by Tim Lillak

Almost every manufacturing plant that I have visited owns at least one impact wrench. Impacts are great, inexpensive tools in terms of power but lack any kind of torque control. To improve the quality, a click wrench is often used but the process then becomes manual and slow. In addition, the click wrench has no ability to inform the user if the impact has over-torqued the fastened joint.

With the growing demands our customers put on us to produce quality products, it is imperative that our screws, nuts and bolts are all fastened correctly. Fortunately, there are power tools now available that provide torque control and other features to help error-proof the assembly process.

It is difficult at times to know what tool to select as there are many options on

the market with different error-proofing features. To help ease the decision, it’s best not to get caught up in tool features but rather to think about the overall significance of a given joint to the functioning of your product.

In fact, most fastening applications can be classified into one of three categories:

Class C: Incorrect installation results in customer annoyance;
Class B: Incorrect installation results in major product malfunction
Class A: Incorrect installation can result in serious or mortal injury

Once you have correctly classified your joint, it is then easy to select the proper tool type for the application.

JOINT CLASS C – TORQUE ACHIEVED
Tools recommended for this class will have some kind of pre-set mechanical clutch. When specified torque is reached, the clutch will roll over and eliminate power to either the electric or pneumatic motor driving the tool. In general, these clutches have good torque accuracy and repeatability. Appliance manufacturers tend to be large proponents of these tools as there are many screws to install but the significance of each screw tends to be very low.

To illustrate, if you purchased a new washing machine with one loose screw on the metal shell, you may be disappointed with the quality of the unit, but it would hardly prevent the machine from washing your clothes.

JOINT CLASS B: TORQUE AND ANGLE ACHIEVED
The ability to measure both torque and angle from a power tool has become a major advancement in the assembly industry. DC electric power tools with torque measurement devices and angle encoders provide a whole new level of error-proofing. Why is angle important? Imagine what happens in a cross-threaded fastener. The torque may be achieved but the fastener has likely not even rotated a full turn, let alone seated.

By measuring torque and angle of rotation of the fastener and comparing the result against acceptance windows, we now have the ability to detect not only cross-threads, but also stripped fasteners, missing parts and even an operator slipping off the tool trigger before the job is complete.

Based on the result, the operator can be informed that there is an error with the assembly using either lights or buzzers and thus force him to take action. The installation of tap valves in beer kegs is one of the most unique applications we have encountered. By only using torque and angle were we able to confirm correct installation of the rubber seals. Obviously, this process was considered critical to the brewery as delivery of flat beer would be unacceptable to its customers.

JOINT CLASS A: TORQUE AND ANGLE ACHIEVED AND STORED
We typically call fasteners in this class Safety Critical joints. Because the risk of injury is high, we need 100% proof that the assembly was completed correctly. Tools recommended for this joint class are also DC electric tools similar to those for joint class B with two major differences: First, the tools have torque transducers which can be calibrated to ensure integrity of the result. Second, the torque and angle data can be stored and exported so that complete records of the assemblies are kept. By combining this data with a time, date and even the serial number of the product, you can provide complete proof to your customers that the assembly was performed correctly.

The automotive industry led the adoption of these tools in the early 90s, particularly on air bags and seat belts, but their use has since grown exponentially. These tools have been used on lawn mower blades, rail cars, hydraulic cylinders, electrical panels, landing gears and even barbeques.

We are now actively working with many of our customers to apply a joint class to every threaded assembly in their production. By doing this we can simplify tool selection, catch errors when they occur, and not rely on quality audits later in production. The result? Reduced costs, increased customer satisfaction and potentially saved lives.

Tim Lillak (tim.lillak@ca.atlascopco.com) is the GI business llne manager with Atlas Copco Tools & Assembly Systems Canada.