How to Choose a Sensor for Gear, Shaft or Conveyor Speed

• time：2026-10-10 14:59:11
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Speed sensing on gears, shafts and conveyors is a five-part specification: define the target (a ferromagnetic gear tooth, a toothed wheel, a magnet or a mark), determine the pulses per revolution the controller needs, verify the sensing gap and speed range against the model, decide whether zero-speed or slip monitoring is required, and match the output to the controller. KJT Sensors' gear speed sensors detect gear teeth without contact using magnetoresistive or Hall elements, output a standard square-wave pulse for PLCs and counters, support near-zero-speed detection with active sensing, and include models with a 0–25 kHz frequency response and a −40 °C to +125 °C operating range — with speed range, output and mounting confirmed for the specific model.

Key Takeaways

• Target first: gear speed sensing requires a ferromagnetic target — iron, carbon steel or electrical steel. Non-magnetic materials (aluminum, brass, plastic) need a special solution.

• Pulses per revolution = speed resolution. More teeth give finer speed resolution and faster underspeed detection, but demand a higher-frequency sensor and cleaner signal at top speed.

• Air gap is a controlled quantity: a typical installation gap of 1–2 mm, adjusted for gear module and radial runout per the model's instructions (manufacturer-stated).

• Zero-speed and slip duties change the choice: active magnetic sensing supports near-zero-speed detection; underspeed/slip monitoring compares measured pulses against a reference and belongs to the conveyor-protection decision.

• KJT Sensors gear-speed anchors (manufacturer-stated): 0–25 kHz frequency response on some products, −40 °C to +125 °C wide-temperature models, 5–28 VDC input range, NPN/PNP/push-pull/digital outputs.

Step One: Define the Target and Its Geometry

The speed sensor answers the question "how fast is this rotating thing moving" by watching a periodic feature pass. What that feature is decides the sensor family:

KJT Sensors' selection guidance for gear applications confirms four target parameters up front: gear module, tooth profile, material and tooth-tip width. Smaller modules require higher sensor sensitivity and tighter installation-gap control (manufacturer-stated) — the first place a marginal application fails.

Step Two: Calculate Pulses per Revolution

The controller computes speed from pulse frequency: speed (rpm) = pulse frequency × 60 / teeth. Two requirements pull in opposite directions:

1. Resolution at low speed. To detect underspeed or zero speed quickly, more pulses per revolution give the controller more information per rotation. A 60-tooth wheel at 1 rpm produces 1 Hz — countable; a 1-tooth target at the same speed needs a full minute per pulse.

2. Frequency headroom at high speed. Every added tooth multiplies the pulse frequency at top speed. The sensor must cover the full range: KJT Sensors lists a 0–25 kHz frequency response on some products (manufacturer-stated). A 60-tooth wheel at 3,000 rpm already produces 3 kHz; the same wheel at 25,000 rpm would exceed it — the arithmetic must be done per application.

The worksheet at the end of this article walks the calculation. The output frequency and the smallest speed that must be resolved are the two numbers that constrain everything downstream.

Step Three: Verify Gap, Speed Range and Environment

Air gap. A typical installation gap is 1–2 mm, adjusted for gear module, radial runout and the model's instructions (manufacturer-stated). Runout matters: a shaft that wanders ±0.3 mm effectively consumes gap margin. Where radial runout is present, allow additional safety clearance.

Speed range. Confirm the model covers both ends: the top speed's pulse frequency (within the sensor's frequency response) and the bottom speed (within the sensor's low-speed capability). Passive variable-reluctance sensors historically lose signal at low speed; active magnetic sensing — the approach KJT Sensors' gear sensors use — supports near-zero-speed state detection for startup, stopping and low-speed monitoring (manufacturer-stated). Zero-speed output capability must be confirmed for the specific model.

Environment. KJT Sensors' gear-speed engineering describes waterproof cylindrical or threaded metal enclosures for oil, dust, water and vibration; stainless-steel structures; optional overvoltage and reverse-polarity protection; and shielded metal cables to reduce interference from variable-frequency drives and large motors (manufacturer-stated). Wide-temperature models cover −40 °C to +125 °C for wind-power, engine-compartment and heavy-industry environments.

Step Four: Decide on Zero Speed and Slip Monitoring

Two duties go beyond plain speed measurement:

Zero-speed detection — positively knowing the machine has stopped — requires a sensor that signals at (or near) zero speed. KJT Sensors' active magnetic sensing supports near-zero-speed detection, with zero-speed output capability confirmed per model (manufacturer-stated).

Slip and underspeed monitoring — comparing belt speed against drive speed, or speed against a threshold — adds logic to the speed signal: a speed switch or slip detection device that trips when the ratio or the absolute speed leaves its window. On conveyors this belongs to the protection-device selection, where slip, tear and plugged-chute devices are chosen together ({{URL_D03}}), and where startup conditions that cause false trips have their own analysis.

Step Five: Match Output and Interface

The standard output is a square-wave digital pulse whose amplitude depends on the power supply and output circuit, usable for high-speed PLC counting or rotational-speed calculation (manufacturer-stated). KJT Sensors' interface options:

• NPN or PNP — suits short control runs to standard PLC inputs; the input-matching discipline is the same as for any switching sensor ({{URL_F10}});

• Push-pull — universal NPN/PNP compatibility in one device;

• Digital communication outputs — for longer distances or high-interference environments, per the manufacturer's interface guidance.

A 5–28 VDC wide input range (manufacturer-stated) covers common industrial supplies. For pulses that go missing or appear extra in service, that is a diagnosis with its own sequence ({{URL_T07}}).