A Journal Bearing, or Fluid Film/Sleeve Bearing, is a type of bearing in which the shaft is suspended in a pressurized regime of oil. These are used on very large or powerful rotating machines in order to provide damping of the vibration in order to pass through critical speeds and suppress instabilities caused by loads or natural frequencies of the housing. This damping of the vibration protects the rest of the production line from the extreme force generated by the rotating element, but it also makes it difficult for a regular accelerometer to get accurate vibration data of the shaft while mounted on the machine housing.

Inductive Proximity Probes (Eddy Current Probes) are non-contact displacement sensors used to determine the absolute displacement between the tip of a sensor and a conductive target material. These probes use the fluctuations induced in an electromagnetic field generated by the probe system to determine the bearing shaft position relative to the bearing casing and the dynamic vibration of the rotating shaft. Industrial Proximity Probes are most commonly used in petrochemical and energy production, specifically in turbines and reciprocating compressors that utilize Journal Bearings.

Proximity Probes are able to provide very accurate information on the position and vibration of the journal bearing shaft relative to the rest of the machine housing. For typical radial condition monitoring applications, two probes will be mounted perpendicular to the shaft 90° apart from one another to measure the X and Y axis. The vibration of the shaft will be measured as a variable DC voltage that simulates an AC vibration signal.

Using this position/vibration data from both axis, analysts can create an orbit that can be used to measure the total vibration of the shaft’s centerline as it rotates in the bearing sleeve. An orbit will provide the peak-to-peak displacement and direction of the vibration relative to the shaft centerline. The location of the shaft centerline within the bearing sleeve can also be measured with radially mounted probes. The DC portion of the voltage signal is proportional to the space between the probe tip and shaft surface. This measurement is critical in knowing the location of the shaft within the bearing housing and preventing metal contact between the shaft and bearing sleeve which could have catastrophic results.

Diagram showing shaft rotating inside a journal bearing

diagram of radially mounted X and Y Probes on Journal Bearing


What is a Complete Proximity Probe System?

Eddy Current Proximity Probe Systems are an effective way to monitor Journal Bearings. A complete Proximity Probe System consists of a Proximity Probe, Extension Cable, and a Proximity Probe Driver.

picture of a proximity probe, extension cable, and driver


Standard vs. Hazardous Rated Systems

All PRO Line Proximity Probe Systems are available in both standard and hazardous rated configurations.

Optional regulatory approvals for hazardous rated systems include CSA, ATEX, IECEx, and CE. Please note - these certifications require the intrinsically safe barrier driver to be mounted in a NEMA 4x and/or IP6x enclosure.


Selecting Your Ideal Proximity Probe System

CTC offers a complete line of proximity probe systems designed, built, and tested to endure prolonged use in the harshest industrial environments. PRO Line Proximity Probe Systems are available in FFvTM 5 mm, 8 mm, 11 mm, and 25 mm systems. The variety of probe sizes offered allows for accurate and precise displacement measurements as well as shaft speed/phase analysis. Complete PRO Line Proximity Probe Systems are compatible with all standard condition monitoring software, including Bently Nevada® software.

Probe Tip Sizes

Selecting the best probe tip size depends on the size of the displacement gap you are monitoring. The displacement gap refers to the space between the probe tip and the shaft you are monitoring. Please refer to the selections below to determine the probe size that is best for you.
side view of an 8 millimeter proximity probe tip

8 mm Probe Tip

For displacement gaps of 10-90 mils

Output Sensitivity:
-200 mV/mil (7.87 V/mm)

side view of an 11 millimeter proximity probe tip

11 mm Probe Tip

For displacement gaps of 20-180 mils

Output Sensitivity:
-100 mV/mil (-3.94 V/mm)

side view of a 25 millimeter proximity probe tip

25 mm Probe Tip

For displacement gaps of 25-525 mils

Output Sensitivity:
-20 mV/mil (-7.87 V/mm)

side view of an FFv 5 millimeter proximity probe tip

FFvTM 5 mm Probe Tip

For displacement gaps of 10-70 mils

Output Sensitivity:
-200 mV/mil (-7.87 V/mm)

Note - FFvTM 5 mm probes may be used for any applications within their linear range when clearance is an issue. They are the only probes we recommend for shafts with a diameter less than 2 inches. They are also used for many general purpose applications for larger shafts. 

System Length

The length of a proximity probe system is measured from the probe tip to the connector on the driver. The system length is then factored into the proximity probe driver calibration.

The length of a proximity probe plus extension cable must equal one of the system lengths allowed for your probe size selection.

