Categories
Hydraulics
Hydraulics is a drive technology that uses fluid under pressure. This fluid is usually oil.
Hydraulic systems can be divided into various categories:
- Simple systems, such as those used for tail lifts.
- Mobile systems, such as those used on excavators.
- Industrial systems, which require high efficiency and long service life.
- Special systems.
The parts used in simple systems are generally simple themselves and often small, such as gear pumps, vane pumps, and vane motors.
A hydraulic system essentially consists of:
- Generating part (the pump, powered by electric motor or combustion engine)
- Control system with lines/hoses
- Actuating part (hydraulic cylinder(s) and/or motor(s))
- Fluid, usually an oil
In the generating part the drive power (rotational speed and torque) is converted to hydraulic power (flow and pressure). The hydraulic power is then converted to power and speed (hydraulic cylinder) or rotational speed and torque (hydraulic motor) in the actuating part of the drive system.
Hydraulic drive
Hydraulic systems bring movable components in motion. A hydraulic drive puts components in motion by means of fluid pressure. Hydraulic oil is usually used for this. Energy is required to bring a vehicle or part thereof into motion and keep it in motion. This must not only be supplied where needed but in many cases must also be converted into energy of a different kind. For example, an electric motor converts electrical energy into mechanical energy and supplies it to a component such as a hydraulic pump.
Advantages of hydraulic drive are:
- Transfer of large forces or torques, in relatively small spaces. The pressure in a hydraulic system can be high. For tail lifts the pressure varies between 180 and 240 bar. The consequence of this is that high pressure liquid can be used to develop large forces.
- Overload protection: the pressure in a hydraulic system can be easily regulated and limited by a pressure relief valve.
- Hydraulic systems speeds can be steplessly regulated. This is a particular advantage when raising and lowering the platform.
Hydraulic parts
Haco Tail Lift Parts sells hydraulic parts under its own HACO label that are 100% interchangeable with the hydraulic parts of all brands of tail lifts.
These include:
The hydraulic pump together with the electric motor produce hydraulic energy, through the conversion of mechanical or electrical energy for example. In the hydraulic systems of tail lifts only gear pumps are used. These have a flow rate (in m3/sec) that is more or less independent of pressure. Most pumps have a constant yield, yet more expensive types sometimes provide adjustable displacement.
Motors
These include both:
An electric motor converts electrical energy into mechanical energy with which a tool or machine can be powered. Electric motors are further divided into DC and AC motors. In addition to the common motors that produce a rotary motion, there are also linear motors. An electric motor is used in machines where something has to move, often something with a circular movement.
Hydraulic motors are used to obtain a rotary drive. The vast majority of hydraulic motors have a constant displacement. Hydraulic motors used in hydraulic systems are always of the positive displacement type. This means that the speed has a direct relation to the displacement of the motor and the supplied flow of fluid, while the torque has a direct relationship to the pressure difference over the motor and the displacement. Slide-under tail lifts are put in motion by means of hydraulic motors or hydraulic cylinders.
There are three different types of hydraulic motors:
- Gear motors (tail lifts).
- Vane motors (construction machines).
- Axial plunger motors (construction machines).
Cylinders
There are two types of hydraulic cylinders:
- Single-acting cylinder (pressure is only applied to one side of the cylinder)
- Double-acting cylinder (pressure can be applied to either side of the cylinder)
Double-acting means that the cylinder actively extends and retracts, under pressure from oil. In the case of single-acting cylinders, the cylinder only extends under the pressure of oil; the cylinder is then pushed back in by the weight of the tail lift.
For moving the tail lift, we use the following types:
- Plunger type (cylinder where the piston does not have seals but rather openings, or where the piston is omitted. A plunger cylinder has a relatively thick rod, because this is the surface with which it pushes. A plunger cylinder is always single-acting, in the pushing direction).
