By Joji Parambath
The book describes the design aspects of hydraulic systems systematically. It highlights the essential parameters and specifications of hydraulic components in SI units.
By Joji Parambath
The book highlights the essential parameters and specifications of hydraulic components in English units.
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A hydraulic system must be designed to meet all the functional requirements of an application safely and efficiently. It is also essential to prepare the circuit diagram of the system using the correct symbols, according to the norms in one’s region. The system must provide the required performance level, withstand operational hazards, and ensure its life expectancy. Safety must be built into the system by incorporating interlocks, power-failure locks, and an emergency shutdown feature.
General Design Principles
Industrial hydraulic systems are designed with correctly-sized components, such as a power pack, pressure relief valve, actuators, and control valves using pipes, tubes and hoses. The use of undersized components and conductors in a system can cause excessive pressure losses resulting from friction, and as a consequence, the operating cost increases significantly. In contrast, the use of oversized components and conductors can impose higher capital and installation costs.
A typical design approach consists of following a set of critical steps. These essential design steps are: (1) System analysis and drawing specifications, (2) Circuit/Control system design, (3) Component selection and sizing, (4) Software simulation and analysis, (5) Development of system prototype, and (7) System performance evaluation & optimisation.
The design of a hydraulic system involves the following basic steps: (1) selection and sizing of components, (2) determining the system operating pressure and flow rate, and (3) finding the component specifications to meet the design objectives. The manufacturers offer wide options for the selection of pressure and flow rate combination for a particular power rating (kW or hp).
Therefore, there are many possible solutions for designing a hydraulic system as there are many components available in the market with varying specifications and quality. Optimum design of a hydraulic system for a project must try to synchronize the specifications and quality of components required with the specifications and quality of components available in the market. Here, a few examples of designing hydraulic systems are presented, purely for educational purpose.
Critical Design Steps
The following steps may be followed for finding the basic parameters while designing an industrial hydraulic system with a pump and cylinder.
An analysis of the system to be designed would reveal the application requirements of output force (F) or torque, speed (v), and output power (Pout).
For example, these parameters for a cylinder are governed by the relation (In the SI system Units):
Pout (kW) = F(N) x v (m/s)/1000
The volumetric efficiency (ηvc) and mechanical efficiency (ηmc) of the cylinder can be assumed, and its overall efficiency (ηoc) can be calculated from the relation
The volumetric efficiency (ηvp) and mechanical efficiency (ηmp) of the pump can be assumed, and its overall efficiency (ηop) can be calculated from the relation
ηop=ηvp x ηmp
Next, the hydraulic power (Phyd) involved in the hydraulic power transmission system can be calculated using the following equation:
Phyd = Pout / ηoc
The mechanical power input to the pump (Pinput) corresponds to the electric motor power rating, and the required power input can be calculated using the following equation:
Pinput = Phyd / ηop
The flow rate (QAp), the maximum pressure rating (Pmax), and prime mover speed (Np) of the pump can be selected from the manufacturer’s datasheet. In this way, it is possible to synchronize the system parameters with the parameters of the component available in the market.
The actual pump flow rate (QAp) is the same as the actual cylinder flow rate (QAa) and can be taken as (QA).
Calculate the theoretical flow rate of the cylinder (QTc) from the following equation:
QTc = QA x ηvc
Find the piston area (Aext) of the cylinder from the following equation.
Aext = QTc / v
Find the piston diameter (D) of the cylinder from the following equation and reconcile with the data from the manufacturer’s domain:
Aext = ∏D2/4
Select the standard size cylinder with a diameter equal to or greater than the calculated value from the data on the manufacturer’s domain. If required, modify the piston area (Aext) and reselect the pump flow rate (QAp) and check and revise the values as per the calculations given in the above sections.
Also, select the required piston-rod diameter from the manufacturer’s datasheet.
Next, find the pressure (P) required in the hydraulic line to develop the necessary force from the following equation:
F = P x A
The working pressure can be calculated by taking into account the pressure drops (Maximum 15%) in the hydraulic power transmission system. Check the hydraulic power (Phyd) and reconcile. This pressure should be less than the maximum pressure rating of the pump or any other component that is selected. Remember, this pressure can be set by using a pressure relief valve.
Next, calculate the theoretical pump flow rate (QTP) from the following equation:
QTP = QA/ ηvP
Find the volumetric displacement of the pump from the following equation:
VDp = QTP/Np
(1) Further Reading: The designs of four hydraulic systems, in detail, are systematically explained in the textbooks as mentioned above, using typical examples in the SI system units and the English system units, respectively. The initial chapters provide the information at the component level. This information can then be used for the system-level design given in the final chapter of the book.
(2) It may be noted that the article is intended for general educational purpose to illustrate the essential principles. The optimum design of a hydraulic system always depends on the exact operating and environmental conditions, amongst other factors.
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