Hose selection must ensure compatibility of the hose tube, cover, couplings, and O-rings with the fluid used. The characteristics shown in Table 2 are for some of the most common hose stocks. Consult the individual hose manufacturer's chemical resistance tables to determine hose compatibility.

Table 2: Comparison of hose stock types
Chemical name
Neoprene
Nitrile
Butyl
Hypalon
EPDM
CPE
ASTM-SE designation
SC
SB
R
TB
R
None
SAE J14 & SAE J200
BC
BG
AA
CE
AA
None
Flame resistance
Very good
Poor
Poor
Good
Poor
Good
Petroleum base oils
Good
Excellent
Poor
Good
Poor
Very good
Diesel fuel
Good to excellent
Excellent
Poor
Poor
Poor
Very good
Resistance to gas permeation
Good
Good
Outstanding
Good to excellent
Fair to good
Good
Weather
Good to excellent
Poor
Excellent
Very good
Excellent
Good
Ozone
Good to excellent
Poor for tube
Good for cover
Excellent
Very good
Outstanding
Good
Heat
Good
Good
Excellent
Very good
Excellent
Excellent
Low temperature
Fair to good
Poor to fair
Very good
Poor
Good to excellent
Good
Water-oil emulsions
Excellent
Excellent
Good
Good
Poor
Excellent
Water-glycol emulsions
Excellent
Excellent
Excellent
Excellent
Excellent
Excellent
Diesters
Poor
Poor
Excellent
Fair
Excellent
Very good
Phosphate esters
Fair for cover
Poor
Good
Fair
Very good
Very good
Phosphate ester based emulsions
Fair for cover
Poor
Good
Fair
Very good
Very good

Variables such as high temperature, fluid contamination, and concentration will affect the chemical compatibility of the tube and fluid. Most hydraulic hoses are compatible with petroleum-based oils, but new environmental fluids may present a problem for some hoses.

Caution also must be exercised in selecting hose for applications that require specialized oils or chemicals that could permeate can through the hose tube. Permeation can manifest itself as either blisters between the tube and cover, created by trapped liquid or gas, liquid on the surface of the cover, or the escape of the fluid or gas into the atmosphere.

Hybrid hose stocks

Tube and cover stocks may be upgraded to enhance performance. The hybridization of rubber and synthetic thermoplastic compounds into abrasion-resistant hydraulic hose offers several enhancements over traditional hose materials, such as Neoprene and Nitrile.

A primary cause of hose failure on fluid power equipment is cover abrasion resulting from cuts, friction caused by other moving parts, or from mechanical impacts. For example, changes in fluid pressure can cause hoses to either elongate or contract, depending on their construction. The elongation or contraction can cause the hose to repeatedly impact adjacent machine elements or other hoses that eventually will wear away the hose cover to expose the underlying reinforcement. Exposed hose reinforcement is susceptible to rust and accelerated damage leading to failure.

Hybridized covers made of thermoplastics have a slick surface with a low coefficient of friction. This results in a cleaner appearing cover that resists dirt and oil residues. Some abrasion-resistant hose covers-have been tested and found to last up to 300 times longer than standard rubber covers. This feature increases service life, lowers maintenance, and eliminates the need for costly hose protectors such as guards, sleeves, and bundling.

Also under development are elastomeric tubing compounds that are more readily compatible with a new generation of environmentally friendly hydraulic fluids and additives.

Choose the right connections

The couplingtohose interface must be compatible with the hose that is selected. Follow the hose manufacturer's coupling recommendations only. The proper mating thread end must be selected to ensure a leak-free seal can be made to mating components.

Coupling selection can be more complicated if the end connection undergoes a high degree of motion, vibration, or both. Split flange couplings — or other couplings that use an O-ring for sealing — perform well under vibration. Couplings that use O-rings also are preferred in applications that undergo extreme temperature fluctuations.

Threadless connectors are becoming an attractive zero-leak alternative to conventional threaded fittings commonly used in hydraulic systems. The male and female halves simply push together — without wrenches or special assembly tools — to form a leak-free connection.

Several manufacturers have introducedthreadless connectors that yield the performance of Oring seals, but eliminate the cost of O-ring installation for hydraulic systems with working pressures to 6000 psi. As with JIC fittings, these connectors also provide a metalto-metal seal, but they don't have to be tightened to a specific torque.

Threadless connectors are used at the hydraulic port or manifold on steering units, valve blocks, pumps and motors. They offer several key benefits to equipment manufacturers:

  • leak-proof connections that reduce the cost of warranty claims and fluid leaks from equipment used in the environmentally sensitive applications
  • easier, quicker connections that reduce assembly costs
  • tactile feedback and visual confirmation that the connection has been made
  • assurance that connections cannot be accidentally made with other threadless couplings
  • easy serviceability, and
  • easy replacement in the field with traditional and widely available threaded connectors.

Some types of threadless connectors eliminate the need for adapters, which not only saves space, but somewhat offsets the higher cost normally associated with these types of hose end fittings.

The biggest advantage of threadless connectors is the reduced time and expense needed to install hydraulic hose and tubing assemblies and a greater accessibility for subsequent maintenance.

Installations with conventional threaded fittings can take several minutes or even hours, and they are subject to leaks because of tightening inconsistencies and other variables. On the other hand, threadless connectors allow even confined, difficult-to-reach connections be completed in minutes.

Instead of screwing on the threaded connector and tightening it with a wrench, threadless connecting halves — the male end and the adapter — simply push together to form a leak-free connection. A snap ring positively engages the male and female halves, without any assembly tools, although some designs do require a special tool for removal.

Avoid using couplings and hoses from different manufacturers interchangeably. This is because different manufacturers use different materials, which can result in a variety of tube styles. SAE allows a wide range of materials. An improperly matched or coupled hose will likely fail, causing downtime and injury. Therefore it is important to follow only the crimp and assembly recommendations of the manufacturer of the products being used.

Temperature considerations

Fluid and ambient temperatures must be considered when selecting hose and couplings. The hose and couplings must be capable of withstanding both minimum and maximum temperatures of the system. All hoses are rated with a maximum working fluid temperature of 200° to 400° F. Using a hydraulic hose at a temperature of 18°F above maximum rated temperature of the hose may cut the hose life in half.

On the other hand, minimum temperature must also be considered. Depending on materials used, hose may accommodate temperatures as low as -65° F (Hytrel and winterized rubber compounds) or as high as 400° F (PTFE).

When hoses are exposed to high external and internal temperatures concurrently, there will be a considerable reduction in hose service life. Insulating sleeves can help protect hose from hot equipment parts and other high temperature sources that are potentially hazardous. In these situations, an additional barrier is usually required to shield fluid from a possible source of ignition.

For more information, visit www.gates.com/fluidpower or call (800) 777-6363.