They answer different questions, so most rotating equipment needs both. A speed sensor tells you how fast something turns — rpm, underspeed, overspeed, slip, direction, zero speed. It's a control and protection signal. A vibration sensor tells you how healthy the machine is — imbalance, misalignment, looseness, bearing wear developing over weeks. It's a diagnostics signal. Speed is what the machine is doing; vibration is how it feels about it. Add eddy-current displacement probes when the machine is critical enough that shaft position itself matters. KJT Sensors publishes all three product families — speed sensors, vibration sensors and eddy-current displacement sensors — and the sections below map each one to the machines and failure modes where it earns its keep.
Key takeaways
- Speed monitoring is protection: underspeed, slip, overspeed and zero-speed detection interlock the process.
- Vibration monitoring is prediction: trend the signal and you catch bearing and balance problems weeks before failure.
- Measuring acceleration, velocity and displacement are three different jobs — most general machine health work trends velocity (mm/s).
- Gear-tooth speed sensors with active magnetic detection keep working at near-zero speed, where passive designs go blind.
- The industry's widely used reference for judging machine vibration severity is the ISO 10816/20816 series — a useful yardstick, but your baseline trend matters more than any table.
What Does a Speed Sensor Actually Tell You?
Speed sensors convert rotation (or linear motion) into a signal: a pulse train whose frequency tracks rpm, or a switched output that trips at a set threshold. The KJT Sensors published speed-sensor range covers the main technologies, each with a reason to exist:
- Hall-effect gear sensors — active devices that read a gear or toothed wheel. Because they sense the magnetic field actively, they keep outputting clean pulses right down to near-zero speed. That's the feature that matters for conveyors and winches, where "still moving at 2 rpm" and "stopped" are different process states.
- Passive magnetoelectric (variable-reluctance) sensors — no power supply needed, tough and simple, but their output shrinks with speed. Fine for engine and turbine rpm; a poor choice for creeping shafts.
- Photoelectric speed sensors — read a mark or slotted disk optically. Accurate, but the optics need a reasonably clean environment.
- Speed switches — the sensor and threshold logic in one housing. Set the trip point, wire the contact to your interlock, done. Conveyor underspeed and slip protection is the classic use, and it's the same device family we covered in the belt-conveyor protection guide.
The manufacturer's published materials note that its speed line spans standard and custom models for harsh environments — railways, power generation, hydraulics, hazardous areas — with speed range, output type, temperature rating and mounting all confirmed per model. One detail from the engineering notes worth repeating: the active magnetic designs were developed specifically because low-speed and near-zero signals get weak or lost with passive approaches. If your shaft creeps, that sentence is your selection criterion.
What Does a Vibration Sensor Actually Tell You?
A vibration sensor converts mechanical shaking into an electrical signal you can trend, alarm and analyze. Three measurands, three jobs:
- Acceleration (g) — best for high-frequency content: bearing defects, gear mesh problems, cavitation. Raw accelerometers feed analysis systems.
- Velocity (mm/s or in/s) — the workhorse of general machine health. Severity correlates well with how much damage the machine is doing to itself across the mid-frequency range, which is why evaluation standards like the ISO 10816/20816 series express their zones in velocity.
- Displacement (µm) — low-frequency, large-motion problems: slow rollers, structural sway, shaft orbits.
KJT Sensors' published vibration line includes velocity sensors, piezoelectric sensors, shock accelerometers, vibration transmitters and three-axis integrated transmitters, aimed at motors, pumps, compressors, fans, winders and structural monitoring. A transmitter is worth singling out: it conditions the signal to a 4–20 mA loop proportional to overall vibration, so a PLC can trend machine health with no special vibration hardware. For most plants, "a transmitter on each critical motor-pump pair, trended in the SCADA" is the highest-value first step — analysis-grade waveform systems can come later for the machines that justify them.
Where Does Eddy-Current Displacement Fit?
Speed and casing vibration both miss something important on critical rotating machinery: what the shaft is doing inside the machine. Eddy-current displacement probes mount close to the shaft and measure the gap without contact — giving you shaft vibration, axial position (thrust), and radial position directly. That's the measurement that catches thrust-bearing wear and rotor instability on compressors, turbines and large high-speed machines.
KJT Sensors describes its eddy-current line as measuring static and dynamic displacement, vibration, and axial/radial position on conductive metal targets with high linearity and resolution, for rotating and reciprocating machinery diagnostics. If your machine has a gearbox the size of a car or a compressor whose trip costs a day of production, this layer stops being optional.
