When a rotating machine needs non-contact vibration or axial position monitoring, the proximity sensor selection often comes down to three Vibro-Meter VM600 series probes: TQ402, TQ412, and TQ902. They look similar in catalogs, but they serve different mechanical, thermal, and signal-chain requirements. Choosing the wrong one can create false trips, weak signals, or installation problems.
This guide compares the three probe families in practical terms. It focuses on what matters during replacement, upgrade, and new project planning.
TQ402, TQ412, TQ902 at a Glance
TQ402 is the standard general-purpose eddy current proximity probe. It is the first choice for most radial vibration, axial thrust, and shaft displacement measurements on motors, pumps, fans, generators, and compressors. TQ402 works with IQS450 signal conditioners and EA402 extension cables. It is stocked in multiple cable lengths and thread configurations under part numbers such as 111-402-000-012 and 111-402-000-013.
TQ412 is a compact derivative used where space is limited or the machine is smaller. It appears in light-duty pump monitoring, small gearboxes, and auxiliary rotating equipment. TQ412 is not simply a smaller TQ402. It has a different mechanical profile and a different measuring loop baseline, so it cannot be blindly swapped into a TQ402 application.
TQ902 is the high-temperature M12 threaded probe. It is specified for gas turbines, precision spindles, and engine positions where mounting space is tight and surface temperatures exceed standard industrial limits. The M12 thread allows installation in confined bearing housings and machine frames.
Quick Comparison Table
| Selection Factor | TQ402 | TQ412 | TQ902 |
|---|---|---|---|
| Typical Use | General-purpose radial vibration, axial position | Small pumps, light-duty gearboxes, compact machines | High-temperature gas turbines, precision spindles, tight engine locations |
| Mounting Profile | Standard eddy current probe body | Reduced-profile body for small machine geometry | M12 threaded high-temperature probe |
| Temperature Suitability | Standard industrial range | Standard industrial range | Extended high-temperature range |
| Common Part Number Examples | 111-402-000-012, 111-402-000-013 | 111-412-000-012, 111-412-000-013 | 111-902-000-011 |
| Main Selection Risk | Must match target surface and gap setting | Do not drop-in replace TQ402 without reconfiguration | High-temperature signal attenuation and cable rating |
When to Choose TQ402
TQ402 is the default for most VM600 rotating machinery protection loops. Use TQ402 when:
- The target is a large motor shaft, generator shaft, compressor rotor, or fan shaft.
- The installation allows a standard probe body and standard target area.
- The ambient temperature is within normal industrial range.
- The signal chain uses IQS450 204-450-000-001 or 204-450-000-002 with EA402 extension cable.
TQ402 part numbers include cable length suffixes such as H05, H10, E050, E090, or C160 in some configurations. The exact suffix determines cable length, armor, connector type, and certifications. Always match the full part number when replacing an existing probe.
For large motor shaft end monitoring, the configuration TQ402 111-402-000-013 with suitable cable length is common. The probe is also used with IPC704 signal conditioning when the application requires multi-channel processing.
When to Choose TQ412
TQ412 is not a replacement for TQ402 only because the part number starts with TQ4. It is a separate mechanical and electrical variant.
Choose TQ412 when:
- The machine is a small pump, small fan, or light-duty gearbox.
- The mounting location cannot accept the standard TQ402 probe body.
- The application is cost-sensitive and the measurement duty does not require the larger TQ402 target area.
TQ412 111-412-000-012 with E050 cable length is often used for small pump economic monitoring. The probe still connects into the same VM600 rack, but the IQS450 configuration and gap voltage baseline must be set for TQ412, not TQ402.
A key issue seen in field replacements is baseline mismatch. If a maintenance team removes a TQ402 and installs a TQ412 on the same IQS450 channel, the DC gap voltage can shift. The system may read a false gap or interpret the offset as vibration. Do not mix TQ402 and TQ412 on the same loop without re-baselining and verifying the signal conditioner configuration.
When to Choose TQ902
TQ902 is selected for two reasons: high temperature and tight mounting space.
Typical applications include:
- Gas turbine bearing positions where surface temperature exceeds standard probe ratings.
