Editor’s Note: This is the second in a multi-part series on materials used in fluid power fittings. We will cover carbon steel, stainless steel, brass and plastic.
By Josh Cosford, Contributing Editor
Brass is an alloy primarily made of copper and zinc, and depending on the formulation, it may also contain small amounts of lead or other elements to enhance machinability. The specific composition of brass gives it several advantages for fluid power fittings, particularly where reliability, cost, and corrosion resistance are key considerations.
One of the main benefits of brass fluid power fittings is their excellent corrosion resistance. Unlike carbon steel fittings, which are prone to rusting without protective coatings, brass naturally resists corrosion in most environments, including exposure to water, air, and many hydraulic fluids. This makes brass an ideal material for pneumatic systems, water-based hydraulic fluids, or systems operating in humid or corrosive environments. In fact, brass is still the number one alloy option for DOT-approved brake lines, making them popular in the on-highway machinery industry as a whole.
Compared to steel, cast iron, and stainless steel, brass is relatively soft and extremely easy to machine. Because of the low wear the brass material exerts on CNC tooling, it’s one of the only materials where fittings are machined from billets rather than castings or forging blanks. CNC machines can work using high feeds and speeds, making them very economical to produce in high quantities.
Brass is primarily used in pneumatic systems when strictly discussing fluid power, although it is suitable for low-pressure hydraulic systems. The soft nature of the material and its thread are not up to the task of high-pressure hydraulics, not because the fitting is likely to blow up, but because the threads are likely to fail. A failed thread has a chance of popping out of the port if the threads sheer entirely, but it’s more likely to simply leak.
Brass fluid power fittings are more frequently equipped with NPT threads. Because high-pressure, leak-free operation is not a primary factor for pneumatic systems, there’s no reason to use positive sealing such as O-ring boss or BSPP. In many cases, with reasonable installation torque, brass fittings with NPT frequently do not require thread sealing technology, especially if both the male and female thread are brass. The malleable nature of brass allows for enough deformation to seal well the first time, although subsequent reinstallations could use sealant to be safe.
Pneumatic fittings are unique in the fluid power world because they use push-to-connect fittings and compression fittings. The ferrule-style compression fitting uses a brass ferrule installed around the OD of the tube, which a brass nut compresses into the fitting body. However, compression fittings are being displaced by one-touch push-to-connect fittings, which offer excellent sealing with an easy-to-use design.
Brass is not without limitations, of course. Being so soft, it’s easy to over-torque the fittings, not just potentially ruining the threads, but risking snapping the fitting in half entirely. When aligning the fittings to a necessary orientation, be careful not to tighten excessively. Also, brass is more likely to erode over time simply from fluid flow, especially if there is a caustic fluid passing through. Despite the minor limitations, brass is one of the most popular fitting materials for pneumatic systems, and rightfully so.
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