Slidmekanismer og slidforebyggelse

Transcription

1 Slidmekanismer og slidforebyggelse Niels Bay DTU-Mekanik FMV Temadag om Slid på Metaller Scandic Hotel, København 15. november 212. Mechanisms of wear Primary mechanisms 1. Adhesive wear 2. Abrasive wear 3. Corrosive wear 4. Fatigue wear Secondary mechanisms 5. Fretting 6. Erosive wear 7. Cavitation wear 1

2 Adhesive wear Arises when two rather smooth surfaces slide against each other and particles from one surface are torn out adhering to the other one. Adhesive wear is due to strong attractive forces appearing when the atoms approach each other.. Adhesive wear Size of wear particles 2

3 Abrasive wear Arises from the cutting action of a hard surface sliding on a softer material (2-body abrasive wear) or when loose debris particles trapped between the sliding surfaces are penetrating the softer surface and scratching a wear groove in the harder one (3-body abrasive wear) Abrasive wear Severe journal wear Severe wear of bronze shaft due to failing soft packing Severe tearing of bearing surface 3

4 2-body abrasive wear 3-body abrasive wear Corrosive wear (Corrosion is the degradation of a surface by chemical reaction with the environment) Corrosive wear arises when two surfaces slide against each other in a corrosive environment. If sliding did not take place, the corrosive products would form a protective film on the surface impeding further corrosion, but the sliding wears out the corrosive film thus allowing the corrosion to continue. 4

5 Corrosive wear Removal of lead phase from lead-bronze slide bearing Fatigue wear Arises when a surface is loaded cyclically due to repeated sliding, rolling or impacts. The repeated loading and unloading causes crack formation in the surface or subsurface and subsequent breaking off fragments from the surface resulting in pit formation. 5

6 Fatigue wear Shear stresses in the subsurface layer Fatigue wear 6

7 Fretting Fretting occurs when two surfaces in contact under load and nominally at rest with respect to each other are subjected to slight oscillating tangential movement with small amplitude. Typical examples are vibrations in: Poorly aligned spline coupling Loosely bolted machine parts Riveted joints Press fits Surgical implants Initial adhesive wear forms wear debris which may oxidize to form abrasive wear particles, which cannot readily escape due to close fit of the surface. Often surprisingly large wear rates. Fretting Scavenge pump failed by fretting fatigue at centre 7

8 Fretting Riveted joint Screw joint Erosive wear Damage experienced by a solid body, when a fluid or gas containing solid particles impinges on to the surface of the body. 8

9 Erosive wear Compressor blade in Ti-alloy Erosive wear of leading edges due to sand particles Cavitation wear Cavitation wear arises when a solid and a fluid are in relative motion, and bubbles formed in the fluid become unstable and implode against the surface of the solid. The implosion creates a chock wave which can tear out particles from the surface. Cavitation wear is closely related to fatigue wear and as such materials which are hard and ductile are resistive to cavitation wear. 9

10 Cavitation wear Ships propellers, centrifugal pumps Modelling of adhesive wear During sliding small asperities may come into contact and during passage there is a small possibility that separation does appear in the original interface. 1

11 Adhesive wear Archards model for adhesive wear P p A r All contacts assumed to have same size and diameter d. The total number of contacts N : 2 d P A r N 4 p 4P N p d 2 Adhesive wear Archards model for adhesive wear Total number of contacts 4P N p d 2 Every contact assumed to exist during a sliding length of d. If N contacts remain under load the number of new formed contacts per unit sliding length is: N 4P n n 3 d p d 11

12 Adhesive wear Archards model for adhesive wear 4 P (n number of new formed p d n 3 contacts per unit sliding length ) k: probability of formation a wear fragment k 1 v: volume of wear fragment dv k n v dx Fragment shape is assumed to be semispherical dv dx dv dx 3 d k n 12 4P k p d dv k P dx 3 p 3 d 12 3 P x V k 3 p Archards model for adhesive wear P x V k The dimensionless wear coefficient k 3 p Combination in dry contact Wear constant k Zinc on zinc Low carbon steel on low carbon steel Copper on copper Stainless steel on stainless steel Copper on low carbon steel Low carbon steel on copper.51-3 Bakelite on bakelite

13 Archards model for adhesive wear The dimensionless wear coefficient k P x V k 3 p Surface condition Material combination Like Unlike Clean (dry contact) Poor lubrication Average lubrication Excellent lubrication Archards model for adhesive wear Prevention of adhesive wear P x V k 3 p Small normal pressures Small sliding lengths Hard materials Combine materials with small interaction (small k) Chose one material to be non-metallic Chose materials with low mutual adhesion energy Apply lubrication 13

