As most of us know, crimping is the most frequent way to assemble a hydraulic fitting to the hose. But before even selecting a fitting and hose combination, we first need to ask ourselves what the proverbial S.T.A.M.P. is: size, temperature, application, materials/media, and pressure.
After establishing S.T.A.M.P., let’s ask ourselves these fundamental questions:
#1 - One- or two-piece hose fittings?- One-piece hose fittings have a ferrule that is pre-crimped onto the hose tail. The primary advantage is that the collar will not slip out of alignment. One-piece hose fittings are good for basic applications and quick assembly.
- Two-piece fitting ferrules allow greater flexibility in hose selection and connection, including 1SN, 2SN and multi-spiral hoses which permit multiple two-piece fitting and ferrule options to accommodate the specific application. SS 316 two-piece hose fittings are the best selection to ensure corrosion resistance and long service life, as well as for critical and exceptionally high-pressure applications, particularly when there is the possibility of extreme hose vibration and/or pressure surges.
#2 - Are the fittings and hose compatible?
The connection is the weakest link in the hose assembly. First, it’s imperative that the fitting and hose have compatible pressure ratings because the working pressure of the system is based on the component with the lowest rated pressure. So, you wouldn’t match a low-pressure brass fitting with a 5,000 PSI spiral-wire hydraulic hose. It would be like wrapping two sections of garden hose together with packing tape — it’s going to burst as soon as you turn the spigot up all the way. Of course, it won’t be as dangerous as the hydraulic example.
#3 - To skive or not to skive — that is the question!
Skiving is the removal of the outer cover at the end of a hydraulic hose where a fitting will be crimped. The outer rubber of the hose is peeled off to accommodate the fitting for medium pressure applications using a special tool or automatic skiving equipment. This is considered external skiving (or single skive) for the ferrule and fitting to mate correctly. However, many high-pressure applications require the hose ID to be skived along with the outer cover so the hosetail will have a more secure fit.
- One-Piece Hose Fittings
One-piece hose fittings have a ferrule that is pre-crimped onto the hose tail. The advantage (is) being that the collar will not slip out of alignment. This style also ensures the correct ferrule is matched to the correct fitting. One-piece hose fittings are good for basic applications and quick assembly.
The hose is not skived when a one-piece fitting connection is used. It is ready to assemble without requiring the skiving process, and therefore the cover is not removed from the hose.
- Two-Piece Hose Fittings
Higher pressure applications may require internal and external skiving of the hose to ensure a more secure fit of a two-piece fitting, which would typically be made of 316 stainless steel.
Two-piece fittings have a ferrule which allows greater flexibility in hose selection and connection, including 1SN, 2SN and multi-spiral hoses which allow for multiple two-piece fitting and ferrule options to accommodate the specific application. Stainless 316 two-piece hose fittings are the best selection to ensure corrosion resistance and long service life, especially in chemical and offshore applications.
Two-piece hose fittings are used when severe conditions are present near the fitting components and are extremely important for providing a powerful assembly of the fitting and hose. In these high-pressure applications, the hose is skived in the inside and outside (two skive), creating a metal-to-metal connection on both the inside and outside of the hose.
#4 - What fitting material should you select?
Like hoses, fittings are made from many different materials and selected for hose compatibility and the specific application. Fittings are most commonly made of brass, carbon steel, stainless steel, or plastic.
Brass fittings are not as strong and durable as other materials, such as stainless steel. However, they are suitable for non-severe conditions. Brass fittings can be used in temperatures ranging from -65°F (-54°C) to 400°F (204°C) and are capable of withstanding pressures of up to 3000 psi, though lower pressure ranges are usually recommended.
Carbon steel fittings are very strong and durable, possessing a high resistance to heat-- they can withstand temperatures ranging from -65°F (-54°C) to 500°F (260°C). Made from an alloy of iron and carbon, they are usually alloyed with other metals to increase strength and durability.
Stainless steel fittings are commonly used in very extreme applications, capable of withstanding temperatures ranging from -425°F (-254°C) to 1200°F (649°C) and pressures up to 10,000 psi. For highly critical applications, stainless steel fittings can be made with special characteristics rating them for up to 20,000 psi. Stainless is also the best choice for highly corrosive environments.
Plastic fittings are typically more resistant to corrosion but are much less durable than their metal counterparts. This makes them less useful in hydraulic applications, though they are usually used in pneumatics.
To learn more about the importance of proper instrumentation fittings, check out our BluPrint!
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