Aluminum bolts is different from steel bolts because aluminum bolts are soft and will stretch more easy than steel bolts. If you apply too much torque to an aluminum bolt, the aluminum bolt will yield and fail. Due to the way that aluminum bolts can easy fail, it is essential to use a torque chart to ensure that you are applying the correct amount of torque to each bolt.
There is various types of aluminum alloys. Each type of alloy have a different strength and requires a different amount of torque to be properly fasten. For instance, 2024-T4 aluminum has a tensile strength of 62,000 PSI and is used in aircraft skin and joints.
How to Tighten Aluminum Bolts Correctly
6061-T6 aluminum has a tensile strength of 42,000 PSI and is used in marine fittings, bicycles, and RVs. 7075-T6 aluminum has a tensile strength of 77,000 PSI and is used in race car and drones. Each of these alloys has a different strength, therefore you must identify each type of bolt prior to determining the proper amount of torque.
The size of the bolt and the type of aluminum alloy being use impacts the amount of torque that must be applied to each bolt. For instance, a 1/4-20 bolt will require 62 inch pounds of torque if it is made of 2024-T4 aluminum but only 42 inch pounds of torque if it is made of 6061-T6 aluminum. If the bolt is made of 7075-T6 aluminum, however, it will require 77 inch-pounds of torque to be properly fasten.
Aluminum bolts will require 30 to 40 percent of the amount of torque that is required for steel bolts due to the lesser strength of aluminum. Another factor that will impact the amount of torque that should be applied to aluminum bolts are the thread engagement and the properties of the aluminum itself. Aluminum bolts requires more thread engagement than steel bolts.
The engagement should be 1.5 times the diameter of the bolt. If the thread engagement is not correct, the threads of the aluminum bolt will strip. Additionally, you should not re-use aluminum bolts once they have been torqued.
If you torque aluminum bolts, they will stretch permenantly. Aluminum bolts can also suffer from galvanic corrosion if they are in contact with other metal. Using anti-seize compound on aluminum bolts will prevent this corrosion.
The type of lubrication for the bolt will also impact the amount of torque that must be applied to the bolt. If the threads are dry, use the full amount of torque indicated on the torque chart. If there is light oil on the threads, reduce the torque by 25 percent.
If you use copper anti-seize, reduce the torque by 30 percent. If you use nickel-based anti-seize, reduce the torque by 20 percent. Any amount of lubrication will reduce the amount of torque that should be applied to the bolt.
If the proper amount of torque is not adjust for lubrication, the bolt will over-clamp on the bolt, causing it to yield. Certain tool can be used to apply the required amount of torque to the bolts. Impact wrench should not be used on aluminum bolts as they can easy shatter the aluminum.
Click wrenches and beam type wrenches can be used. Digital torque wrenches provide the highest accuracy when applying the correct amount of torque. Preset T-handle wrenches can be used for bicycle applications.
Safety measures should always be used when working with aluminum bolts. Aluminum bolts should never be re-used. If any means stretched aluminum bolts they will fail.
Use a two-pass star pattern when installing bolts to ensure even distribution of the force on the bolt. Always inspect the bolts before installing them. Reject any bolts that have cracks, nicked thread, or bent shanks.
To convert inch pounds to foot pounds, divide the number of inch pounds by 12. To convert foot pounds to Newton meters, multiply the number of foot pounds by 1.356.
