Tips for selecting and replacing hydraulic hose assemblies

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.?

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 [email protected] Whatsapp 0086-18033758486

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