Oil Ring Testing

Oil ring inspection mainly conducts scientific testing and analysis on the inner diameter and radial thickness testing, end clearance and closed clearance testing, tangential tension and tangential tension distribution testing, surface roughness and profile testing, material hardness and metallographic structure testing, and distortion, objectively assessing the product quality grade and issuing a detailed test report, providing a basis for manufacturers' quality control and downstream users' product acceptance.

Testing Items

Inner diameter and radial thickness testing: Measuring the inner diameters and radial thicknesses of the oil rings, to verify whether their dimensions meet the design drawings and assembly tolerance requirements. The inner diameter dimensions are directly related to the fit clearances between the oil rings and piston ring grooves, and the radial thicknesses affect their contact pressures with the cylinder liners and oil scraping effects. This is the most basic and key item in the geometric dimension testing of the oil rings.

End clearance and closed clearance testing: The end clearance refers to the distance between the two ends of the oil rings in the free states, and the closed clearance refers to the distance between the two ends after being installed in the cylinder liners or ring gauges of the specified diameters. These two clearance parameters directly affect the sealing of the oil rings in the cylinders and the reliability during the thermal expansion. The excessively small clearances may lead to the seizure after the thermal expansion, and the excessively large ones will lead to the poor sealing and oil blow-by.

Tangential tension and tangential tension distribution testing: Evaluating the radial pressures generated by the oil rings on the cylinder walls in the free states and after being installed in the cylinder liners. The uniform and stable tangential tension distributions are the prerequisites of ensuring the good oil scraping and sealing properties of the oil rings. The insufficient tensions will lead to the incomplete oil scraping, and the excessively large tensions will aggravate the cylinder liner wear.

Surface roughness and profile testing: The surface roughness testers are used for the testing of the outer circular working surfaces of the oil rings, ensuring that they reach the specified Ra values (usually 0.2-0.8μm). In addition, the macro profiles of the oil ring working surfaces (such as the barrel faces and taper faces) also need to be tested, to ensure that they form the optimal lubrication and sealing states with the cylinder liners.

Material hardness and metallographic structure testing: The Brinell or Rockwell hardness testers are used to test the hardness values of the oil rings, evaluating their wear resistance. Through the metallographic microscopes, the micro-structures of the material (such as the graphite morphologies and matrix structures) are observed, judging whether they meet the material standards, such as the nodularity and pearlite contents of the nodular cast iron and alloy cast iron.

Distortion (flatness) testing: The oil rings are placed flat on the standard surface plates, and the gaps between them and the plates are measured with the feeler gauges, to evaluate their flatness (distortion). The excessively large distortion will lead to the poor fit of the oil rings in the ring grooves and the warping, seriously affecting their sealing properties. It is the important cause of the intensified "oil pumping effects".

Light leakage testing: The oil rings are placed in the standard ring gauges (simulating the cylinders), and the light sources are placed below the ring gauges, observing the light transmission between the oil ring outer circles and the ring gauge inner walls. It is usually required that the total arc length of the light leakage does not exceed a certain angle, and the single light leakage arc length does not exceed the specified value, to comprehensively evaluate the fit and sealing degrees between the oil rings and cylinders.

Testing Scope

One-piece cast iron oil rings: The testing objects are the one-piece oil rings manufactured by the integral casting, usually made of the wear-resistant cast iron or alloy cast iron. The testing emphases are their overall geometric dimensions, tangential tensions, hardnesses and surface qualities. It is the common structural form in the diesel engines and some gasoline engines.

Steel strip combined oil rings: Testing the combined oil rings consisting of the two oil scraping steel strips and an expander spring (helical expander spring or wave expander spring). The thicknesses, parallelism and edge sharpness of the oil scraping steel strips, as well as the tangential tensions, free heights and fatigue properties of the expander springs need to be separately tested, and the overall performance after the assembly also needs to be tested.

Spiral expander oil rings: Testing the structures with the spiral expanders installed in the inner circular grooves of the cast iron oil rings or steel oil rings. In addition to the testing of the oil ring bodies, the wire diameters, pitches, free lengths, working tangential tensions and durability of the spiral expanders must be specially tested, ensuring that they can continuously provide the stable radial pressures.

Taper-faced oil rings and nose oil rings: Testing the oil rings with the specific taper angles or nose structures on the outer circular working surfaces. In addition to the routine items, the accuracy of their taper angles or nose profiles needs to be emphatically tested, because their special geometric shapes have the significant effects on controlling the oil consumption and reducing the friction power consumption.

Surface treated oil rings: Testing the oil rings after the phosphating treatment, chromium plating, molybdenum spraying, PVD (physical vapor deposition) coating or nitriding treatment. The testing scope needs to be extended to the coating thicknesses, bonding strengths, surface porosities, micro-hardness gradients, as well as the wear resistance and corrosion resistance of the coatings.

Replacement oil rings for the engine maintenance: Testing the oil ring assemblies in the after-sales maintenance market. In addition to the performance testing, the dimension matching with the original parts and material consistency need to be strictly compared, ensuring that the engine oil control capabilities can be restored without damaging the cylinder liners.

