Unit - 6

System Analysis and Design

6.1 Calculation of Piston Velocity and Acceleration or Deceleration

Depending upon purpose of operations, the hydraulic cylinder may be subjected to various accelerations or decelerations steps. The piston of cylinder is accelerated to reduce cycle time that means to minimize idle time and the piston of cylinder is decelerated to reduce impact loads and provides proper cushioning.

The following equations are used to study the dynamics of piston-cylinder mechanisms.

Following equation is used to find out the thrust or force generated

The other element which also plays a role in dynamics of piston and cylinder mechanics are the load inertia, seal friction, load friction, etc.

Dynamic thrust = 0.9 x Static thrust

Hydraulic Forces/Thrust, Pressures and velocities under static and dynamic applications, considering friction and inertia loads.

Figure shows the various force generated in the cylinder

Let,

Fp = Hydraulic Force Available for work

Fb = Back Pressure force

Ff = Frictional force generated

Fd = Dynamic Force

For smooth working,

m.a = Inertia force

m.g = Gravity Force

Hydraulic Force = Static Force + Dynamic Force

Friction Force

Seal and packing friction causes major loss of useful hydraulic force.

Let,

Ac = Circumferential Area of seal =

D = diameter of piston or inner diameter of cylinder

P = pressure of oil

µ = coefficient of friction

Fpre = Pre-load applied on seal

The friction force can be given as

Effective force

The cylinder force available for operation is called as effective force.

Effective force is some percent of theoretical force

Therefore, effective force is given as

The pressure drop inside the cylinder is given by,

Total Pressure drop

Pressure drop in inlet line/ pressure line

This is by considering a double acting cylinder.

Flow Velocities

Let,

Vf = Velocity of piston for forward stroke

Vr = Velocity of piston for return stroke

Qp = Pump flow rate

Velocity of piston during forward stroke can be calculated as

Velocity of piston during return stroke can be calculated as

Example

An 8 cm diameter hydraulic cylinder has a 4 cm diameter rod. If the cylinder receives flow at 100 LPM and 12 MPa, find the (a) extension and retraction speeds and (b) extension and retraction load carrying capacities.

Solution

Let us first convert the flow in LPM to before we calculate forward velocity =100

LPM=100/(1000*60)=1/600

Now

DC= diameter of cylinder =8cm =8*

=diameter of piston rod=4cm=4*

P=12MPa=12*N/or Pa

(a)  Forward velocity is given by

Return velocity is given by

(b) Force extension is given by

Force during retraction is given by

fret=p(Ap-AR)

=42238.9N=45.24 KN

6.2 Design Consideration in selection of hydraulic cylinder

There are so many factors involved in sealing elements working principle. Ignoring any of these factors could increase the possibility of a design mistake. Hydraulic cylinders have different working conditions. For example, the cylinder used in injection machine and loader both have different seal design criteria in terms of material and sealing elements.

Four main variables should be taken into account while choosing a sealing element.

1. Temperature:

Media and working condition temperature should be considered when choosing suitable sealing element.

2. Pressure:

System pressure and pressure type should be considered when choosing suitable sealing element.

3. Sliding Speed:

System sliding speed should be considered when choosing suitable sealing element.

4. Media:

Media type should be considered when choosing suitable sealing element.

The technical values related to these 4 main variables can be found in our technical catalogue for each sealing element. Apart from these 4 main criteria, there are other variables which also affect the working of hydraulic cylinders. Alp Seals consider these following conditions to be the most important ones.

5. Working ConditionsThis is one of the most important variables for hydraulic cylinder design. There are so many factors on this stage which deserve attention.

Main factors are as follows;

• Working in an open or closed area and dirty working conditions are most important criteria for choosing a wiper. It may be necessary to use an ice scraper in winter for cylinders in outside working conditions.
• Radial loads and working positions that affect hydraulic cylinders should be taken into consideration when deciding on the hydraulic cylinder design. For example; rain drops should be prevented from entering into the vertical functioning hydraulic cylinders that are in outside working conditions.