Which Machines Need Which Sensors?
| Machine / duty | Speed monitoring | Vibration monitoring | Displacement |
|---|---|---|---|
| Belt conveyor drive | Underspeed/slip switch on the pulley — protection | Optional, on the drive motor | No |
| Critical pump/fan/motor set | Optional (if process needs rpm feedback) | Yes — velocity transmitter, trended | No |
| Winder, hoist, crane travel | Yes — active gear sensor for low-speed certainty | On gearbox if critical | No |
| Large compressor, turbine, high-speed gearbox | Yes — rpm plus overspeed protection | Yes — casing vibration | Yes — shaft vibration and thrust position |
| Slow roller, kiln, large agitator | Zero-speed/rotation confirmation | Low-frequency displacement or low-frequency-rated velocity sensor | Case by case |
| Engine, wheel, rail axle | Passive magnetoelectric or Hall, per speed range | Optional | No |
Conditional conclusion: protection-critical motion gets speed monitoring, failure-expensive machines get vibration trends, and machines where the shaft itself is the asset get displacement probes. Budget follows consequence — ask "what does an undetected failure cost here?" and the sensor bill writes itself.
Getting the Installation Right
Vibration measurements are unforgiving of sloppy mounting. Stud-mount to a flat, spot-faced surface whenever the machine justifies it; adhesive pads are a decent second choice; magnetic bases are for walk-around checks, not permanent trending — the high-frequency content you need for bearing analysis rolls off through every mechanical joint. Mount as close to the bearing housing as the machine allows, in the direction the load acts, and record where you put it so the next person trends the same point.
Speed sensors have their own rules: hold the sensing gap to the middle of the model's specified range, use a target with enough ferrous material (gear teeth work; a painted shaft doesn't), and shield the cable on runs near VFD outputs — the manufacturer's notes on its gear speed sensors call out shielded cabling and filtering specifically because EMI false-triggering is the classic field complaint. Low-speed applications should also confirm the sensor is an active design; a passive probe on a 5-rpm shaft will cost you a commissioning day.
FAQ
Can I measure speed with an ordinary proximity sensor?
Yes — pointed at a gear, keyway or bolt head, a proximity sensor produces one pulse per pass, and frequency converts to rpm. It's a legitimate method for control feedback at moderate speeds. Dedicated speed sensors earn their price when you need specified low-speed behavior, hazardous-area ratings, or a built-in trip threshold (speed switch).
Transmitter or raw accelerometer — which do I start with?
Ask who's going to look at the data. If the answer is "the PLC and the SCADA trend," start with 4–20 mA transmitters — simple, robust, immediately useful for alarming on overall level. If you have (or plan to hire) vibration analysis capability, add raw accelerometers on the machines where spectrum analysis will pay. KJT Sensors' published line covers both directions.
How many monitoring points does a machine train need?
Minimum useful coverage for a motor-pump set: one velocity point per bearing housing in the direction of greatest load — typically four points for the train, plus axial on the pump. Critical machines justify horizontal, vertical and axial at each bearing. Start with the bearings that have failed before; every plant has that list.
What's a realistic first step for a plant with zero monitoring today?
Pick the five machines whose unexpected failure hurts most. Put speed/underspeed protection where the process needs interlocking (conveyors first), and vibration transmitters on the rest, trended in whatever SCADA you already have. A simple trend that someone actually looks at beats a sophisticated system nobody opens.
Conclusion
Speed and vibration are complementary, not competing: one protects the process from what the machine does, the other protects the machine from what it's becoming. Conveyors and hoists need dependable low-speed and slip detection; pumps, fans and motors need trended vibration; the few truly critical machines need displacement probes watching the shaft itself. KJT Sensors publishes all three families — including active gear sensors for near-zero speed, vibration transmitters for straightforward 4–20 mA trending, and eddy-current probes for shaft diagnostics — so the monitoring architecture can grow from a few protected belts to a full predictive program without changing suppliers. The best monitoring program isn't the one with the most sensors; it's the one where every alarm has an owner and a response.
Next step: List your five most failure-critical machines with their speed ranges and bearing arrangements, and send them to the KJT Sensors engineering team for a point-by-point monitoring recommendation.
Sources
- KJT Sensors — Speed sensors product line (Hall gear, passive magnetoelectric, photoelectric, high-temperature, speed switches): https://www.kjt-sensors.com/list-sdcgq.html (accessed September 2026; specifications to be confirmed per model page)
- KJT Sensors — Vibration sensors product line (velocity, piezoelectric, transmitters, three-axis): https://www.kjt-sensors.com/list-zdcgq.html (accessed September 2026)
- KJT Sensors — Eddy-current displacement sensors (shaft vibration, axial/radial position): https://www.kjt-sensors.com/ (accessed September 2026)
- ISO 10816 / ISO 20816 series — Mechanical vibration evaluation of machine vibration, International Organization for Standardization. Standard reference for vibration severity evaluation cited above.
Content Notice
This article is brand content published by KJT Sensors. Product capabilities are described according to the manufacturer's published product information; sensitivity, frequency range, speed range, temperature ratings, output types and protection ratings must be verified against the documentation of the specific model before specification. Monitoring configurations for hazardous areas must use appropriately certified models selected by qualified personnel.
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