Why Are Test Methods So Important?

Although diamond friction shims are small‑size components, they directly determine the connection reliability of high‑end equipment. Common confusions in engineering practice:

• Suppliers claim “friction coefficient of 0.7”, how can this be verified?
• Test data varies drastically across different test methods; which result is credible?
• There exists a large gap between laboratory data and real‑world service conditions; how to narrow such deviation?

Core principle: Without standardized test methods, there will be no credible performance data. This article systematically introduces key test methods and standards for diamond friction shims, helping engineers establish a scientific quality‑evaluation system.

I. Friction‑Coefficient Test: Verification of Core Performance Indicator

1.1 Why is friction‑coefficient testing so complicated?

The friction coefficient is not an intrinsic material property, but a system characteristic of the friction pair. Even for the identical diamond shim, drastically different test results may be obtained under varied conditions:

• Different counter‑part materials (steel / aluminum / coated surfaces) yield different friction coefficients
• Different surface roughness yields different friction coefficients
• Dry friction versus oil lubrication yields different friction coefficients
• Different contact pressures yield different friction coefficients

Therefore, discussing friction coefficients without specifying test conditions bears no engineering significance.

1.2 Comparison of Mainstream Test Standards

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1.3 Detailed Introduction to ISO 16047: Friction‑Coefficient Test for Bolted Joints

Test Principle Within the friction pair composed of bolt‑nut‑washer, measure torque (T) and preload force (F) to calculate total friction coefficient (μ_tot):

μ_tot = T / (F × d / 2)

Where d stands for nominal bolt diameter.

Equipment Requirements

• Torque‑sensor accuracy: ±1%
• Preload‑force‑sensor accuracy: ±1%
• Bolt size: M8‑M20 (M10, M12 commonly used)
• Test speed: 5‑20 rpm (simulating real‑assembly speed)

Sample Preparation

• Bolts / Nuts: Grade 8.8 or higher complying with ISO 898‑1
• Counter‑part: Metal plates of the same material as test shim (thickness ≥10 mm)
• Surface roughness: Measured per ISO 4287, Ra ≤1.6 μm

Test Procedures

1. Assemble specimens and apply target preload (e.g. contact pressure of 50 MPa).
2. Tighten at constant speed and record torque‑preload curves.
3. Calculate friction coefficient as average value from 3‑5 replicate measurements.
4. Report standard deviation to evaluate data dispersion.

1.4 Detailed Introduction to ASTM G99: Pin‑on‑Disk Friction‑and‑Wear Test

Test Principle Fix shim specimen on rotating disk; apply normal load via standard pin (6 mm diameter); measure friction force to derive friction coefficient.

Test Parameters ‑Normal load: 10‑50 N (simulate practical contact pressure) ‑Rotational speed: 200‑800 rpm (corresponding linear velocity: 0.1‑0.4 m/s) ‑Test duration: 30‑60 min (or specified wear distance) ‑Environment: dry friction or oil‑lubricated condition

Data Processing ‑Record real‑time friction‑force curves ‑Calculate average friction coefficient ‑Measure pin wear volume ‑Plot friction‑coefficient‑versus‑time curve

Limitation: The pin‑on‑disk setup uses point contact, which differs significantly from surface contact in real bolted connections. Its data is only for preliminary screening and shall NOT be directly adopted for engineering design.

1.5 DAOLER Friction‑Coefficient Test Solution

DAOLER adopts a dual‑track testing strategy: ISO 16047 as primary method, ASTM G99 as auxiliary method.

‑ISO 16047 (engineering‑application level):  Equipment: Hahn+Kolb torque‑preload test system (Germany)  Specimen: Grade‑8.8 M12 bolt with standard steel flange  Contact pressure: 50 MPa (simulate real assembly)  5 replicates per group; report mean ± standard deviation  Typical result: μ_tot = 0.55‑0.72 (dry friction) for DAOLER diamond shims

‑ASTM G99 (material‑R&D level):  Equipment: CSM Instruments pin‑on‑disk tribometer  Normal load: 30 N  Rotational speed: 400 rpm  Duration: 60 min  Purpose: material‑formula optimization & coating‑process validation

II. Vibration‑Fatigue Test: Durability Verification

2.1 Why is vibration‑fatigue testing necessary?

The core value of diamond shims lies in long‑term anti‑loosening performance. How can “long‑term performance” be quantified? Laboratory vibration‑fatigue test serves as the key means for durability verification.

