The torque from a jet pipe servo valve’s torque motor directs the jet to one receiver or the other, creating an imbalance in spool end pressures. The main spool will keep moving until the feedback spring, positioned between the main spool and the jet, pushes the jet pipe back to a near-neutral position. The main spool’s position will then correspond with the coil current.
The flapper nozzle comes in two designs: the rigid design, where the force from the nozzle flow is minimal compared to the spring and torque motor forces, and the flexible design, where the torque motor and nozzles are sized so the nozzle flow exerts a significant force on the flapper. The flexible design is considered more tolerant of certain contamination issues. The rationale is that when both fixed orifices are fully open and unobstructed, the unpowered flapper will center itself due to the combined effects of fluid-momentum force, the restoring force of a light spring, and the magnetic force in the torque motor.
When current enters the Automotive Lift Repair Tampa Florida torque-motor coil (a), it rotates the armature against a stiff feedback spring. The flapper, attached to the armature, blocks nozzle A and relieves nozzle B, which increases pressure PA and decreases pressure PB. This pressure imbalance shifts the spool to the left. The system eventually reaches a state where the flapper is nearly centered, the pressures are almost equal, and the spool settles at a position proportional to the torque (coil current).
If one Automotive Lift Repair Tampa Florida nozzle or fixed orifice becomes partially obstructed, the reduced flow creates less force on the blocked side of the flapper. The torque motor’s current then moves the flapper around a shifted neutral, but the pressure does not reach an extreme level. Consequently, the main spool might not fully shift in one direction.
The current in the Automotive Lift Repair Tampa Florida torque motor of a jet-pipe servo valve directs a jet nozzle, creating a pressure difference between two collector ports. For example, if the A-port pressure is high, the main spool shifts to the right. Simultaneously, the feedback spring pulls the jet nozzle toward the center, approximately equalizing the collector pressures. Thus, the main spool is positioned according to the coil current.
The swinging wand, depicted in Figure 6 above path D, features a proprietary mechanical-to-hydraulic interface. It comes in two versions:
1. Dual Nozzle: Two fluid streams are deflected off the wand’s outer edges.
2. Single Nozzle: A single fluid stream passes through a central hole in the wand.
In the Automotive Lift Repair Tampa Florida dual-nozzle version shown in Figure 9, the fluid streams from the pilot head are collected in opposing receiving ports. When the torque motor’s current causes the wand to swing, one receiving port’s pressure increases while the other’s decreases. As with the jet pipe and flapper nozzle pilots, this pressure difference moves the valve’s main spool.
The swinging-wand pilot stage creates a differential pressure in receiver ports C1 and C2 by deflecting two fluid streams off each edge of the wand. An unseen torque motor moves the wand according to the current, resulting in a pressure difference between C1 and C2 that reflects the coil current. Port pressures can be equal (a), C1 pressure higher (b), or lower (c).
The Automotive Lift Repair Tampa Florida single-nozzle version has a centrally bored hole in the wand through which the fluid stream passes. When the wand is centered, the two receiving ports have equal pressures. As current flows into the coil, the wand shifts, deflecting the fluid stream off the central hole’s edge, creating different pressures in the receiving ports. This differential pressure moves the main spool.
This swinging-wand design has a supply pressure limitation as the pilot head must be sized for a specific pressure range. Excessive flow can pin the wand against the receiver side. To address this, an orifice matched to the supply pressure and pilot stage requirements can be installed in series with the nozzle side, allowing for an approximate 2:1 change in supply pressure.
Force motors are similar to torque motors but operate linearly and use permanent magnets. The direction of motion depends on the input current, as shown in Figure 12. Only one U.S. manufacturer, Fema Corp. in Portage, MI, produces force motors. Each permanent magnet creates attractive forces that counterbalance each other when centered, while a stiff centering spring prevents the armature from moving.
When current is applied (Figure 12), the resulting electromagnetic fields strengthen the magnetic fields in some air gaps and weaken them in others, causing the armature and poppet to move. State-of-the-art force motors provide about 5 lbs. of stall force, approximately 0.02 inches of travel (without load), at around 5W of power.
A permanent magnet generates equal fluxes in the four air gaps of an electromagnetic force motor, resulting in no net force on the armature. Applying current to the coil strengthens the flux in gaps B and D while weakening it in gaps A and C, creating a net force to the left that pushes the poppet against the nozzle. Thus, force control through current adjusts the output pressure.
Reviewing Proportional Solenoid Valves: Automotive Lift Repair Tampa Florida Proportional solenoids, which have been around for about 30 years, are produced by various companies globally. Their performance specifications are similar across the board. State-of-the-art designs typically feature:
The trapezoidal air gap in a proportional solenoid is designed to produce a relatively constant force regardless of armature position when current is constant. Without permanent magnets, the force always acts in one direction (left in this case), regardless of current direction. Therefore, bidirectional valves require two proportional solenoids.
Typical force versus armature position curves show a region of proportional solenoid travel where force remains relatively constant at a given current. Valve designers must operate the armature in this proportional region, which is about 0.10 inches wide with current technology.
Comparing Automotive Lift Repair Tampa Florida Solenoid and Motor Devices: There are notable differences between proportional solenoids and force/torque motors. Solenoids generally require higher electrical input power, produce more mechanical travel and force, have higher stiction, operate with greater hysteresis, and generate force independent of current direction. Consequently, a 4-way directional valve needs two proportional solenoids but only one force/torque motor.