Hoses and couplings are key components of every hydraulic system. They transfer hydraulic fluid from the pump to machine components such as valves, motors and actuators that use fluid pressure and flow to generate the machine motion and forces needed to do their jobs.
The importance of selecting or replacing hydraulic hoses and fittings is often overlooked ...... until problems arise.
Before selecting a hydraulic hose assembly, engineers must consider several factors.
Hoses are manufactured and tested according to industry specifications such as SAE and EN. Engineers should be familiar with the equipment specifications associated with the hoses they design. They provide guidelines for sizes, material properties and minimum performance characteristics for the major types of hoses and couplings.
Tubing carries fluids and is usually made of rubber compounds and plastics. It is critical that the tubing be able to resist the fluids it carries and comes in contact with; chemical permeation; and high and low temperatures.
The coupling makes a leak-free connection to the hose (coupling end) and connects the hose to different components (terminals) in the hydraulic system. Most couplings have two parts, a stem and a collar.
Couplings on the market include one-piece and two-piece designs. In the one-piece version, the stem and collar are pre-connected or secured together as one component. The two-piece design has separate stems and collars that are connected by crimping.
There are several hydraulic system characteristics that help engineers design hoses and fittings to maximize service life: size, temperature, application, material and pressure.
Size: The two key dimensions of a hose are the inside diameter (ID) and the overall length (OAL). The ID must be sized to keep the fluid velocity within the recommended range. If the ID is too large or too small, it will alter fluid flow and impair machine performance by causing erosion, excessive pressure drop and power loss, excessive turbulence (heat generation), pump cavitation, or noise.
The OAL must be sized to provide proper routing. Too short will put too much pressure on the hoses and fittings and prevent them from bending and stretching due to pressure pulses. If the hoses are too long, they may rub against each other or against nearby components, or get caught on something.
When replacing hoses, you can determine the ID by checking the label. Do not use the hose OD to identify the inside diameter of the hose if it is painted or worn. Hose OD varies by construction and manufacturer and is not directly related to ID. A better option is to cut the hose and measure the inside diameter. Remember to record the overall length and fitting orientation before cutting the hose.
Temperature: Too high or too low a temperature can reduce hydraulic system performance and hose life
Overheating can cause the hose cover to crack and dry out. Internal tube cracks are also commonly found, but only after dissection of the failed hose. Cracked hoses no longer protect the reinforcement and should be replaced.
Hypothermia damage usually occurs when a hose bends below its negative temperature rating. This usually occurs when moving the machine before the hydraulic fluid is warmed up, or when removing and straightening a refrigerated coiled hose.
It doesn't take much extra heat to shorten the life of a hose. A good rule of thumb is that for every 18°F increase in the maximum temperature rating of a hose, the life of the hose will be reduced by 50%.
The preferred option for managing temperature is to ensure that the heat of the hydraulic system and external heat sources are kept within acceptable limits. The next best option is to choose hoses and fittings that can withstand extreme temperatures. Be sure to choose hoses with an upper limit well above the machine's operating temperature.
Another way to protect hoses from heat is to install insulating sleeves on the components.
Applications: Engineers need to document any special features of the machine they are designing, including safety and environmental risks, as well as the extreme conditions the hose is expected to handle.
Seventy-five percent of all hydraulic failures are caused by contaminated fluids. If left unchecked, particles as small as invisible can reduce the efficiency of a hydraulic system by as much as 20 percent.
If you are replacing a hose, ask some questions, not just replicate the original.
How did the original hose fail? Are there signs of casing wear or temperature cracks? Does the machine create pressure fluctuations, or is it more of a static application? Make sure you find the replacement hose that best meets the requirements of the application, not just the original part.
Over the past few years, TOPA has developed hoses and couplings that far exceed the performance and constructability of SAE specifications. These include higher pressure and temperature capabilities and greater flexibility, as well as the ability to bend twice as much as standard hose.
Materials: Most hydraulic fluids are petroleum-based. Others are water-based, water-glycol or synthetic-based (such as phosphate esters). In the past, hydraulic fluid spills contaminated the soil and polluted the water supply. As a result, the industry is shifting to more environmentally friendly fluids.
Since penetration can expose the entire hose assembly to fluid, it is important to check not only fluid compatibility with the tubing, but also with reinforcements, caps, fittings and seals. This also applies to assemblies handling special oils or chemicals.
