Many systems such as marine autonomous vehicles need actuators that will work reliably in a wet environment, whether those actuators will be submerged or at the surface. It is tempting to use one of the hundreds of "waterproof" hobby servos. These are inexpensive, widely available in a wide range of torques and speeds, and have been proven to be extremely reliable in dry conditions. But if your application involves anything more than just an occasional light splash of water, this will not be a viable solution. The vast majority of these "waterproof" servos are actually water-resistant, but they will not survive prolonged exposure to water or submersion in water, especially if that water is salt water.
So how can we make a servo truly waterproof? There are three main leak points that need to be sealed: the output shaft, the cable entry, and any joints or seams in the servo case.
All servos have an output shaft that rotates, usually with an arm and pushrod attached to the output shaft. To seal against water intrusion, the output shaft needs a “dynamic seal” (a seal that’s on a moving component). Because the output shaft rotates relatively slowly, it's actually fairly simple to seal the output shaft using a simple O-ring or "quad-ring" (which is just an O-ring with a four-lobed cross-section). An O-ring or quad-ring seal can work reliably under hundreds of psi of pressure, but the geometry and surface finishes of the output shaft and servo case have to be within tight specs. And the effectiveness of a seal depends not only on the seal itself but on the design of the "gland" or groove in which the seal resides. For this reason, a robust dynamic seal on the output shaft inevitably requires a larger amount of physical volume where the output shaft exits than a typical hobby servo provides. Most if not all "waterproof" hobby servos attempt to fit the dynamic seal into a standard servo case, thus compromising the robustness of the seal. Blue Trail Engineering's waterproof servos utilize quad-ring seals residing in a properly-sized and tightly-toleranced housing, with the titanium output shaft being supported by dual ball bearings to keep the shaft concentric with the seal.
The cable entry is another possible leak point. In order to be truly waterproof, it is not sufficient to merely place a blob of sealant where the cable exits. This may work for occasional splashes or even short-term submersion, but is not robust in the long term. For details on how to make a truly waterproof electrical connection, visit this Tech Tip. Again, the hobby servos advertised as "waterproof" do not dedicate the required amount of space (or expense, frankly) for a truly waterproof cable entry.
The last possible leak point is the joint or seam in the servo case. Sealing these joints requires a "static seal", so named because there is no motion between the mating parts of the case. Static seals are quite simple, usually consisting of an O-ring in a groove. However, as with any other sealing situation, the geometry of the groove is key. And again, a typical hobby "waterproof" servo does not dedicate the amount of space required for a proper O-ring goove. Blue Trail Engineering's waterproof servos use 3.5 mm thick cases which give enough room for a properly-sized O-ring and O-ring groove (and also make for an extremely robust package).
For further information about O-rings and O-ring groove design, take the time to read the Parker O-ring Handbook. This is an absolutely essential guide for anyone working with waterproof components.
In addition to the problem of sealing a waterproof servo, another key consideration is materials. For long-term exposure to water, especially salt water, the materials must be chosen very carefully. Any non-stainless steel will almost certainly corrode, and even common stainless steel will corrode over time. In the marine industry, 316 stainless steel is considered to be one of the few acceptable stainless steels. Titanium, although expensive, is perhaps the ultimate metal for use in salt water, as it is virtually corrosion-proof. Among plastics, nylon is an extremely common plastic but is typically a poor choice for high-moisture applications since it absorbs water, swells, and softens. A close look at "waterproof" hobby servos reveals that the materials used for the fasteners, the servo arms, the servo cases, and the output shafts (with the exception of titanium output shafts) are not suitable for long-term use in water.
In the above discussion, I do not intend to be critical of hobby servos. The truth is that a high-quality hobby servo is an extremely high-performance, high-reliability device. But to be truly waterproof, it needs to be enclosed in a dedicated case with proper seals, an appropriate electrical connection, and the proper materials. The servos we provide through Blue Trail Engineering take advantage of the performance and reliability of the R/C servo by packaging it in a proper waterproof enclosure using proper materials, at a reasonable cost.