Directional Control Valves - Part 3

More on Coils

Solenoids are devices that are capable of changing electrical energy into mechanical, or linear, energy. The simplest type of solenoids assemblies used in hydraulic valve applications relies on two main aspects for their function: an insulated (or enamelled) wire, shaped into a tight coil, and an armature of varying designs also colloquially called the “iron core” which contains a solid rod or pin of either iron or steel. The iron or steel pin is ferromagnetic, a property that allows it, when exposed to electrical current, to function as an electromagnet. The solenoid uses the magnetic field created from an electrical current as the trigger for the production of a push or pull that drives mechanical action into the pin. Solenoids that rely on electrical current fall into two main categories-- solenoids that rely on AC (alternating current) as the source of power and solenoids that rely on DC (direct current) as the power source. 

While AC and DC solenoids use different types of current, they both work in the same basic manner. When the insulated, coiled wire of the solenoid receives electrical current, the magnetic field produced strongly attracts the pin which pushes the spool inside the valve to change the flow path (“on”). The spool is attached to a compression spring on the opposite side which is compressed till the current is stopped. When the current is turned off, the compressed spring forcefully snaps the pin and spool back into its original position (“off”).

Armature Design:

Two common designs for solenoids are (a) the air gap design; and (b) wet armature design. In the air gap design, the two sections (armature and spool) are isolated from each other using dynamic seals.The downside of this design is that the seal wears off eventually causing leakages from the valve into the armature. The wet armature design is more common with the entire armature assembly submerged in oil from the valve. The solenoid magnetically moves the armature while it is submerged in oil. This design provides a lower leakage due to the absence of wearable dynamic seals. The only sealing in wet armature designs is the O-ring that seals the threaded connection between the iron core and the body and, if present, the one on the manual over-ride push pin which, although a dynamic seal, only acts when the valve has to be manually reset once in a while. (Figure 16) Another advantage of this design is that the movement is naturally dampened by the presence of oil. This provides a smoother, more quite movement with a longer service life. The viscosity of

the oil, however, means that the wet armature design needs 60% more power to actuate. 

Voltage Selection:

There are four main types of voltages used with solenoid valves: 12 VDC, 24 VDC, 110-115 VAC and 220-230 VAC. Although the 115 VAC finds hardly any usage outside North America, the remaining three are quite common in the rest of the world. Now, though the type of current (Direct or Alternating) is the prerogative of the designer, the voltage rating of the solenoids entirely depend on the power required to shift the spool. 

Both the categories (AC and DC) have their own set of advantages and disadvantages. DC solenoids are quieter and require less maintenance than AC coils. On the other hand, they function more slowly than AC  solenoids and are also less powerful than AC solenoids. In AC solenoids, the current that runs through the solenoid starts with a first rush of extremely strong current, then drops to a lower, normal level as the solenoid gap reduces. Thus if the spool gets stuck in the open (full-current) position for too long in the body due to contamination particles in the land areas, it receives too much of this first wave of maximum current and it can permanently damage the device by allowing the coil to burn. By contrast, DC solenoids experience no alteration in currents and do not run the risk of being damaged by the current. Off late, all solenoids are made to function on DC voltage. An AC Solenoid would have a DC Coil rated to a voltage close to the required input AC voltage (195 VDC coil for a 220 VAC input) and a rectifier plug that would convert the incoming AC current into DC (Figure 17). Although DC circuits can be utilized with AC solenoids without a problem, DC solenoids cannot be used on other circuits without becoming noisy and overheated and possibly burning out due to excess current. 

Due to the inherent problems with AC Solenoids, a lot of Original Equipment Manufacturers are rethinking their designs and opting for DC Solenoid Valves. Using the same voltages that are predominantly used in their equipment’s PLCs, it is less of a hassle giving longer service lives free from maintenance issues. 

Conclusion

In conclusion, the selection of a proper directional control valve is as important as its use in the circuit. The above article will help new initiates in hydraulics to select a proper valve for their circuit while refreshing the fundamentals of people already well versed in hydraulics. It’s best to check requirements of the system and decide the best valve for the job. Some of the circuits are highly traditional and outdated and need a major revamp on spool and solenoid selection. Some applications become a lot easier with the help of poppet valves instead of sliding spool valves.

Click Here for Directional Control Valves - Part 1
Click Here for Directional Control Valves - Part 2