Introduction to Hydraulic units (Motor, Cylinder, Accumulator)

Hydraulic Motors

A hydraulic motor converts the hydraulic energy into mechanical energy i.e. a rotating shaft. Torque and rotation are generated by utilizing the hydraulic pressure and flow.

Hydraulic motors can be used for many applications, such as winches, crane drives, self-driven cranes, mixer and agitator drives, excavators, roll mills, etc.

The design of a hydraulic motor and a hydraulic pump are very much similar. Hence, some pumps with fixed displacement volumes can may also be used as hydraulic motors.

Different Types of Hydraulic Motors

The hydraulic motor should be geared to hydraulic system requirements; the issues such as load, range of load, speed, serviceability, etc. must be taken into consideration. There are different types of hydraulic motors, which are listed below:

Hydraulic Gear Motors

• Hydraulic Gear Motor
• Epicyclic Gear Motor

Hydraulic Piston Motors

• Axial Piston Motor
• Radial Piston Motor

Hydraulic Vane Motors

• Balanced Vane Motor

Part-turn Actuators

• Rotary Actuator
• Rack and Pinion Actuator

The power generated by a hydraulic motor is determined by the flow and the pressure drop of the motor. The displacement and pressure drop of the motor determines the torque it produces. The power output is therefore directly proportional to the speed of the rotor. Generally, the motors range from high speed motors of up to 10,000 rpm to low speed motors with a speed of 0.5 rpm.

It should be noted that low speed hydraulic motors are designed in such a way that large torques are produced at low speeds. High speed motors possess better operational characteristics at the speeds that are at least higher than 500 rpm.

Hydraulic Cylinder

Hydraulic cylinders generates mechanical energy from hydrostatic energy. They are taken as motors or actuators capable of producing work. However, hydraulic cylinders differ from hydraulic motors in the sense that they carry out a linear movement instead of a rotary movement. Thus, the cylinders are also termed as linear motors.

The maximum cylinder force ‘F’ depends on the maximum operating pressure p and the effective area of the piston ‘A’. At constant pressure, a cylinder with a larger effective area A will produce a larger force F. At higher pressures, the force ‘F’ will also become larger.i.e.

F = P • A

Hydraulic cylinders are needed to perform certain types of work: for example, in applications where loads must be lowered, lifted, locked or moved.

Different Types of Hydraulic Cylinders

There are two functional types of hydraulic cylinder: double-acting and single-acting cylinders. Only rapid traverse cylinders and telescopic cylinders are available in both single-acting and double-acting types. Due to their function, it is possible to categorize hydraulic cylinders as below:

Single Acting Cylinders

• Push action cylinders
• Pull action cylinders
• Telescopic cylinders
• Rapid traverse cylinders

Double Acting Cylinders

• Differential cylinders
• Double rod cylinders
• Telescopic cylinders
• Rapid traverse cylinders
• Tandem cylinders

Single-acting cylinder has one effective area, located at the bottom side, which exerts force in one direction only. In most of the hydraulic systems, these are designed as plunger cylinders and an external force is applied to return the piston to its starting position: e.g. upstroke presses, lifting devices, etc.

If there is no force to return the piston to the starting position, springs must be used. This is only possible with small cylinders due to the limited force and stroke length of the return springs. Examples of spring return single-acting cylinders are assembly tools and installation work.

Telescopic cylinders are used for hydraulic lifting applications, or where large amplitudes are necessary but the space available is limited, e.g. tipper trucks. The single or double-acting differential cylinder is normally used in industrial and mobile hydraulic systems.

Hydraulic Filters

Every minute, approximately one million particles that are larger than 1 micron (0.001 mm or 1 μm) can enter into a hydraulic system. These particles can damage the hydraulic system components because hydraulic oil can easily get contaminated. The wear of hydraulic system components depends on this contamination, and the presence of metal parts in hydraulic system oil (iron and copper are mainly powerful catalysts) accelerates the degradation of hydraulic parts.

Hydraulic filters remove dirt and particles from the fluid in a hydraulic system.

A hydraulic filter helps to remove the dust particles and clean the oil on a continuous and regular basis. The performance of hydraulic filter is measured by its contamination removal efficiency, i.e. high dirt-holding capacities. Almost every hydraulic system has more than one hydraulic filter.

Different Types of Hydraulic Filters

The different types of hydraulic filters as mentioned below:

• Suction filters
• Pressure filters
• Bypass filter systems
• Return line filters
• Fillers and breathers
• Filter elements

Accumulators

A hydraulic accumulator holds fluid under pressure and serve a number of functions within a hydraulic system.

Accumulators can take a fixed amount of fluid under pressure and store it. The fluid is then released when a specific task is to be performed in the hydraulic system. Accumulators can offer several functions, such as:

• Energy storage
• Compensation fluctuations of temperature
• Emergency operation
• Compensation of leakage oil
• Cushioning of pressure shocks which may occur during sudden switching of the valves
• Swell compensator (in marine hydraulics)
• Dampening vibrations

Accumulators normally contain two compartments: one of the compartments is filled with gas and the other compartment, connected to the hydraulic circuit, is filled with the fluid. The accumulator shell is made up of carbon steel, stainless steel or aluminum. Depending on the separating elements, there are three types of hydraulic accumulators:

• Bladder accumulators
• Piston accumulators
• Diaphragm accumulators

Accumulator Accessories

Accumulators are the pressure vessels and therefore must be handled with more care. For the safe mounting of the accumulator, special fixing elements are available. Also, an accumulator should never be installed without safety and shut-off control block.

There are special testing devices to test pressure of the nitrogen gas. Testing should be done at least once a year. Mobile nitrogen charging devices are available to refill the gas or change the pressure of the prefilled gas.

References

1. Technical book, “Construction Machinery Training”, Instate, Imlambad
2. Harris, F. and McCaffer, “Management of Construction Equipment”, Macmillan Education Ltd. London, UK.
3. Erich J. Schulz, “Diesel Equipment I and II”, Mcgraw-Hill book co.
4. Frank Harries, Ronald McCaffer, “Construction of Plant Excavating and Material Handling”, Granda Publishing.
5. SAE Handbook Volume 4
6. “Caterpillar performance Handbook”, Edition 33, Caterpillar Inc, Peoria, Illinois, USA.