Review and summary

REVIEW AND SUMMARY

• The four important factors that should be considered while designing any fluid power circuit are:

(i) Safety of operation,

(ii) Performance of desired function,

(iii) Efficiency of operation, and

(iv) Cost.

• A hydraulic circuit is the graphic representation of the hydraulic components in a hydraulically operated machine.

• In this chapter, the design, operation, and purpose of the following hydraulic circuits have been discussed:

(i) Control of a single-acting hydraulic cylinder,

(ii) Control of a double-acting hydraulic cylinder,

(iii) Regenerative circuit,

(iv) Regenerative cylinder circuit for machine tool applications,

(v) Pump-unloading circuit,

(vi) Double-pump hydraulic circuit,

(vii) Counterbalance valve application,

(viii) Sequencing of two double-acting cylinders,

(ix) Automatic cylinder reciprocating system,

(x) Synchronizing circuits,

(a) Synchronizing hydraulic cylinders connected in parallel

(b) Synchronizing hydraulic cylinders connected in series 

(c) Synchronizing hydraulic cylinders with flow control valves 

(xi) Fail-safe circuits

(a) Prevention from inadvertent cylinder extension

(b) Overload protection

(c) Two-handed safety circuit

(xii) Speed control circuits

(a) Meter-in circuit

(b) Meter-out circuit

(c) Bleed-off circuit

(xiii) Speed control of a hydraulic motor

(xiv) Hydropneumatic circuits

• Regenerative circuits are used to speed up the extending speed of the double-acting cylinder.

• Sequencing circuits are used for sequencing operations when two or more cylinders are employed.

• Fail-safe circuits are designed to safe-guard the operator, the machine, and the workpiece.

• Speed control circuits are designed to control the speed of the hydraulic cylinders.

• In meter-in-speed (or flow) control circuit, the flow control valve is located in between the pump and actuator. Thereby this circuit controls the amount of fluid • flowing into the cylinder.

• In meter-out speed (or flow) control circuit, the flow control valve is located in between the actuator and the oil reservoir. Thereby this circuit controls the fluid flowing out of the actuator.

• In bleed-off speed (or flow) control circuit, the flow control valve is located in between the pressure line and return line. Thereby this circuit controls the fluid by bleeding off the excess not needed by the actuator.

• In some applications, the hydraulic and pneumatic circuits are coupled to get best use of the advantages of both oil and air mediums. This combination circuit is known as hydropneumatic or pneumohydraulic circuits. Example: Air-over-oil circuit. 

• Hydrostatic transmission is special case of energy transmission system where the mechanical energy of the input drive shaft is converted into pressure energy in the nearly incompressible working fluid and then reconverted into mechanical energy at the output shaft.

• At the end of this chapter, automotive power-steering application of hydro- mechanical servo system and electro-hydraulic servo system are presented.

KEY TERMS ONE SHOULD REMEMBER

Fluid power circuits 

Hydraulic circuits 

Regerative circuits 

Pump-unloading circuit

Double-pump hydraulic

circuit

Sequencing circuits

Synchronizing circuits 

Fail-safe circuits 

Speed control circuits 

Meter-in circuit 

Meter-out circuit

Bleed-off circuit

Hydropneumatic circuits

Pneumohydraulic circuits 

Air-over-oil circuit 

Hydrostatic transmission system 

Mechanical hydraulic servo system 

Electrohydraulic servo system