Two Marks Q&A

 two marks questions and answers: Hydraulic Actuators and Control Components - Hydraulic Actuators and Control Components

TWO MARKS QUESTIONS AND ANSWERS

1. What are fluid power symbols ?

Fluid power symbols are used to represent individual components in fluid power circuit diagrams, which identify components and their functions uniquely.

2. Draw the graphic symbols for the following pumps :

(a) Fixed displacement bidirectional pump, and

(b) Variable displacement unidirectional pump. 

Ans.

3. Draw the ANSI symbols for the following motors : 

(a) Fixed displacement unidirectional motor, and 

(b) Variable displacement bidirectional motor. Ans.

Ans.

4. Give the hydraulic symbols for (a) single-acting and (b) double-acting cylinders. 

Ans.

5. Give the fluid power symbols for the 3/2- and 4/3-way valves. 

Ans.

6. How can you indicate the direction of flow (i) for hydraulic fluid, and (ii) for pneumatic fluid.

7. For the following graphic symbols shown, identify their mode of operation/actuation.

Ans.

 (a) Push button actuation,

(b) Pedal actuation,

(c) Lever actuation, and

(d) Solenoid actuation.

8. Draw the ANSI symbols for the following hydraulic components :

(a) Reservoir,

(b) Filter, 

(c) Cooler, and

(d) Accumulator.

Ans.

9. The following graphic symbols are used for indicating some measuring devices. Identify them.

Ans.

(a) Filling level indicator, 

(b) Thermometer,

(c) Pressure gauge, 

(d) Flow meter.

REVIEW QUESTIONS

1. Draw ANSI fluid power symbols for the following cylinder types :

(a) Single-acting cylinder with spring return,

(b) Double-acting cylinder with through piston rod,

(c) Differential cylinder, and

(d) Double-acting telescopic cylinder.

2. Draw the graphic symbols for a pressure relief valve and a 3-way pressure regulator.

3. Draw ANSI symbol for adjustable flow control valve with throttle.

4. Give the fluid power symbols for the following non-return valves :

(a) Spring-loaded non-return valve, and

(b) Pilot controlled non-return valve.

5. Draw the ANSI symbols for the following coupling types :

(a) Coupling with check valve," and

(b) Half of quick release-coupling without valve.

6. Draw the graphic symbols for hydraulic power pack.

Pump Performance

 The performance characteristics of a pump can be represented in terms of overall efficiency.

PUMP PERFORMANCE

The performance characteristics of a pump can be represented in terms of overall efficiency. Overall efficiency, in turn, has two components: 'volumetric efficiency' and 'mechanical efficiency'. These three efficiencies are presented below.

1. Volumetric Efficiency

• Definition: It is the ratio between the actual flow rate produced by the pump and the theoretical flow rate that the pump should produce. 

• Formula:

• Significance: The volumetric efficiency indicates the amount of leakage within the pump. The lower the internal slip losses, the higher the volumetric efficiency. For zero slip, the volumetric efficiency is 100%.

2. Mechanical Efficiency

• Definition : It is the ratio between the theoretical power required to operate the pump and the actual power delivered to the pump.

• Formula :

Mechanical efficiency can also be calculated in terms of torques as follows: 

• Significance: The mechanical efficiency indicates the amount of energy lost due to friction in bearings and other mating parts, and energy lost due to fluid turbulence. In other words, the mechanical efficiency indicates the amount of energy losses that occur due to reasons other than leakages.

• Since the amount of power required to overcome friction rises with increased liquid viscosity, mechanical efficiency decreases as liquid viscosity decreases.

• Power losses in timing gears, bearings and seals reduce mechanical efficiency.

3. Overall Efficiency

• Definition: It is the ratio between the actual power delivered by pump and the actual power delivered to pump.

• Formula:

Mathematically, the overall efficiency can also be written as

• Significance: The overall efficiency indicates the amount of energy losses by all means.

Note 

In our calculations, we may use the terms 'brake power' and 'hydraulic power'. The actual power delivered to a pump from a prime motor (like electric motor) via a rotating shaft is often called by the term 'brake power'. Similarly the actual power delivered by a pump to the fluid is called 'hydraulic power'.

4. Comparison of Various Positive Displacement Pumps

Table 4.9 compares the various types of positive displacement pumps.

Table 4.9. Comparison of various positive displacement pumps

Pump Characteristics

 The behaviour of a pump under varying conditions is shown graphically by the curves known as characteristic curves of the pump.

PUMP CHARACTERISTICS

1. What are Characteristic Curves?

• The behaviour of a pump under varying conditions is shown graphically by the curves known as characteristic curves of the pump.

• The characteristic curves are used to identify and interpret the following parameters of the hydraulic pumps:

• Delivery at various pressures (with constant speed).

• Delivery at various speeds (with constant pressure).

• Volumetric efficiency, mechanical efficiency, and overall efficiency (with constant speed).

