Experiment No: 02 Demonstration of Hydraulic Circuits on Hydraulic Trainer Kit

 

Experiment No :02 Demonstration of Hydraulic Circuits on Hydraulic Trainer Kit

 Following experiments to be done on hydraulic trainer:

 a. Regenerative circuit                                     b. Speed control circuit

 c. Sequencing circuit                                       d. Traverse and feed circuit

Aim: To study various hydraulic circuits to control single-acting and double-acting cylinders

Apparatus:   Hydraulic trainer [ L=900mm,W =600mm,H =1400mm , Max Working Pressure = 20kg/cm²  , Tank Capacity =30 Liter, Hydraulic Oil Enclo-68 ]

Precautions:    

ü  CHECK oil Level before starting the unit

ü  Ensure all the instruments/ equipment are proper and calibrated.

ü  Do not operate Pressure Gauge above 60% of the full range

ü  Also avoid sharp bends which could pinch or weaken the hoses

ü   Also avoid sharp bends which could pinch or weaken the hoses.                    

 

Theory :

A hydraulic circuit is a group of components such as pumps, actuators, control valves, conductors and fittings arranged to perform useful work. There are three important considerations in designing a hydraulic circuit: 

1. Safety of machine and personnel in the event of power failures.

2. Performance of given operation with minimum losses.

3. Cost of the component used in the circuit. 

Following experiments to be done on hydraulic trainer:

 a. Regenerative circuit                                     b. Speed control circuit

 c. Sequencing circuit                                       d. Traverse and feed circuit

a.      Regenerative circuit :

 

Figure 1: Regenerative circuit

 

Figure 1 shows a regenerative circuit that is used to speed up the extending speed of a double-acting cylinder. The pipelines to both ends of the hydraulic cylinder are connected in parallel and one of the ports of the 4/3 valve is blocked by simply screwing a thread plug into the port opening. During retraction stroke, the 4/3 valve is configured to the right envelope. During this stroke, the pump flow bypasses the DCV and enters the rod end of the cylinder. Oil from the blank end then drains back to the tank through the DCV.  When the DCV is shifted in to its left-envelope configuration, the cylinder extends as shown in Fig. 1. The speed of extension is greater than that

 

for a regular double-acting cylinder because the flow from the rod end regenerates with the pump flow Q_P to provide a total flow rate  Q_T .

 

The total flow rate Q_T  entering the blank end of the cylinder is given by

Q_T =  Q_{P  +}  Q_r

where is the pump flow rate Q_P  and is the regenerative flow or flow from the rod end Q_r. Hence,

Pump flow rate Q_P  = Q_T - Q_r

But the total flow rate acting on the blank rod end is given by

Q_T = A_p * V_ext

Similarly, theflow rate from the rod end is given by

Q_r  = (A_p -A_R) V_ext

So pump flow rate is

Q_P   = A_p * V_ext  - (A_p -A_R) V_ext

Q_P   = A_R V_ext

 

Thus, a small area provides a large extending speed. The extending speed can be greater than the retracting speed if the rod area is made smaller. The retraction speed is given by

The ratio of extending and retracting speed is given as

When the piston area equals two times the rod area, the extension and retraction speeds are equal. In general, the greater the ratio of the piston area to rod area, the greater is the ratio of the extending speed to retraction speed.

We do not obtain more power from the regenerative cylinder during extension because the extension speed is increased at the expense of reduced load-carrying capacity.

 

Fig :  Arrangement of Component for Regenerative Circuit on Hydraulic Trainer

b. Speed control Circuit :

In hydraulic operations, it is necessary to control the speed of the actuator so as to control the force, power, timing and other factors of the operation. Actuator speed control is achieved by controlling the rate of flow into or out of the cylinder.

Speed control by controlling the rate of flow into the cylinder is called meter-in control. Speed control by controlling the rate of flow out of the cylinder is called meter-out control.

Meter-in systems are used primarily when the external load opposes the direction of motion of the hydraulic cylinder. When a load is pulled downward due to gravity, a meter-out system is preferred. If a meter-in system is used in this case, the load would drop by pulling the piston rod, even if the FCV is completely closed.

 

 

A meter-out flow control system is one in which the FCV is placed in the outlet line of the hydraulic cylinder. Thus, a meter-out flow control system controls the oil flow rate out of the cylinder. One drawback of a meter-out system is the excessive pressure build-up in the rod end of the cylinder while it is extending. In addition, an excessive pressure in the rod end results in a large pressure drop across the FCV. This produces an undesirable effect of a high heat generation rate with a resulting increase in oil temperature.

C. Sequencing circuit :

In this circuit, two cylinders are connected in parallel. Without the sequence valve, these cylinders would extend together as they are both unloaded. In order for this circuit to function properly, the clamp cylinder must extend completely before the bending cylinder begins to extend. The sequence valve accomplishes this by not allowing flow into the bending cylinder branch of circuit until the clamp cylinder has reached the end of its stroke. When the clamp cylinder extends completely, the pressure rises and opens the sequence valve,thus allowing the bending cylinder to extend. The sequence valve must be set high enough so that it opens only after the complete extension of the clamp cylinder.

During the retraction of cylinders, the check valve allows the sequence valve to be bypassed. The sequence valve has no effect on the circuit in this situation. Both cylinders retract together because both are unloaded and split the pump flow.

 

Fig : Sequencing circuit

 

 

 Fig :  Arrangement of Component for Sequencing Circuit on Hydraulic Trainer

 

D. Traverse and feed circuit

It provides mid-stroke stop and hold of the hydraulic cylinder (during both the extension and retraction strokes) by deactivation of the four-way, three-position DCV. It provides two speeds of the hydraulic cylinder during the extension stroke:

ü  When the three-way, two-position DCV is unactuated in spring offset mode, extension speed is normal.

ü  When this DCV is actuated, extension speed increases by the regenerative capability of the circuit.

 

Fig : Traverse and feed circuit

 

Fig :  Arrangement of Component for Traverse and feed Circuit on Hydraulic Trainer

Conclusion  : Studied various hydraulic circuits such as speed control, regenerative, sequence ,traverse feed circuits  to control single-acting and double-acting cylinders successfully