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11/30/2018

How RPM Impacts Torque Output

Synopsis


Details

During the assembly of parts using an electric torque screwdriver there are many things to consider to achieve proper torque control. Is the joint hard or soft? The material that is being used? Is the screw lubricated or treated with a locking patch? One factor that's often overlooked when using an electric screwdriver is the RPM setting of the tool.


Will changes in RPM have a net effect to the torque applied to a joint?
The answer in short is yes. RPM settings can be a contributor as to the torque applied to a joint. There are a number of variables to consider. A best practice is to document the settings and ensure they are not altered after all has been set and validated. It is important to ensure the same settings are used when validating calibration and making correlations.


A few of the variables are as follows:
- Higher RPM, less energy is applied at the joint as the force is present for a shorter period of time.
- Higher RPM may result in increased inertia, although the net differences are dependent on the mass of the force being generated.

- Lower RPM, more energy is applied at the joint, as the force is present for a longer period of time.
- Lower RPM can result in less inertia, although the net differences are dependent on the mass of the force being generated.


In some scenarios, the net differences may be negligible, while in other scenarios, the net differences may be more significant. Due to these and other variables, it is best to check the residual torque at the joints and develop a formal and consistent plan for setting and validation.


Three Proven Methods of Verifying Torque Specifications
Once a torque specification is determined, the joint should be audited to verify the product has been fastened to the specified torque. It is important to audit the joint for accuracy and to ensure your product's quality, safety and reliability isn't compromised. The failure of a three-cent fastener that isn't properly tightened can lead to catastrophic or latent failures. Fasteners that are insufficiently torqued can vibrate loose and excessive torque can strip threaded fasteners. It is important for many companies to ensure that proper torque is being applied and maintains gauge requirements associated with the ISO 9001 Quality Standard.


To perform this test, there are three common methods that have been established to provide an accurate reference to the applied torque.


1) First Movement Test - Once the fastener has been tightened, employ the use of torque measuring tool. Mark the tightened fastener and surrounding application. In the tightening direction, begin to slowly apply force to the tool until the first movement in the fastener is noted. The reading recorded is a good indication of the original torque applied to the joint. This is the best way to determine residual torque.

2) Loosening Test - This is a similar process to the first movement test described above, except instead of the tightening the fastener, the torque is applied in the direction that loosens the fastener. At the point the fastener breaks loose, the torque reading is recorded. The torque value to loosen the fastener is the approximate torque that was applied to the joint.

3) Marking Test - Once the fastener tightened, mark clearly the surface of the fastener, nut or bolt and continuing the mark onto the surface being clamped for reference. This time loosen the fastener and retighten until the marks on both application and fastener are aligned. The torque required to return the fastener to its original location is the reference to the original torque applied to the fastener.


What is Residual Torque? It is the amount of tension that remains in a joint after fastening a threaded fastener.


Many users may want to verify residual torque. By checking the torque after assembly, you not only verify adequate torque was delivered to the fastener, but may also detect missed or loose fasteners, or joint relaxation. But since the application is already seated and friction during rundown is different than the friction in a static joint, the torque reading will vary from those in the tool crib and from the dynamic values. These differences will need to be accounted for when engineering a residual torque specification.


The equipment used for these testing methods would be:


Dial Screwdrivers
Dial Wrenches
Torque tester with a Rotary Torque Sensor, or Torque Screwdriver Sensor to move the fastener