Hydraulic lines used in pressurized hydraulic systems are: hoses, tubes and fittings. Hoses are rated by their inside diameter (ID) while tubes are rated by their outside diameter (OD) and wall thickness. Tube fitting sizes are based on the tubes outside diameter and thread size. Hose fittings are based on the hose size (hose ID) at one end and the thread size at the other.
In general, there are four types of tubes.
Hydraulic (fluid line) tubing is a metal tube. The most commonly used metals for hydraulic tubing are: low-carbon steel, stainless steel, alloy steel, copper, aluminum and copper-nickel alloy. Steel tubing is normalized to achieve softness necessary for ease of bending and flaring. Low-carbon steel tubing has one or more corrosion resistant coatings. Hydraulic tubing is produced to OD and wall thickness dimensions.
Pneumatic tubing can be made from metal or non-metal materials: copper, aluminium, nylon, polyethylene or PVC. Nylon tubing is the most popular because of its flexibility and low cost.
Mechanical tubing is steel tubing for structural applications. It can have round, rectangular or a square shape.
Cylinders tubing is mechanical tubing with a finished inside diameter (ID) manufactured to be ready to use for hydraulic cylinders. The ID has tight
tolerances and surface finishing requirements. The cylinder tubing is produced to OD and ID dimensions.
Mechanical and cylinder tubing will not be discussed in this book because they are not used for connectors. When the term “tubing” or “tube” are further used, they will refer only to hydraulic fluid line tubing.
Hydraulic line selection is an important part of the design process.
There are two important parameters- fluid velocity and maximum pressure- taken under account when selecting the size of the connectors. Few other parameters: temperature, construction and fluid type must also be taken into account. In addition, the effect on the system stiffness must be considered when selecting the type of connector. Elasticity of the flexible hoses reduces the dynamic stiffness of the system.
Pressure
All hydraulic fluid lines have rated pressure. Lines (for medium, normal and high pressure systems) are rated with a safety factor of four. That means that their safety (burst) pressure is four times the working pressure. Pressure rating varies in accordance with the different materials used. For example: copper tubing rating is about 10 MPa, steel tube rating is more than 25 MPa.
Fluid velocity
Fluid line size is selecting on the basis of fluid velocity inside the connector. The fluid velocity affects the pressure drop and the fluid Reynolds number. In general, pressure losses decrease when the diameter increases. In high- pressure systems big diameters are not economical because of the increased cost of the connectors, fittings and all other components. Bigger pressure hoses are very stiff, require bigger bending radiuses and take more space. Also, it is more difficult to deal with larger fluid volume because it would require a larger reservoir. On the other hand, exceeding recommended maximum velocity of the fluid may cause turbulence in the flow. Small suction lines can increase the vacuum and create cavitation in the pump inlet. The cavitation damages the pump surfaces and causes noise and mechanical damages.
The optimum recommended velocities in the connectors are: • Suction line v= 0.5 to 1.5 m/s
• Pressure lines
Maximum Pressure Recommended velocity p < 50 bar (5MPa) v = 4 m/s
p = 50 to 100 bar v = 4 to 5 m/s p = 100 to 200 bar v = 5 to 6 m/s p > 200 bar v = 6 to 7 m/s • Return lines v= 2 to 3 m/s
It has been estimated that 80% of hose failures are caused by external physical damage to the hose. In order to increase durability of the hose, the hose manufactures use special hybrid compounds for the cover material instead of standard rubber covers. This more durable compound increases the service life, lowers the maintenance, and eliminates the need for costly hose protectors such as guards and sleeves. The pressure ratings for hoses are provided by the manufacturers.
Design principles
• Maximum pick pressure in the system must be below maximum rated pressure for the hose. Hose burst pressure should not be used for hose selection. The burst pressure is only for safety purpose. • Hoses used for the suction line must be able to withstand vacuum
and pressure;
• Hose routing must ensure minimum length and number of bends, avoid twisting and avoid external heat sources;
• Hose sizes (inside diameters) are selected according to recommended velocity;
• Hoses have to be chemically compatible with the fluid in the system; • Hoses must be protected from rubbing against metal edges or hard
objects, snagging, cutting, pulling, bending and twisting; • Use proper end fitting;
• Hose cleanliness has to match the system’s cleanliness level. After a hose is cut to size and fittings are assembled, the hose assembly should be cleaned and plugged.
• It is recommended that mating connectors (tubing, end hose connectors and the fittings) are made from the same material.
