Internal combustion engines — Piston rings — Part 2: Inspection measuring principles
1Key Takeaways
This document defines the measuring principles to be used for measuring piston rings; it applies to piston rings up to and including 200 mm diameter for reciprocating internal combustion engines. This document can be used for piston rings for compressors working under analogous conditions.
2Expert Interpretation
A comprehensive interpretation of the measurement principles of 23 key inspection indicators of piston rings in the ISO 6621-2:2020 international standard, including standardized measurement methods and technology evolution analysis of parameters such as radial wall thickness, tangential force, and ovality, which is suitable for quality inspection of reciprocating internal combustion engine piston rings with a diameter of ≤200mm.
Analysis of the core content of the standard
ISO 6621-2:2020 standard systematically specifies 23 key characteristic test methods for piston rings, which are applicable to reciprocating internal combustion engine piston rings with a diameter of ≤200mm. Compared with the 2003 version, the third edition of the standard mainly updates the diameter range of 3.0mm and 3.5mm width oil rings to 160mm.
Key Technical Parameters Measurement System
| Measurement Parameters | Measurement Principle | Typical Tolerance | Instrument Requirements |
|---|---|---|---|
| Radial Wall Thickness (a1) | 4mm Radius Roller Radial Measurement (3-10N Force) | ±0.02mm | Spherical Probe Radius 1.5mm±0.05mm |
| Tangential Force (Ft) | Steel Strip Method/Hoop Method (0.08-0.1mm Thick Steel Strip) | ±10% | Vibration frequency 40-50Hz (multi-piece ring) |
| Ovality (U) | Vertical diameter difference measured after flexible steel belt is closed | 0.001×d1 | Measurement force ≤1N |
Detailed explanation of innovative measurement methods
Evolution of tangential force measurement technology
The standard compares three measurement schemes: the traditional steel belt method (Figure 13) using a 10mm diameter roller, the hoop method (Figure 14) achieved through a precision loading mechanism, and the surrounding steel belt method. For multi-piece rings, 40-50Hz vibration is applied to eliminate the influence of friction, and the measurement repeatability can reach more than 90%.
Nitriding layer depth detection
Use a microhardness tester to measure the HV0.1 hardness value on the annular section (Figure 34-35), and use 700HV0.1 as the critical point to determine the thickness of the nitriding layer. The standard specifically points out that there is currently no reliable non-destructive testing method, and recommends taking three measurement points 15mm away from the back of the ring.
Implementation recommendations
- Measurement environment control: All measurements should be performed on the reference plane. No additional force is allowed except for 4.2.19 (unevenness) and 4.2.20 (spiral)
- Instrument resolution requirements: not more than 10% of the measured dimension tolerance
- Special treatment for oil rings: The gap area must be thoroughly cleaned before measurement, and the spring gap should be placed at 180°
- Dispute resolution mechanism: It is recommended that manufacturers and customers jointly determine the arc length for waviness evaluation (the standard recommends excluding the 30° area on both sides of the gap)
Standard application case
A certain model