• Volumetric efficiency, mechanical efficiency, and over efficiency (with constant pressure).

• Input power at various delivery and pressure conditions (with constant speed).

2. Pump Discharge Pressure Vs Rate of Discharge

• Fig.4.22 depicts the relation between the pressure (or head) produced by the pump and the rate of discharge when the pump is running of constant speed.

• From Fig.4.22, it is clear that less the pressure to be delivered, more the rate of discharge.

3. Pump Discharge Pressure Vs Power, Efficiency, and Rate of Discharge

• Fig.4.23 tells us how much power must be supplied to the pump to obtain different rates of discharge against different discharge pressures.

• From Fig.4.23, it is clear that more power is required to deliver higher discharge pressure. These curves also give information about the volumetric efficiency and mechanical efficiency at different discharge pressures.

4. Rotary Pump Characteristics

• Fig.4.24 depicts the typical efficiency curve, power curve, and rate of discharge curve for the rotary pump at constant operational speed.

• From Fig.4.24, we can interpret the following conclusions :

• Rotary pumps usually run more efficiently at lower delivery pressures.

• The power necessary to run a rotary pump increases almost directly, as does the pressure produced by the pump at constant speed.

• There is relatively little difference in the rate of discharge for delivery pressures.

5. Centrifugal Pump Characteristics

• Fig.4.25 depicts the typical efficiency curve, power curve, and rate of discharge curve for the centrifugal pump at constant operational speed.

• From Fig.4.25, we can interpret the following conclusions:

• Centrifugal pumps usually run more efficiently at higher rate of discharge.

• The rate of discharge of centrifugal pumps drops quickly as discharge pressure increases.

6. External Gear Pump Characteristics

• The typical characteristics curves of an external gear rotary pump is shown in Fig.4.26. These curves show the capacity and power input for an external gear pump at various speeds.

• The curve showing the relation between pump discharge pressure and pump capacity is often termed the head-capacity or HQ curve.

• The curve showing the relation between the power input and the pump capacity is termed the power-capacity or PQ curve.

• From Fig.4.26, we can interpret the following conclusions:

• At any given speed, the capacity characteristic is nearly a flat line. 

• The power input increases with both operating speed and discharge pressure. 

• As the speed of a gear pump is increased, its discharge rate also increases.

Pump problems

 Noise is a significant factor used to determine the performance of the pumps.

PUMP PROBLEMS

1. Pump Noise

• Noise is a significant factor used to determine the performance of the pumps. 

• Any increase in noise level normally indicates wear and danger of failure of pump. 

• Normally noise is measured in units of decibels (dB).

• Generated noise levels depend on many factors such as the pump type, pump component materials, pump mountings, rigidity, manufacturing and fitting accuracies of the pump elements, size and flow capacity, pressure, speed of rotation, pressure pulsations, and the other components in the circuit.

• Table 4.10 presents the approximate noise levels for various pump designs. Generally speaking, external gear and the piston pumps are the noisiest while screw pumps are very quiet; vane and internal gear pumps have noise levels somewhere in between a piston and screw pumps.

Table 4.10. Noise levels for various pump designs

2. Slip

• Slip is the leakage occurs between the discharge and suction sides of a pump through the pump clearances.

• The extent of this leakage depends on the width, length and shape of the clearances, the viscosity of the pumped liquid, and the pressure difference between the discharge and suction sides of the pump.

• Pump speed does not influence slip. But slip increases with increasing liquid viscosity.

3. Cavitation

• Definition: The formation, growth and collapse of vapour filled cavities or bubbles in a flowing liquid due to local fall in fluid pressure is called cavitation.

• Phenomenon: For smooth operation, pumps should be completely filled with liquid. When liquid does not completely fill the pump chamber, a loss of capacity results. This may occur through the vapourization of some of the process liquid in the suction line or the pump chamber. The vapour bubbles are carried into higher-pressure regions of the pump where they collapse, resulting in noise and vibration. This phenomenon is called pump cavitation.

• Effects: The pump cavitation may cause severe erosion of the pump components (also called pitting) and reduce the pump life.

• Cavitation can be avoided by ensuring that the suction pressure is always greater than that required by the pump.

4. Pump Ripple

Small variations of fluid flow that takes place during pumping (due to some design problems) are called ripple.

Selection of hydraulic pumps

 Pumps are selected by considering number of factors into account. Main factors among these considerations are presented in Table 4.11.

SELECTION OF HYDRAULIC PUMPS

(FACTORS INFLUENCING THE SELECTION OF PUMP TYPE)

Pumps are selected by considering number of factors into account. Main factors among these considerations are presented in Table 4.11.

