Torque are the amount of rotational force that is apply to a fastener. The torque is used to create clamp force, which is the force that hold the parts together. However, if a wrong amount of torque is applied, the clamp force will not be correct.
If too much torque are used, the screw will strip the threads. If too little torque is applied, the fastener will loosen due to vibration. The material of the fastener and the material of the part that is being fastened will dictate the amount of torque that should of be use.
How to Tighten Screws Correctly
For instance, an #6 screw made of Grade 8 steel requires eighteen inch pounds of torque to be applied to the screw. However, if the screw is made of aluminum, seventy percent less torque are required because aluminum is a softer metal then steel. Metals like brass are also soft in relation to steel, requiring sixty percent of the torque that is required for steel.
Plastic is another soft material that will also require only half of the standard torque to be apply. Additionally, if a light coat of oil is apply to the threads, approximately thirty percent less torque will be required due to the reduction of friction between the screw and the part being fastened. The drive head of the screw will dictate how much torque can be applied to the fastener.
For instance, if Phillips head screw are used, the head of the screw will slip or “cam out” with over-tightening of the screw. Thus, no torque can be transmitted to the screw with this type of head. However, if Torx or hex socket head screws is used, the heads will not slip as easy as Phillips head screws.
Additionally, slotted drive screws will slip easily when being tighten. Therefore, the drive head should be chosen for the specific job being perform to avoid damaging the heads of the screws. The tools that are use to apply the required torque to the screw also have an impact on the amount of torque that is applied to the fastener.
For instance, hand screwdrivers are use for tiny electronics and require only between zero and ten inch pounds of torque to be applied. However, calibrated torque drivers provide more precision for screws of different size. Additionally, electric and air screw drivers may increase the speed at which the screws can be tightened.
However, they may not be as accurate as hand tools; the clutches on electric and air screwdrivers may drift over time. Therefore, you should calibrate torque drivers once a year or after five thousand cycle of the tools have been used. The grade of the screw will indicate the strength of the screw.
For instance, if there is no marks on the screw head, it is a Grade 2 screw. Grade 2 screws are for light-duty application. If the screw has three radial lines on the screw head, it is a Grade 5 screw.
Grade 5 screws have more strength than Grade 2 screws. If the screw has six radial lines on the head, it is a Grade 8 screw. Grade 8 screws are for heavy load.
Metric screws have different numbers for the same grades; for instance, a 10.9 metric screw will have the same strength as a Grade 8 steel screw. In order to avoid damaging the fasteners, certain procedure must be followed. For instance, you should hand start every fastener by rotating the fastener in a counterclockwise direction before it is tightened.
If the fastener is started in the clockwise direction, it can lead to cross threading. Cross-threading will damage the threads of both the fastener and the part that it is being use on. Additionally, if multiple bolts are being used, they should first be hand tightened prior to the tightening of the bolts with a star pattern.
You should tighten the bolts in this star pattern to ensure that the parts will not warp. Finally, washers should be use for added protection for the parts. Flat washers will distribute the load of the fastener over a larger area of the part.
Additionally, if a lock washer is use on the fastener, it will help to prevent the fastener from loosening due to vibration.