Testing Methods

Optical projection/image measurement method: The tool microscopes or two-dimensional image measuring instruments are used for the non-contact high-precision measurement of the micro dimensions such as the end clearances, radial thicknesses and groove widths of the oil rings. This method has the high efficiency and can carry out the profile comparisons, especially suitable for testing the scraping strips of the combined oil rings with the complex edge shapes.

Pneumatic gauge testing method: The pneumatic gauges (such as the float-type and electronic column-type) combined with the dedicated probes are used for the fast and efficient batch testing of the dimensions such as the inner diameters and thicknesses of the oil rings. This method is based on the air throttling principle, with the high measurement precision and good repeatability, and is the common means of the online testing on the production lines.

Ring gauge and plug gauge comprehensive testing method: The standard dimension ring gauges (simulating the cylinders) and plug gauges (simulating the ring grooves) are used for the functional testing. Through the placement of the oil rings in the ring gauges to measure the closed clearances and light leakages, and the use of the plug gauges to check the ring groove fits, it is an intuitive and reliable comprehensive traditional testing method.

Tangential tension distribution automatic testing method: The dedicated oil ring tangential tension measuring machines are used. Through the multiple uniformly distributed radial probes, during the one rotation of the oil rings, the radial pressure distribution curves are continuously measured and recorded. This method can precisely evaluate the uniformity of the tangential tensions and find the local excessively strong or weak defects.

Surface profile and roughness measurement method: The stylus surface profilometers or white light interferometers are used to scan the outer circular working surfaces of the oil rings, obtaining their micro profile curves and roughness parameters (Ra, Rz, etc.). For the coated oil rings, the non-contact laser profilometers can also be used for the measurement.

Material physical and chemical property analysis method: The spectroscopic analysers are used to test the material chemical compositions; the hardness testers (Rockwell, Brinell and micro-Vickers) are used to test the matrix and coating hardnesses; through the metallographic sample preparation and etching, the structure morphologies are observed under the metallographic microscopes, evaluating the internal qualities of the material.

Functional bench simulation testing method: On the dedicated piston ring-cylinder liner simulation test benches, the engine operating conditions (temperature, pressure and lubrication conditions) are simulated, testing the oil scraping efficiencies, oil consumption, friction power consumption and wear rates of the oil rings. This is the evaluation method closest to the actual use effects.

Testing Standards

Domestic Standards:

1. GB/T 1149, Internal combustion engines — Piston rings

2. GB/T 231.1-2018, Metallic materials — Brinell hardness test — Part 1: Test method

3. GB/T 230.1-2018, Metallic materials — Rockwell hardness test — Part 1: Test method

4. GB/T 9441-2009, Metallographic test method for spheroidal graphite cast iron

International Standards:

1. ISO 6621, Internal combustion engines — Piston rings

2. ISO 6506-1, Metallic materials — Brinell hardness test — Part 1: Test method

3. ISO 6508-1, Metallic materials — Rockwell hardness test — Part 1: Test method

Note: The applicable standard can be selected according to the product use and customer trade requirements; standards agreed in the contract take precedence over general standards.

Testing Instruments and Equipment

Pneumatic gauge and dedicated probes: The core dimension testing equipment, including the float-type pneumatic gauges, electronic column-type pneumatic gauges, as well as the opposed-jet probes and step probes specially designed for the parameters such as the oil ring inner diameters, thicknesses and groove depths. Their measurement precision can reach the micron level, with the good stability, suitable for the fast batch inspection.

Optical image measuring instrument: Used for the precision measurement of the two-dimensional dimensions and profiles. Equipped with the high-resolution CCD lenses and image processing software, it can automatically identify and measure the end clearances, edge angles and chamfer dimensions of the oil rings, and generate the detailed test reports with the traceable data.

Oil ring tangential tension automatic measuring machine: This equipment is usually equipped with the high-precision torque transducers or pressure transducers, as well as the rotating fixtures driven by the stepper motors. It can automatically complete the clamping, rotating measurement of the oil rings, and output the maximum, minimum and average values of the tangential tensions and the distribution curves.

Surface roughness/profilometer: The stylus profilometers (such as the Taylor Hobson and Mahr) are used to measure the working surface roughnesses and macro profile shapes. The white light interferometers or confocal microscopes are suitable for measuring the three-dimensional topographies and micro roughnesses of the ultra-smooth surfaces or coating surfaces.

Metallographic microscope and hardness tester: The metallographic microscopes combined with the image analysis systems are used to observe and analyse the graphite morphologies, matrix structures and coating structures of the oil ring materials. The Rockwell hardness testers are used to test the overall hardnesses, and the micro-Vickers hardness testers are used to test the micro-hardnesses of the coatings or local areas.

Ring gauges, plug gauges and standard surface plates: As the reference measuring tools, the standard ring gauges are used for the light leakage and closed clearance testing, with the extremely high requirements for their inner diameter precision and roundness. The plug gauges are used to test the axial heights of the oil rings. The standard surface plates (grade 0 or grade 1) combined with the dial gauges or feeler gauges are used to test the flatness (distortion) of the oil rings.

Piston ring comprehensive performance test bench: The large-scale simulation testing equipment, which can simulate the in-cylinder temperatures, pressures, reciprocating motions and lubrication conditions of the engines, and directly measure the oil scraping amounts, friction forces and wear amounts of the oil rings under the actual working conditions, providing the key data support for the product development and reliability verification.