Table 4.11. Factors affecting the selection of hydraulic pump

1. Safe and maximum operating pressures.

2. Maximum flow rate/delivery requirements.

3. Pump drive speed.

4. Type of control.

5. Fluid compatibility.

6. Fluid contamination.

7. Operating environment.

8. Tolerable pump noise level.

9. Compactness and weight-to-power ratio.

10. Efficiency.

11. Cost and economic factors.

12. Availability and interchangeability.

13. Ease of maintenance and spares.

1. Safe and maximum operating pressures: 

• The hydraulic pump is selected based on the safe and maximum operating pressure of the hydraulic system for the particular application.

• In general, when the operating pressure is higher, the flow rate for a given system power will be lesser. This results in smaller pumps, smaller pipe sizes and smaller components. But, at high pressures, the compressibility of the fluid adversely affects the load control.

• Table 4.12 presents the operating pressure ranges for different types of pumps. 

Table 4.12. Comparison of various performance factors for pumps

* Moderately higher than other pumps.

† General industrial requirement is 25-30 W.

2. Maximum flow rate/delivery requirements:

• The second important consideration in selecting a pump is its flow rate capacity or maximum delivery.

• The pump should be capable of delivering the maximum flow rate required by the system.

• If the hydraulic system requires constant delivery, a fixed displacement pump is selected.

• If the system requires varying delivery at fixed levels, a multi-pump system is chosen.

• If the system requires varying delivery, a variable displacement pump can be chosen.

• Table 4.12 presents the flow rate ranges for different types of pumps.

• Usually a pump with a capacity about 10% more than that required is selected (in order to compensate for leakage losses that are caused due to increased operating pressure).

3. Pump drive speeds: 

• The pumps are usually driven by electric motor or internal combustion engine. Since the delivery rate depends on the drivig speed of the pump, which in turn depends on the pump drive speeds.

• Table 4.12 presents the speed ranges for different hydraulic pumps.

• It may be noted that higher the pump speed, the shorter will be its life."

4. Type of control: 

• The various types of pump controls are manual, servo control, pressure compensated control, constant power control and constant flow control.

• A suitable pump control is chosen based on the application requirements such as complexity, accuracy of control, cost, type of machining operation, etc.

5. Type of fluid (fluid compatibility): 

• The pump should be selected based on the type of fluid used in the hydraulic circuit.

• The compatibility between fluid type, pump type, and pump seals are to be taken into consideration.

6. Fluid contamination:

• Depending on the nature of fluid contamination and particulate size of contamination of the hydraulic circuit, the pump type should be selected. 

• For example, if a contaminated (with more dirt) fluid has to be pumped, then non- precision gear pumps such as lobe pumps and gerotor pumps can be selected.

• Irrespective of pump type, pump suction line must have a filter fitted.

7. Operating environment: 

• The effect of operating environment should also be considered while selecting a pump.

• This involves effects of ambient temperature, altitude, humidity and extremes of operating temperatures.

8. Tolerable pump noise level: 

• The operating noise level for a given type of pump varies with the workmanship and make.

• Usually the pump noise level increases with higher operating speed and pressure.

9. Compactness and weight-to-power ratio: 

• It is the ratio of pump weight to the hydraulic power delivered by the pump, expressed as kg/kW.

• This weight-to-power ratio factor is of critical importance in aerospace and military engineering applications.

• The weight-to-power ratio should be as low as possible.

• Table 4.13 presents the weight-to-power ratio ranges for different pumps.

Table 4.13. Comparison of weight-to-power ratio factor for pumps

10. Efficiency: 

• Though the actual pump efficiency depends on design, operating pressure, speed and fluid viscosity, ideally the pump with higher overall efficiency should be chosen for the system requirement.

• Table 4.12 presents the range of volumetric and overall efficiencies for different types of pumps.

• As could be seen from Table 4.12, the reciprocating pumps usually have higher efficiencies than rotary pumps.

11. Cost and economic factors: 

• The pump cost is also a very important factor in selection of a pump.

• The operating and maintenance costs of the pumps are also to be taken into consideration, in addition to its capital cost.

• Usually gear pumps are cheaper; vane and piston pumps are more expensive.

The factors such as availability and

12. Availability and interchangeability: 

• interchangeability are also considered while selecting a pump.

• Now-a-days almost all pump manufacturers adapt international standards, which facilitates direct interchangeability between pumps of different manufacturers.

13. Ease of maintenance and spares: 

• Since pump components worn out after a time and they need to be replaced. Thus the ease of pump maintenance and availability of spare components are also should be considered while selecting a pump.

• Usually gear pumps when worn-out are replaced, whereas in vane pumps, all wearing parts are replaced as a set.

Note 

1. In general, gear pumps are the least expensive but also provide the lowest level of performance. Since the gear pumps are simple in design and compact in size, therefore they are most commonly used in fluid power systems.

2. The efficiency and the cost of vane pump lies between those of gear and piston pumps. However, clean oil with good lubricity is necessary for its continued satisfactory performance.

3. Among all pumps, piston pumps are the most expensive but provide the highest level of overall performance. They produce a non-pulsating flow and can operate at the highest pressure levels. However, because of their complex design, piston pumps cannot be repaired in the field.