Wrijving en slijtage van kunststofproducten
Jan Spoormaker
Spoormaker-Consultancy in
Reliability and Liability of
Plastic Products
Dictaat Hfdst. 3 tekst
Friction
Friction is a force that resist relative movements between two surfaces in contact.
Wear
Wear is a process in which interaction of surface(s) or bounding face(s) of a solid with the working environment results in the dimensional loss of the solid, with loss of
material.
Friction and wear are system properties
Tribology
Science about friction, lubrication and wear
Tribotechnology
Tribological System according to Czichos
Relative movement Gliding, rolling Load Fn Velocity v Ambient temperature Friction force Fw Friction coefficient f, μ Wear h Friction temperature Noise Loads & environment Loads & environment Tribological quantities Surface parameters Roughness Macro geometry 1. Body 2. Body 3. Substance 4. Environment Dictaat Hfdst. 3 Fig. 3.1- polymerisatie
- ketenopbouw en ketenlengte
- fysische eigenschappen
- kristallisatie
IO-2040-20081201Polymeren
Plastics are fantastic, they can fail in many
unexpected ways
Polymeriseren en ketenlengte
additiepolymerisatie
Polymeriseren van etheen C
2
H
4
PE
IO2070-20100215
onvertakt
Ketenlengte
Paraffine
(500 g/mol )lijmtube
(105 g/mol)UHMW-PE
(5.106 g/mol)Molecuulmassa - sterkte en viscositeit
IO2070-20100215
Kristallisatiesnelheid
wrijving
slijtage
contactoppervlak
vervorming
cohesie en adhesie
IO-2040-20081201Normal force, drag force and friction force
Material combination μ, f
PA6 - PA6 (no lubrication) 1.20
PA6 – POM (no lubrication) 0.60 PA6 – PTFE (no lubrication) 0.10 POM – POM (no lubrication) 0.55
Wear
Material combination wear factor k
POM – steel (Ra = 0.1 μm) 2 10-15 m2/N
POM – steel (Ra = 0.6 μm) 80 10-15 m2/N
h
k
p v t
Micro contact surface
Deformation
Physical processes cohesion and adhesion
Bonding forces Cohesion Adhesion
ruwheid
snelheid
stick-slip effect
IO-2040-20081201
Friction coeeficient as a function of roughness
High yield strength Low yield strength
Surface roughness Surface roughness
Friction coefficient as a function of roughness for
POM and HDPE
Roughness Ra (μm) Roughness Ra (μm)
Friction coefficient as a function of speed
Speed
Mass-spring model
Stick-slip effect
time
deformatie
adhesieve slijtage
abrasieve slijtage
pitting
IO-2040-20081201
Boundary and deformation zone
• Boundary zone • Deformation zone Hard roughness asperity top Dictaat Hfdst. 3 Fig. 3.15Adhesive wear
Abrasive wear
Shear stress for rolling contact
Pitching as a result of fatigue
Fatigue cracks Pitting
slijtage – ruwheid
meten van slijtage
vlaktedruk – pv waarde
warmte-ontwikkeling
PTFE
Hardheid
IO-2040-20081201Lagers
Wear as a function of roughness for POM and HDPE
Wear as a function of roughness and pressure for PA6.6
Wear as a function of pressure
Different wear tests
Pin on disk
Calculation of average bearing pressure and wear
p43MPah
k
p v t
F
T D
p
ExtraHeat flux and Average Bearing Pressure
2(W/m )
hf
p v
- heat flux f – friction coeficientp – average bearing presure v - velocity
Critical pv-values
Prediction of wear
h
k
p v t
k – wear factor
p – average bearing presure v – velocity
t - time
Coefficients of friction and wear rates
1 1
h
k
p v t
h
2k
2p v t
ExtraPTFE & silicone to reduce friction and wear
Effect of shaft finish on wear
(micro-inches!)
warmte-ontwikkeling
vermoeiing
IO-2040-20081201
Failure analysis
Failure analysis
Guidelines for plastic bearings
• Metal bearing surfaces must be smooth (< 1 μm), hard and corrosion resistant.
• Wear particles must be collected in grooves.
• Bearing clearance must account for high thermal expansion and absorption of water (PA)
• The average bearing pressure should preferably be not higher than 1 N/mm2
and at most 5 N/mm2.
• The out-of-roundness of bearings should be as small as possible and film gates are preferred.
• Bearing plastics with PTFE are preferred especially for applications without lubrication.
• Boundary lubrication is must better than running dry.
• Grease or oil can affect plastic bearings (swelling & ESC)
