When it comes to precision bolting tools, there are two types of pneumatic torque wrenches that get the job done; shut-off nutrunners and stall torque nutrunners. But what's the difference?
A shut-off nutrunner is a reliable tool that can deliver different torque (depending on the model) with a constant air pressure (90 PSI / 6.3 Bars) – based on what you need for your application.
As the name suggests, once the set-up torque is reached, the tool shuts itself off. A clear shut-off signal tells the operator when the tightening cycle is complete, therefore, you get the same accurate result regardless of the operator and their level of experience. Not only does this ensure accuracy, but it also boosts productivity because the operator can immediately move on to the next bolt without having to spend too much time on checking.
A stall torque nutrunner is also a continuous rotation tool, but unlike the shut-off, the torque delivered by these tools varies depending on the air pressure.
Stall torque describes the torque produced by a mechanical device with output rotational speed equal to zero. The torque load could also cause the output rotational speed of a device to become zero, meaning it caused stalling. Stall torque nutrunners are very strong, compact and lightweight tools, ideal for maintenance precision bolting. To obtain the right tool's torque, the operator should set up the appropriate air pressure (all information is provided by the manufacturer in the tool's torque chart, available on the tool test certificate. The air pressure level can go from 21.7 PSI (1.5 Bars) to 90 PSI (6.3 Bars).)
This set-up allows the tool to be versatile, and offers a wide range of torque, which can go up to 7375.6 ft.lbs (10000 Nm); however, any kind of air pressure variations will modify the final torque applied. Therefore, in order to ensure that the set-up torque is reached, a check is recommended.
“Before making a choice, make sure you identify the torque you need, and the frequency and conditions of use. For example, on applications such as wheel changing on heavy trucks or buses, the frequency of use is high, and therefore a shut-off nutrunner may be best. Contrarily, for flange assembly in the oil & gas industry, I would recommend a stall torque nutrunner, which will provide a higher torque range and better user comfort.”
Providing a safe working environment for operators is essential, especially when using pneumatic tools and compressed air. If there is a failure in the air network and the hose is not adequately clamped, the resulting whiplash could be devastating. Failure to follow best practice can result in injuries, associated production downtime and decreased productivity.
There is no industry standard to measuring and reporting on torque values, and torque is presented differently by tool manufacturers. The various methods of measuring torque have ultimately led to much confusion amongst tool users as to which values are important when specifying and operating equipment.