One fastener does not fit all. Do you have the right ones for your needs?

Many of the screws we sell work for a wide range of applications. Examples of good general-purpose fasteners include our line of metric pan screws, which come in slotted, Philips or Torx varieties. If you don’t have, say, a truss, or binding head screw, you can most likely drop one of these in as a substitute. Other screws, however, work well in one type of situation but not in another.

Sometimes it is the material that makes the difference. Nylon, for example, is great for electrical assembly work because it doesn’t conduct electricity and won’t corrode. But for large-scale structural fastening, nylon just isn’t strong enough. Stainless steel screws, on the other hand, are both strong and corrosion-resistant for those big outdoor industrial applications. But environmental considerations can mean more than just “indoors versus outdoors.” Fasteners used in boats, for instance, must stand up against seawater, as our stainless A-4 metric hex bolts do.

And what about head types? Do you know when you need socket or socket shoulder screws, button head screws, or pipe plug screws, and what application each kind of screw performs best? Chances are you don’t — and that’s why you need Mr. Metric, they live and breathe fastener technology. Mr. Metric is the leading metric fastener specialist in North America. Well regarded as experts in metric, Mr. Metric is known for hard to find metric items at competitive prices. The staff at Mr. Metric are happy to advise you personally on the ideal screws, nuts or bolts for your specific needs. If you have any doubts on what fasteners you need, contact Mr. Metric.

Not only make sure you order the right fastener for the job, but also make sure you are using the proper torque tool for the fastening application. Whether you are using a torque wrench, torque driver, electric screwdriver or a pneumatic screwdriver, controlling torque is essential for companies to ensure their 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. Using a quality torque tool has become increasingly important for many companies to ensure that proper torque is being applied and maintains gauge requirements associated with the ISO 9001 Quality Standard.

These general guidelines are to help identify potential pitfalls relating to the tightening of bolted joints.

Use a calibrated Torque Tool: Ensure that a calibrated torque tool is used and that a torque value is specified on the tightening specification. Be aware that certain automatic tightening tools, such as impact wrenches, can result in significant variations occurring in the torque value and the bolts preload. A calibrated torque tool should therefore be used for the final tightening operation or inspection.

Specify the correct tightening torque: Whenever feasible, specify the tightening torque based upon actual test results rather than a theoretically calculated value. Experimental determination of the tightening torque can be established by measurement of bolt extension, strain gauges or by the use of a load cell embedded in the joint.

Specify a Tightening Sequence: The majority of joints consist of more than one bolt and bring together surfaces that are not completely flat. The sequence of tightening bolts can have a major influence on the resulting preloads. With such joints, consideration should be given to specifying the sequence in which the bolts are to tighten. Because the joint surfaces compress, tightening one bolt in the vicinity of another will affect the preload generated by the first bolt tightened.

A good tightening sequence ensures that an even preload distribution is achieved in the joint. Since joints containing conventional gaskets have a comparatively low compressive stiffness, bolt preloads in such joints are particularly sensitive to the tightening sequence. Based on experience, if the bolts are in a circular pattern, a cris-cross tightening sequence would normally be specified. For non-circular bolt patterns, a spiral sequence starting at the middle would normally be specified.

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Generally, in the majority of applications, the reliability of the joint is dependent upon the bolts ability to clamp the parts together. Adequate clamping prevents relative motion between parts of the joint and leakage from joints containing gaskets. Measuring a bolts clamp force is difficult, especially under production assembly conditions.

The clamp force generated by a bolt can be indirectly controlled by regulating the applied torque. This method, known as torque control, is by far the most popular method of controlling a bolts clamp force. The initial clamp force generated by the bolt is frequently called preload.

There is a link between the torque applied to a bolt and the resulting preload. A problem exists in that friction has a large influence on how much torque is converted into preload. Besides the torque required to stretch the bolt, torque is also required to overcome friction in the threads and under the nut face. Typically, 10% to 15% of the torque is used to stretch the bolt. Of the remaining torque, typically 30% is dissipated in the threads and 50% to 55% under the nut face. Because friction is an important factor in the relationship between torque and preload, variations in friction have a significant influence on the bolts preload. Different bolt surface finishes generally have different friction values.

The torque required for a socket head screw will not be the same as that required the same size standard hexagon bolt. The larger bearing face of the standard bolt will result in an increased torque being required compared to a socket headed screw. This is because more torque is being dissipated between the nut face and the joint surface.

Stresses indicated into a Bolt: When a bolt is tightened the shank and thread sustain a direct (tensile) stress due to it being stretched. In addition, a torsional stress is induced due to the torque acting on the threads. These two stresses are combined into a single equivalent stress to allow a comparison to be made to the bolts yield strength. In order to effectively utilize the strength of the bolt, yet leave some margin for any loading the bolt would sustain in service, an equivalent stress of 90% of yield is commonly used. It is this approach that is used in this guide.

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Position Control torque arms are designed to reduce the risk of improperly fastened screws, ensuring that every screw is in the correctly tightened in the correct sequence. Using a Position Control torque arm is like putting the eyes and ears of a quality control manager where they are needed most – right on the assembly area.

“Sequence based fastening is critical to proper process control and a quality fastening result for many applications,” said Brad Mountz, President & CEO of Mountz, Inc. “If an assembly exhibits cross talk, a phenomenon where torque to one fastener changes the result to another in the in fastening pattern, altering the sequence is often necessary to achieve proper results. An encoded torque arm is perfect because it guides the operators sequence and provides feedback if done incorrectly.”

The new EZ-Glider Position Control torque arms by Mountz Inc. help manufacturers detect and eliminate costly screw-fastening errors during the assembly process. Assembly sequences are easily programmed for the torque arm from an easy-to-use control box. Up to nine sequence programs can be stored and are manually or automatically selected for easy recall.

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