Unitized sealing solutions feature over-molding and precision machining to eliminate challenges inherent in standard piston and gland designs.
Contributed by Andrew Iddeson, Product Technology Director, Advanced Sealing Technologies and Chuck White, Global Product Development Manager
Hydraulic cylinders are a key component in efficiently harnessing the power created by a fluid power system into usable linear motion. This motion is used in many applications, enabling various modes of operation. From excavation to lifting operations, hydraulic cylinders play a key role in a wide range of equipment found in the market today.
The recent drive for more sustainable solutions means that demand for cylinders to perform in harsher conditions is growing, challenging cylinder manufacturers to rethink common designs. The sealing system is a critical element of a hydraulic cylinder. Although seals usually make up less than 5% of the cost to manufacture the cylinder, they are essential.
Hydraulic cylinder seals are typically manufactured in high-grade thermoplastic polyurethane and other thermoplastic elastomers, which range in shore hardness between 90A and 72D.
The seals must be flexible enough to respond to changing pressure, maintaining seal function, which results in movement of the cylinder, and yet durable enough to minimize wear and creep over long periods of time. Several challenges can threaten the system’s performance. Let’s explore three of the most common challenges in more detail.
Common performance challenges
Extrusion gap: Extrusion is one of the leading causes of reduced seal performance or failure. Extrusion occurs when the system pressure forces the seal material into the gap that exists between the dynamic surface being sealed and the clearance diameter of the gland or piston. The extrusion gap is created due to the machine tolerances of the various components, resulting in a “least material condition” (LMC) and a “maximum material condition” (MMC). If the LMC is too large, this could result in an unacceptable extrusion gap, allowing the seal material to deform into the area when under pressure. If the MMC is too small, this could potentially result in metal-to-metal contact between the dynamic and static components of the cylinder.
Coaxial Alignment: All sealing systems require some level of guidance to ensure the seals are not biased to one side or crushed under heavy side load conditions. This is typically achieved using wear bands that are installed near the seal in a machined groove. To accommodate these grooves, the hardware (e.g., piston or gland) requires sufficient length to accommodate these wear bands, resulting in heavy components and a high cost of raw materials. Also, the wear bands and the grooves have tolerances, which add to the LMC and MMC stacks that we described earlier on. Also, consideration must be given to the compressibility of the material being used for the wear bands to ensure that metal-to-metal contact does not occur. The compressing strain is also to be considered in the LMC and MMC calculations making it even more challenging.
Side loading: As mentioned, the hydraulic cylinder industry is seeing an increased demand for harder-working cylinders in certain applications, such as excavation. When the system works, any side load created in the cylinder begins to challenge the wear bands due to the loss of straight-line operation. Extreme side loading may result in failure of the wear bands, allowing metal-to-metal contact to occur. Operating seals with a lateral bias can cause uneven seal wear, resulting in premature failure over time. Highly engineered wear band materials may mitigate some of the effects of side loading but tend to be expensive and are not always 100% effective.
An innovative complete solution
The unitized piston and gland solution addresses all the challenges discussed. It offers an innovative approach of over-molding the product and then, by precision machining the wear material, eliminates the sealing challenges inherent in standard piston and gland designs.
To address the extrusion gap, a unitized sealing solution is over-molded and then precision machined to the desired OD (piston) or ID (gland). By doing this, the LMC and MMC are minimized as the tolerances of the wear band grooves are no longer a factor. The precision of the machined bearing results in up to 70% reduction in the extrusion gap compared to typical configurations with separate bearings. This results in longer seal life and the ability of the seal to resist higher operating pressures. Since the unitized products’ introduction, many customers have benefited. Here are two cases.
Case A: A customer faced challenges in high-pressure environments, struggling to reduce the extrusion gap sufficiently for the seals to function effectively. The extrusion gap was narrowed to a mere 0.004 in. at the “least material condition” (LMC), enabling the seals to withstand pressures exceeding 10,000 psi. This significant improvement not only eradicated seal-related warranty claims but also substantially prolonged the operational lifespan of the cylinders in their demanding applications.
