HYDRAULIC

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Hydraulic Cylinders:

 

Pressure (Psi) = Force (lbf) / Area (in2)

 

Cylinder speed (ft/s) = 231 x GPM / (720 x net area (in2))

 

Cylinder volume capacity (Gal.) = Pi x radius2 (inches) x stroke (inches) / 231

 

Oil flow rate to cylinder = 720 x velocity (ft/s) x net area (square inches)

 

Figures 1 and 2 show how several hydraulic cylinder terms are defined

 

Figure 1 Typical cylinder, clevis at both ends

 

 

Figure 2 Typical cylinder, clevis end and flat base-end

 

Table 1: Pushing-force for hydraulic cylinders (thousands of pounds)

Bore Dia. (in.)

Bore Area (in2)

Pressure (Psi)

1500

2000

2250

2500

3000

2

3.14

4.71

6.28

7.07

7.85

9.42

2.25

3.98

5.96

7.95

8.95

9.94

11.93

2.5

4.91

7.36

9.82

11.04

12.27

14.73

2.75

5.94

8.91

11.88

13.36

14.85

17.82

3

7.07

10.60

14.14

15.90

17.67

21.21

3.25

8.30

12.44

16.59

18.67

20.74

24.89

3.5

9.62

14.43

19.24

21.65

24.05

28.86

3.75

11.04

16.57

22.09

24.85

27.61

33.13

4

12.57

18.85

25.13

28.27

31.42

37.70

4.25

14.19

21.28

28.37

31.92

35.47

42.56

4.5

15.90

23.86

31.81

35.78

39.76

47.71

4.75

17.72

26.58

35.44

39.87

44.30

53.16

5

19.63

29.45

39.27

44.18

49.09

58.90

5.25

21.65

32.47

43.30

48.71

54.12

64.94

5.5

23.76

35.64

47.52

53.46

59.40

71.27

 

Table 2: Pulling-force table for hydraulic cylinders (thousands of pounds)

 

Subtract the appropriate value given below from the pushing force value given in the pushing-force table to determine the pulling capacity of the cylinder based on the rod diameter

Rod Dia. (in.)

Rod Area (in2)

Pressure (Psi)

1500

2000

2250

2500

3000

0.5

0.20

0.29

0.39

0.44

0.49

0.59

0.75

0.44

0.66

0.88

0.99

1.10

1.33

1

0.79

1.18

1.57

1.77

1.96

2.36

1.25

1.23

1.84

2.45

2.76

3.07

3.68

1.5

1.77

2.65

3.53

3.98

4.42

5.30

1.75

2.41

3.61

4.81

5.41

6.01

7.22

2

3.14

4.71

6.28

7.07

7.85

9.42

2.25

3.98

5.96

7.95

8.95

9.94

11.93

2.5

4.91

7.36

9.82

11.04

12.27

14.73

2.75

5.94

8.91

11.88

13.36

14.85

17.82

3

7.07

10.60

14.14

15.90

17.67

21.21

3.25

8.30

12.44

16.59

18.67

20.74

24.89

3.5

9.62

14.43

19.24

21.65

24.05

28.86

3.75

11.04

16.57

22.09

24.85

27.61

33.13

4

12.57

18.85

25.13

28.27

31.42

37.70

Hydraulics Calculator

Cylinder Push/Pull Force  
  1. Enter bore size (inches)  
  2. Enter Rod diameter (inches)  
  3. Enter Pump pressure (psi)  
     
  Push Force (lbs)  
  Pull Force (lbs)  
 
Cylinder Speed  
  1. Enter bore size (inches)  
  2. Enter Rod Diameter (inches)  
  3. Enter Pump Flow (GPM)  
     
  Extension Speed (inches/minute)  
  Retraction Speed (inches/minute)  
 
Pump Displacement  
  1. Desired Flow Rate (GPM)  
  2. Operating Speed (RPM)  
     
  Displacement Required (cubic inches)  
 
Horsepower to drive a Pump  
  1. Enter Flow Rate (GPM)  
  2. Enter System Pressure (PSI)  
     
  Horsepower Needed (HP)
 HP is for electric motors, double this for gas engines.
 
Hydraulic Motor Speed  
  1. Enter Pump Flow (GPM)  
  2. Enter Motor Displacement (cubic inches)  
     
  Motor RPM (RPM)  
 
Hydraulic Motor Torque  
  1. Enter Motor Displacement (cubic inches)  
  2. Enter System Pressure (PSI)  
     
  Motor Torque Output (in/lbs)  

 

Hydraulic Reservoirs:

 

Hydraulic reservoir sizing: One gallon of capacity per one GPM of pumping capacity.

 

Reservoirs should be tall and narrow rather than short and wide so as to reduce vortex effects and improve heat dissipation.

 

Hydraulic Pumps & Motors:

 

Pumping: 1 Hp = 1 GPM x 1500 Psi (linear relationship i.e. 2 GPM @ 1500 Psi = 2 Hp)

 

Displacement (in3 / rev) = GPM x 231 /RPM

 

Hp to drive hydraulic pump  = Psi x GPM / 1714

 

Typically assume hydraulic pump/motor efficiency of 85%

 

Hydraulic motor torque (in-lb) = Pressure (Psi) x motor displacement (in3/rev) / (2 x Pi)

 

Hydraulic motor speed (RPM)= 231 x GPM / motor displacement (in3/rev)

 

Hydraulic motor power (Hp) = Torque (in-lb) x RPM / 63025


Hydraulic Valve Terms:

 

Open-center, open-center-power-beyond and closed-center are terms used to describe hydraulic valves in the neutral position.

 

Open-Center: Typically used with a fixed-displacement pump, allows oil to free-flow back to the tank in neutral position. Shifting the spool redirects oil to the selected work port.

               

Open-Center-Power-Beyond: Same as Open-Center valve except oil flows to downstream circuit in the neutral position instead of returning to the tank.

               

Closed-Center: Typically used with a variable-displacement pump, oil flow is blocked at the valve until the spool is shifted from neutral.

 

Motor-spool: In the neutral position fluid is allowed to flow back to the tank. This allows the operator to run a hydraulic motor under load and, when the valve is shifted to stop flow to the motor, allows the motor to coast to a stop.

 

Cylinder-spool: In the neutral position fluid is blocked from flowing to the tank. This effectively locks the load in place and should be used in applications where a load is to be raised and held aloft with a hydraulic cylinder.

 

Three-way valve: Typically used to control single-acting circuits

 

Four-way valve: Typically used to control double-acting circuits

 

Hydraulic Hose Sizing:

 

Hose Purpose

Recommended Flow Speed (ft/s)

Intake/suction

2-4

Return

4-13

Pressure

7-18

               

Flow speeds at the lower end of the ranges should be used when designing for a continuous duty system. The inside diameter of the hoses should be chosen such that the needed flow rates can be supplied at fluid speeds within the acceptable ranges listed above. The following equation can be used to relate flow desired flow rate to hose diameter:

 

Q = D2 *V

                       4*231

 

Where Q is fluid flow rate in gallons per minute, D is the inside diameter of the hose in inches and V is fluid speed in feet per second.