How to Choose and Maintain Your Next Torque Wrench
- Gray Tools Official Blog
- 16 May, 2016
The torque wrench is probably the most important tool for professionals working in industries such as aerospace, automotive and industrial, who are responsible for securing critical parts to exact tolerances with zero margin for error.
Choosing a reliable torque wrench will not only help you perform the basic fastening job. It will also provide the peace of mind that everything was done according to required specifications.
What Is a Torque Wrench?
A torque wrench is a precision instrument used to apply an exact amount of torque to a fastener. Once the desired torque value has been achieved, the operator is notified through a visual and/or audible alert, thus preventing under or over torqueing.
Choosing the appropriate torque wrench for your specific needs requires a good understanding of the options available, your usage requirements, maintenance, and re-calibration.
Pre-set versus Adjustable Torque Wrenches
If you are working on an application that requires torqueing to a specific value that remains constant, then a pre-set torque wrench is a good choice. Pre-set torque wrenches are factory-calibrated to a specific value. Once this value has been reached, it becomes impossible to apply additional torque.
Pre-set torque wrenches are used to prevent operator adjustment of the torque settings. If your project requires a change in the torque value, the manufacturer can re-set the wrenches to the new value. Pre-set torque wrenches are also ideal in assembly line environments or applications where the wrench will only be used in a single repeated application.
Adjustable torque wrenches offer the operator flexibility in setting the desired torque value according each specific application. These torque wrenches feature a scale, usually expressed in imperial and metric values.
The most commons scales are inch/pound, foot/pound and newton/meter. Regardless of scale adjustable torque wrenches allow the selection of desired torque value within a lower and upper limit, in specific increments.
For example, a torque wrench with a capacity of 30-250 ft/lb, with 1 lb increment will accommodate torque values between 30 ft/lbs and 250 ft/lbs, in increments of 1 lb.
Torque Wrench Styles
Click-type torque wrenches are the most commonly used type of torque wrench. They have the same functionality as a ratchet, with the added advantage of a distinct breaking action when the desired torque value has been achieved.
Available torque values are displayed on a dual scale (metric, expressed in Newton Meters, and Imperial expressed in foot/pound or inch/pound) on the barrel of the tool. Torque wrench capacity is indicated by the minimum and maximum value on the scale. The operator sets the desired torque value by unlocking the wrench and twisting the handle, which moves a precision indicator along the scale.
Just like a ratchet, the head of click-type torque wrenches houses the gear and paw mechanism. The repetitive force applied to its mechanism leads to the need for regular maintenance and re-calibration.
Dial-Type torque wrenches are considered the most accurate torque wrenches available.
These wrenches feature a double-end square drive that can be used for both tightening and loosing fasteners. The torque value is displayed on an easy-to read, analog, dual-scale dial (hence their name). Inside the dial, there are two memory needles. The first needle is set to the desired torque setting while the second needle is set to zero. As torque force is applied, the second needle moves along the dial. When the two needles meet, the desired torque has been met.
Due to their simpler torque mechanism, dial-type torque wrenches do not require as much maintenance and re-calibration as the click-type torque wrenches. Most models feature an audio-visual alert that signals the operator when the target torque is achieved.
Dial-type torque wrenches are widely used in quality control applications, to ensure the actual torque applied in the production process complies with the stated specifications.
Electronic torque wrenches have become very popular in recent years and are now available in traditional and dial type design shapes. They use an electronic sensor to measure torque and display the torque value on a digital display. An audible and visual indicator informs the user that they are approaching the desired torque value, and when the exact value has been reached.
Electronic torque wrenches provide the option to download the information into a computer via a wired or Bluetooth connection for future reference.
Digital torque wrenches are very accurate instruments. However, their more delicate construction requires extra care while handling. They also require batteries in order to operate.
Digital torque wrenches are the most expensive of styles noted in this article however they can be considered 4 torque wrenches in one, due to their ability to set torque values in four scales; ft/lbs, in/lbs, Nm and kg./cm.
