5 key considerations for designing underwater linear systems

Underwater applications present a variety of unique challenges to design engineers not found in other application areas. If these challenges are not thoroughly understood and planned for, the system will likely fail. 

Read on to learn about five of the most important considerations for designing underwater linear systems, and some of the key components igus® provides that can operate in splash water and underwater applications. 

Application requirements

Requirements for underwater applications can vary greatly, depending on the level of water exposure, operating depth, fresh vs. saltwater, and more. Understanding exactly what your linear system will be experiencing in an application is essential to choosing the optimal components.

The test data below demonstrates the effectiveness of anodization on our aluminum pillow blocks in resisting corrosion stemming from saltwater exposure. Both clear anodized materials entirely resisted corrosion over the course of the 1,000-hour test, showing only minimal color changes and small surface dents by the end of the test. Both hard anodized aluminum options and our die-cast stainless steel showed no signs of corrosion, wear, or discoloration by the end of the test.

The test was a 1000-hour salt spray that was applied to various aluminum grades. Clear anodized aluminum 6060 and clear anodized aluminum 5083 saw changes in color, however they saw no corrosion by the end of the 1000 hours. Hard anodized aluminum 6060 and hard anodized aluminum 5083 saw no changes to the product after the test was completed. During this test, we also included our two stainless pillow block materials. The machined 1.4571 stainless saw color variations after an hour and began corroding after 2 hours, while the 1.4408 cast stainless did not experience any discoloration or corrosion.  Both our hard anodized and our die-cast 1.4408 stainless pillow blocks will perform well in saltwater environments based on the testing results.

Bearing material

When selecting bearings for underwater linear systems, the right bearing material will make the difference between successful, long-lasting performance, and premature failure of the entire system. Bearing materials with a low moisture absorption rate are essential, and the rest of the system needs to account for the specific absorption rate of the bearing material to avoid seizing.

igus offers multiple plastic bearing liners with particularly low rates of moisture absorption, such as iglide® A160, iglide X, and iglide J. When fully submerged, the moisture absorption rates are as follows:

All three of these materials are also resistant to seawater, and do not experience any noticeable change in mechanical properties or appearance due to exposure to seawater. For applications submerged in a chlorinated swimming pool, igus would recommend our iglide X material as it is resistant to chlorinated water. iglide J should be avoided in chlorinated applications while iglide A160 is conditionally resistant, meaning it could have some changes to its properties but still function. 

Water acts as a lubricant with igus engineered plastics, reducing the coefficient of friction and thus having a correction factor of 4 on the tolerated pv value for each material.

Additionally, we have extensive chemical resistance charts for our liner materials, so for applications submerged in water/chemical mixtures or other liquids, resistance data may be available.

Application example: Bearings in underwater torque tool

Shaft material

Like our liners and housing materials, choosing the correct shaft material is essential to a successful underwater linear system. Stainless steel is a common choice for these applications due to its corrosion resistance, but there are various types of stainless steel to choose from. It’s also important to choose a shaft and bearing material that will work well in tandem, rather than choosing each separately with no consideration for how they will pair together.

igus offers shafting in 316 stainless steel, which provides significant resistance to corrosion when submerged in water and is often the preferred choice for underwater applications. Our J, X and A160 liners all perform well and see little wear when used against stainless shafting. Our pillow blocks also come in 316 stainless along with our iglide A160, X, or J liners. Each of these materials has a low moisture absorption rate and pairs well with stainless steel shafting.

Aluminum shafting can also be used in underwater applications. If an aluminum shaft is fully hard anodized, this will offer the most protection against oxidation as seen in our salt spray test results above. While steels will rust and can flake away with exposure to water, aluminum will oxidize, creating a white protective layer over the raw aluminum. Unlike rust, this does not flake away and will remain as a protective barrier against water. For applications where aesthetics is not a factor, oxidation may be a tolerable result to water exposure for added benefits such as cost savings and reduced weight. 

Driving solution

Most linear systems will require a driving component. In some cases, primarily where the application only experiences splash water, this can be handled manually. However, many applications will require a motor to drive the system. Choosing a motor for underwater applications may seem daunting, but with the help of an Ingress Protection Ratings Chart, the process is made much simpler.

The second number in an ingress protection rating (example: the 5 in IP65) conveys the level of protection a motor offers against water, with 1 offering the lowest level of protection and 8 offering full protection against water immersion for long periods. 

Motors being considered for underwater use should have special casings that, while more expensive, leave no open gaps or pores in the surface, preventing ingress and leakage. Components entering the motor such as the shaft or power source should be sealed. 

Both IP65 and IP68 NEMA23 motors are available from igus, capable of operating in splash water and fully submerged up to 10m depths respectively. These motors are easily paired with our drylin® linear actuators, along with most common motor controls, to create reliable and effective underwater systems. 

igus also offers online configurators with options to select environmental factors including splash water and underwater to help narrow searches down for your specific needs.

Environmental impact

Sustainability is a leading consideration in all mechanical applications, not just underwater. However, underwater applications are one area where sustainability is especially important due to the impact of all bodies of water on our day-to-day life. Choosing components that protect the environment should therefore be a top concern for designers working on underwater systems.

drylin components offer multiple environmental benefits over alternative solutions like ball bearings, but the biggest is the lack of lubricating grease. drylin systems are completely dry-running and utilize embedded solid lubricants rather than grease. This eliminates the possibility of grease leaching into the surrounding water and causing environmental damage.

Application example: Remotion underwater ship treatment robot

drylin bearings are also particularly quiet in operation, which can help avoid the disruption of nearby wildlife. The plastics used in drylin products are also free of PFAS to prevent the harmful chemicals from entering the environment, where it becomes nearly impossible to contain or remove them.

White Paper: PFAS in focus: Planned restrictions, challenges, and solutions for the industry

Conclusion

Designing underwater linear systems requires a significant amount of preplanning to account for the variables not present in other applications. Understanding the materials you’re using, the requirements of the application, and how the system could impact the environment are all essential to the successful design of linear systems in underwater environments.