2.2 Mainstream Vibration‑Test Standards

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2.3 Detailed Introduction to Junker Transverse‑Vibration Test

Test Principle Mount bolted‑joint specimens on transverse‑vibration tester; apply alternating transverse load to simulate real‑world vibration environments.

Typical Test Parameters ‑Amplitude: ±0.3 mm (adjustable) ‑Frequency: 12.5 Hz (adjustable) ‑Preload: set according to ISO 16047 (e.g. 20 kN for M10 bolt) ‑Cycles: 100 000 cycles (industry baseline) or 300 000 cycles (DAOLER enterprise requirement)

Evaluation Metrics ‑Preload retention rate = (post‑test preload / initial preload) × 100 %  Excellent: >90 %; Acceptable: 70‑90 %; Fail: <70 % ‑Loosening angle: relative rotation angle of nut against bolt  Excellent: <1°; Acceptable: 1‑5°; Fail: >5°

Equipment ‑Proell Junker vibration tester (Germany) ‑Preload sensor (accuracy ±1 %) ‑Angle sensor (resolution: 0.01°)

2.4 DAOLER HYB‑ProTest® Special Test for Humanoid‑Robot Joints

Targeting extreme service conditions of humanoid‑robot joints, DAOLER establishes the proprietary HYB‑ProTest® enterprise standard.

Test Conditions ‑Angular velocity: 120°/s (simulate high‑speed motion) ‑Swing range: ±45° (simulate joint movement stroke) ‑Equivalent load: 50 kg (simulate full‑robot dead weight) ‑Thermal cycle: −20 °C ↔ 80 °C, one cycle every four hours (simulate temperature fluctuation) ‑Cycles: 300 000 cycles (3× industry baseline)

Significance Hip joints of humanoid robots endure impact loads 2‑3 times per second, accumulating >100 000 cycles daily. The conventional 100 000‑cycle test cannot reflect real‑service conditions. HYB‑ProTest® with 300 000 cycles plus thermal cycles better replicates vibration loads over the robot’s full service life.

III. Coating Inspection: Guarantee of Quality Consistency

3.1 Coating‑Thickness Measurement

Importance Coating thickness directly governs friction coefficient and wear life. Insufficient thickness leads to poor diamond‑particle embedding and low friction coefficient; excessive thickness impairs dimensional accuracy and causes assembly difficulty.

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DAOLER Inspection Regime ‑Outgoing inspection: XRF test for every single piece ‑Arbitration sampling: metallographic‑microscopy test on 3 pieces per batch ‑Typical total coating thickness (including substrate): 0.13‑0.23 mm

3.2 Coating‑Adhesion Test

Importance Coating adhesion determines service life of diamond shims. Poor adhesion triggers coating delamination under vibration, followed by rapid friction‑coefficient degradation.

‑Method 1: Cross‑cut test (ISO 2409)  Cut 100 grid cells (10 × 10) into coating with cross‑cut knife.  Apply standard adhesive tape and peel off rapidly.  Rating scale: Grade 0 (no delamination) ~ Grade 5 (>65 % delamination).  DAOLER acceptance criterion: ≤Grade 1 (<5 % delamination).

‑Method 2: Tensile test (ASTM C633)  Bond coating surface to pull‑off stud via adhesive.  Apply axial tensile load on tensile tester.  Record load at coating‑delamination moment and calculate adhesion strength (MPa).  DAOLER typical value: ≥30 MPa.

3.3 Diamond‑Particle Size & Distribution Inspection

Importance Diamond‑grain size influences both friction coefficient and surface roughness. Larger grains raise friction coefficient yet accelerate counter‑part wear; finer grains produce smoother surfaces but may deliver insufficient friction coefficient.

‑Scanning‑Electron‑Microscopy (SEM) observation  Magnification: 1 000‑5 000 ×  Observed items: diamond‑grain morphology, distribution uniformity, embedding depth  Evaluation indicators: particle density & agglomeration ratio

‑Laser‑particle‑size analysis  Measuring range: 1‑100 μm  Evaluation indicators: average grain size & particle‑size‑distribution width

DAOLER typical grain grade: G10‑G50 (10‑50 μm, adjustable).

