Merits and Demerits of industrial hydraulic systems

Advantages / merits of Hydraulic System :

        The advantages of hydraulic system over other methods of power transmission are:

1.         Freedom of location of input and output power converters such as prime movers, pumps & actuators. This ensures flexibility.

2.         Infinitely variable control of output force, output torque, output speed and actuator position. This provides control of speed & forces over wide range.

Actuators (linear or rotary) of a hydraulic system can be driven at infinitely variable speed by either varying the pump delivery or by using a flow control valve.

A pressure adjusting / regulating valve can be used to vary the operating pressure to adjust the output force or output torque.

3.         Extremely high output forces and force multiplication is possible by means of the “hydraulic lever”.

4.         Drive system in hydraulics is reversible.

A hydraulic actuator can be reversed instantly while in full motion without damage. A 4 way direction control valve or a reversible pump provides the reversing control.

A pressure relief valve protects the system components from excess pressure occurring during instantaneous reversible.

(Other types of drives like electric motor or I.C. Engine etc., have to be slowed down, stopped completely before reversing)

5.         Safety and overload protection of the system components can be achieved by automatic valves, to prevent system breakdown.

6.          Low inertia and ease of shock absorption during actuator motion start and stop operation is smooth & quiet. Vibration is kept to minimum.

7.         Emergency power can be stored in an accumulator.

8.         Greater efficiency and economy due to low friction losses and high system reliability (efficiency is approx. 70% -80%).

9.         Design of system is simpler. In most cases few pre-engineered components will replace complicated mechanical linkages.

10.     Small packages – The system has high power to weight ratio, ensuring compactness.

11.     The hydraulic drive system can be stalled without damage when overloaded and will startup immediately when the load is reduced. During stall, the relief valve simply diverts pump delivery to tank which results only in loss of Horsepower.

12.     Hydraulic systems are self lubricating and power can be diverted to alternative actuators.

Demerits / limitations of a hydraulic system :

1.         Precision parts are necessary in fluid power system.

2.         Precision parts are exposed to unsympathetic climates and dirty atmospheres and hence call for very cautious and regular system maintenance.

3.         Initial system cost is high. But high efficiency, minimum frictional losses keeps the cost of power transmission at minimum. This offsets the disadvantage of high initial cost.

4.         Contamination of hydraulic fluid is inevitable & is the most serious disadvantage of using fluid power. To prevent clogging of precision clearances in the fluid actuators and valves requires extensive filtering systems to maintain cleanliness in oil.

5.         Leaking problems, both internal and external, are sure to occur. Special provisions must be made to accommodate such leakages.

Line ruptures and bursts can cause hazard due to high velocity oil jets. Oil might also be flammable posing fire hazards.

6.         Controllability of hydraulic systems is not as easy as electrical systems.

Comparators

Comparators are those instruments which can be used to compare, generally, linear dimensions of similar components with a slip gauge standard.

Features:

Comparators have no built in standards of their own; they indicate only the differences in size between the nominal dimension set on the comparator by SLIP Gauges and the dimension of the component being measured.

They have precisely engineered mechanisms used in the principle of its operation.

They can be built to give a magnification as high as X50000

They are so designed to allow smallest scale division of 0.00025 mm

In principle a comparator works on relative measurements, i.e., it gives only dimension difference in relation to a basic dimension.

So, a comparator is to compare the unknown dimensions of a part with some standard or a master setting which represents the basic size and the dimensional variations from the master setting have to be amplified and measured.

The function of a comparator is to detect to a high degree of precision the small difference, if any, between a suitable length standard ad the measured part.

In construction, a comparator consists of three essential elements:

1. A sensing device, usually, a plunger, which faithfully senses the input signal represented in this case by a change of length or a surface displacement.
2. A magnifying or amplifying system to increase the signal to suitable magnitude.
3. A display system (usually a scale and pointer) which utilizes the amplified signal to provide a suitable readout.

Desirable Characteristics of a good comparator:

The fundamental requirements o be fulfilled by every comparators are as follows:

1.      Robust in design and construction: in order to withstand ordinary usage to maintain consistent measuring accuracy.

2.      Indicating device used shall be such as to show readings at least possible time, for which the magnification system should be such as to get readings dead beat.

3.      Provision for compensation against temperature effects.

4.      Scale must have linear characteristic.

5.      Though sensitive, the instrument should be rigid enough to accommodate mishandling to some extent without permanent harm.

6.      Should have maximum versatility, to accommodate to be used for measuring wide range of dimensions.

Uses of Comparators:

a)    In mass production, where components are to be checked at very fast rate.

b)    As laboratory standards from which working or inspection gauges are set and calibrated/correlated.

c)    For inspecting newly purchased gauges.

d)    As working gauges, in some machines, for stage inspection of parts produced. This prevents rejection and ensured maintenance of tolerances at various stages of manufacturing.

e)    For grading of parts into groups depending on their tolerances, for use in selective assembly of parts.  

Hydraulics

Basic Hydraulic Circuit

BASIC CONCEPT OF A HYDRAULIC SYSTEM

Hydraulics is the engineering science of liquid pressure and liquid flow. Hydraulic power transmission systems are concerned with the generation, modulation and control of fluid pressure and flow.

A Hydraulic system consists of:

A Pump, which converts, available electric or mechanical power from the prime mover to hydraulic power at the actuator. A pump moves the liquid in the system.  The intake of the Pump is connected to a liquid source, usually called the Tank or Reservoir. Atmospheric pressure, pressing the liquid in the reservoir, forces the liquid into the pump. When the pump operates, it forces the liquid from the tank into the discharge pipe at a suitable pressure. The flow of the pressurized liquid discharged by the pump is controlled byValves.

Three control functions are used in most of the hydraulic systems to control and regulate the fluid flow.

a)      Control of the liquid pressure.

b)      Control of the liquid flow rate, and

c)      Control of the direction of liquid flow.

The fluid/liquid discharged by the pump in Fluid Power System is directed by valves to a Hydraulic Motor for developing rotary force (torque) and motion or to a Hydraulic Cylinder when linear motion is desired. Both, hydraulic motor and hydraulic cylinder uses pressurized liquid as their energy sources. Hydraulic motors operate in a reverse manner to that of a pump. Actuators convert hydraulic power to usable mechanical power output at the point required.

The Direction Control of the piston in hydraulic cylinder or the direction of rotation of the hydraulic motor depends on, to which end of the cylinder or motor the liquid enters. The Direction Control Valves (DCV) carries out this work of directing the liquid to proper ports of the actuators. As pressurized liquid enters one end, the existing liquid on the other end must be drained. The drained liquid is led back to the reservoir.

Connectors connecting various system components provide power conductors for the fluid under pressure and flow return to tank (reservoir). Fluid storage and conditioning equipment ensure sufficient quality and quantity as well as cooling of the fluid.

Hydraulic system is not a power source. Power source is the prime mover such as Electric motor or as an Engine, which drives the pump.