Proximity Probe Transducers (Proximity Probes) are
the vibration transducer of choice when installing vibration monitoring on Journal Bearing
equipped rotating machinery. Proximity Probe Transducers are the only transducers that
provide Shaft Relative (shaft relative to the bearing) vibration measurement.
methods are usually available for the installation of Proximity Probe Transducers, including
internal, internal/external, and external mounting.
Before selecting the appropriate method of mounting Proximity Probe Transducers, special
consideration needs to be given to several important installation considerations that will
determine the success of your monitoring program.
Theory of Operation
Proximity Probe Transducers work on the proximity theory of operation. A Proximity Probe System
consists of a matched component system: a Probe, an Extension Cable and an
Oscillator /Demodulator. A high frequency RF signal @2 MHz is generated by the
Oscillator/Demodulator, sent through the extension cable and radiated from the Probe tip.
Proximity Probes are generated in the surface of the shaft. The Oscillator /Demodulator
demodulates the signal and provides a modulated DC Voltage where the DC portion is
directly proportional to gap (distance) and the AC portion is directly proportional to
vibration. In this way, a Proximity Probe Transducer can be used for both Radial Vibration
and distance measurements such as Thrust Position and Shaft Position.
Number of Transducers
All vibration transducers measure motion in their mounted plane. In other words, shaft
motion either directly away from or towards the mounted Proximity Probe Probe will be
measured as radial vibration.
On smaller less critical machines, one (1) Proximity Probe Transducer system per bearing
may be adequate for machine protection.
The single Proximity Probe Probe will then measure the shaft's vibration in that given
plane. Therefore, the Proximity Probe Probe should be mounted in the plane where the greatest
vibration is expected.
On larger more critical machines, two (2) Proximity Probe Transducer systems are normally
recommended per bearing. The Probes for this type of installation should be mounted 90° apart from each other. Since the Probes will measure the vibration in their respective
planes, the shaft's total vibration within the journal bearing is measured. An
"Orbit" or cartesian product of the two vibration signals may be viewed when
both Proximity Probe Transducers are connected to an SKF-CM Information System or an
Several versions of Proximity Probe Transducers are available with a variety of Linear Ranges
and body styles. In most cases, SKF-CM's CMSS68 with a linear range of 90 mils
(0.090") is more than adequate for Radial Vibration measurements...
1/4"x28 UNF 1" to 5" Length
3/8"x24 UNF 1" to 9" Length
1" x 12 UNF 1" to 5" Length
Target Material/Target Area
Proximity Probe Transducers are calibrated at the factory for 4140 Steel unless specified otherwise.
As Proximity Probes are sensitive to the permeability and resistivity of the shaft material
any shaft material other than 4000 series steel must be specified at the time of order.
In cases of exotic shaft material, a sample may need to be supplied to the factory.
Proximity Probe Transducers are also sensitive to the shaft smoothness for Radial Vibration.
A smooth (64 micro-inch) area approximately 3 times the diameter of the Probe must be
provided for a viewing area. The prepared journal area on most shafts are wider than the
bearing itself allowing for Probe installation immediately adjacent to the bearing.
Since Proximity Probe Transducers are sensitive to the permeability and resistivity of the
target material and the field of the transducer extends into the surface area of the shaft
by approximately 15 mils (0.015"), care must be taken to avoid non homogeneous
viewing area materials such as Chrome.
Another form of electrical runout can be caused by small magnetic fields such as those
left by Magna-fluxing without proper degaussing.
Perpendicular to shaft centerline
Care must be exercised in all installations to ensure that the Proximity Probe probes are
mounted perpendicular to the shaft center-line. Deviation by more than 1-2 degrees will
effect the output sensitivity of the system.
Orientation of Transducer(s)
As most machine casings are horizontally split, transducers are commonly found mounted at
45° both sides of vertical, and 90° apart.
If possible transducer orientation should be consistent along the length of the machine
train for easier machine diagnostics. In all cases, orientation should be well documented.
Transducer (Probe) side clearances
The RF Field emitted from the Probe tip of a Proximity Probe Transducer in approximately a
45° coned shape. Clearance must be provided
on all sides of the Probe tip to prevent interference with the RF Field. As an example, if
a bearing is drilled to permit installation, the hole must be counter bored to prevent
side clearance interference. Care must also be taken to avoid collars or shoulders on the
shaft that may thermally "grow" under the Probe tip as the shaft grows from
Proximity Probe Probe tip to tip clearances
Although Probe tip to tip clearances are not normally an issue on most machines, it should
be noted that Proximity Probe Probes radiate an RF Field larger than the Probe tip itself. As
an example, Model CMSS65 and 68 probe should never be installed with less than one (1)
inch of Probe tip to tip clearance. Larger Probes require more clearance. Failure to
follow this rule will allow the Oscillator/Demodulator to create a "beat"
frequency which will be the sum and difference of the two Oscillator/Demodulator RF
System Cable Length and Junction Boxes
Proximity Probe Transducer Systems are a "tuned" length, and several system lengths
are available. Length is measured from the Probe tip to the Oscillator/Demodulator, and is
measured electrically which can slightly vary the physical length. For example, the Model
CMSS65 and 68 are available in 5 and 10 meter system lengths. Care must be taken to ensure that the proper system length is ordered to reach the required Junction Box.