Electronic torque wrenches are ideal in automotive and transportation settings as they can be linked to hardware such as laptops, tablets and printers. In many heavy truck repair environments it is mandatory that printed proof of wheel torque adjustments be available as part of certified repair service.
Beam torque wrenches are the most simplistic torque wrenches available. They are difficult to read and the least accurate. Consequently, they have virtually been eliminated from use in professional working environments.
Factory Certificate of Calibration - A Must Have Document
A torque wrench should include a certificate of factory calibration proving the tool has been inspected and tested at the manufacturer’s facility prior to final packaging and is within the stated accuracy range. Torque wrenches with missing or expired certificates of calibration should never be used.
For critical applications you should invest in third party calibration and certification that will provide piece of mind the torque wrench is properly calibrated.
In many companies and industries quality processes such as ISO demand and specify scheduled calibration intervals. In sensitive industries such as aviation and transportation, daily checks of each onsite torque wrench’s tolerance are conducted.
A valid certificate of calibration should include the following information:
- Torque wrench type, manufacturer, model number, and range
- Calibration equipment used to perform the calibration and its date of last calibration
- Torque reading clockwise and counter clockwise (if applicable)
- Calibration date
- Name of the person who performed the calibration and his/her supervisor
- A statement indicating that the torque wrench was calibrated to meet the accuracy in specifications as stated in ASME B107.14M-1994, GGG-W-686, Type 2, Class A Style 1, 2 and ISO 6789.
- A premium quality torque wrench will also include text indicating the wrench was calibrated on a torque standard traceable to the National Institute of Standards & Technology (N.I.S.T).
As a general rule of thumb all torque wrenches should be re-calibrated at least once per year or every 5000 cycles. More frequent calibration is strongly suggested if the tool is used regularly. As torque wrenches are stored in their protective cases at their lowest setting when they leave the factory the one year calibration guideline does begin until the tool’s first use.
Every torque wrench operator has to be able to present a valid certificate of calibration whenever required. This document becomes an important piece of evidence that will prove the operator took all the necessary precautions and worked with proper tools, in the event of an unexpected failure.
Torque Wrench Selection Checklist
With the fundamentals of torque wrench types and styles covered the next step is choosing the ideal torque wrench(es) that best suit your application. An important step is determining your needs. The list below is an excellent guide:
- do you need to tighten fasteners to multiple or single torque values
- what is the lowest and highest torque value you will be setting fasteners to
- what drive size are the sockets you will most commonly be using (¼ ", ⅜”, ½”, ¾” or 1”)
- do you need the torque wrench to operate clockwise and counter clockwise directions
- what level of precision do you require; expressed as accuracy +/-
The most important consideration in the selection process is understanding the torque range you will most commonly be working within. Torque wrenches work best within 40-80% of their working scale.
Therefore, do not choose a wrench that will be primarily used at the bottom or top end of the scale. You may be tempted to do so to save money but inaccurate torque output could be serious outweighing any savings.
Do’s and Don’ts of Torque Wrench Use and Maintenance
Good quality torque wrenches are an important investment for any professional user. Due to their role in ensuring critical parts are properly fastened proper use and maintenance is required. Here are some things you need to take into account:
- Never use a torque wrench with missing or expired certificate of calibration.
- Never use a torque wrench to break fasteners loose.
- Always perform the preventive maintenance required by the manufacturer.
- Never set the wrench at values higher or lower than those indicated on the scale.
- When not in use, clean and keep the torque wrench in its storage case at its lowest torque setting.
- Handle the torque wrench with care. Dropping a torque wrench will cause loss of calibration.
- Never disassemble a torque wrench yourself. Repair and re-calibration should only be done by an accredited laboratory, as per manufacturer’s instructions.
- If you buy a used torque wrench, properly calibrate it before the first use.
- Never exceed the rated torque of the tool as over torquing will result in wrench or part failure.
- Do not use a cheater bar or any other type of extension on the handle to apply extra torque. If extra torque is required consider a larger torque wrench of the use of torque multiplier
With proper care and maintenance, a torque wrench should provide reliable service for many years.