One of our members wrote me this question:
“One of our customers has a new wheeled excavator. The auxiliary circuit on this machine is controlled by an electronic proportional foot pedal which sends its signal to an ECU and this ECU then sends a variable voltage to 2 proportional solenoids located on auxiliary circuit sandwich on the main DCV. But the operator dislikes this foot control of the auxiliary circuit and would like to have it controlled by ON/OFF switches on the joysticks. Luckily enough, this new machine has pre-installed electric circuitry which enables simple ON/OFF control of whatever function directly from joystick switches. So from a practical point of view it is not a problem to disconnect the original electrical connectors to the proportional solenoids and plug in the ON/OFF electrical connectors instead.
However, I know proportional solenoids are not designed to be supplied with full battery voltage. So I am concerned about solenoid life if we connect them to constant, full battery voltage. On the other hand, I also know that many field mechanics do this commonly when the proportional solenoids’ electronic controller fails and the customer does not want to invest in a new one. In these cases, connecting the proportional solenoids to constant DC voltage is common practice. What is your opinion to this practice as far as the useful life of these proportional solenoids is concerned?”
If a proportional solenoid is connected to supply or battery voltage, the life of the proportional solenoid will depend on the coil’s resistance and the magnitude of the difference between battery voltage and the solenoid’s normal maximum operating voltage. And this varies.
On mobile sandwich valve sections it’s common for electro-proportional control to be achieved using an electro-proportional, pressure reducing valve to vary a hydraulic pilot signal, which shifts the valve spool. According to the data for a Rexroth FTD2REK, 24 Volt proportional pressure reducing valve — see inset pic, which can be used to actuate their mobile sandwich DCV sections, coil resistance is 12 ohms and current draw at maximum output pressure (max. pilot signal) is 0.8 Amps.
Using the formula for Ohm’s Law: V = IR (Volts = Current x Resistance) the solenoid’s normal maximum operating voltage is 9.6V (0.8 x 12). If battery voltage of 24V is connected directly to this proportional solenoid, then using the formula I = V/R, the coil will see 2 Amps (24 รท 12). This is 2-1/2 times normal current. Suffice to say the solenoid coil will get hotter at this voltage (and current), something which will certainly shorten its life. Excessive heat is the enemy of every solenoid coil because it breaks down the insulation on the wire in the winding. And when this happens the coil eventually shorts out.
With this in mind, for the machine owner who doesn’t like the foot pedal operating the auxiliary circuit, the better solution is to switch the proportional solenoids ‘ON’ by doing it through the ECU. Assuming the foot pedal is sending an analogue control signal to the ECU, for example +/- 10V, then it’s a relatively simple exercise to disconnect the foot pedal and use the joystick switches to switch in and out fixed resistors that correspond to the maximum control signal the foot pedal generates.
Of course, this option is not available when the ECU or amplifier card driving the proportional solenoid is faulty. In this situation, directly connecting battery voltage to the solenoid coil can be a mistake that turns out to be more costly in the long run. And to discover six other costly mistakes you want to be sure to avoid with your hydraulic equipment, get “Six Costly Mistakes Most Hydraulics Users Make… And How You Can Avoid Them!” available for FREE download here.