Hose ratings and constructions
Hose pressure ratings used in the industrial truck applications are:
High-pressure hose is used for pressures of 20 bar to 400 bar. These type hose has three components: internal tube, steel mesh layers (four or six) and cover. The tube is made from synthetic rubber. Steel mesh is made from high-tensile still wire. The cover could be thermoplastic, synthetic rubber or fabric mesh.
Medium-pressure hoses are used for pressures of 20 bar to 200 bar. This type hose has the same three components. The only difference is that the steel reinforcement consists of one layer still mesh.
Low-pressure (suction or return) hoses are used for pressures from 0.6 bar to 20 bar. These hoses should be rigid enough to resist compression when the absolute pressures are below 1 bar (the hoses experience vacuum). This type has synthetic non-rigid reinforcement.
High, medium and low-pressure hoses are rated for temperature range from -40 to 100 ºC.
Connector restraining
Restraining is achieved by clamping the connectors to rigid surfaces of the machine. Proper clamping increases the life of the tube and hose assemblies.
Tube clamps can have non-metal (rubber or nylon) dampeners, which protects the tube from vibrations and mechanical shocks.
Hose clamps are usually metal. The purpose is to protect the hose from twisting and rubbing against other surfaces or edges.
Tubes and hoses can be attached to each other by a floating clamp. This clamp ensures that connectors are not rubbing against each other by creating a space between them. The clamp is called “floating” because it is not fastened to a rigid structure.
Fittings and Couplings
The couplings in mobile forklifts have to meet the requirements for high flow and pressure, high and low temperatures and different fluids. Coupling connection has to be able to handle pressure pulsations and spikes higher than the rated pressure. These spikes appear when the work tool (the forks) hit a hard object or cylinder piston hits the hard stop at the end of the stroke. Tube fittings and adapters are cycle tested for endurance to 133% of the work pressure and with minimum torque. If the fittings are over tightened, their cycling endurance is reduced.
Most commonly used types of couplings are:
37° flare (JIC) threaded couplings is one of the most commonly used. It has good performance and low cost. Both mating parts have coned surfaces which fit against each other to form a seal. Because the sealing is ensured by metal-to-metal contact, this type is torque sensitive. If they are under-tightened, there is leak path between the sealing surfaces. Over- tightening can damage the treads and also causes leaks. It is recommended in extreme low or high temperature applications.
O-ring coupling is a very popular threaded connection. There are O-ring face seal, O-ring ISO and O-ring SAE. The difference between ISO and SAE are: at machined ports ends, ISO uses metric thread, tube size and hut hex for metric wrenches while SAE has inch thread, tubes and hex. The male component has a rubber O-ring which is compressed against the machined seat in the female component. This rubber-to-metal seal is less torque sensitive. It is recommended for high-vibration and high-pressure applications. The O-ring coupling is not recommended for extreme temperature conditions because rubber ages and deteriorates at very low or high temperatures.
Flanged connection (four-bolt split flange type) is designed to avoid the use of threaded connections. The fitting has two parts: a flanged head fitting and a flange clamp (one-piece or sprit version). Flange connection has high pressure capability because of the larger sealing surface and the lower torque requirements. Flange heads and clamps are made of plated or coated carbon steel. Flanged connections are designed to maximum inside diameter of the hole. The sealing is achieved by O-ring or by seal-plate which are compressed between the mating surfaces. This connection is
recommended for systems with high pressure and dynamic (pulsating) pressures.
Pipe thread couplings are sealed by metal-to-metal contact. They have tapered male threads. The sealing is achieved by tightening both parts to certain torque. The thread surfaces deform and flatten against each other blocking the flow passage. For better sealing, a sealant material is added to the mating surfaces. This connection is used for low cycle pressure and high static (non-pulsating) pressure applications. The main limitations of it are: 1) shaped fittings (elbows) cannot be oriented in desired positions; 2) high risk for leaks at dynamic applications; 3) risk of thread cracking at extreme temperatures because of material expansion or contraction.
Cutting face coupling is used for tube fittings. It has four components: body, cutting ring, collar and a nut. The cutting ring has two cutting edges. The ring slides on the tube. The ring geometry is design so that when the nut is tighten, the front edge cuts into tube surface first and the second edge follows. The collar ensures that all forces are equally distributed.
Quick acting (connect/disconnect) is threadless coupling. Both components are pushed together by hand. A clip ring locks the male and the female parts. These couplings are usually used to ease the operator when additional attachments need to be attached to the truck.
Flare tube end is used in tube connectors for low and medium (up to 200 bar) pressure systems. It has a flared tube which is clamped against a flared nose fitting. Clamping is achieved by a tube nut screwed to fitting.