IV. Salt‑Spray Test: Corrosion‑Resistance Verification

4.1 Why is salt‑spray testing required for marine / outdoor applications?

Offshore wind turbines, marine equipment and coastal industrial hardware operate under heavy salt‑fog environments. Metal shims are prone to corrosion, leading to unstable friction coefficients and joint failure.

4.2 Neutral Salt‑Spray (NSS) Test (ISO 9227 / ASTM B117)

Test Conditions ‑Salt solution: 5 % NaCl ‑Chamber temperature: 35 ± 2 °C ‑Spray volume: 1‑2 mL / 80 cm² · h ‑pH: 6.5‑7.2 ‑Test duration options: 24 h / 48 h / 96 h / 240 h / 500 h / 1 000 h

Evaluation Indicators ‑Appearance rating per ISO 10289: assess corroded‑area ratio  Rating: Grade 0 (zero corrosion) ~ Grade 10 (>50 % corroded area)  DAOLER criterion: ≤Grade 3 after 1 000 h (<5 % corroded area) ‑Friction‑coefficient retention rate = (post‑salt‑spray friction coefficient / initial friction coefficient) × 100 %  DAOLER typical value: ≥94 % retention after 1 000‑hour NSS test

4.3 Copper‑Accelerated Acetic‑Acid Salt‑Spray (CASS) Test

For harsher corrosion‑environment assessment.

Test Conditions ‑Salt solution: 5 % NaCl + 0.26 g/L CuCl₂, pH adjusted to 3.1‑3.3 with glacial acetic acid ‑Chamber temperature: 50 ± 2 °C ‑Duration: typically 1/4 of NSS duration (e.g. 1 000 h NSS ≈ 250 h CASS)

DAOLER Test Results ‑Appearance rating ≤Grade 2 after 250‑hour CASS test ‑Friction‑coefficient retention rate ≥92 %

V. Dimensional‑Accuracy Inspection: Assembly‑Reliability Assurance

5.1 Key Dimensional Parameters

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5.2 Batch‑to‑Batch Consistency Control

Sampling scheme complying with ISO 2859‑1: ‑Batch size ≤500 pcs: sample 20 pcs ‑Batch size 500‑5 000 pcs: sample 50 pcs ‑Batch size >5 000 pcs: sample 80 pcs

Acceptance criteria: ‑Dimensional pass rate ≥98 % ‑Friction‑coefficient dispersion (standard deviation) ≤0.05 ‑Coating‑thickness dispersion ≤±0.01 mm

VI. Sourcing Guidance: How to Evaluate Suppliers’ Test Capabilities

6.1 Supplier‑Capability‑Assessment Checklist

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6.2 Data‑Verification Workflow

Request suppliers to provide complete documentation: ‑Friction‑coefficient test report (including test conditions, specimen info, statistical data) ‑Vibration‑fatigue test report (including preload‑retention curves) ‑Coating‑inspection report (thickness & adhesion‑strength data) ‑Salt‑spray‑test report (appearance rating & friction‑coefficient retention rate) ‑Dimensional‑inspection report (sampling scheme & acceptance criteria)

Key verification points: ‑Are test conditions aligned with your service conditions? ‑Is data dispersion within reasonable limits? ‑Is there third‑party‑lab certification? ‑Are test instruments periodically calibrated?

Summary

Performance validation for diamond friction shims requires systematic testing: ‑Friction‑coefficient test: ISO 16047 reflects real‑world assembly; ASTM G99 suits material screening. ‑Vibration‑fatigue test: Junker test is industry baseline; HYB‑ProTest® targets humanoid‑robot scenarios. ‑Coating inspection: coating thickness, adhesion strength and diamond‑particle distribution are critical for quality consistency. ‑Salt‑spray test: 1 000‑hour NSS serves as threshold requirement for marine & outdoor applications. ‑Dimensional accuracy: batch‑to‑batch consistency is the foundation for mass‑production deployment.

When selecting products, do not rely solely on single figures such as “friction coefficient 0.7”. Instead, focus on full test reports, defined test conditions and data dispersion. Scientific test methodology underpins quality assurance and directly demonstrates a supplier’s technical competence.

Related Readings

‑Selection Guide for Precision Friction Shims: Anti‑Loosening & Wear‑Resistant Solutions for High‑End Industrial Equipment ‑Friction‑Shim Selection Guide: Full Logic from Preload & Friction Coefficient to Installation Torque ‑Dissection of Core‑Patented Process for DAOLER® Diamond Friction Shims