Case B – Due to an excessive extrusion gap in the design, a customer was using a high-cost, four-piece piston seal with anti-extrusion rings. The unitized sealing solution provided the advantage of a compact two-piece piston seal, reducing the cost while also reducing the length of the piston, resulting in less material and increasing the stroke length of the cylinder by 1.5 in. Ultimately, these changes culminated in improved performance and longer cylinder life in the application.
With a smaller extrusion gap, there is also an improvement in coaxial alignment. The standard piston could see upwards of 0.010 in. movement side to side from the center line of the rod or piston, while the unitized product will see a maximum of 0.003 in. side-to-side movement. This keeps the wear on the seal package equal by not allowing the piston or the rod to continually bias one side which is especially important in horizontal operating applications. The following customer cases put this in perspective.
Case C – A cylinder operating in a horizontal plane was experiencing seal failures due to uneven wear on the rod seal, resulting in premature leakage. The customer was using ductile iron for the gland, but it could not achieve a tight enough tolerance to counter the leak. The unitized gland corrected the issue by improving the alignment of the rod, which prevented uneven wear on the rod seal and improved its life.
Case D – A high level of dirt ingression was getting past a customer’s rod wiper. The cylinder operated in an environment with high levels of airborne dirt and sand, which, once inside the cylinder, would embed in the rod and piston seals. A review of the cylinder and application revealed that a lack of coaxial alignment allowed the rod to bias to one side, reducing the preload of the rod wiper to less than 1%. The unitized piston and gland combination corrected the coaxial alignment. Also, it provided a barrier with over-molded wear surfaces that trap any contamination before infiltrating the seal package.
Finally, with the unitized sealing design, it is possible to increase the bearing surface of the wear band as it covers a much larger part area. In the case described below, over-molding the surface versus using wear bands increases the overall bearing surface by 72%, whereas over-molding a gland would increase the bearing surface by 147%. Because of the significant increase in bearing surface and the fact that metal is no longer exposed, it is now possible to evaluate shortening both the piston and gland while still having more bearing surface. The customer case illustrates how the piston can be shortened by 0.500 in. and still have a bearing surface that is 22% greater than the original piston. The rod gland can be shortened by 0.880 in. and maintain a bearing surface that is 46% greater than the original gland.
Case E – A customer using cylinders in a loader application reported seal failures and metal-to-metal contact. Improper use of the equipment meant that instead of using the loader to lift and dump media, the customer was inverting the bucket down and using it as a scraper to level hard surfaces, resulting in high-frequency shock on the seal package and excessive side loading to the rod and piston. In some cases, this load was so high that metal-to-metal contact began to damage the cylinder. The unitized piston and gland eliminated the metal-to-metal contact, protecting all metal surfaces. Also, the reduced extrusion gap protected the seals from the high-frequency shock load, delivering improved cylinder performance and longevity of the seal package.
Additional benefits underscored by sustainability
Using the unitized sealing solution in a cylinder system can reduce the piston and rod gland lengths, reducing the material required to produce them and the cost.
With shorter pistons and glands, the same stroke length may be achieved with a shorter tube rod combination, resulting in less material and reduced cost. The reduction in length can also be used to maximize the stroke length of the hydraulic cylinder, which may improve performance in the application.
Another advantage is that the unitized solution is delivered to the customer fully assembled, with the seal package ready to assemble to the cylinder. This speeds up the assembly and production process and reduces inventory space.
Finally, the unitized solution contributes to a more sustainable hydraulic cylinder system. By reducing the weight of these components, less energy is required from the fluid power system, and less weight is carried on mobile hydraulic equipment. They also solve the challenges presented by extrusion gap, coaxial alignment, and side loading, protecting the sealing system and significantly increasing its longevity. Metal and some bearing materials utilize recycled products and can also be recycled at the end of their lives.
In summary, unitized pistons and glands benefit hydraulic cylinders by increasing their longevity while protecting the seal package. These products represent a sustainable solution due to their impact on the longevity of the overall cylinder and the ability to address more aggressive operating conditions. They require less energy to operate because they are lighter in weight than a standard solution, which means they require less power from the fluid power system. Lastly, there are clear cost savings to the customer due to the requirement of less steel and a significant impact on reducing the cylinder production process time by installing a fully machined and assembled part.
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