Hydraulic drive

What does hydraulic mean?

A hydraulic drive uses fluid under pressure to create motion through hydraulic cylinders or hydraulic motors. It is widely used due to its high power density, reliability, and precision in various industries.

Different possibilities to arrange your hydraulic system:

• Simple systems; for example, used in tail lifts
• Mobile systems; for example, used in excavators
• Industrial systems; have high efficiency and require a long service life
• Special systems

Simple systems use small components such as gear pumps and vane pumps.

A hydraulic system consists of:

• Generating part; the pump, driven by an electric motor or combustion engine
• Control with piping
• Motor part; hydraulic cylinder(s) and/or motor(s)
• Fluid; usually oil

In the generating part, the drive power, consisting of speed and torque, is converted into hydraulic power, measured in flow rate and pressure. This hydraulic power is then converted into force and speed in the motor part of the drive. For example, in a hydraulic cylinder, or in speed and torque, as in a hydraulic motor.

Hydraulic drive

Hydraulic systems provide motion to individual components through fluid pressure, often using hydraulic oil. To set a vehicle or its component in motion and maintain it, energy is required, which not only needs to be supplied but often needs to be converted into another type of energy. For example, an electric motor converts electrical energy into mechanical energy, which is then transferred to a hydraulic pump.

Advantages of hydraulic drive include:

• Transferring large forces or torques is possible in relatively small spaces thanks to hydraulic systems. In these systems, the pressure can be significantly high, such as in tail lifts, where the pressure varies between 180 and 240 bar. This high pressure allows generating large forces with hydraulic fluid.
• Overload protection is possible because the pressure in a hydraulic system can be easily regulated and limited using a pressure relief valve.
• Hydraulic systems offer the ability to control speeds continuously, which is particularly beneficial in lifting and lowering platforms.

Hydraulic components

Haco Tail Lift Parts offers hydraulic parts under its own HACO label that are fully compatible with the hydraulic parts of all tail lift brands. Some examples of these are:

Pumps

The hydraulic pump and the electric motor work together to generate hydraulic energy by converting mechanical or electrical energy, for example. In tail lift hydraulics, mainly gear pumps or vane pumps are used. The flow rate of these pumps, i.e., the amount of fluid per unit time (expressed in m3/sec), is generally more or less constant, regardless of the pressure. However, some more expensive pump types can be set so that the displacement volume is adjustable.

Motors

There is a distinction between:

• Electric motors
• Hydraulic motors

An electric motor converts electrical energy into mechanical energy, which then drives a tool. These motors are divided into direct current and alternating current motors. In addition to traditional motors that provide rotary motion, there are also linear motors. Electric motors find their application in machines where motion is required, often of a rotating nature.

Hydraulic motors are used for rotational drive. Most hydraulic motors have a fixed displacement and belong to the displacement type, which means that the speed is linked to the displacement volume and the supplied fluid flow. The torque is directly related to the pressure difference across the motor and the displacement volume. In the case of tail lifts, hydraulic motors or slide cylinders are used for motion.

There are three different types of hydraulic motors:

• Gear motors (intended for tail lifts)
• Vane motors (intended for construction machinery)
• Adjustable axial motors (intended for construction machinery)

Cylinders

Hydraulic cylinders come in two types:

• Single-acting cylinder (cylinder is pressurized from one side)
• Double-acting cylinder (cylinder is pressurized from two sides)

A double-acting cylinder extends and retracts under oil pressure. In single-acting cylinders, the cylinder is only extended by the pressure of the oil; the retraction is done by the weight of the tail lift.

When moving the tail lift, we use the following types:

• A rod cylinder, characterized by a thick rod, typically has no seals on the piston but openings or where the piston is completely absent. This rod acts as the surface against which the cylinder can exert pressure. Rod cylinders are typically single-acting, exerting pressure.
• Piston cylinder has the piston and the rod as one piece. The rod is thinner than in a rod cylinder because the piston fulfills this function. The piston rod is moved by the inflow and outflow of oil.

Sealing sets for the cylinder

Seals ensure that the cylinder is protected against external influences.
We have two types of sealing sets:

• Single-acting sealing sets
• Double-acting sealing sets

Hydraulic hoses

Hydraulic components are connected by hydraulic hoses or pipes, which serve to transport oil between the various parts. Hydraulic hoses are flexible in nature and are usually black.

Valves

Valves are used to regulate the flow direction of the oil.
We have four types of valves:

• Solenoid valve
• Control valve
• Flow regulation valve
• Hand valve

What functions do the valves have?

Solenoid valve

• Single-acting valves can be identified by one support ring next to the O-ring; they allow oil pressure to pass through when unactuated and block the oil.
• Double-acting valves can be identified by two support rings on either side of the O-ring; these block the oil flow in both directions.

Solenoid valves ensure safety by shutting off the oil supply when a hose breaks after the control has been released. In emergencies, the oil supply can be manually controlled by pressing a pin (2/2 valves). The solenoid valve interrupts the oil supply as soon as a hydraulic hose and the control stop break. It is recommended not to use the tail lift until the problem is fixed.

Control valve

This type of valve regulates the oil flow to the desired components. The tail lift industry uses hose burst valves (2/2 valves) or 4/2, 4/3, 3/2 valves, for example. A 4/2 valve, for instance, has 4 ports and 2 positions.

Flow regulation valve

A flow regulation valve regulates the speed at which the tail lift moves by slowing down the flow of oil. In tail lifts, this is done, for example, by the lowering speed control valve. This valve is often integrated into a cylinder or in the valve block.

Hand valve

The hand valves are manually operated and ensure that the desired components receive the oil flow.

Oil pressure switch

The oil pressure switch provides an electrical signal when the preset pressure is reached. For example, placed on the lift cylinder, the oil pressure switch activates the leveling when it has reached the set pressure.

Hydraulic oil

The oil in a hydraulic system has the following functions:

• Transporting energy
• Lubricating moving parts
• Protecting metal surfaces in contact with the fluid
• Disposing of oil-generated heat from internal leakage and friction
• Removing contaminants

Many hydraulic oils are based on mineral oils. Leakage can contaminate the soil, which is why bio-hydraulic oils have been developed, made from vegetable oils and fats. Oils based on synthetic esters, which have a mineral origin but are biodegradable, are also used.

The hydraulic oil used in tail lifts has a high viscosity index, meaning it is less sensitive to temperature changes. Hydraulic systems are equipped with oil filters to ensure that the oil remains clean.

Oils are used with two different viscosity levels: 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 or VG15.

Maintenance of hydraulic parts

• Avoid dirt on the (quick) couplings, clean them if they are dirty before connecting.
• Ensure that hoses and couplings are in good condition. The hydraulic system operates at high pressure, which can reach up to 240 bar, and it is dangerous if hoses burst.
• There should be enough hydraulic oil in the reservoir. In addition to leaks, oil is sometimes lost when connecting the quick couplings. Therefore, the oil level should be checked regularly.
• Regularly check the control or regulating elements. Control and regulating elements are used between the pumps and motors (linear and rotary) to control the hydraulic system. These elements are generally called valves (valve blocks), which are parts that, in their original form, have a valve on a seat (check valves, safety valves, pressure reducing valves, balancing valves, etc.).
• Directional slides and valves are the components that control and regulate fluid flows. The latter components are often electrically operated (cartridges), sometimes on/off, sometimes adjustable. “Flow control valves” are not valves in that sense, but flow regulation valves.