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Purpose of different types of bolt/screw heads


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What is the purpose of different types of bolt/screw heads (I am more concerned of bolt heads).I have seen that bolts having same material of construction, area of application(temperature, structural or non structural application etc.) are having different bolt head design (hexagon, double hexagon, Tee head etc.). Does it have some relation to the torque applied or constraints in tool used to tight them?

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Interesting subject, I am currently involved in the analysis of a complex assembly build and the most common specified fixing is a button head set screw, followed by a cap head.

Obviously a button head is more user friendly due to it's lower profile but I imagine it would lack the ultimate head strength of a button head due to the reduced cross sectional area.

A circular button or cap head will give a more uniform clamping force distribution, even when either a button or hex head is used with a washer.

I know there is a limit to the size range offered with socket head screws so for high torque requirements the option has to be a hexagonal head bolt, material spec to match the tensile stress required and the conditions of use.

Any comments on washer configurations? If we use a split washer to help prevent a fixing coming loose, is the normal practice to use it with a plain flat washer? In this case most of the fixings are 304 stainless steel (unlubricated) going in to a mild steel zinc plated captive nut (Nylock is not usually an option), and the profiles being fixed are aluminum. Sizes are small, generally M3 up to M8.

 

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On 10/30/2020 at 4:17 AM, Vinay.S said:

What is the purpose of different types of bolt/screw heads (I am more concerned of bolt heads).I have seen that bolts having same material of construction, area of application(temperature, structural or non structural application etc.) are having different bolt head design (hexagon, double hexagon, Tee head etc.). Does it have some relation to the torque applied or constraints in tool used to tight them?

Vinay, the different types of bolt/screw heads have specific applications and some are cosmetic. Most are for different types of applications. For example, Countersunk heads are low profile and allow for placement in limited spaces, but due to its characteristics hole to hole tolerances would have to very tight and they are not for high load applications which might cause pull thru.  The you have x-recess which also a low load applications where the strength is only what your hand can apply. Then we have the hex patterns where are mostly used in high load application. If you want to go even higher in torque you have the 12 point, etc.  So the head is important if not cosmetic. Now, there is more to screws than stated, you also have the pitch-size-class, which give you combinations based upon what the loading is commonality and application.  In the aerospace field you have the UNJC and UNJF series, these classes are to allow for sealing. As the screws are tightened to torque, they form a seal in the threads.  I can go on and on, but important factor is that the head is important for structural application and cosmetically for non-structural applications.

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On 11/10/2020 at 9:59 PM, Everett Tsosie said:

Vinay, the different types of bolt/screw heads have specific applications and some are cosmetic. Most are for different types of applications. For example, Countersunk heads are low profile and allow for placement in limited spaces, but due to its characteristics hole to hole tolerances would have to very tight and they are not for high load applications which might cause pull thru.  The you have x-recess which also a low load applications where the strength is only what your hand can apply. Then we have the hex patterns where are mostly used in high load application. If you want to go even higher in torque you have the 12 point, etc.  So the head is important if not cosmetic. Now, there is more to screws than stated, you also have the pitch-size-class, which give you combinations based upon what the loading is commonality and application.  In the aerospace field you have the UNJC and UNJF series, these classes are to allow for sealing. As the screws are tightened to torque, they form a seal in the threads.  I can go on and on, but important factor is that the head is important for structural application and cosmetically for non-structural applications.

Hi Everett,

Thank you for your reply.

I have design problem where i am using a screw and helicoil insert to attach two components.

The functional requirement is to hold the assembly during the transportation. There aren't any tensional loads acting on the screw. So which thread series should  is use here., UNJC or UNJF. This is an aerospace application.

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Not knowing the application and loads. The most likely screw sizes would be #10-32 UNJF-3A. This will require using MS21209F1-15L at minimum. If you using Boeing specs that would callout the holes to BACD2079N030B3R. If Airbus, I would have to look that up.

Note: There would always be tensional loads. All Screws/Bolts are designed to have tensional loads. There are some that are shear loaded and those have a solid shank larger than the threads that require match reaming to shear load. The most common application for that method would be the seat tracks or structural framing.

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On 11/13/2020 at 1:00 AM, Everett Tsosie said:

Not knowing the application and loads. The most likely screw sizes would be #10-32 UNJF-3A. This will require using MS21209F1-15L at minimum. If you using Boeing specs that would callout the holes to BACD2079N030B3R. If Airbus, I would have to look that up.

Note: There would always be tensional loads. All Screws/Bolts are designed to have tensional loads. There are some that are shear loaded and those have a solid shank larger than the threads that require match reaming to shear load. The most common application for that method would be the seat tracks or structural framing.

Hi Everett,

This component is used in the rear fan case of gas turbine engine. The component is further divided in to 2 subassemblies. One made of titanium alloy and the other made of aluminum alloy

Functions

  • Titanium alloy subassembly : To provide necessary stiffness for the composite fan case.
  • Aluminum alloy subassembly :  To provide attachment points for thrust reverser unit clamps.

Loads acting

  • Titanium alloy S.A : Fan blade off loads, maneuvering loads.
  • Al Alloy S.A : Weight of the thrust reverser unit doors.

The Al alloy S.A will be attached to Ti Alloy S.A using screws and inserts and this assembly will be attached to the whole engine.

Later when the engine is assembled on the aircraft, the thrust reverser unit and both the sub assemblies will be fastened together. After this the screw and inserts used for attaching the Al Alloy S.A with Ti Alloy S.A will not  be having any functional role in the assembly.

So here i am trying to say that the purpose of screw and insert is to hold the Al alloy S.A to Ti Alloy  S.A during the transportation of engine from manufacturing plant to aircraft assembling center.

 

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