Refer to the chart below to determine the system length that is best for your application:
picture of a 5 m proximity probe and an 8 m proximity probe extension cable
chart showing allowable lengths for each system

Other Important Considerations

PRO Line Proximity Probes have a wide variety of ancillary selections to better tailor your proximity probe set to your application:
picture of two proximity probe cables, one with standard blue FEP jacket, and the other with metal armored jacket

Cable Jacket

Standard FEP Jacket or Armor Jacket available

picture of two proximity probe cable connectors with connector protector

Connector Protectors

Required for hazardous applications

Provide an added layer of durability for the connection between your probe and extension cable

picture of a black mounting bracket, a reverse mount housing, and a metal mounting bushing

Mounting Style

Standard internal and external brackets and bushings as well as external housing for "reverse mount" probe systems available

Proximity Probe Mounting

There are two main ways to mount proximity probes - a mechanical method and an electrical method.
Mechanical Method
In this method, the probe will be threaded into the bushing, bracket, or bored-out hole in the machine housing and screwed down until the probe tip is close to the target shaft. A non-metallic feeler gauge is required and should be the size of the desired gap between the probe and the target. For example, 50 mils is a common gap selection for 8 mm probe systems since it sits near the midpoint of the system's linear range of 10-80 mils. The probe should be carefully turned into the housing until it is lightly touching the feeler gauge. Once the probe gap is set, the probe should be locked into place with the locknuts on the probe casing.
Electrical Method
The first step for this method is to connect the Proximity Probe to its extension cable and driver. Apply -24V to the driver and connect a digital multimeter to the voltage output terminal. Insert the probe into the tapped mounting hole. As the probe tip is threaded through the hole, the sensor's output voltage will remain low or give a false reading because it is sensing the surrounding materials of the machine housing. As the probe extends through the housing, the output voltage will increase to its maximum point and then decrease as it approaches the observed surface. The output voltage from the probe system should correspond with the calibration curve that is published on the calibration certificate. Use the linearization table on the calibration certificate to gap the probe to the desired distance.

Proximity Probe Driver Selection

PRO Line Proximity Probe Drivers feature a rugged powder-coated aluminum case and isolated BNC for in-field dynamic voltage proportional to gap displacement. 

PRO Voltage Drivers also come supplied with a three-wire output terminal block to connect to a protection system.

CTC also offers four-wire Proximity Probe Drivers with a scaled 4-20 mA output proportional to either gap displacement, radial vibration, and axial position.

The dynamic voltage signal is available on all proximity probe drivers via the BNC Jack located on the unit.

Reference the following diagrams to determine the best driver selection for your application:


picture of DD10080 proximity probe driver

The voltage drive has a dynamic voltage output that is compatible with industry-standard continuous vibration monitoring systems and is the format specified in API Standard 670. The dynamic voltage output is proportional to the DC gap between the target material and probe tip.

View DD100180

4-20 mA DRIVERS:

4-20 mA Drivers Proportional to DC Gap:

Picture of a proximity probe tip mounted to machinery

Probe showing proportional output for the 4-20 mA signals. With shaft surface at 50 mils, 4-20 output is 12 mA. If the gap increases to 70 mils, 4-20 output will be 16 mA.

*Please note: The numbers referenced above are for use with an 8 mm probe.

Radial Application Drivers:

picture of a radial application proximity probe mounted to a machine

In radial applications, the probe drivers select the average shaft surface distance and the 4-20 is proportional to the overall peak to peak vibration in mils around the average surface of the shaft.


Axial Application Drivers:

picture of an 8 millimeter probe tip facing a shaft

In the thrust position the probes auto range to the face of the shaft or thrust collar, and the 4-20 is proportional to the positive or negative vibration away or toward the probe, as shown.


Proximity Probe Driver Enclosures

CTC offers our PXE Proximity Probe Driver Enclosures in both fiberglass and stainless steel options. PXE Enclosures are NEMA 4X rated to ensure your proximity probe drivers are protected from dirt, dust, oil, and water.

24V power supply and/or terminal blocks are also available as configurable options for the PXE150 and PXE250.

image of an open, fiberglass PXE150 enclosure with 6 mounted drivers

Proximity Probe Driver Enclosure, Fiberglass, NEMA 4X (IP66) Rated, Channel Options: 1 to 6


image of an open PXE150P fiberglass enclosure with 6 mounted drivers and a 24V power supply

Proximity Probe Driver Enclosure with Power Supply Included, Fiberglass, NEMA 4X (IP66) Rated, Channel Options: 1 to 6


image of an open PXE150T fiberglass enclosure with 6 mounted drivers and terminal blocks

Proximity Probe Driver Enclosure with Terminal Blocks Included, Fiberglass, NEMA 4X (IP66) Rated, Channel Options: 1 to 6


image of an open PXE250 stainless steel enclosure with 6 mounted drivers.

Proximity Probe Driver Enclosure, Stainless Steel, NEMA 4X (IP66) Rated, Channel Options: 1 to 6


image of an open PXE250P stainless steel enclosure with 6 mounted drivers and a 24V power supply.

Proximity Probe Driver Enclosure with Power Supply Included, Stainless Steel, NEMA 4X (IP66) Rated, Channel Options: 1 to 6

Ready to Purchase Your PRO Line Proximity Probe System?

View Proximity Probe Systems



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