The most critical hose and fitting material consideration is chemical resistance, so verify which fluids will flow through the hose. What fluids or gases may penetrate the tubing and potentially weaken the reinforcement? What materials will come in contact with the hose cover during normal use or cleaning?
What corrosive chemicals will contact the coupling bores, collars, and terminations? Do the terminations use O-rings or elastomeric seals? Are internal system fluids compatible with the seal assembly?
Do these chemicals come in contact with hoses and couplings continuously or intermittently? What is the concentration of the chemicals? Are their properties affected by temperature?
There are several resources that provide detailed information on the chemical resistance of materials. The first is the Chemical Resistance Guide, which describes the general properties of tubing and covering compounds.
The second resource is a chemical resistance chart. Most hose and fitting manufacturers list common chemicals and their compatibility with hose materials.
If you want to learn more, please visit our website MASTER.
The third resource is a hose and fitting catalog or engineering specification.
Pressure: Working pressure and burst pressure are the two most common pressure ratings for hoses.
Working pressure helps select the correct hose based on hydraulic pressure. System pressure should never exceed the working pressure of the hose. Burst pressure is the maximum pressure a hose can withstand before a catastrophic rupture.
For most hoses, the burst pressure is usually four times the working pressure.
Do you know of any other points to keep in mind when selecting and replacing hoses and couplings? Or comment if you have more knowledge to share, or contact our sales staff at -
NPT (National Pipe Tapered) style pipe threads have been widely used for over 100 years. NPT is a U.S. standard for tapered threads used on pipes and fittings. They are used to effectively seal pipes for fluid and gas transfer. The nominal pipe size can be identified by physically measuring the thread diameter, then subtracting 1/4″.
They are available in iron or brass for low-pressure applications and carbon steel and stainless steel for high-pressure.
NPTF (National Pipe Tapered Fuel) style connections are widely used in fluid power systems. They have a tapered thread by which a seal is made by deformation of the threads. NPTF Threads are measured at the thread diameter and subtracting 1/4-inch to establish the nominal pipe size.
NPSM (National Pipe Straight Mechanical) connections are also often found in fluid power systems. The female component incorporates a straight thread with an inverted 30° seat. The male component has a straight thread and a 30° internal chamfer. A seal is made by compression of the 30° seat on the chamfer. This is considered a mechanical connection. If an NPTF male is properly chamfered it will also seal with an NPSM female connection.
Learn more about NPT Fittings>>
Society of Automotive Engineers Thread (SAE)
SAE J Straight Thread O-Ring Boss (ORB) is recommended by the National Fire Protection Association (N.F.P.A.) for leak prevention in medium and high pressure hydraulic systems. The male connection is a straight thread with an O-ring. The female port has a straight thread and a machined surface to provide a smooth, flat surface (minimum spotface), along with a chamfer where the O-ring seats. It seals when the O-ring is compressed into the chamfer when mating the male connection. This is also considered a mechanical connection.
SAE J514 JIC/37° Hydraulic connections are common in most fluid power systems. Both male and female components have 37° seats. The seal is made by establishing contact between the male flared and female coned seat. This is also considered a mechanical connection.
SAE J512 45° connections are used in automotive, refrigeration and truck pipe systems. These connectors are typically brass material. The male and female connections have 45° seats, where the seal is make where the male flare and the female cone meet. This is a mechanical connection also.
NOTE dash sizes: -02, -03, -04, -05, -08, and -10 of SAE 37° and SAE 45° have the same threads, but NOT the same seat angles. Intermixing the two different types of fittings will result in leakage, so use care in measuring seat angles.
SAE J (ORFS) O-ring Face Seal connections are considered the best for leak control. The male connector has a straight thread and an O-ring in the face. The female has a straight thread and a machined flat face. The seal takes place by compressing the O-ring onto the flat face of the female, similar to the split flange type fitting. The threads maintain the connection mechanically.
SAE J512 Inverted connections are typically used in automotive systems. The male connector is either a 45° flare within the tube fitting or a 42° seat in the machined adapter. The female incorporates a straight thread with a 42° inverted flare. The fittings are sealed at the flared surfaces. These threads also maintain a mechanical connection.