6.3 Design and analysis of Hydraulic/Pneumatic Circuit for practical application

To analyze the given circuit, following steps are followed

Step I: Draw circuit diagram as it is

Step II:label the circuit diagram

Step III:List the components

Step IV:Explain the sequence of operation

Example

Analyze the given hydraulic circuit.

Solution

Step I: Drawing the circuit

Step II: Labelling

Sr.no.	Component	Notation used	Specification/function
1.	Tank		To store hydraulic oil
2.	Pump		To pressurize hydraulic oil .
3.	Pressure relief valve		Normally closed valve , opens when system pressure crosses set limit
4.	Direction control valve		4/3 spring centered ,solenoid operated directional control valve
5.	Check valve (pilot operated)		Restrict flow in one direction and allows in other
6.	Sequence valve		Start operation two once operation one is over .
7.	Cylinder		Double acting cylinder.

Step IV: Sequence of operation:

1) For the forward position of valve V1 (DCV) (position I) fluid flows pump to the piston side of cylinder C1 and C3, which activates forward stroke of the system. Once the cylinder C1 and C2 reaches certain forward distance, more resistance encounters which turns pressure in pressure line to higher level. Due to this valve V3 (Sequence valve) gets open against the spring force and additional pressure/force is applied through the cylinder C2 and hence all three cylinders offers synchronized force to carry out operations. The return fluid from cylinder C1 and C3 passes through valve V4 (RPV) towards tank.

2) For the return position of valve V1 (DCV) (position II), fluid from pump flows to the rod side of the cylinder C1 and C3 through check valve V5. Hence return stroke is executed. The return fluid from cylinders C1 and C3 directly goes to tank through valve V1(DCV). As the pilot signal or valve V2

(check valve) is taken from main pressure line of the return stroke, the valve V2 (check valve) opens, which send return fluid from cylinder C2 to the tank.

6.4 Selection of components

The components are selected from manufacturer’s catalogue, which is given below:

1)     Suction Strainer

It is selected on the basis of capacity in terms of liters per minute (lpm).

2)     Oil Reservoir

The oil reservoir is selected on the basis of storage capacity required for particular operation

One can select the reservoir capacity 2 to 3 times the max flow rate. Generally it is min 4 times the max flow rate.

3)     Pressure gauges

Pressure gauge is to be selected on the basis of working pressure.

Range is to be twice the working pressure.

4)     Selection of pump

The hydraulic pump is selected on the basis of the pressure to be generated and the flow rate or delivery of the pump.

In case of vane pump, following table is selected

5)     Pressure relief valve

Pressure relief valve are chosen on the basis of the working pressure and the flow rate of the pump.

6)     Flow control valves

Selected on the basis of max working pressure and flow range

7)     Direction control valve

Selected on the basis of max working pressure and flow capacity.

8)     Check Valve

Selected on the basis of max working pressure and flow capacity.

9)     Pilot operated check valve/sequence valve

Selected on the basis of max working pressure and flow capacity.

10) Cylinder (maximum working pressure: 210 bar)

Selected on the basis of rod diameter and bore diameter.

Increasing bore diameter, cylinder becomes safe but also bulky.

11) Motor

Selected on the basis of Pressure and volume displacement.

Note: Prefer the selection on the basis of capacity.

6.5 Selection procedure of hydraulic system

Step I: Design of cylinder

Calculate Apfrom the above formula

And find the diameter of piston using

Select the model corresponding to the piston diameter.

Sometimes, pressure is not mentioned in the problem, so in such cases, select the model of cylinder firstly and then calculate the working pressure.

Step II:Flow rate / Discharge

From the given condition calculate the velocities of the extension stroke. Stroke may be divided into different parts giving different velocities.

Select the maximum velocity to calculate the flow rate.

This flow rate must be considered for further calculations.

Step III:Repeat the same calculation for return stroke

And

From the above formulas calculate max working pressure and flow rate for return stroke.

