Directional Control valves

Transcription

1 Chapter 10.1 Chapter 10 Directional Control valves 10.1 Check Valves Figure 10.1 Check valve wo conditions: free flow or "checked" flow Usually has a cracking pressure of 69 a 104 a (10-15 psi) If used with a large spring the cracking pressure is 520 a (75 psi) (typically) 10.2 Examples of using check valves: Use of a check valve as a pilot pressure source. o pilot lines is at least 520 a (75 psi) Figure 10.2 Check valve as a pilot pressure source his application ensures that as long as the pump is running, a pilot pressure of 520 a is available for other circuits even if the main line pressure is lost ilot Operated C.V. here are two types: internal and externally drained and they have a significant effect on sizing and application. Internal Drain External Drain

2 Chapter ilot operated check valve, internally drained A = A A 3 1 S S Detail A 1 D A 3 A 1 Figure 10.3 NOE: his small poppet can be used to decompress a large volume before fully opening ilot operated check valve Force Balance A 3 = D (A 3 -A 1 ) + s A 1 + Kx = ( s - D ) A 1 A 3 + D + Kx A 3 Note A 3 /A 1 is called the ILO RAIO and is labeled as R: It is used to determine the pressure at which the pilot will open. = ( s- D) R + D + Kx A 3 NOE: If D =/ 0, problems can occur since the opening pilot pressure will be a function of both S and D. his is OK his is not OK his is OK but not for locking his is OK Figure 10.4 ilot operated check valve configurations

3 Chapter ilot operated check valve, externally drained A 4 S A 2 D A1 A 3 A 1 Figure 10.5 ilot operated check valve, externally drained In this configuration, the chamber of A 4 is at tank pressure. hus is relatively insensitive to D ilot ratio is thus A 3 A 1 = R Opening forces A 3 + D A 1 Closing forces S A 1 + D A 4 + Spring or A 1 A 4 = ( S - D ) A + D 3 A + C, 3 where C is the spring force so = S- D R + D A 4 A 3 + C Compare to internal drain = S- D R + D + C Since A 4 <<A 3, is not as dependent on D for external drain Example of using pilot checks he following examples show how a pilot operated check valve is often used.

4 Chapter 10.4 Cross-over check valves Figure 10.6 ilot operated check valve applications Example A ratio = 1.125/1 S = 3000 psi mg Figure 10.6 System for example calculation We will use this example using imperial units.

5 Chapter 10.5 Internal Drain: = 3000psi (Assume 3:1 pilot Ratio and 75 psi spring) = 1075 psi ilot pressure is 1075 psi to open. Now: his means that must > 1075 psi. his will increase the s to 3000 psi + A ratio 1075 = 4200 psi. But, will now increase since is also a function of s, that is = s- D R + D + C = = 1478 So must increase to 1478 psi - But that increases s and so on. Eventually, a balance will occur. We must modify our equation to reflect this situation. = L + C 1 - AR c D = 0 L = Load induced pressure (original) C = spring constant factor AR v = ilot ratio area blind end AR c = area rod end In our example, L = 3000 C = 75 AR v = 3 AR c = = = 1720 psi Note: ressure s will now be AR c = 4935 psi DANGER!! his is even larger if a down stream pressure exists. If there is a residual pressure D due to a valve downstream, then = L - D + D + C 1 - AR c

6 Chapter Summary AR C D S AR V AR C S AR V D = = = = L - D + D + C 1 - AR c L - D + D A4 A3 + C L - D + D + C 1 (1 - ARc ) L - D + D A4 A3 + C 1 (1 - ARc ) s = L + (ARc) s = L + (ARc) s = L + ARc s = L + ARc Some special considerations. After a check valve (CV), one must use either a counterbalance valve (CBV) or flow control to prevent erratic free fall of load. If a pilot operated C.V. is to be used, then the pilot line must be at tank sometime so that the CV can close. When using a pilot C.V., always use a floating center valve to prevent drift in the actuator. V Figure 10.8 example

7 Chapter 10.7 Consider Figure 10.7 Recall that most pilot C.V. have a small poppet inside the main poppet as illustrated. Figure 10.9 small poppet his small poppet can be used to decompress a closed volume slowly and then move fluid through the main poppet at a lower pressure. Large volume Use a small pilot to open small poppet (to decompress). Use a larger pilot to open large poppet for free return of platten at low pressure. Figure ilot check valve to decompress a large volume ressure settings can become a problem since the pilot pressure necessary to open the small poppet is < main poppet wo-way Valves - Are spool type or rotary - Are actuated by solenoids, mechanically, hydraulically

8 Chapter hree-way Valves Figure hree way spool valve 10.5 Shuttle Valves ort out ort 1 in ort out port 2 in ort 1 in Figure Shuttle valve

9 Chapter Four-way Valves Figure Four way valve hese valves can be shifted manually, or by solenoid (direct and pilot) Figure Manually Operated Four-Way Valve coil spool Armature push rod Figure Solenoid-operated spool valve In many cases, two stage or pilot valves are used to actuate the main spool. his is shown in Figure 10.14

10 Chapter B A B A Figure ilot operated spool valve- basic principle Figure Hand operated spool valve

11 Chapter Figure Solenoid-pilot operated four-way valve For pilot operated valves, a higher pressure must be ensured for switching. his can be accomplished by using a check valve. to other pilot circuits Figure Maintaining positive pressure for pilot operated valves. In many cases, three positions are available in directional valves. he centered positions can vary and are demonstrated below. ypes of centers

12 Chapter Open centre Figure Centre types for valves

13 Chapter ank ort A s ort B andem centre Figure Centre types for valves (cont.)