- Precision spindle monitoring where an M12 threaded body is needed.
- High-temperature environments where standard probes would drift or fail.
TQ902 111-902-000-011 uses M12 thread. Installation in tight spaces requires checking the target surface size and gap. The smaller thread does not remove the need for a proper eddy current target. The target surface must still be flat, conductive, and large enough relative to the probe tip.
High-temperature probes also have signal attenuation characteristics. As the probe body heats, the conductor and insulation behavior changes. The signal conditioner must be configured to compensate for this. If the system uses IQS450 or IPC704, the correct channel parameters and cable compensation must be selected. Otherwise, the measured gap and vibration amplitude will be inaccurate.
Probe Gap Setting and Target Surface Guide
For all three probes, correct gap setting is essential. The probe measures displacement by the change in impedance between the probe tip and a conductive target. This requires:
- A flat, smooth, clean target surface.
- A minimum target diameter. As a rule, the target should be at least 1.5 to 2 times the probe tip diameter. For TQ402, check the exact tip diameter on the datasheet and ensure the shaft or collar target is sufficient.
- No keyways, drilled holes, or edges inside the target area.
- Gap set to the center of the linear range specified by the signal conditioner. Actual DC voltage depends on the IQS450 or IPC704 configuration.
- Torque control during installation. Overtightening can damage the probe body or distort the thread.
If the target surface is too small, the probe output becomes nonlinear. The system may pass a static gap check but fail under dynamic vibration because the target is leaving the probe field.
Common Mistakes in Selection and Replacement
- Replacing TQ402 with TQ412 without reconfiguring the signal chain. This creates a baseline offset and can cause false alarms or missed trips.
- Using a standard EA402 extension cable with TQ902 in high-temperature zones. The cable must be rated for the installed environment.
- Ignoring the cable length suffix. Longer cables change the high-frequency response and may require different signal conditioner tuning.
- Selecting a probe based only on thread size. Thread compatibility does not mean signal compatibility.
- Skipping gap voltage verification after installation. Always check the static gap voltage before returning the machine to service.
Sourcing and Spare Parts Considerations
When requesting a quote or replacement, do not send only “TQ402 probe.” The full part number matters:
- TQ402 111-402-000-013 vs 111-402-000-012
- TQ412 111-412-000-012 vs 111-412-000-013
- TQ902 111-902-000-011
The suffix defines cable length, thread, connector, and certifications. A wrong suffix can delay installation and risk downtime.
Joyoung International Trading Co., Limited supplies TQ402, TQ412, TQ902 and supporting VM600 system modules including IQS450, IPC704, EA402, MPC4, CPUM, and rack components. We support global clients with system matching, technical consultation, and fast delivery for industrial automation spare parts.
For current stock, configuration support, or a replacement quote, contact:
- Email: [email protected]
- Phone/WhatsApp: +86-181-5013-7565
FAQ
What is the main difference between TQ402 and TQ412?
TQ402 is the standard general-purpose probe for medium and large machines. TQ412 is a compact variant for smaller machines. They are not electrically interchangeable without reconfiguration of the signal conditioner.
Can I replace a TQ402 with a TQ412?
Only if you re-baseline the IQS450 or IPC704 channel and confirm the target surface, cable length, and gap setting match the TQ412 requirements. In most existing installations, it is safer to replace with the same model.
When should I choose TQ902?
Choose TQ902 when the application has high surface temperature or requires an M12 threaded probe in a tight mounting location. Typical examples are gas turbines and precision spindles.
What happens if I use the wrong target surface size?
The probe output becomes nonlinear. You may see correct static gap voltage but incorrect dynamic vibration readings. Always confirm the target diameter meets the probe specification.
How do I read the TQ402 part number?
The full part number includes model, version, thread, cable length, and certification codes. Always quote the complete number shown on the existing probe label.
If you’re interested, check out these related articles:
Epro PR6424 002 100 Proximity Probe Replacement Specs
GE IS200AEPAH1A Mark VI Analog I O Module Sourcing Guide
Oil Gas Automation Spares SIS DCS PLC Reliability
Industrial I O Module Types Digital Analog Communication Explained
DCS Migration or Legacy Parts Strategic Industrial Choices
