Machine screw and bolts can appear to be teh same to many peoples, but there are actual difference between the two. One of the main difference is that machine screws and bolts comes in different strength grades. These different grades of strength mean that there are different amount of force that can be borne by each machine screw or bolt before it begin to fail.
In order to select the appropriate screw or bolt for a given application, it is necessary to use a strength chart to determine the strength that is required, and to ensure that the screw or bolt that is selected will be able to handle the loads that will be placed upon it. The strength of a machine screw or bolt is defined by three different value: proof load, yield strength, and tensile strength. Proof load is the maximum force that can be placed on a machine screw or bolt before it begin to stretch permenantly.
How to Choose the Right Machine Screw or Bolt
Yield strength is the value at which a machine screw or bolt will begin to deform permenantly. Tensile strength is the force at which a machine screw or bolt will break. The grades of metal that are used for machine screws and bolts has different strengths.
For instance, low grades of carbon steel have tensile strengths of around 74,000 PSI. Medium grades of carbon alloys have tensile strengths 120,000 PSI or more, and are used in components like automotive mounts or machinery frames. It is important to not treat machine screws and bolts as if they are all the same strength, because using a low-grade machine screw or bolt in a component that is subjected to dynamic loads will eventually lead to fatigue of that bolt.
Another important term related to machine screws is the concept of the thread stress area. The thread stress area is not to be confused with the shank diameter of the bolt. The thread stress area is the area at the threads of the bolt, which is the area at which the bolt will fail.
Fine threads (UNF) have a larger thread stress area than Coarse threads (UNC) of the same size. The design limit of a bolt is created by multiplying the proof strength of the bolt by a safety factor, and then applying that factor to the thread stress area of the bolt. If the shank area is used instead of the thread stress area, the calculations will be incorrect.
In addition to tensile strength, another critical value of a machine screw or bolt is its shear strength. Shear strength is the force at which a bolt will separate from a joint. The shear strength is approximately 57% of the tensile strength of the material (calculated using von Mises math).
For instance, a Grade 8 quarter-inch machine screw or bolt will have a shear load of 2,700 lbs. This value increases to approximately 5400 lbs. In a double shear joint.
In cases in which the force that is to be exerted on a bolt is transverse or shear forces, it is necessary to use a strength chart to determine the strength and diameter of the bolt. Additionally, if the bolts will be subject to vibration, there is a risk of shear failures in those bolts. One of the factors to consider when selecting a screw or bolt is the material of the screw.
Each metal has different property regarding strength and corrosion. For instance, carbon steel is a strong metal with low cost, but will rust quick if used in outdoor applications. Stainless steel is very resistant to corrosion in marine environments, but has a lower yield strength than alloy grades of steel.
Finally, titanium is used in aerospace applications because it is lightweight, but is more expensive relative to other metals. It is important to ensure that the material of the machine screw or bolt is compatible with the other metals in the application. If not, corrosion will occur between the metals.
This type of corrosion is known as galvanic corrosion. In galvanic corrosion, one metal sacrifice itself to protect another metal. For instance, steel can be plated with zinc or hot-dip zinc to protect it from moisture.
However, care must be taken in utilizing this for high strength machine screws or bolts because zinc plating will lead to hydrogen embrittlement in the metal. Hydrogen embrittlement can lead to the metal snapping. Another consideration for the design of a bolt is the torque that should be applied to the bolt.
The torque that is applied to a bolt will create the clamp force, but the two are not the same. Only about 10% of the amount of torque that is applied to a bolt will create clamp force. The remaining 90% of the torque is used to overcome the friction between the threads.
If the threads of the bolt are lubricated, however, 33% more preload can be achieved at the same level of torque. For example, 8 foot-pounds of torque is required to achieve the clamp force of a dry Grade 5 bolt quarter-inch screw. A lubricated Grade 5 bolt of the same size will only require 6 foot-pounds of torque to achieve the same clamp force.
The formula for calculating the amount of torque that should be applied to a bolt is: T = K x D x F. T is the value of the torque to be applied. K is the friction coefficient between the bolt and the mating parts. D is the bolt diameter.
F is the desired clamp force. Machine screws or bolts can fail in a variety of different ways. One of the most common ways is due to tensile failures, in which the bolt will snap across the body of the machine screw.
Another type of failure is shear failures, which occur when the bolt is sliced diagonally through the body of the bolt. Other failures include stripped threads, which can occur if the nuts that are used with bolts are too soft, fatigue failures due to excessive stress on the bolt, and vibration, which can cause the bolts to loosen over time. It is important to use safety factors when designing a bolted joint.
Safety factors of 1.5 to 2.0 can be used for static (non-moving) loads that are known to the designer. Safety factors of 2.0 to 2.5 can be used in the same situation, but the static load is unknown. If the bolts are to be subjected to vibration or shock loads, safety factors of 3.0 or higher are recommended.
The preload of the bolt should be set to 75% of the proof strength of the bolt. This ensures that the bolt remains within its elastic range. The grade of the bolt can easily be identified by the markings on the head of the bolt.
For example, three lines indicate a Grade 5 bolt, while six lines indicate the use of a Grade 8 bolt.
