Direct Control Valves (DCVs) that aren’t working correctly can cause problems with your customer’s hydraulic system. You can troubleshoot some DCV problems on the fly, when the hydraulic system is online. This article provides information on 3 DCV problems your end-user should watch for.
There are many ways for DCVs to degrade system performance. Most common are Sticking/will not shift, Internal Leakage and External Leakage.
Problem #1: Sticking/will not shift
Most DCV valves use a sliding spool/bore design. When a valve sticks, the spool cannot change positions when an operator applies normal force. Several factors can cause this sticking action, including contamination, silting, mechanical failure or operator failure. Let’s look at these common causes.
When hard particle contamination gets between the bore and the spool, the normal amount of force required to move the spool increases beyond the limits of the operator. The spool becomes jammed and will not move unless the contamination is removed. Forcing the spool to move will damage the bore and spool, causing gouges and wear. This sets up a circle of contamination, which causes increasing damage until the valve begins to leak excessively. Taking apart the valve and removing the contamination before it causes more damage is the best way to fix the sticking action.
Silting is similar to contamination but usually involves soft contamination like sludge and varnish. Silting causes the sliding force to move the spool to increase and the operator does not generate enough force to move it past the silt in the hydrostatic lands and bores. Taking the valve apart and cleaning all the parts is the best way to correct a silting action.
There are many small parts inside of a DCV and some are prone to breakage. Springs, pins, washers, and detent devices can break and cause the valve to jam or stick. Replacement of the broken parts or the complete valve will be required.
The DCV uses some type of external signal to cause it to change position. The most common types of operators are electrical (solenoids), hydraulic (pistons), mechanical (levers and rods), and pneumatic (pistons). The first thing to determine is if the sticking action is occurring by something inside the hydraulic portion of the DCV or the operator. Most valve operators have a small mechanical device called a manual override so the mechanic can manually operate the valve. If the valve shifts when the manual override is used but not when the normal signal is used, then more than likely, it is an operator issue. If the valve will not shift when the manual override is used, the problem is more than likely a hydraulic or mechanical problem inside the DCV. A failure of the hydraulic section of the valve can cause solenoid coil burn out which will need to be corrected when the DCV is disassembled and rebuilt.
Problem #2: High Internal Leakage/Heat
Sliding spool valves are prone to wear, especially in hydraulic systems that have high levels of contamination. As the spool and bore wear, more pressurized fluid is lost through these increased clearances to return to the reservoir without providing any useful work. The energy generated by moving from high pressure to tank pressure is released as heat. As the valve temperature increases, it will cause even more leakage. The most common symptoms of high leakage in DCVs are slower cycle times, slower actuator speeds, and drifting or moving of actuators when they should not be moving. The only fix for DCVs with high leakage is complete replacement.
Problem #3: External Leakage
External leakage in DCVs is usually caused by seal failure, spool wear, pushpin wear or a solenoid core tube failure. Seals, push pins and solenoids can be replaced but if the seal area of an exposed manually operated spool becomes worn or damaged, it cannot be repaired and will have to be replaced.
The best way to prevent Directional Control Valves from failing is to prevent system contamination. Keeping the oil clean, cool, and dry (preventing water contamination) will prevent most DCV failures and promote longevity of the DCVs in the hydraulic system.
Steve Downey is an Adjunct Fluid Power Instructor at Henry Ford Community College and Macomb Community College. He has worked in the Fluid Power Industry for 30 years in both Industrial and Mobile Hydraulics. Steve Holds 11 International Fluid Power Society certifications.