A residual magnetism strength of 0.05 millitesla (mT), in some precision balance applications, is enough to prevent the armature (Armature) of a push-pull solenoid from resetting smoothly. This phenomenon is especially common in open-frame structures because the magnetic circuit (Magnetic circuit) loses more magnetic flux in the air, leading to non-linear force output efficiency, which is a physical detail engineers often overlook in initial designs.
The Physical Truth of Magnetic Circuit Efficiency: Differences in Leakage Flux Between Push-Pull Solenoids and Tubular Solenoids
We must admit that in open-frame structures, magnetic field lines diffuse outwards from the core (Core), creating a noticeable magnetic leakage phenomenon. When disassembling failed parts, we found that when the U-shaped frame of a push-pull solenoid has insufficient magnetic permeability, energy dissipates into heat rather than mechanical energy. This directly leads to a situation where, at the same input power, the initial pulling force of a frame-type structure is often weaker than that of a closed-type design.
- Magnetic Circuit Path: Push-pull solenoids primarily rely on U-shaped iron components for magnetic guidance, resulting in a longer magnetic path with multiple air gaps.
- Magnetic Flux Density: Tubular solenoids use an integrated steel tube as their casing, where magnetic field lines are completely enclosed within the tube wall, leading to higher magnetic flux density.
- Residual Magnetism: As closed magnetic circuits are more prone to accumulating charge, tubular designs must control the ‘residual magnetism’ elimination mechanism more precisely, for example, by adding non-magnetic shims.
Impact of Open Structures on Magnetic Force Curves
On automated packaging lines, we observed that push-pull solenoids perform exceptionally well in short strokes (Stroke), but after the stroke exceeds 5mm, the force attenuates exponentially. This is because the magnetic resistance (Magnetic resistance) of a frame-type structure increases sharply with distance, making it more suitable for high-frequency switching applications with extremely short strokes.
Key Considerations for Push-Pull Solenoid Selection: Why Frame-Type Structures are Suitable for Short Stroke, High Burst Force Applications
The magnetic circuit design of open frame solenoids (Open Frame Solenoid) is relatively straightforward, and their greatest advantages lie in cost control and excellent instantaneous burst force. These products typically use stamped U-shaped frames, which have a large heat dissipation surface area, making them suitable for operation under intermittent duty (Intermittent duty) conditions, allowing the coil to cool down quickly after high loads.
| Performance Metric | Push-Pull (Frame Type) | Tubular Solenoid |
|---|---|---|
| Magnetic Circuit Efficiency | Medium (More leakage flux) | Very High (Closed magnetic circuit) |
| Heat Dissipation Capacity | Excellent (Large open surface area) | Good (Relies on casing conduction) |
| Durability (Cycles) | Approx. 500,000 to 2,000,000 cycles | Over 20,000,000 cycles |
| Dust Protection Rating | Low (Coil exposed) | High (Fully enclosed structure) |
The Secret to the Long Lifespan of Tubular Solenoids: Balance Between Closed Magnetic Circuits and Heat Dissipation Efficiency
Why are tubular solenoids (Tubular Solenoid) almost the only choice in medical devices requiring over a million cycles? The answer lies in their precise sliding bearing design and perfectly symmetrical magnetic field distribution. Because the armature experiences minimal radial side load (Side load) when moving inside the tube, this significantly reduces internal friction loss, thereby extending mechanical lifespan.
Unlike open-frame structures, tubular solenoids use a tubular casing to form a complete closed magnetic flux loop. This design is like building a highway for magnetic field lines. When current flows through the coil, the magnetic field is confined within a very small space, allowing it to maintain relatively stable torque output even with long strokes. When the operating stroke exceeds 10mm, the torque stability of tubular solenoids is usually more than 25% higher than that of push-pull types. This is due to the uniform magnetic flux distribution provided by its pole piece (Pole piece), which effectively reduces impact vibrations at the end of the stroke.
Customized Solenoid Solutions for High-Precision Equipment
Three core indicators—stroke, force, and duty cycle—determine whether your solenoid selection will become a future maintenance burden. When assisting clients with ‘solenoid customization,’ we first evaluate the dust and humidity of the working environment, because the tubular structure inherently offers better physical protection, preventing fine dust from entering the sliding area and causing jamming, whereas push-pull types require additional dust boot designs.
While standard products can meet 80% of automation needs, the remaining 20% often determines a product’s brand value. Shih Shin Technology (世僖科技), when dealing with cases of high-frequency reciprocal motion, specifically optimizes materials for ‘residual magnetism’ treatment by selecting electrical steel with high permeability (Permeability), ensuring the armature springs back at millisecond speeds upon power disconnection to prevent mechanical hysteresis.
Solenoid selection should not be based solely on nominal suction force values, but also on the compatibility of the magnetic circuit structure with mechanical lifespan. Push-pull types are suitable for low-cost, high-heat-dissipation, short-stroke applications, while tubular types are the preferred choice for high precision, long lifespan, and harsh environments. Understanding the physical boundaries of these two structures is crucial for finding the most stable balance between cost and performance.
It is recommended that before selection, you first measure the required initial torque of the mechanism at its ‘longest stroke’ and reserve a safety factor of 1.5 times to offset the force attenuation caused by increased coil resistance during high-temperature operation.
