Compressibility of liquid lubricants at high pressures. Mohd Hafis Sulaiman

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1 Compressibility of liquid lubricants at high pressures Mohd Hafis Sulaiman

2 Introduction When the pressure between surfaces increases, a confined lubricant in closed pockets will be a factor determining possible hydrodynamic and hydrostatic lubrication mechanism. How is the lubricant carry the load at different pressures? Are they any difference between a lubricant with and without boundary lubrication as the oil pressure increases in the pockets? Increased knowledge about the lubricant compressibility is required in order to design tailored surfaces for metal forming. p re τ re Tool q τ re p re Lubricant pocket p oil Workpiece Boundary layer v 2

3 Introduction Bulk modulus is a property that measures the compressibility of a fluid. Bulk modulus The product of fluid volume at any specified pressure. K t = V dp dv F 1 F 2 Δh p oil poil 3

4 High Pressure Equipment Built-in high pressure equipment at DTU-MEK. F Upper punch Bridgman seal Punch Test lubricant Ø37 H o Lower punch Bridgman seal (<500MPa): - Copper (Brown) - Teflon (White) - Trelleborg (Black) Oil pressure relief Pressure sensor Punch (static) Oil pressure relief tubing Oil relief nut Pressure sensor 4

Oil Pressure p oil (MPa) Pressure limit check (by Numerical simulation) Before Punch stroke ΔL Upper punch Prestressed die container After Material

Prestressed die container: Vanadis 4E, E = 206 GPa Lubricant properties: Bulk modulus: K = 1500 MPa. K = 2200 MPa.

5 Oil Pressure p oil (MPa) Pressure limit check (by Numerical simulation) Before Punch stroke ΔL Upper punch Prestressed die container After Material properties: Punches: Unimax, E = 213 GPa. Punch cap: AISI 316, E = 193 GPa. Prestressed die container: Vanadis 4E, E = 206 GPa Lubricant properties: Bulk modulus: K = 1500 MPa. K = 2200 MPa. Upper punch cap Test lubricant Lower punch cap Lower punch Analytical (K=2.2 GPa) Numerical (K=2.2 GPa) Analytical (K=1.5 GPa) Numerical (K=1.5 GPa) Pressure threshold Punch stroke ΔL (mm) 5

6 Pressure limit check (by Numerical simulation) No marked expansion on the die and the punches. Oil Pressure Von Misses Stress X-displacement p oil 700 MPa at ΔL 26 mm 6

7 Volumetric Errors (%) Pressure p oil (MPa) Die expansion x-dir (mm) Deflection (Punch/Die) (%) Pressure limit check (by Numerical simulation) Numerical (Rigid materials) Numerical (Elastic materials) x 2 = MPa x 1 = Pressure p oil (MPa) Pressure p oil (MPa) Diff. in Upper Punch Stroke Diff. in Die Expansion Pressure p oil (MPa) Pressure threshold 500 MPa y 1 = y2 = MPa Analytical (K=2.2 GPa) Numerical (Rigid materials) Numerical (Elastic materials) Punch stroke y (mm) 7

Bridgman seal check (by Numerical simulation) Ring

Unimax E = 216 GPa Die Vanadis 4E E = 206 GPa Ring

54 σ o = 1275.ε 0.45 Ring 2 Steel Ma8 σ o = 636.

8 Bridgman seal check (by Numerical simulation) Ring 1 (Copper) 3 2 Ring 1 Die Pressing ring 3 down to 0.5mm 1 Punch ε eff = 0.56 Part Material Mechanical Properties Punch Unimax E = 216 GPa Die Vanadis 4E E = 206 GPa Ring 1 Copper, or Stainless steel 304 σ o = 315.ε 0.54 σ o = 1275.ε 0.45 Ring 2 Steel Ma8 σ o = 636.ε 0.23 Ring 3 Stainless steel 304 σ o = 1275.ε 0.45 Ring 1 (Stainless steel) ε eff = 0.15

9 Bridgman seal check (by Numerical simulation) Modified design of ring 1 Design 1 Ring 1 (stainless steel) Pressing ring 3 down to 1 mm σ xx 1.1 GPa σ xx 1.8 GPa Sharp edge Sharp edge σ xx = 1.8 GPa > p liq 1 GPa Design 2 Ring 1 (stainless steel) Pressing ring 3 down to 0.5mm σ xx 1.1 GPa σ xx 0.8 GPa Blunt edge Blunt edge σ xx = 0.8 GPa < p liq 1 GPa

10 Test Lubricants There are 3 types: 1) Water (for verification with the published data). 2) Plain mineral oils (Thick and thin version). 3) Good boundary lubricants (Thick and thin version). Oil type Product name Kinematic Viscosity η 40ºC [cst] - Water Naphthenic plain mineral oil CR5 660 Plain mineral oil* CR5-Sun Mineral oil with additives Rhenus LA Mineral oil with additives Rhenus LA Chlorinated paraffin oil TDN *50 wt. % mixture oil Houghton Plunger CR5 (η=660 cst) and Sunoco Sun 60 (η=10 cst). 10

11 Bulk Modulus K (MPa) Results Verification of the built-in equipment at DTU-MEK Bulk modulus of water was compared with the established result Water (Ref: Hayward, 1967) Water - Laboratory test Oil Pressure p oil (MPa) 11

12 Bulk Modulus K (MPa) Results Bulk modulus at different pressures TDN81 Rhenus 800cSt Rhenus 300cSt CR5 CR5-Sun60 Water Oil Pressure p oil (MPa) The boundary lubricants has a marked influence on compressibility when compared to the thin plain mineral oil. Meanwhile, the thick plain mineral oil shown a good compressibility in comparison to that of the boundary lubricant (medium viscous version). 12

13 Results A light damage on the seals caused by testing of plain mineral oil (a thin oil version). Extruded copper ring Extruded copper ring After cleaning Extruded copper ring 13

14 Results Heavy damage on the punch cap and the seals caused by testing of plain mineral oil (a thick oil version). Extruded copper ring Oil leakage through the sealing After cleaning Deformed punch cap 14

15 Summary The compressibility test results shown a similar trend when evaluating the same oil types on textured tool surfaces in strip reduction testing. The compressibility of the lubricant may be an influential factor in determining possible hydrodynamic and hydrostatic lubrication mechanism. The influence of the lubricant compressibility need to take into account for a future work in modelling trapped lubricant behavior in metal forming operation. 15

Fundamental Course in Mechanical Processing of Materials. Exercises

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