Understanding the TQ402 Eddy Current Probe
The TQ402 (part number 111-402-000-013) is a non-contact eddy current proximity probe used in rotating machinery protection systems. It measures shaft vibration, axial position, and speed by sensing the gap to a conductive target surface. The probe outputs a voltage proportional to the physical distance, which is then processed by a signal conditioner (such as the IQS450 204-450-000-001) and a monitoring card like the MPC4.
Proper gap setting is critical for accurate measurement and machine protection. A gap that is too large leads to weak, noisy signals; a gap that is too small risks probe-tip contact and mechanical damage. This guide explains target surface preparation, step-by-step gap adjustment, and verification so you can install the TQ402 reliably.
Target Surface Preparation
The target surface directly determines measurement linearity and repeatability. For the TQ402, the target must meet these requirements:
Material: Electrically conductive and non‑magnetic. Stainless steel (300‑series), aluminum alloys, and most non‑ferromagnetic metals work well. Avoid carbon steel or materials that attract a magnetic field, as they cause non‑linearity.
Surface finish: Smooth, with a roughness better than Ra 3.2 µm. Scratches, pits, or heavy oxidation create false vibration signals and should be polished out.
Shaft diameter: A minimum of three times the probe tip diameter is recommended. For the TQ402, a shaft diameter ≥ 30 mm gives acceptable linearity; larger diameters improve performance.
Runout: Total indicated runout (TIR) should be below 0.05 mm. Excessive runout appears as a cyclic gap change and may trigger false alarms.
Coatings: Avoid insulating coatings such as chrome plating, thermal spray, or paint unless they are thin (< 0.5 mm) and electrically conductive. Any non‑conductive layer acts as an extra gap and degrades the signal.
Step‑by‑Step Gap Setting Procedure
Pre‑installation checks
Confirm the probe, extension cable (e.g., EA402 913-402-000-013), and signal conditioner (IQS450 204‑450‑000‑001) are correctly matched. Install the probe holder or bracket rigidly so it does not vibrate.
Wiring and power
Connect the probe and extension cable to the conditioner, then apply the required supply (typically +24 V DC). Allow a 10‑minute warm‑up for the electronics to stabilize.
Initial mechanical positioning
Screw the probe into its holder until you feel light contact with the shaft, then back off half a turn. Never force the probe tip against the shaft; the tip is fragile.
Monitor the gap voltage
Measure the DC output voltage from the conditioner (usually at the OUT terminal). In the VM600 system, the TQ402 linear range is typically –2 V to –18 V (or –2 V to –10 V, depending on configuration). The recommended operating point is near –8 V to –12 V, which corresponds to a gap of about 0.8 mm to 1.0 mm. Refer to the probe’s calibration certificate for the exact voltage‑to‑gap slope.
Adjust to the optimal gap
Rotate the probe further in (smaller gap → more negative voltage) or back out (larger gap → less negative voltage) while watching the multimeter. Set the voltage to approximately –10 V, then lock the nut. After locking, re‑check the voltage—mechanical tightening often shifts the reading by 0.1–0.2 V.
Verification and Functional Check
Rotate the shaft slowly; the DC gap voltage should remain steady. The AC output should show only normal runout.
Perform a bump test or slide a thin shim between the probe and target to confirm the system reacts.
Verify that the conditioner’s “OK” or system‑healthy LED is on solid.
Common Issues and Troubleshooting
Gap voltage drifting: Check cable connectors for looseness or moisture. Inspect the EA402 extension cable for shield damage.
Voltage outside linear range: Re‑adjust the gap. Voltage more negative than –18 V means the probe is too close; voltage between 0 V and –1 V indicates the probe is too far or the target is missing.
Noise on the signal: Confirm surface finish quality, check for electrical interference, and ensure the cable shield is grounded only at the conditioner end.
False trip from the monitoring card: Review the MPC4 threshold settings and the mechanical condition of the machine before adjusting alarm levels.
Spare Parts and Engineering Support
Joyoung Industrial Automation Parts keeps TQ402 probes (111‑402‑000‑013 and 111‑402‑000‑012), matching EA402 cables, and IQS450 signal conditioners in stock. We ship globally with fast delivery to minimize downtime. For technical questions or urgent replacements, contact Li Hao directly at chen@htechplc.com or +86‑181‑5013‑7565.
Browse more proximity probe options on our website. Our engineering team can help you select the right probe, cable, and conditioner for your machine protection architecture.
If you’re interested, check out these related articles:
GE IS200AEPAH1A Mark VI Analog I O Module Sourcing Guide
Epro PR6424 002 100 Proximity Probe Replacement Specs
Industrial I O Module Types Digital Analog Communication Explained
Foxboro FBM202 Sourcing Reliable AO Field Bus Module Replacements
Recommended Models
EA402 913-402-000-013 A1-E090-F0-G000
IQS450 204-450-000-001 A1-B21-H05-I0
IQS450 204-450-000-002 A1-B21-H10-I0
IQS450 204-450-000-001 A1-B23-H05-I0
IQS450 204-450-000-002 (A1-B21-H10-I1)
VM600 CPUM 200-595-067-114
VM600 CPUM 200-595-063-314
TQ902 111-902-000-011 A1-B1-C50-D2-E1000-F0-G0-H05-I0
TQ902 111-902-000-011 A1-B1-C50-D2-E1000-F0-G0
TQ902 111-902-000-011 A1-B1-C70-D2-E1000-F0-G0-H10
TQ402 111-402-000-013
EA402 913-402-000-013
EA402 913-402-000-012
MPC4 200-510-071-113
MPC4 200-510-017-017
TQ412 111-412-000-012
TQ402 111-402-000-012
TQ402 111-402-000-012 A1-B1-C155-D000-E050-F2-G050-H05
TQ402 111-402-000-012 A1-B1-C045-D000-E010-F0-G000-H10
EA402 913-402-000-013 A1-E040-F0-G000
