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In several applications, such as a secondary seal in a floating ring seal of a nuclear power plant's feed water pump or as a seal at a temporary screw-in connection for automobile air conditioning systems, the O-ring has shown to be an effective and affordable sealing element. These are only a few of the numerous instances when O-rings have fulfilled the strictest standards for leak tightness, regardless of the medium or temperature—from -60 °C to 300 °C. O-rings are simply mounted, widely accessible, and just need a basic seal housing. Because of this, design engineers find it particularly frustrating when O-rings must be rejected as sealing components in order to solve an issue because of excessive measurement deviation. It will often cost the consumer extra to rely on other sealing.
Especially for smaller O-ring applications, the DIN 3771 part 1 common tolerances provide a significant limitation on their applicability. This study illustrates the causes behind the manufacturing process variations in O-ring measurements and the functional consequences that follow. It outlines a brand-new, very accurate optical measuring method for O-rings that is being resold as a service for precise tolerance-based measurement and sorting.
Due mostly to the production process, there are three determining factors:
-Variations in mold temperature when O-rings are curing
-Variations in rubber's and other chemical components' shrinking behavior
-Mold offset as a result of the guide bolt's diameter clearance increasing with running time owing to wear.
Temperature variations can cause significant differences in diameter measurements even within a single batch, particularly when large molds are used to create O-rings. Batch-related variations in compound composition and modest batch-related temperature variations can cause clearly visible changes in inner diameters even in highly temperature-controlled procedures, such as those that can be achieved with tiny injection molds. Another crucial influencing element for the cable thickness, an O-ring's functional dimension that is normally more significant, is the mould offset. Figure 1 uses a straightforward top view of an O-ring (upper mould half = red, bottom mould half = turquoise) to show how variations in cord thickness can happen on an O-ring as a result of the offset of the higher and lower halves. DIN 3771 part 4 states that although this offset is often less than 0.05 mm on fresh injection molds, wear can cause it to greatly rise, adding up to 0.05 to 0.1 mm.
The factors discussed earlier have resulted in the measurement tolerances specified by DIN 3771/part 1 (figure 2), which most O-ring manufacturers adhere to. However, finding suppliers willing to significantly narrow these tolerances (while maintaining consistency) can be a challenge. This is particularly pertinent for small O-rings, where users often find the allowed tolerances too lenient.
To illustrate, let's compare two standard sizes: The first O-ring, with dimensions (inner diameter x cord thickness) of 2.0 +/- 0.13 × 1.80 +/- 0.08, exhibits considerably wider tolerances compared to the second O-ring, sized 20.0 +/- 0.22 × 3.55 +/- 0.1. For the smaller O-ring, the tolerances translate to approximately +/-6.5% for the inner diameter and +/-4.4% for the cord thickness. In contrast, the larger O-ring features much more favorable tolerances, with only approximately +/-1.1% for the inner diameter and +/-2.8% for the cord thickness.
This disparity becomes even more pronounced when considering a smaller cord thickness, such as 1 mm, as manufacturer tolerances typically do not decrease substantially, further exacerbating the proportion of unfavorable tolerances for smaller O-rings.
The sealing effectiveness of O-rings relies on creating surface pressure by deforming the cord thickness of the O-ring. Since rubber materials exhibit non-ideal elastic behavior, O-rings require a minimum deformation of approximately 6% of the cord thickness. If the deformation falls below this threshold, compression can quickly reach 100%, especially with smaller cord thicknesses. Typically, cord thickness compression ranges from 20% to 29% for static radial seals and from 12% to 22% for dynamic seals [1]. These values are based on an O-ring with a cord thickness of 1.78 mm and a nominal diameter of 20 mm, with components positioned concentrically for sealing. Additionally, considering maximum possible eccentricity, the variation range can widen further, depending on the diameter clearance.
