Introduction
The Bently Nevada 84140-01 is a Magnetic Speed Sensor, also commonly known as a Magnetic Pickup (MPU). It is a fundamental component used for measuring rotational speed and providing a Keyphasor® reference signal in machinery protection systems, often interfacing with Bently Nevada's monitoring systems like the 3500 series.
This passive sensor operates on the variable reluctance principle. It consists of a permanent magnet with a coil wound around it. When a ferromagnetic target (such as a gear tooth or a notch on a shaft) passes in front of the sensor's face, it causes a change in the magnetic flux. This change induces an AC voltage pulse in the coil. The frequency of these pulses is directly proportional to the rotational speed of the shaft. It is a simple, robust, and reliable sensor that requires no external power, making it ideal for critical speed and overspeed protection applications.
Parameters / Specifications
The key technical specifications for the Bently Nevada 84140-01 are summarized in the table below.
| Parameter | Specification |
|---|---|
| Part Number | 84140-01 |
| Product Line / Type | Bently Nevada Magnetic Speed Sensor / Magnetic Pickup (MPU) |
| Operating Principle | Variable Reluctance (Passive) |
| Thread Size | 5/8-18 UNF |
| Tip Diameter | Approx. 0.31 in (7.9 mm) |
| Output Signal | AC Voltage (sine wave), amplitude proportional to speed and gap. |
| Power Supply | None (Self-generating / Passive) |
| Sensitivity | Varies with speed and gap; typically produces a usable signal from 1-2 Hz up to 10,000+ Hz. |
| Recommended Gear | 60 teeth or higher, made of ferromagnetic steel. |
| Standard Gap | 0.020 to 0.060 in (0.51 to 1.52 mm) from the gear tip. |
| Electrical Connection | 2-pin, Military-style (MS) connector (standard for this model). |
| Cable Length | Typically 10 feet (3.05 meters) (integrated). |
| Housing Material | Stainless Steel |
| Dimensions (Sensor Body) | Length: ~3.5 in (89 mm); Diameter: 5/8 in (15.9 mm) |
| Weight (Approx.) | 0.55 lbs (0.25 kg) |
| Operating Temperature | -50°C to +120°C (-58°F to +248°F) |
Application Scenarios
The 84140-01 Magnetic Speed Sensor is a versatile and critical component used for speed measurement and timing reference across various industries.
Typical application scenarios include:
Turbine and Compressor Overspeed Protection:
The primary application. Mounted near a gear on the main shaft of gas or steam turbines and centrifugal compressors to provide a reliable speed signal to the monitoring system (like a Bently Nevada 3500/42M). This signal is used for continuous speed monitoring and to initiate an emergency shutdown if a dangerous overspeed condition is detected.
Keyphasor® Reference for Vibration Analysis:
Used with a single, once-per-revolution notch on the shaft to generate a Keyphasor pulse. This pulse provides a timing reference that is essential for measuring vibration phase, which is critical for rotor balancing and diagnosing faults like unbalance and misalignment.
Zero-Speed Detection:
Used to monitor when a machine has completely stopped rotating, which is important for sequencing auxiliary systems like turning gears or for safety lockout procedures.
General Speed Monitoring on Pumps, Fans, and Motors:
Providing a reliable speed feedback signal for a wide range of rotating equipment in power generation, oil & gas, and water treatment plants.
Industries of Use:
Power Generation (Gas, Steam, and Hydro Turbines)
Oil & Gas (Pipeline Compressors, Pumps)
Petrochemical & Refining
Marine Propulsion
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