Solving fastening problems is about controlling variables or eliminating potential causes that create an unwanted fastening event. Let’s take a look at common unwanted fastening events. Cross threading, thread or component stripping, unseated fasteners, missing screw, nut or bolt (omissions) these are all evident events of failure that we can see with our eyes. Thank goodness for our eyesight, because these are easily recognized. Fastening problems that we cannot see are our biggest risk. Latent failure can be a big problem depending on the product or exposure to risk or liability. Latent failure is caused not only by under torque, but largely by over torque events. Most all of us have opened a product package and discovered a screw or nut that came loose in transit. We might have also found one on the floorboard of our car. Many times these screws or nuts are not critical, they may come from the exterior of a product. However, they still concern us. They should. It makes us wonder if something more critical might also fail. Every time we find one of these rogue fasteners something did not go as planned, thus a torque related problem or unwanted fastening event has occurred. These occurrences are frustrating, time consuming and many times create delay and cost. You can get rid of them!
Let’s not over complicate things when we study fastening variables. In our experience there is only a handful of really complicated fastening problems and most of our customers don’t experience many of these all that often. Generally, it is a simple set of variables that need to be discovered, isolated and removed to solve fastening related headaches. It’s important to thoroughly investigate in a scientific manner what may be wrong with the fastening scenario. More important is not to freak out and think that everything is wrong, and try to change too many things too fast. The process of elimination is a great tool for solving fastening failure. Fasteners are generally made to a specification that defines physical criteria. The information is public and validating dimensions is not hard. You can do it on line or in most any hand book.
The clamping force is the main factor that is needed in controlling a fastened joint. Unfortunately, for large-scale production, no particular method has yet been developed to allow us to do this. Therefore, instead we must control other parameters that influence the clamping force. In this day in age, the most common method of controlling the clamping force is to control the installed torque to which the fastener is tightened.
Controlling the Installed Torque
The reasons for the widespread use of this method are mainly historic. In the past, accurate control of the clamping force was not so critical because products tended to be over engineered (by today’s standards), controlling the tightening torque proved to be a satisfactory method of ensuring joints did not fail. Also, it has been a tradition that most quality checks have been made separate from, and after, the assembly operation. The torque is a parameter that quality inspectors can check after the event.
Relationship Between Installed Torque and the Clamping Force
It’s important to realize that in a typical joint only about 10% of the energy spent in turning the fastener is actually converted into the clamping force. The other 90% is lost in friction. It’s very important to be aware that any changes of friction in the joint have a great effect on the clamping force. For example, if the average coefficient of friction in the joint changes from 0.10 (lightly oiled) to 0.18 (dry). For the same installed torque, the clamping force is reduced by 50%.
Designed for high production environments, the E-DRIV BF-Series feature a high performance brushless motor design that provides durability and reduces the standard maintenance costs for electric screwdrivers. Mountz offers a variety of models including BF brushless electric screwdriver with an adjustable RPM setting directly on the tool as well as a selectable “Soft Start” option.
The adjustable speed control provides manufacturers with the versatility to quickly change the tool settings as production projects change. The selectable “Soft Start” mode is for applications that require the power tool to slowly ramp-up to its operating speed, perfect for plastic or sensitive applications.
A dial indicating torque screwdriver is designed for torque evaluation and torque verification. It is a low cost tool that can utilized for QC, R & D and assembly operations. The Mountz dial indicating torque screwdriver can be used to monitor torque as it increases or decreases or display peak torque applied. The dial torque screwdrivers, by Mountz, measure torque through the use of known twisting force needed to distort a torsion bar. The amount of torque applied is measured on a dual scale dial display. This tool can be used in two ways, it can either be set in track mode so it can constantly track the applied torque or set in memory mode, which is done by withdrawing the slider and resetting the bezel ring to zero before using. This tracks the applied torque as before, but leaves the memory pointer at the maximum torque position.
Determine Torque Requirements in R & D
When determining correct torque specifications, the engineer must consider the maximum load placed on the fastener, the strength of the material joined, and whether the joint is hard or soft. A hard joint connects materials directly. In this case, the fastener rotates very few degrees to develop full clamping force after it encounters the material. Since a soft joint contains a gasket or involves compressible materials, it requires additional tightening after the fastener makes contact, to achieve full clamping force.
One recognized method is to perform a destructive test with a calibrated torque control tool on the actual material and fastener to be joined. An evaluation is usually conducted with ten parts, ten fasteners, and a calibrated torque control tool with a transducer. First the fastener is tightened to the point of failure, then repeated several times to verify the consistency of the failure point. Now another series of tests is begun whereby the joint is torqued to 75% of the failure point. Depending on how the parts will be used, the tightening can be reduced by any degree necessary. If parts on a machine are subject to heavy vibration, maybe 85% of the total force is necessary for good torque control.
Electric Torque Screwdrivers are designed for precision torque control and ensure product quality, cost savings and a reduction in overall assembly failures. 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 or damage parts. Using a quality assembly torque tool is important for many companies to ensure that proper torque is being applied and maintain requirements associated with the ISO 9001 quality standard.
Listed below is quick reference guide for selecting from the various common electric torque screwdrivers available.
Direct Plug-in Electric Screwdrivers These tools plug directly into a power outlet without the need for a transformer (power supply). A direct plug-in electric screwdriver is intended for low to medium cycle applications. The electric screwdriver is designed to install a fastener then rest for 3.5 seconds before you install the next fastener. The reason is that all the electronics are built into the power tool and it may over-heat and damage the tool. These reliable power tools automatically shuts-off when the preset torque has been reached.
DC Powered Electric Screwdrivers For high production applications, an electric screwdriver with a transformer (power supply) is built for these applications. Low voltage electric screwdriver automatically stops when the preset torque has been reached. Various models and styles.
Brushless Electric Screwdriver: Designed for high production environments, the electric screwdriver features a high performance brushless motor design that provides durability and reduces the standard maintenance costs for electric screwdrivers.