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3. Calibration Environment Requirements

3.1 Environmental Conditions

Temperature:(20 ± 10) °C

Relative Humidity: ≤ 80%

Environment: The calibration site must be free from corrosive media, strong electromagnetic fields, strong winds, or intense light interference. Outdoor calibration should be avoided.

Power Supply:(220 ± 22)V

3.2 Measurement Standards and Other Equipment

Standard Torque Meter or Hydraulic Torque Wrench Calibrator (hereafter referred to as the “calibrator”)

The calibrator’s display must be clear, stable, and complete, providing real-time torque readings.

The Maximum Permissible Error (MPE) of the calibrator must not exceed 1/5 of the MPE of the hydraulic wrench being calibrated.

10% of the calibrator’s upper measurement limit must be less than 20% of the wrench’s rated value. The calibrator’s upper measurement limit must be at least 130% of the wrench’s rated value.

Hydraulic Power Source

The hydraulic source used for calibration must operate stably with clear pressure indicators and meet the requirements of the wrench under test.The absolute MPE of the hydraulic source’s pressure gauge must be better than 1/4 of the absolute MPE of the gauge typically used with the wrench.During calibration, when the pressure is adjusted to a preset point, the gauge reading fluctuation must not exceed 0.5% of the preset value. This pressure must be held for at least 3 seconds before a value is recorded.

Stopwatch: Resolution better than 0.1 s.

Safety Equipment: Necessary protective measures must be in place during the calibration process to prevent accidental injury.

Where:

Lk — Internal leakage, %;

Pmax — Rated pressure value (MPa, kPa, etc.);

▲P — Absolute value of the maximum change in hydraulic pressure due to internal leakage (MPa, kPa, etc.).

4.2.3 Calibration of Torque Indication

(1) Before calibration, start the hydraulic power source under no-load conditions. Drive the hydraulic wrench to its rated pressure for three pre-loading cycles to ensure the system is operating normally.

(2) Calibration points should generally be selected within the range of 20% to 80% of the wrench’s rated pressure (or within a specific range requested by the customer), preferably at whole-number intervals. There must be at least five calibration points, including the upper and lower limits of the range, distributed evenly and calibrated point-by-point.

(3) Zero the calibrator, start the hydraulic source, and load the system to the preset calibration point until it stabilizes.

(4) Once the stability requirements are met (pressure fluctuation must not exceed 0.5% of the preset value for at least 3 seconds), record the hydraulic source pressure reading and the calibrator’s torque reading. Then, release the load.

(5) Repeat steps (3) and (4) three times for each point.

(6) Perform the calibration for all remaining points by following steps (3) through (5) in sequence.

4.2.4 Repeatability of Indication

The repeatability of the torque indication is calculated using Formula (2):

Where:

R — Repeatability of the hydraulic wrench reading at the i-th calibration point, %;
Timax — Maximum value of the calibrator reading at the i-th calibration point, Nm;
Timin — Minimum value of the calibrator reading at the i-th calibration point, Nm;
Ti — Arithmetic mean of the calibrator readings at the i-th calibration point, N.

4.2.5 Calibration Equation

The calibration equation is calculated using Formula (3), where the coefficient a is determined by Formula (4) and the coefficient b is determined by Formula (5):

Where:

y is the fitted value;

a is the slope of the fitted curve;

x is the pressure value;

b is the intercept of the fitted curve;

xi is the pressure value at the i-th calibration point;

x is the arithmetic mean of the pressure values ​​at all calibration points;

yi is the torque measurement value at the i-th calibration point;

y is the arithmetic mean of the torque measurements at all calibration points.

4.2.6 Linearity Error

The linearity error is calculated using formulas (6) and (7):

Where:

Δǒj — The residual at the j-th calibration point, i.e., the difference between the measured value and the fitted value at that calibration point, Nm;

Tj — The arithmetic mean of the measured values ​​at the j-th calibration point, Nm;

Tej — The fitted value at the j-th calibration point, Nm;

ǒ — The linear error of the hydraulic wrench, %;

Tefs — The fitted value corresponding to the rated pressure, Nm;

Δǒmax — The maximum value among the residuals at each calibration point, Nm.

