By: David Marlowe 

Because of the numerous advantages seamless steel tubing possesses over other tubing—apart from the added cost—seamless steel tubing is widely used in hydraulic systems. Seamless steel tubing is easier to bend than steel pipe, so it does not require the number of fittings and can be reused. High-volume applications are probably one of the only limited areas to the use of tubing.

Tubing size is measured on the outside diameter of the tube. All sizes are available in a variety of wall thicknesses. The application and use of tubing, (material, diameter and maximum pressure) will determine the wall thickness of the tubing used. Sizes range from 1⁄8 to 2 in. in fractional sizes.
Considering the application, internal pressures, weight, environmental conditions and whether you are going to use flared or flareless tubing are all considerations you must know to help you in material selection. Good quality tubing must be selected to ensure a good seal.

Tubing comes in a variety of materials, including:

• Seamless carbon steel, which is the most commonly used. It has a tensile strength to 47,000 psi.
• Stainless steel is used when external conditions require noncorrosive materials. It also has a tensile strength to 47,000 psi.
• Plastic can be used in low pressure applications.
• When selecting a material for your specific application, don’t focus only on the price tag. The manufacturer’s information is the most reliable and should be used when selecting the material based on allowable stress and working pressures.

Fittings
Tubing does not use threaded fittings. The fitting attachment is accomplished by either a component or another tube. The attachment is made by the following:

• Flared fittings are used primarily in low to medium pressure applications.
• SAE standard J533B covers specifications for both single and double flared (37° and 45° flare) tubing. Double flared tubing is used with thin walled tubing.
• SAE 37° angle is the standard angle used for hydraulic system tubing.
• SAE 45° angle is used with soft copper tubing for applications such as refrigeration, fuel lines, etc.
• The maximum wall thickness recommended by SAE standard should be checked prior to flaring a tube.
• Flareless Fittings, also commonly known as compression fittings, are used primarily in medium- to high-pressure applications.
• As system pressures increase, you require a thicker wall thickness. Higher pressures also affect the ability to use a flared type fitting.
  • Pressure capacity can be calculated using the following formula:
    • Pressure (P) = 2 x S x T / D,

Where
P – pipe internal pressure in psi
T – wall thickness
D – O.D. tube
S – stress on tube material in psi
Flareless type tubing fittings are available in
two common styles.

• Ferrule or Sleeve Type: The ferrule is used to seal against the tapered seat of the fitting and the tube. When tightening the nut a sealing pressure is felt on sealing surfaces causing the tube to deform
• O-ring type: Sealing is obtained by the installation of an internally mounted O-ring. The O-ring type uses a split ferrule to clamp the tube.
  It is important to remember to never mix from different manufacturers and always refer to the manufacturers specifications (torque recommendations) prior to installation.
• Welding fittings are used primarily in high pressure, vibration and fixed applications.
  Butt and sockets welds are both used to connect the fitting to the tube, so material type wall thickness and welding skills must be considered when using a welding fitting.

Flexible hose

• 

  As stated previously, pipe and tubing are rigid fluid conductors and therefore not suitable when there is movement of one end of the conductor while the opposite end is fixed.

• The benefits of flexible hose over rigid pipe and tubing is that flexible hose is simple to route, withstands vibration and thermal expansion or contraction.
• Flexible hose consists of the following:
  • Inner tube
  • Reinforcement
  • Outer protective cover

The inner tube is made of either oil resistant synthetic rubber or thermoplastic material. Reinforcement amount and type is determined by the hose’s pressure rating. As the requirement of reinforcement increases from a single fiber/wire braid to multiple fiber/wire braids, the hose loses its flexibility. Outer protective cover is manufactured to be oil and weather resistant. Primarily, the outer protective cover is made of a synthetic rubber or thermoplastic material. The Society of Automotive Engineers (SAE) has set standards for hydraulic hose specifications. Most manufactures of hydraulic hose meets or exceeds SAEJ513 requirements.

Final note: Because the hose meets standards it is extremely important to carefully match the end fittings to the hose selected