Now, for selecting the components, consider

1. Max working pressure from return and extension stroke
2. Max flow rate from return and extension stroke

Step IV:Calculate power

Calculate power considering max Q and max p

Step V:Selecting other components

From the above calculated values, select the other components by using manufacturer’s catalogue given in section 6.5.

Example:

A machine slide is moved by means of hydraulic cylinder. The motion of the cylinder is as follows,

a)     Initially it moves through a distance of 250 mm against a load at 15000N in about 5 sec.

b)    It is followed by a working stroke of 100 mm against an effective load of 35000 N. The feed rate during this part of stroke is required to be between 0.5 to 1 m/min.

c)     The return stroke is to be as fast as possible.

A meter-out circuit is to be used. Draw a circuit which will fulfil these requirements.

Solution:

Step-1

First meter-out circuit need to be usedv and forward stroke is split two parts.

Step-11: Design stage

Given    ,    , ,

A Calculation for forward stroke:

It is mentioned in the problem that initial movement during forward stroke is through a distance of 250mm against the load of 15kN in 5 seconds. So extension speed will be,

The second part of the forward stroke is through 100mm against the effective load of 35000N. The feed rate is 0.5 to 1 m/min. For safer side taking higher feed rate which is 1m/min.

Selection:- Selecting model “A5” of cylinder for getting highest safety.

From data given,

Bore diameter=piston diameter==100mm

Rod diameter=

Now, calculating pressure and flow rate for both part of forward stroke,

a)     For first part of stroke:

Flow rate for this part,

b)    For second part of stroke:

Flow rate for this part,

B) Calculation for return stroke:

In order to calculate return speed (which is as fast as possible), the maximum flow rate from forward stroke is considered.

Also for return stroke effective area for pressure will be

From this we can calculate the time required for the return stroke to cover 350mm(250+100) distance.

Time=5.24sec

Put the maximum working pressure and maximum flow rate(discharge) together.

System parameters:

a)     Maximum working pressure:

   (during second part of forward stroke)

b)    Maximum flow rate:

(during first part of forward stroke)

From above, let us calculate the power required by the system,

Power=P=p*Q

Step-III Selection of components

Following are the model selected from the design,

1)     Oil reservoir: Model:T2:

By thumb rule, reservoir capacity should be 2 to 3 times the maximum flow rate.

Capacity 70.68 liters

So T2 model is selected,

2)     Solution strainer: Model:S1:

From maximum flow rate of 23.56 lpm, we select the model S1.

3)     Hydraullic pump: Model:P5:

From the table 16.4 (b), selecting model for P5, for the flow rate of 35.6 lpm at the pressure of 70 bar.

4)     Directional Control Valve (DCV): Model:D2:

Both the DCV’s used are selected from Table 16.7 on the basis of working pressure (44.56 bar) and maximum flow rate (23.56 lpm). So selected model is D2.

5)     Pressure Relief Valve (PRV): Model: R3:

The satisfying model of pressure relief valve is R3 which is selected from table 16.5

6)     Flow Control Valve: Model:F4:

The flow control valve satisfying the system conditions is selected from Table 16.6. The model selected is F4.

7)     Cylinder:Model:A5:

The cylinder model “A5” is already selected, considering safety aspect.

Finally you can put the selected models in tabular form:

Component	Model selected	Model Capacity
Oil reservoir	T2	100 lit.
Suction strainer	S1	38 lpm
Hydraullic pump	P5	35.6 lpm, 70 bar
Directional control valve	D2	38 lpm, 210 bar
Pressure control valve	R3	30.4 lpm, 70 bar
Flow control valve	F4	0-24.6 lpm, 70 bar
Cylinder	A5

References:

1. Pipenger J.J, Industrial Hydraulics, McGraw Hill

2. Pinches, Industrial Fluid Power, Prentice Hall

3. Yeaple, Fluid Power Design Handbook

4. Andrew A. Parr, Hydraulics and Pneumatics, Elsevier Science and Technology Books

5. ISO - 1219, Fluid Systems and components, Graphic Symbols

6. Standard Manufacturer’s Catalogues