Hydraulic Circuit Design

Hydraulic Circuit Design & Analysis Dr. Sunil Jha

MEL334: Low Cost Automation

1

Design Considerations 

Safety of Operation 

Pressure and Temperature ratings

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Interlocks for sequential operations

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Emergency shutdown features

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Power failure locks

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Operation speed

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Environment conditions


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Meet functional requirements 

Meet required performance specification

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Life expectancy same as machine

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Facilitate good maintenance practice

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Compatibility with electrical and mechanical components

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Withstand operational hazards


3


Efficiency of Operation 

Keep system Simple, Safe and Functional

Access to parts need repair or adjustment

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Design to keep min operational cost

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Design to prevent and remove contamination.


4

Linear Circuits 

Simple reciprocating circuit • Reservoir • • • • • • • •

Strainer Pump Flexible coupling Electric Motor Connectors Relief valve DCV – 4 way Hyd. Cylinder


5

Circuit Design Approach 

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What are the specifications of the job? 

Force requirement – 8 kN

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Length of work stroke – 15 cm

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Speed of piston and rod assembly – 0.5 sec

What Size of Cylinder is needed? 

Force known. Cylinder Area & Operating Pressure must be selected. F = PA

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Standard bore and rod size cylinders MEL334: Low Cost Automation

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Cylinder Selection Reasoning 

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Large Diameter Cylinder 

Operates at Low Pressure

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Requires Bigger pump for speed

Small Diameter Cylinder 

Operates at High Pressure

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Smaller pump give speed

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Cylinder selected = 50 mm bore

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Find Pressure and then select suitable pump MEL334: Low Cost Automation

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What Capacity Pump is needed? 

Max Cylinder Speed required

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Flow rate = (Cyl Area)x(Stroke)/(time)

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Pump selection

What size of Electric Motor needed? 

Calculate power required to run Pump

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Consider efficiency of pump

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Shaft size, type and electric source MEL334: Low Cost Automation

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What Size reservoir should be used? 

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2.5-3.0 times the pump capacity

What size of Electric Motor needed? 

Calculate power required to run Pump

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Consider efficiency of pump

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Shaft size, type and electric source


9


Size of pump inlet? 

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Size of Discharge tubing? 

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Inlet flow velocity = 60-150 cm/sec Outlet flow velocity = 200 – 450 cm/sec

Relief valve selection Pressure range  Valve port size 

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Direction Control valve 

Based on function MEL334: Low Cost Automation

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Control of a Single Acting Hydraulic Cylinder Two Position Three Way Manually Actuated Spring Offset DCV


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Control of a Double Acting Hydraulic Cylinder Three Position Four Way Manually Actuated Spring Centered DCV


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Regenerative Circuit Qt 

Pressurized fluid discharge returned to system

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Speed up extending speed

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Retraction bypass DCV

Extend

Retract

Qr

Qp

Qt = Qp + Qr


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Drilling Machine Application 

Spring centered position – Rapid spindle advance

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Left envelope – Slow feed

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Right envelope – Retracts piston


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PUMP Unloading circuit 

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Unloading valve unloads the pump at the ends of extending and retracting strokes As well as in spring centered position of DCV

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Double Pump Hydraulic System  

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Punch Press Initial Low Pressure high flow rate req. When punching operation begins, increased pressure opens unloading valve to unload low pressure pump.

Relief Valve

Unload Valve


16

Counterbalance Valve 

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To keep vertically mounted cylinder in upward position while pump is idling. Counterbalance valve is set to open at slightly above the pressure required to hold the piston up.


17

Hydraulic Cylinder Sequence Circuit 

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Left Env: Left Cyl extends completely and then Right Cyl extend. Right Env: Right Cyl retracts fully and then Left Cyl retracts. MEL334: Low Cost Automation

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Automatic Cylinder Reciprocating System 

Two sequence valve sensing strokes completion by corresponding pressure build up.


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Locked Cylinder using Pilot Check Valves 

Lock Cylinder so that piston can not move by external load.


20

Cylinder Synchronizing Circuit

   

Cylinders connected in parallel Loads identical – Moves in exact synchronization Loads Not exactly Identical (practical situation) Cylinders also not exactly identical (packing Friction) MEL334: Low Cost Automation

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  

Cylinders connected in Series For two cylinder to be synchronized Piston Area of Cyl 2 = Piston Area of Cyl 1Rod area MEL334: Low Cost Automation

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Pump pressure should overcome load acting on both cylinders. P1 Ap1- P2(Ap1-Ar1) = F1 P2 Ap2- P3(Ap2-Ar2) = F2 MEL334: Low Cost Automation

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Cylinder Synchronizing Circuit Q. For the Cylinder Synchronizing Circuit, what pump pressure is required if the cylinder loads are 22kN each and cylinder 1 has a piston area of 65 cm2?

Ap2 = Ap1-Ar1 P3 = 0 P1 Ap1 = F1+F2


24

Fail Safe Circuit 

Designed to prevent injury to operator or damage to equipment. 

Prevent Cylinder from accidentally falling on an Operator in the event of: 

Hydraulic line ruptures

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Person inadvertently operates manual override on Pilot actuated DCV when pump not operating


25

Fail Safe Circuit with Overload Protection 

DCV-1 controlled by Push button valve-2.

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When Overload Valve -3 is in spring offset mode, it drains the pilot line of valve 1.

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If Cyl experience excessive resistance, Valve-4 actuates overload valve -3. This drains pilot line of Valve1, causing it to return to spring offset mode.

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Nothing happen if push button 2 pressed unless overload valve shifted manually into blocked configuration.


26

Two hand Safety Circuit 

Designed to protect an operator from injury.

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For circuit to function, operator must depress both manually actuated valves.

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Any one button prevents operation.


27

Speed Control of a Hydraulic Motor 

Hydraulic Motor – Rotary Motion

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Spring Center Position 

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Left Envelop 

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Motor hydraulically locked Rotates Clockwise

Right Envelop 

Rotates Anticlockwise MEL334: Low Cost Automation

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Speed Control of a Hydraulic Motor


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Hydraulic Motor Braking System 

  

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Hydraulic motor may be driving Machine having a large inertia. Creates Flywheel effect. Stopping motor, acts as a pump. Circuit is designed to provide fluid to the motor while it is pumping. Provisions should be made for discharge fluid from motor to be returned to Tank. This would stop motor without damaging. MEL334: Low Cost Automation

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Hydraulic Motor Braking System


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Hydrostatic Transmission 

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Open Circuit Drives 

Pump draws fluid from reservoir

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Pump output directed to Hydraulic Motor

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Discharge from Motor into reservoir

Closed Circuit Drive 

Exhaust oil from the motor returned directly to pump inlet.


32

Closed Circuit One-Direction Hydrostatic Transmission 

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Closed Circuit that allows only one direction of motor rotation. Motor speed varied by changing pump displacement. Torque capacity of motor adjusted by pressure setting of the relief valve.


33

Closed Circuit Reversible Direction Hydrostatic Transmission


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Hydraulic Circuit Design

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