Torque is the combination of tangential force acting at a distance from the axis of rotation. Proper torque is absolutely necessary if you want optimal performance from your valves. Proper torque prevents your cartridge valve from backing out under high pressures/high cycles, ensures proper operation of the valve's spool and prevents leakage. However, you must take caution when applying torque to a valve as over-zealous torque is one of the more common start-up or post-service issues (along with contamination, which was discussed in a previous post).
The solenoid tube is a thin walled stainless steel tube that acts as a guide for the armature (plunger). When the coil is energized, the magnetic force causes the ferrous armature to move inside the tube causing the valve to shift. The wall thickness of the tube is just over 0.050" and the clearance between the inside diameter of the tube and the outside diameter of the armature can be as little as 0.002".
The red area in the drawing below shows the internal clearance separating the tube and the armature (plunger).
When you are screwing a valve into a cavity, applying excessive torque can cause the valve to bind. Not only can this prevent the valve from shifting, but it can also significantly degrade the hysteresis traits of the valve. This happens because the material has been stretched. And what happens when you stretch a material? When you stretch a tube the material yields and the diameter decreases and pinches the armature. Because of the pinched armature, the spool of the valve cannot operate properly, and therefore the valve does not shift. This can make for a frustrating troubleshooting experience because valve performance issues caused by over torque or improper torque may not be visible to the naked eye (see below). The valve must be replaced if the tube stretches to the point where permanent deformation occurs.
Shown above is a line-up of valves that have been over-torqued. Valves on the far right of each group are new/properly operating valves, the others have torque related deformation...kind of hard to tell the difference visually....
The good news is that this problem can be easily remedied by ensuring that you always use the correct coil nut and by making sure that the valve is properly torqued in the first place. Recommended torque values for valves and coil nuts are shown on all individual HydraForce data sheets and catalog pages. You can find the appropriate part numbers for all coil nuts HERE (see pages 18 and 19). In addition, the latest torque information for our entire range of products can be found HERE.
About the Author:
Craig Sinnott is a Regional Manager at HydraForce with more than 15 years of hydraulic experience. Contact Craig