc. Roloff Matek

Page 9: Presentation3 aangepast

Design rules preventing loose bolts

• Bolt can elongation, Lk/d>5• Do not over constrain parts• Tolerance play of the physical properties at

cryonic or elevated temperatures

Page 10: Presentation3 aangepast

Procedure

• Sequence of tightening– Cryonic vessel (liquid N -273°C)

Page 11: Presentation3 aangepast

Broken boltsRoot cause

• Physical properties at cryonic and elevated temp change ( bake out “eliminating bacteria”)

• Low (unexpected) friction, Use of Alcohol/ Grease/ adhesives.

• Hydrogen embrittlement, breaking of pretensioned bolts> between 1-24 hrs after install base

• SCC, breaking of bolts. After > 24 hrs• Self made bolts! Fatigue strength. • To high applied torque.

Page 12: Presentation3 aangepast

Broken bolts

Customized screw is used, ( turned) during installation is a loctite been applied. ( low friction than expected)

Page 13: Presentation3 aangepast

Design rules preventing breaking bolts

• Calculate the bolted connection with help of Roloff Matek• Use standardized fasteners• Do not use additional cleaning methods before mounting• When use: of adhesive/ grease-> in the threaded hole

Page 14: Presentation3 aangepast

Galling phenomena

AISI 316 ( A4) bolts in AISI 316 counter materials

Page 15: Presentation3 aangepast

Stuck Bolts (Galling)

Root cause of Galling • Material choice, (clean surfaces) and high pressure• High Hertzian contacts, use of Metric thread for

hoisting/lifting (screw jack-translation thread)!• Bending of the screw during mounting• Damaged threaded parts• Surface deposition technologies ( e.g. NiP)• Misalignment ( perpendicular fit)• Burrs/ chips

– Use of STD fasteners

1

Page 16: Presentation3 aangepast

Preventing stuck bolts

• Use different alloys for the connection, prevent materials which have similar hardness

• Prevent blind holes• Don’t excessive re-use the fastener• Don’t use metric thread for translations• Do ensure the female threaded part is

perpendicular towards the bolted connection• Use a lubrication (sealed!)

Page 17: Presentation3 aangepast

Human Safety

Page 18: Presentation3 aangepast

Electrical conductivity• Electrical discharges in metallic joints: cause

failure of electronic assemblies (PCB) and human safety.

Root cause: • Oxidation layers ( Anodized layers)• Adhesives/ grease• Material choice

Page 19: Presentation3 aangepast

Design rules, provide electrical contact

• Do not use adhesives/ grease.• Do use suitable materials, electrical

conductive– Oxidation layers ( Al2O3)

• Do use earthed washers only ones!

Page 20: Presentation3 aangepast

Summary

• Provide pretension such that such no slip will occur• Calculate with the help of Roloff Matek the critical connection• Include effect of friction and its variation• Define required active thread length• Use STD screw• Provide large lk/D, guide line: lk/D ≥ 3

Bolt calculation tool, Futek.com

Page 21: Presentation3 aangepast

WHAT DETERMINES THE RIGHT TORQUE

QUESTIONS?


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