As the reaction forces of a deformed O-ring increase progressively with relative deformation [2] (see figure 3), this implies a ratio of maximum to minimum deformation, and thus friction force, of approximately 3:1 for dynamic seals in the provided example. Due to this potential variance, O-rings are often not suitable for many dynamic applications due to their significant tolerances. In static applications, these typical variations in deformation forces result in sealing face pressures that depend on tolerances, significantly influencing the leakage rate of O-ring connections [3]. However, since statically applied O-rings are generally deformed to a greater extent than dynamically applied ones, this effect does not pose an insurmountable constraint.
The second part of the O-ring standard DIN 3771 specifies that O-rings should be measured non-contactly with regard to their cross-section, but the number of measuring points remains unspecified. To account for the influence of mold offset, meaningful data for radial cord thickness can only be acquired by averaging numerous individual values across the entire perimeter. According to DIN 2771, measurements using conical test mandrels and mandrels with varying levels are permissible. However, this method is notably unsuitable for small O-rings in terms of accuracy and repeatability.
The O-Ring Test Laboratory Richter has commissioned a specialized measurement and sorting machine for O-rings. This machine combines a line scan camera controlled by a linear drive with a rotatable glass measuring table, enabling highly accurate non-contact measurement of O-rings and other seals with inner diameters ranging from 0 to 400 mm. (Figures 4 and 5 illustrate the measurement principle for small and large O-rings.) Additionally, cord thickness or wall thickness can be measured alongside outer and inner diameters.
Unlike existing measurement machines, this system does not consider out-of-roundness caused by seal flexibility as a measurement error. By evaluating several thousand measuring points around the perimeter and employing a patented evaluation process, it ensures excellent repeatability and high measurement accuracy. In addition to inner and outer diameter measurements, the system provides average values and selective minimum and maximum cord thickness values as results. This facilitates the acquisition of reliable data on O-ring functionality, as it enables the detection of increased offset and flash.
Critical to this measurement process is its capability to perform measurements for dimensions up to an outer diameter of 25 mm at intervals of nearly one second. Moreover, a blowing device allows for cost-effective sorting of good and defective parts. Large seals can also be measured within seconds. Through the correlation of measurement results or the imposition of tolerance limits, standard O-rings can be transformed into high-precision sealing elements.
Technological advancements have significantly enhanced the production of small O-rings. Whether it's the precision in molding tool production or their rheological dimensioning through calculation programs, the temperature control of injection machines using microprocessors, or the implementation of refined deflashing procedures, these developments have led to the achievement of very tight normal distributions for functional dimensions, as illustrated in figures 6 and 7. A study involving various suppliers' NBR O-rings with inner diameters under 20 mm revealed that over 90% of the tested O-rings met significantly narrower tolerances.
Practical tolerances, where over 90% of the parts meet specifications, can average at a radial cord thickness of +/- 0.03 mm and an inner diameter of +/- 0.05 mm concerning nominal size. This would necessitate precise dimensioning of the injection mold for the desired mean and the selection of suitable rubber material, while ensuring the maximal permitted value for mold offset according to DIN 3771 is not fully utilized. By narrowing tolerances in this manner, the potential variation range in cord thickness deformation can be significantly limited, thereby greatly expanding the applicability of these O-rings, especially in dynamic scenarios.
Founded by an O-ring specialist, the O-Ring Test Laboratory Richter provides a range of services, including the measurement and sorting of O-rings and other seals in accordance with specified tolerances, as outlined above. With rapid measurement capabilities, our sorting services are cost-effective even for large batch sizes.
In addition to sorting services, our test laboratory offers comprehensive testing methods for qualifying new seals and conducting incoming goods inspections on serial parts. Our independent consulting and training services assist in swiftly identifying effective sealing solutions, reducing development timelines, and crafting reasonable purchase specifications. Leveraging modern testing technologies and expertise in various rubber materials' performance, we excel in finding economical sealing solutions, even for specialized applications.
