Bently Nevada 991-25-50-01-01 991 Thrust Transmitter

Description

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

Technical Specifications

Connections/Interfaces

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.