The Bently Nevada 991-25-50-01-01 is a two-wire, loop-powered thrust transmitter designed to measure the relative axial position of a rotating machine shaft. It accepts input directly from a 3300 NSv proximity probe and extension cable, transforming the proximity signal into a proportional 4 to 20 mA current output. This device integrates directly with Programmable Logic Controllers (PLCs), Distributed Control Systems (DCSs), and SCADA networks to provide continuous machinery protection and condition monitoring. Typical industrial applications include centrifugal air compressors, small pumps, motors, and fans with fluid-film thrust bearings where a full machinery protection system monitor rack is not required.
Features
Two-wire, 4 to 20 mA loop-powered current output
Designed for non-contacting shaft axial displacement measurement
Compatible with 3300 NSv proximity probe and extension cable
Integrated BNC connector for dynamic vibration signal access and diagnostics
DIN rail or panel mounting configurations
Non-interacting zero and span adjustments
Potted construction for high humidity environments up to 100% condensing
Not OK/Signal Defeat circuit to prevent false alarms due to faulty proximity probes or loose connections
Applications
Centrifugal air compressors
Process pumps and industrial motors
Steam and gas turbines
Electric motors with thrust bearings
Cooling tower fans and blowers
Ordering Information
Option Code
Description
991
Thrust Transmitter
25
Full-scale Option: 25-0-25 mils
50
System Length Option: 5.0 meters (16.4 feet)
01
Mounting Option: 35 mm DIN-rail clips
01
Agency Approval Option: CSA Division 2
Technical Specifications
Category
Parameter
Specification
Manufacturer
Brand
Bently Nevada
Origin
Country of Origin
United States
Physical
Approximate Weight
0.43 kg
Dimensions (H x W x D)
53.3 mm x 73.9 mm x 100.1 mm (2.10 in x 2.91 in x 3.94 in)
Body Height (without DIN clip)
42.6 mm (1.68 in)
Mounting Hole Distance (Width x Length)
63.5 mm x 69.9 mm (2.50 in x 2.75 in)
Mounting Hole Diameter
5.8 mm (0.23 in)
Electrical
Input Voltage Range
12 to 35 Vdc (at the transmitter terminal block)
Current Output
4 to 20 mA (Linear over 50 mils range)
Loop Accuracy
Within ±1.5% over specified full-scale range
Maximum Loop Resistance
1000 ohms including cable at 35 Vdc
Current Limiting
23 mA typical
Dynamic
Frequency Response
DC to 10 Hz
BNC Connector Output
Raw proximity probe voltage buffered (Nominal 200 mV/mil)
Minimum Target Size
9.5 mm diameter
Environmental
Operating Temperature
-35 to +85 Celsius
Storage Temperature
-51 to +100 Celsius
Relative Humidity
100% condensing, non-submerged
Connections/Interfaces
Terminal / Connector
Function
PROX
Proximity Probe Input Terminal Connection
COM
Signal Common / System Ground
+4-20mA
Positive Loop Power / Current Output Terminal
-4-20mA
Negative Loop Power / Current Output Terminal
BNC
Dynamic Signal Output (Buffered Proximity Voltage)
Installation Guidelines
Dimensions and Mounting
The module accommodates both panel (bulkhead) and DIN rail mounting options. When using the panel mount configuration, prepare four mounting holes with a spacing layout of 63.5 mm (width) by 69.9 mm (length). Use the standard 5.8 mm diameter mounting holes or the supplied 6-32 x 1.326 screws for secure bulkhead fixation. For DIN rail configurations, ensure a standard 35 mm DIN rail is rigidly installed before snapping on the transmitter's DIN mount clips.
Grounding and Shielding
Proper grounding is essential to minimize noise interference. The transmitter must be mounted to a grounded plate or a grounded DIN rail. Ensure the cable shield is grounded at only one end, typically at the control room/DCS side, to prevent ground loops.
Probe Gap Adjustment
Before final commissioning, the proximity probe must be physically positioned (gapped) relative to the shaft target. Adjust the gap until the transmitter output reads approximately 12 mA, which corresponds to the midpoint of the linear thrust range (0 mils).
Cable Routing
Run probe and loop cables in dedicated, grounded metal conduits or cable trays. Avoid routing transmitter cables parallel to high-voltage AC power lines or heavy machinery switching circuits to prevent electro-magnetic interference (EMI).
Hazardous Area Precautions
When installed in classified areas, appropriate intrinsic safety barriers or explosion-proof housings must be utilized in accordance with local electrical codes and the specific installation drawings provided by the manufacturer.
FAQ
What is the primary function of this thrust transmitter?
It measures the axial displacement or thrust position of a rotating machine shaft relative to a fixed reference point using a non-contacting proximity probe.
What type of probe system is compatible with this transmitter?
It is designed specifically to interface with a Bently Nevada 3300 NSv proximity probe and extension cable system.
How is the device powered?
It is a loop-powered device operating on a 12 to 35 Vdc supply connected via the 4 to 20 mA current loop.
What does the BNC connector provide?
The BNC connector provides a buffered raw voltage signal proportional to the probe gap distance, allowing engineers to connect diagnostic equipment like oscilloscopes or analyzers without disconnecting the loop.
Can this device be used for radial vibration measurements?
No, this specific model is configured and calibrated for thrust (axial) position measurement. For radial vibration, a 990 Vibration Transmitter should be used.
What is the function of the Not OK/Signal Defeat circuit?
The circuit forces the signal output to drop below 3.6 mA within 100 microseconds of a probe fault or loose connection to prevent false alarms in the control system.
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Expédition express : Expédition le jour même pour les commandes en stock passées avant 14h00 (GMT+8).
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Retours et garantie
Garantie de 30 jours : Retours acceptés pour les produits en stock dans leur emballage d’origine scellé en usine.
Garantie de 12 mois : Chaque composant industriel est couvert par notre garantie technique professionnelle.
Les commandes sont traitées et livrées du lundi au vendredi (hors jours fériés).
Pour connaître les conditions d’éligibilité complètes, les frais de restockage et les détails des retours internationaux, veuillez consulter notre site officiel
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