14 Adhesive wear in space Problems with cargo hatch hinges on space shuttle Adhesive wear in space prevented by ion sputtering of MoS 2 on clean surfaces High duty tribo-elements like balls and gears are Au-plated Abrasive wear Experimental studies of plowing and cutting mechanisms Plowing Cutting Cutting T. Abildgaard Petersen, T. Wanheim 14

15 Model for abrasive wear P p d A r p d 4P p P d p d z tan 2 Projected area of indented cone on plane perpendicular to sliding direction: A rs 2 d tan 4 Volume removed when sliding the distance dx: 2 d tan dv1 dx 4 P tan dx p Model for abrasive wear P Volume removed when sliding the distance dx: d z tan 2 P tan dv1 dx p Summing up for all contact points we get: P tan dv dx p or V ridge tan P x p Assuming that only a fraction β of the load is carried by asperities with cutting action we get: V V wear wear k abr tan P x p k abr P x 3 p 3 tan d p 15

16 Model for abrasive wear V wear k abr P x 3 p k abr 3 tan 2-body abrasive wear:.2 < k abr <.2 3-body abrasive wear:.1 < k abr <.1 Experimental investigations confirms the model as regards the influence of the three main parameters P, x and p Prevention of abrasive wear P x 3 Vwear k abr k abr tan 3 p Ensure that one of the materials is harder than the abrasive particles and the other so soft, that it can bury wear particles. Since these have about the same hardness as the harder surface a hardness ratio of minimum 3 is feasible. Ensure filtering of lubricant Be careful to avoid formation of large wear particles which are more dangerous than small ones. Avoid fatigue wear particles. Chose materials with good resistance against fatigue. Apply very flexible materials like rubber as the softer material. 16

17 Prevention of abrasive wear in hot forging Diminish sliding P x 3 Vwear k abr k tan 3 p abr Lubricated forging Unlubricated forging Tool roughness R a not too small Abrasive wear Mohs hardness scale 1. Talc 2. Gypsum 3. Calcite 4. Fluorite 5. Apatite 6. Felspar 7. Quartz 8. Topaz 9. Corundum 1. Diamond 17

18 Off-line testing of new, environmentally friendly tribosystems for sheet metal forming Objective: To replace environmentally harmful lubricants such as chlorinated paraffin oils Sheet forming of tribologically difficult materials, e.g. AHSS, stainless steel, Al, Ti Partners: Grundfos Uddeholm SSAB Outokumpu Stainless Deep drawing in progressive tool - Grundfos a. Deep drawing b. 1 st redrawing c. 2 nd redrawing Tribologically the most severe operation d. Sharp pressing of flange Step a Step b Step c Step d Workpiece material: AISI 34 Step c E. Ceron, E. Madsen, N. Bay 18

19 Simulation of deep drawing and 2 redrawings LS-DYNA 2D implicit model The blank is transferred from one process to the following updating flow stress and equivalent strain E. Ceron, N. Bay Distribution of radial stress in step c Maximum contact pressure p max = 9 MPa die workpiece [MPa] E. Ceron, N. Bay 19

20 Simulation of BUT test tool strip MPa Round pin with radius R = 3,5 Maximum contact pressure 36 MPa with maximum back tension 3 MPa E. Ceron, N. Bay Distribution of radial stresses in BUT test By modifying the BUT test tool to a 45 contact instead sufficient contact pressure can be reached Maximum contact pressure p max = 1 MPa with back tension 3 MPa [MPa] Workpiece strip tool E. Ceron, N. Bay 2

21 New, universal sheet tribo-tester BUT DBT SRT E. Ceron, N. Paldan, J. Gregersen, N. Bay Universal sheet tribo-tester Automatic PLC controlled running of repeated tests Material feed from coil of more than 1m Adjustable sliding lengths, speed, cycle time and total number of strokes Ensuring appropriate emulation of production conditions with heating and cooling cycle Easy programming by Labview BUT_test_running_detail.avi BUT_test_running.avi E. Ceron, N. Paldan, J. Gregersen, N. Bay 21

22 Second screening test campaign DP 8; Shell Ensis PQ 144 Back tension = 25 MPa; production speed = 85 stroke/min; Vanadis 4E; (test 1) EDS analysis of pick-up Spectrum In stats. C Si S V Cr Mn Fe Total 1 Yes Yes No Vanadium and Chromium in spectrum 1 but Manganese and Silicium. Spectrum 2 is tool surface Second screening test campaign DP 8; Fuchs PLS 1 Back tension = 3 MPa; production speed = 4 stroke/min; Vancron 4 (V4_C3); (test 26) Almost no pick-up on the tool surface Exit edge Entrance threshold Sliding direction 22