SAE J518 4-Bolt Flange* There are two pressure ratings for these connections: Code 61 which is considered the standard series and Code 62 which is the PSI series. The design is the same for each series, yet the flange head diameters and bolt hole spacing are larger for the PSI high pressure Code 62 connection. The female port of the fitting is a smooth, un-threaded port with four bolt holes set in a rectangular pattern around the port. The male is a flanged head with a groove for an O-ring and either split or captive flange halves and bolt holes which match the port. The seal is made where the O-ring is compressed between the flanged head and the flat surface of the port. The connection is held by threaded bolts.
*Excluding bolt sizes, SAE J518, JIS B , ISO/DIS and DIN are interchangeable.
British Standard Pipe (BSP) and BSPT (tapered) connections are comparable to NPT, except most sizes have a different thread pitch, plus the OD’s and thread form are close, but not the same. Sealing takes place by distortion of the threads. Because of this, thread sealants are recommended when securing these connections.
BSPP (parallel) male connection is comparable to NPSM male except most sizes have a different thread pitch. A captive seal is made using metal to metal angled surfaces or a combination of metal to metal and an O-ring. This type of connection is very similar (but not interchangeable) with the American NPSM male. The female swivel BSPP has a tapered nose flareless swivel where the seal occurs on the cone seat of the male connector.
Note: the thread sizes are often expressed as fractional dimensions preceeded by the letters “G” or “R”, where “G” represents a parallel thread and “R” represents a tapered thread. Example: BSPT 5/8-14 can be designated by R 5/8 and BSP 1/16-28 can be designated by G 1/16.
JIS Tapered Pipe (PT) has metric threads per JIS B . These are JIS tapered threads and are comparable to the design of BSPT connections in their dimensions and appearance. JIS tapered thread connections are interchangeable with BSPT connections.
JIS 30° Male Inverted Seat connections are parallel pipe threads per JIS B . JIS parallel connections are comparable to BSPP connections. JIS parallel thread connections are interchangeable with BSPP connections.
JIS 30° Female (Cone) Seat are parallel pipe threads per JIS B . Japanese JIS 30° flare connections are comparable to American SAE 37° flare connections in application and sealing principals. Yet, JIS 30° flare angle and dimensions are different with threads that are similar to BSPP.
JIS B 4-Bolt Flange connections are frequently used in fluid power systems. There are two pressure ratings for JIS B 4-Bolt Flange fittings:
1) Type I Code 61 is the standard series 4-Bolt Flange
2) Type II Code 62 is the PSI series
Each design concept is the same, yet the flanged head diameters and bolt hole spacing are larger for the Type II PSI connection. Metric and inch bolts are each used with these connectors. The male connector has a flanged head with a groove for seating an O-ring and either a captive flange or split flange with bolt holes to correspond to the port. The female port of the fitting is a smooth, un-threaded port with four bolt holes set in a rectangular pattern in around the port. The seal is made where the O-ring is compressed between the flanged head and the flat surface of the port. The connection is held by threaded bolts.
JIS 210 Kgf/cm2 4-Bolt Square Flange incorporates a JIS 4-bolt square flange connection which is comparable to SAE 4-bolt flange connections with one difference – the flange itself is different and the JIS bolt pattern is square.
ISO/DIS 4-Bolt Flange is another common connection found in fluid power systems. There are two pressure ratings for this connection- Code 61: PN 35/350 bar which is considered the standard series and Code 62: PN 415 bar which is the high pressure series. They maintain the same design, yet with the bold hole spacings and flanged head diameters being larger on the PN 415 bar high pressure connection. Inch or metric bolts are found in these connections, however there is an “M” stamped on the port if metric bolts are to be used. The female port of the fitting is a smooth, un-threaded port with four bolt holes set in a rectangular pattern in around the port. The male is a flanged head with a groove for an O-ring to seat and either split or captive flange halves and bolt holes which match the port. The seal is made where the O-ring is compressed between the flanged head and the flat surface of the port. The connection is held by threaded bolts.
ISO Port and Stud Ends with ISO 261 Threads and O-ring Seal though it is similar to the SAE J514 Straight Thread O-ring Boss (ORB), this type of connection incorporates metric threads. The male connector has straight threads with an O-ring. The female port is also straight threads machined surface to provide a smooth, flat, accurately located surface (minimum spotface), along with a chamfer where the O-ring seats. It seals when the O-ring is compressed into the chamfer when mating the male connection. This is also considered a mechanical connection.
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