4.2.7 Interpolation Error

The interpolation error is calculated using Formula (8):

Where:

Ij — Interpolation error of the j-th calibration point, %;
Tj — Arithmetic mean of the measurements at the j-th calibration point, Nm;
Tej — Fitted value at the j-th calibration point, Nm.

5. Contents of the Calibration Certificate

Calibration results must be documented in a Calibration Certificate, which should include at least the following information:

• Title: “Calibration Certificate.”
• Laboratory Information: Name and address of the calibration agency.
• Location: The specific site where the calibration was performed (if different from the laboratory address).
• Unique Identification: A unique serial or certificate number, with identification on every page and a total page count.
• Customer Information: Name and address of the client.
• Tool Identification: Detailed description and clear identification of the hydraulic wrench being calibrated.
• Dates: The date of calibration and, if relevant to the validity of the results, the date the tool was received.
• Technical Specifications: Identification of the technical standards or codes used for the calibration (including names and reference numbers).
• Traceability: A statement confirming the traceability and validity of the measurement standards used during calibration.
• Environmental Description: A summary of the environmental conditions during the test.
• Calibration Results: The measured values and a statement regarding the uncertainty of measurement.
• Technical Requirements: An explanation of the technical parameters required by the calibration standards.
• Deviations: A description of any deviations from the standard calibration procedure.
• Signatures: Signatures of the technician who performed the calibration, the reviewer, and the authorizing official.
• Limitation Statement: A statement that the calibration results apply only to the specific object calibrated.
• Reproduction Rights: A statement that the certificate shall not be reproduced, except in full, without the written approval of the laboratory.

6. Recalibration Interval for Hydraulic Torque Wrenches

Since the length of the recalibration interval depends on several factors—including tool usage frequency, operator handling, and the quality of the instrument itself—the user may determine the specific interval based on actual operating conditions. However, it is recommended that the calibration interval for hydraulic wrenches generally does not exceed 6 months.

If any major components of the hydraulic torque wrench are repaired or replaced in a way that could affect the technical specifications described in Section 2 (Basic Specifications), the tool must be recalibrated.

Summary

Calibration of a hydraulic torque wrench is a rigorous and recurring process. Every calculation and operation throughout the procedure must be performed carefully, and all data must be accurately recorded.

If you lack professional experience in this area, we offer expert technical support, products, and training. You can contact the TorcStark team at any time.

Finally, we hope this calibration tutorial helps you learn how to calibrate your hydraulic wrenches independently. If you have any questions, please feel free to reach out to us.

Technical Terminology

Internal Leakage

A phenomenon where pressure becomes unstable while the hydraulic wrench is holding pressure, caused by poor internal sealing.

Calibration Equation

A mathematical relationship established from a limited set of calibration data between the hydraulic wrench’s output torque and the applied pressure. It is a fitting equation where the pressure value is the independent variable and the torque output is the function.

Rated Pressure

The maximum pressure required for the hydraulic wrench to operate under normal working conditions.

Torque Wrench with Non-Torque Value Display

A wrench that displays output in units other than torque.

Fitted Curve

A curve drawn by establishing a mathematical model based on discrete data points to perform interpolation or approximation.

Interpolation Error

The relative deviation between the arithmetic mean of the measured values at each calibration point and the corresponding fitted value calculated using the calibration equation.

Linearity Error

The ratio of the maximum residual (the difference between the arithmetic mean of the torque measurements and the corresponding fitted value) to the fitted value at the rated pressure.

FAQ

1. Why do hydraulic torque wrenches require regular calibration? What are the consequences of not calibrating?

A hydraulic torque wrench is an indirect measurement tool for applying torque. Its output torque is not measured directly but is calculated based on the parameters of the hydraulic system. Therefore, its accuracy highly depends on the overall condition of the hydraulic system and the mechanical transmission components.

Regular calibration determines the accuracy of all mechanical transmission components within the entire hydraulic system. This includes checking for issues such as internal leakage and wear on seals, pistons, ratchets, or drive mechanisms.

