Shaft vs Casing Vibration: Which Sensor Should You Use?

A shaft vibration sensor and a bearing-housing vibration sensor answer different measurement questions. An eddy current proximity probe observes a conductive shaft without contacting it and measures displacement relative to its mounting reference. A casing-mounted accelerometer or velocity sensor measures motion at the housing or structure where it is attached.

Choose the measurement point before comparing output signals or sensor ranges. NI's overview of vibration monitoring describes the roles of proximity probes, accelerometers and velocity sensors.

Question	Shaft-relative vibration	Casing or bearing-housing vibration
What moves in the measurement?	The shaft relative to the probe mounting reference	The surface carrying the sensor
Typical sensor to review	Eddy current proximity probe and its matched measurement system	Accelerometer or vibration velocity sensor
Physical arrangement	Probe tip faces the shaft across a gap	Sensor is coupled to the housing or bearing cap
What needs installation review?	Accessible target, gap, bracket, probe dimensions and complete electrical system	Measurement direction, mounting surface, attachment method and cable arrangement

Both may be useful on one machine. The required monitoring task determines whether to specify one measurement or a combination.

For a machine with fluid-film or journal bearings, the oil film and machine structure can attenuate the vibration transmitted from the shaft to the outside housing. A low casing reading therefore does not by itself establish that shaft-relative motion is low. Proximity measurement is commonly used when the shaft's motion within bearing clearances is the quantity of interest.

For many rolling-element-bearing machines, a housing-mounted accelerometer or velocity sensor provides a practical measurement path. The appropriate signal and frequency band depend on whether the task is overall trending, impact assessment or waveform analysis. Bearing type is a useful starting point, not a complete sensor specification.

Metrix's pump vibration-monitoring application guidance distinguishes rolling-element-bearing and journal-bearing arrangements. Use the machine design and monitoring requirements to assess your own installation; a pump label alone does not identify its bearing construction.

No. The measurement reference still matters after a signal has been processed into displacement units.

A proximity probe measures the changing distance from its mounting reference to the observed shaft surface. A housing accelerometer can be integrated to obtain velocity and displacement within the usable measurement band, but those derived values still describe the housing's motion. They do not become a direct measurement of the shaft-to-housing gap.

Bently Nevada explains this reference-frame distinction in Accelerometers: What do I use?. When comparing two readings, record the physical location and direction as well as the units, frequency band and amplitude definition. Comparing only a number labeled displacement can conceal two different measurements.

For an eddy current system, review the probe, any extension cable and the matching conditioner together. The probe geometry, usable range, observed target material and required system length affect selection. State whether the task is radial vibration, axial position or another displacement measurement. Check the supplied installation and calibration information for the selected configuration.

For a housing sensor, review the required acceleration or velocity signal, usable bandwidth, mounting and receiving input together. If the instrument needs a waveform, the measurement chain must preserve a dynamic signal. If the control system needs an overall current value, review the transmitter or conditioning path that produces it.

The measurement location and electrical interface are separate choices. For the latter, read the 4–20 mA versus IEPE selection guide.

Measurement requirement	Product to review	Information to check
Non-contact shaft displacement and vibration	YD9800 eddy current displacement sensor system	Probe range and mounting, target, cable-plus-conditioner system length and receiving signal
Bearing-housing vibration velocity	YD33 vibration velocity sensor	Velocity range, frequency band, excitation, mounting and instrument input
Housing acceleration for a compatible acquisition channel	YD30 acceleration sensor	Applicable hardware version, output interface, usable frequency band and mounting

The YD9800 is documented as a probe, extension cable and proximitor system. Its system-selection steps describe the details to review together. Probe-specific limits apply, so a general catalog accuracy figure should not be applied to every probe range.

For YD30, documented sources differ on the sensor principle and specifications. Confirm the supplied version instead of combining values from different sources. These product options identify measurement paths to review; the machine's actual requirements still determine the configuration.

A useful request identifies the measurement point and the information needed from it:

1. Machine type, bearing arrangement and operating-speed range, if known.
2. Shaft-relative motion, housing motion, or both; radial or axial direction as applicable.
3. Required quantity and amplitude definition, such as displacement peak-to-peak or velocity RMS.
4. Measurement frequency band and whether a dynamic waveform is needed.
5. Existing sensor and receiving-instrument models, supply/excitation and available inputs.
6. Mechanical access, mounting drawing, cable length, environment and quantity.

If you do not know the bearing arrangement or signal interface, say so and provide the available drawing or instrument description. Ask CEZHEN about a shaft or housing vibration sensor, or use a product link above for a model-specific request.