- Piston type In a piston type, the rod is an integral part of the piston. The rod is be thinner than that of a plunger cylinder, because the piston serves this purpose. The piston rod is moved by adding or releasing oil.
Seals protect the cylinder against external influences.
There are two types of seal kits:
- Single-acting seal kits
- Double-acting seal kits
Hydraulic components are connected by hydraulic lines or hoses. Hydraulic lines transport oil between the different components. Hydraulic hoses are flexible and often black in colour.
Valves
Valves are used to control the flow of oil.
There are four types of valves:
- Hose burst valve
- Control valve
- Throttle valve
- Manual valve
Hose burst valve
• Single-acting (recognizable from one support ring beside the O-ring) lets oil pressure through when not operated and restricts oil flow.
• Double-acting (recognizable from two support rings, one on either side of the O-ring) blocks the oil flow in both directions.
Hose burst valves ensure safety if the hose bursts; oil flow is prevented when the control is released. The hose burst valve is designed so it can be operated in an emergency by manually depressing a pin (2/2 valves). When a hydraulic hose ruptures, a hose burst valve prevents oil from flowing. From a safety standpoint, it is advisable not to use the tail lift again until the problem is fixed.
Control valve
This type of valve ensures that the desired components receive the flow of oil. In the tail lift industry, hose rupture valves (2/2 valves) or 4/2, 4/3, or 3/2 valves are used. For example, a 4/2 valve has four ports and two positions.
Throttle valve
A throttle valve controls the speed of movement of the tail lift by restricting the oil flow; on a tail lift it is the lowering brake valve that performs this function. A lowering brake valve is often integrated in a cylinder or in the manifold block.
Manual valve
Manual valves ensure that the desired components receive the flow of oil. These valves are operated manually instead of electrically.
The oil pressure switch produces an electrical signal when the set pressure is reached. The oil pressure switch is located on the lift cylinder and produces a signal that is used for angle adjustment (with electronic angle adjustment).
Hydraulic oil
The oil in a hydraulic system has the following functions:
- Transmission of energy.
- Lubrication of the moving parts.
- Protection of metal surfaces the liquid comes into contact with.
- Carrying off oil that has escaped through internal leakage and heat resulting from friction.
- Carrying off contaminants.
Many hydraulic oils are based on mineral oils. When hydraulic oil leaks, the soil can be contaminated. This is why biodegradable hydraulic oils, made from vegetable oils and fats, were developed. Oils based on synthetic esters are also used. These oils are mineral in origin yet are nevertheless biodegradable.
Hydraulic oil used for tail lifts has a high viscosity index. Viscosity is the density of the oil, and oil with a high index is less sensitive to temperature fluctuation. Hydraulic systems are equipped with oil filters so the oil in the hydraulic system remains clean.
Oil of two different viscosities is used:
The German tail lift brands, such as Bär, Cargolift, Dautel, and Sörensen, use oil with a viscosity of ISO VG10. Ama, Anteo, Dhollandia, and Zepro use ISO VG22.
Maintenance of hydraulic parts
- Prevent dirt from getting on the quick couplers; clean them before connection if they are dirty.
- Make sure that hoses and couplers are in good condition. The hydraulic system uses high pressure which can reach 240 bar, and it is dangerous if hoses break.
- There must be enough hydraulic oil in the reservoir. Besides leaks, some oil is occasionally lost when connecting the quick disconnects. Therefore the oil level must be checked regularly.
- Check the regulation or control elements regularly. Between the pumps and motors (linear and rotary) control and regulation elements are used to regulate the hydraulic system. These elements are generally called valves (manifold blocks); these are components which in their original form have a valve on a seat (non-return valves, safety valves, pressure-reducing valves, balancing valves, etc.).
- Valves are the parts that control and regulate the flow of fluid. These parts are often electrically operated (cartridges): sometimes on/off, sometimes adjustable. 'Flow control valves' are actually not valves at all; they are flow restriction orifices.