Oil goes from port A of the control valve lifts the counter pressure valve and continues to the cylinder. It will also produce servo pressure to the pilot piston. Returning oil goes from cylinder over pressure reducer and the poppet of the load-holding valve into port B of the control valve.
Note the connection line T3 to T. It is important for conveying reference pressure from the control valve tank channel to the spring housing of the pressure reducer so that wanted pressure difference 8 bar is maintained without influence of counter pressure in the return line.
HYDRAULIC CONTROL SYSTEMS
HYDRAULIC CONTROL SYSTEMS
H003-3
Function
The working principle is similar to the one we know from a control valve having a built-in port relief valve. The difference is the load-holding valve being fitted directly on the cylinder, which gives a faster reaction. This results in good function for pressure shocks.
Symbol drawing
Cylinder oil at over pressure opens the relief valve. Return oil goes in a separate line to the tank channel of the control valve. At the same time reference pressure is being fed back from the return line
Load-holding valve 91
(Overload)
HYDRAULIC CONTROL SYSTEMS
HYDRAULIC CONTROL SYSTEMS
H004-3
Description
This is a further development of the V91load-holding valve to include load holding for lifting as well as lowering. All functions are built into the top left cartridge. Both pressure relief valves have been fitted with suction valves.
Function
When lifting, the piston side check valve opens as earlier. But at the same time the collar of the load-holding cartridge is pushed upward which makes the seat of the inner poppet open and return oil can pass out through V2. There is no pressure reduction when lifting. In this boom position there is no practical difference compared to a single-acting load-holding valve.
Symbol drawing
The symbol for the complete load-holding valve has been altered from V91load-holding valve so as to make one of the check valves pilot operated.
Load-holding valve DHVL
(Lifting)
HYDRAULIC CONTROL SYSTEMS
HYDRAULIC CONTROL SYSTEMS
H005-3
Function
Oil is directed into V2 where it pushes down the load holding cartridge before going further to the right for build- up of 30 bar counter pressure, then out into C2 and the cylinder. When the pressure builds up the load holding cartridge pushes the pilot piston down opening the small poppet. Locked-in pressure in the reducer and the backside of the large poppet is released and the large poppet is lifted. At the same time the pressure reducer begins releasing oil from the cylinder but at 13 bar pressure level, this being the level at which the reducer piston is balanced. After this, return pressure 13 bar sinks due to pressure drop in the lines down to the required pressure difference 8 bar across the spool before reaching the control valve. In this way ideal conditions for proportional flow control are maintained over the full effective spool stroke even for lowering.
Load-holding valve DHVL
(Lowering)
HYDRAULIC CONTROL SYSTEMS
HYDRAULIC CONTROL SYSTEMS
H006-3
Function
Outward movement of the outer boom is to be prevented. Pressure from the piston rod side distributed into C2 is kept locked in by the check valve of the pressure relief valve and also by the long and narrow poppet of the load-holding valve. The upper part of the load holder cartridge is drained to tank via the dashed passage. This gives a load holding function for the piston rod side also.
Symbol drawing
The pressure is kept locked in by check valves.
Load-holding valve DHVL
(Holding)
HYDRAULIC CONTROL SYSTEMS
HYDRAULIC CONTROL SYSTEMS
� � � � �� H007-3
Description
Figure 7 illustrates the V91 load-holding valve in an overload condition. The valve spool is in neutral.
Function
Oil at overload pressure forces the shock relief valve to open at 330 bar. Vacuum created on the piston rod side when the boom sinks will open the suction valve of the rod side port and oil will be refilled.
Since the load-holding valve is situated directly on the cylinder a rapid response to shock loads is provided. Suction valves take care of the refilling
Symbol drawing
Cylinder oil at over pressure opens the relief valve. In a separate line return oil goes to the tank channel of the control valve. At the same time oil is refilling the rod side by means of the suction valve.
Load-holding valve DHVL
(Overload)
HYDRAULIC CONTROL SYSTEMS
HYDRAULIC CONTROL SYSTEMS
H008-3
Description
Figure 8 illustrates the crossover valve with a double load holding function for the slew function. The valve is shown in two cross-sections. The top one shows the load holding functions, the bottom section the shock functions.
Function
Oil enters V1 where it pushes down the cartridge, lifts the check valve, continues out via port C1 to the left slew cylinder. The pushed-down cartridge at the same time acts upon a piston releasing return pressure and opens the load holding function for return oil. Slewing speed is controlled by the rate of oil fed in without regard to downhill or uphill. Oil from the valve is supplied to the cylinder via the check valve. Oil is allowed to return over the load holding function.
Double load-holding/crossover valve DLC (Slewing)
HYDRAULIC CONTROL SYSTEMS
HYDRAULIC CONTROL SYSTEMS
H009-3
Description
The slewing system is to be held stationary in a certain position. The valve spool in neutral position.
Function
Both pressure and return oil are kept locked-in by poppet type check valves. This results in no slewing creep,even on a steep downhill slope.