Electrical noise is a very serious consideration when installing any vibration transducer,
and special care needs to be taken to prevent unnecessary amounts of noise. As most plant
electrical noise is 60 HZ, and many machines running speed is also 60 HZ, it is difficult
to separate noise from actual vibration signal. Therefore, noise must be kept to an
A 3-wire twisted shielded instrument wire (ie; Belden #8770) is used to connect each
Oscillator/Demodulator to the Signal Conditioner in the Monitor. Where possible, a single
run of wire from the Oscillator/Demodulator (Junction Box) to the Monitor location should
be used. Splices should be avoided.
The gauge of the selected wire depends on the
length of the instrument wire run, and should be as follows to prevent loss of high
Up to 200 feet
Up to 1000 feet
Up to 4000 feet
The following wiring connection convention should be followed:
Common Point Grounding
To prevent Ground Loops from creating system noise, system common, ground and instrument
wire shield must be connected to ground at one location only. In most cases, the
recommendation is to connect commons, grounds, and shields at the Monitor location. This
means that all commons, grounds, and shields must be floated or not connected at the
Occasionally due to installation methods instrument wire shields are connected to
ground at the machine case and not at the monitor. In this case, all of the instrument
wire shields must be floated or not connected at the monitor.
Dedicated conduit should be provided in all installations for both mechanical and noise
protection. Flexible metal conduit should be used from the Eddy Probe to the Oscillator
/Demodulator junction box, and rigid bonded metal conduit from the junction box to the
All Proximity Probe Systems (Probe, Cable and Oscillator Demodulator) should be calibrated
prior to being installed. This can be done by using a SKF-CM CMSS601 Static Calibrator,
-24 VDC Power Supply and a Digital Volt Meter. The Probe is installed in the tester with
the target set against the Probe tip. The micrometer with target attached is then rotated
away from the Probe in 0.005" or 5 mil increments. The voltage reading is recorded
and graphed at each increment. The CMSS601 Calibrator will produce a voltage change of 1.0
VDC +-0.05 VDC for each 5 mils of gap change while the target is within the Systems linear
When installed, Proximity Probe Probes must be gapped properly. In most Radial Vibration
applications, gapping the transducer to the center of the linear range is adequate. For
the Model CMSS65 and 68, gap should be set for -12.0 VDC using a Digital Volt Meter (DVM),
this corresponds to an approximate mechanical gap of 0.060" or 60 mils. The voltage
method of gapping the Probe is recommended over mechanical gapping. In all cases, final
Probe gap voltage should be documented and kept in a safe place.
Internal Mounting is accomplished with the Proximity Probe Probes mounted
internally to the machine or bearing housing with a CMCP805 Bracket or with a custom designed and
manufactured bracket. The Transducer system must be installed and gapped properly prior to
the bearing cover being reinstalled.
Provisions must be made for the transducer's cable
exiting the bearing housing. This can be accomplished by using an existing plug or
fitting, or by drilling and tapping a hole above the oil line. The Transducer's cables
must also be tied down within the bearing housing to prevent cable failure from
For added safety and reliability, all fasteners inside the bearing housing should be
safety wired, or otherwise prevented from working loose inside the machine.
Advantages of Internal Mounting
- Most economical installation.
- Less machining required.
- True bearing relative measurement.
- Usually good viewing surface for Eddy Probe.
Disadvantages of Internal Mounting
- No access to Probe while machine is running.
- Cables must be tied down due to "windage".
- Transducer cable exits must be provided.
- Care must be taken to avoid oil leakage.
External/Internal mounting is accomplished when the Eddy Probes are mounted with a
Mounting Adapter (CMCP801). These adapters allow external access
to the Probe yet allows the Probe tip to be internal to the machine or bearing housing.
Care must be taken in drilling and tapping the bearing housing or cover to ensure that the
Eddy Probes will be perpendicular to the shaft center line.
In some cases, due to space limitations, External/Internal mounting is accomplished by
drilling or making use of existing holes in the bearing itself, usually penetrating at a
oil return groove.
Advantages of External/Internal Mounting
- Eddy Probe replacement while machine is running.
- Usually good viewing area for Eddy Probe.
- Gap may be changed while machine is running.
Disadvantages of External/Internal Mounting
- May not be true bearing relative measurement.
- More machining required.
- Long Probe/Stinger length (Resonance).
Pure external Eddy Probe mounting is usually a last resort installation. The only valid
reason for using this method is inadequate space available within
the bearing housing for internal mounting. Special care must be given to the Eddy Probe
viewing area and mechanical protection of the transducer and cable.
Advantages of External Mounting
- Most Inexpensive Installation.
Disadvantages of External Mounting
- May be subject to "Glitch" or Electrical/Mechanical runout.
- Requires mechanical protection.
- Mounting Type, Internal External/Internal External
- Number of Transducers, X or X&Y
- Target Material, 4140 Other
- Smooth Target Area
- Size of Target Area
- Junction Box Location(s)
- Metal Conduit (Junction Box to Monitor)
- Flexible Conduit (Junction Box to Probe)
- Correct Instrument Wire
- Shielding Convention, Monitor or Machine
- Gap Set