Failure to calibrate leads to a systemic deviation between the actual applied torque and the set value, resulting in bolts being over-tightened or under-tightened. In high-risk industries such as nuclear power, wind power, and petrochemicals, this can cause serious safety and liability accidents, leading to massive economic losses.

Therefore, regular calibration is not only necessary for precision control but is also a fundamental requirement for safety management and quality compliance.

2. What is the principle of hydraulic wrench calibration? How is hydraulic pressure converted into torque?

The operation and calibration of a hydraulic torque wrench are based on the conversion process of: Hydraulic Energy → Linear Force → Rotational Torque.

The tool uses hydraulic pressure to generate linear thrust. This linear thrust is then converted into rotational torque through a mechanical structure, establishing a specific pressure-to-torque relationship.

The essence of calibration is to verify whether this pressure-to-torque conversion still falls within the design or nominal accuracy range and to perform corrections or scaling if necessary.

3. How to correctly connect the hydraulic pump, wrench, and torque tester during calibration?

The connection process during calibration is the same as when using the wrench to tighten bolts. When positioning the wrench, ensure that the contact area between the working surface of the reaction arm and the calibrator’s reaction block is at least 2/3 of the reaction arm’s surface area.

4. Which load points should be selected during calibration?

In engineering practice, calibration load points are generally selected in the mid-to-high range of the tool’s rated torque. Common practice includes:

20% – 30% of the maximum rated torque

50% of the maximum rated torque

70% – 80% of the maximum rated torque

100% of the maximum rated torque (or near maximum)

This distribution verifies the linear relationship and stability of the hydraulic wrench under low, medium, and high load conditions.

If your hydraulic torque wrench is consistently used within a specific torque range for actual construction—for example:

Constant use between 60% – 90% of the rated value

Primarily used for flanges, wind power, or heavy-duty structural connections

Then the calibration points should focus on covering that specific range. If necessary, the density of load points within that range can be increased to improve reliability for your specific application.

5. How to calculate the accuracy of a hydraulic wrench?

During the calibration process, data such as linearity error, repeatability of indication, interpolation error, and internal leakage are calculated. By comparing these results with the data in the “Accuracy Classes and Technical Indicators” table mentioned in Section 2.2, the error rate can be determined.

6. If calibration fails, which component is usually at fault?

If calibration results are unqualified, it is usually due to issues with wear parts, such as sealing rings.

7. How to determine if the hydraulic cylinder has internal leakage through calibration data?

Calculate the value using the internal leakage formula mentioned in the article, then compare it with the data in the Section 2.2 table. If the difference is excessive, it indicates an internal leakage problem.

8. Does ambient temperature affect calibration results?

Yes, it does. Therefore, calibration must be performed according to the environmental conditions specified in the article.

9. How does the support position of the reaction arm affect calibration accuracy?

The support position of the reaction arm directly determines the force geometry and torque balance of the hydraulic wrench. Offset support points, insufficient rigidity, or incorrect angles will introduce systematic errors, reducing calibration accuracy and repeatability. Therefore, it is essential to ensure the reaction arm is supported at the specified position in a rigid and stable manner, maintaining consistency throughout the calibration process.

10. Does using hydraulic oil of different viscosities cause deviations in calibration values?

Hydraulic oil viscosity does not change the theoretical pressure-to-torque conversion relationship. However, it indirectly affects stability and repeatability during calibration by influencing internal flow resistance, leakage, and temperature sensitivity. It is recommended to use hydraulic oil with the viscosity specified by the manufacturer for calibration whenever possible.

11. Difference between dynamic torque testing and static calibration

Static Calibration: Used to verify the pressure-to-torque accuracy of the hydraulic wrench in a stable state. It is the foundation of metrology and quality management and serves as the standard for determining torque output accuracy. Static calibration is mandatory.

Dynamic Torque Testing: Used to evaluate torque changes and peak characteristics during the actual tightening process, reflecting performance under real working conditions. Dynamic testing is a supplementary item used for critical applications or special operating conditions.