2. Content
1. Introduction
What makes the medical industry special
Examples failure modes
2. Loose bolts
Root cause/ Design rules
3. Broken bolts
Root cause/Design rules
4. Stuck bolts
Root cause/Design rules
5. Electric conductivity
Root cause/Design rules
6. Human Safety!
3. 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)
4. Loose bolts I
• Root cause:
Low pretension, due to:
Scatter of high friction
6. 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
9. 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
11. Broken bolts
Root 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.
12. Broken bolts
Customized screw is used, ( turned) during installation is a loctite been applied. (
low friction than expected)
13. 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
15. 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
16. 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!)
18. 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
19. 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!
20. 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
