Finnveden Bulten - BuProx

Welcome to BuProx!

BuProx is short for BUlten bolted joint apPROXimation. It is a tool for making quick assesments on bolted joint applications. BuProx calculations often prove to be highly accurate. Even so, the results should always be seen as guidlines. 

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Tightening torque

µ Friction [µ]: mm Outer dia. bearing surface[D]: mm Inner dia. bearing surface [d]: kN Preload [F]: ° Countersink [V]:

mm Screw diameter: mm Pitch [P]:

Instructions

  • Insert total friction for the joint. If the friction is unknown, use the friction specified for the bolts surface treatment.

  • Specify the bolt head friction diameter by inserting outer diameter and inner diameter of the bearing surface. Note that only inner and outer diameter for the surface that is actually in contact with the bolt head is applicable.

  • Insert the clamp load that you aim for. In this app it is named “Preload” to indicate that the load immediately after tightening is completed will decrease due to relaxation etc.

  • Enter the angle of the bolt head. If flat, press button “flat” or enter 180. The steeper the counter sink, the smaller the angle. 180-0.

  • Enter bolt diameter and pitch. E.g. M14x2 has diameter 14 mm and pitch 2mm.

  • When all parameters are set, the calculation will begin automatically.   

About the calculation 

In this app tightening torque is calculated with the following formula:

Where dm is thread friction diameter, µ1 is thread friction, Dk is bearing surface friction diameter, µ2 is bearing surface friction and P is pitch. There are three terms within the brackets, two describe the part of applied torque that is required to overcome the friction and one how the pitch transforms torque to preload.

D = Bearing surface outer diameter

d = Bearing surface inner diameter

Thread friction diameter, dm, is considered equivalent to pitch diameter.

 

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Preload

µ Friction [µ]: Nm Tightening torque [M]: mm Outer dia. bearing surface [D]: mm Inner dia. bearing surface [d]: mm Screw diameter: mm Pitch [P]: ° Countersink [V]:

Instructions

  • Insert total friction for the joint. If the friction is unknown, use the friction specified for the bolt's surface treatment.

  • Put in the tightening torque used for the joint.

  • Specify the bolt head friction diameter by inserting outer diameter and inner diameter of the bearing surface. Note that only inner and outer diameter for the surface that is actually in contact with the bolt head is applicable.

  • Enter the angle of the bolt head. If flat, press button “flat” or enter 180. The steeper the counter sink, the smaller the angle. 180-0.

  • Enter bolt diameter and pitch. E.g. M14x2 has diameter 14 mm and pitch 2mm.

  • When all parameters are set, the calculation will begin automatically.   

About the calculation 

In this app preload is calculated with the following formula:

Where Dk is the bearing surface friction diameter, dm is the thread friction diameter, µ1 is the thread friction and µ2 is the bearing surface friction. The torque is devided by three parts: Pitch, thread friction and bearing surface friction.

Thread friction diameter, dm, is considered equivalent to pitch diameter.

Bearing surface friction diameter, Dk, is the mean diameter of the screw head surface in contact with the underlying surface.

 

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Torqued tensile load

mm Screw diameter [d]: mm Pitch [P]: MPa Tensile strength [Rm]:

mm² Stress area [As]: kN Max tensile load:

Instructions

  • Enter screw diameter, pitch and tensile strength. Stress area and ultimate tensile load will be auto generated.

  • Tensile strength can be chosen from the preset buttons, or by manually setting a value of choice. 

  • The calculation will begin automatically soon as the input data is set.

About the calculation

Ultimate tensile load = Stress area*Tensile strength

The tensile strength is given by the fasteners property class. The stress area is calculated with the formula stated in ISO 8908-1:

Where d2 is the basic pitch diameter of the thread and d3 is the minor diameter of the thread.

In which d1 is the basic minor diameter and H is the height of the fundamental triangle of the thread.

Torqued tensile load = Ultimate Tensile Load * Torsional Stress Factor 

The torsional stress gained from applying tightening torque to the bolt reduces the maximum tensile force the bolt can achieve before fracture. The Torsional Stress Factor is proportional to the thread friction, µth. E.g. if the thread friction is µth0,14, the Torsional Stress Factor is 0,86 (1-0,14). 

Because of the difficulty in finding reliable thread friction figures, in this app we use Torsional Stress Constant equal to 0,8. I.e. µth0,2.  

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Surface pressure

kN Preload [F]: mm Outer dia. bearing surface [D]: mm Inner dia. bearing surface [d]:

Instructions

  1. Insert the preload that you aim for.

  2. Enter the outermost diameter of the bolt head that will be in contact with the underlying surface.

  3. Enter the innermost diameter of the bolt head that is in contact with the underlying surface. 

  4. Adjust 2 and 3 for chamfers and diameters that may affect the contact surface diameters.

  5. When all parameters are set, the calculation will begin automatically.  

In this app bearing surface pressure is calculated with the following formula stated in VDI 2230:

Where P is bearing surface pressure, F is clamp load, D is bearing surface outer diameter and d is beraing surface inner diameter. D is usually equivalent to dw in fastener drawings. The most accurate way to determine D and d is to tighten and untighten the fastener which will give a measurable marking on the bearing surface.

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mm Screw diameter [d]: HB Hardness external thread:

HB Hardness internal thread:   Safety factor:

Instructions

  • Chose bolt and enter its major thread diameter.

  • Enter the bolt’s maximum hardness. Use the preset buttons if applicable.

  • Enter the female thread’s minimum hardness. The female thread can be a nut or a threaded hole.

  • Chose a safety factor that aligns with your safety need. Safety factor 1 equals no added safety. Factor 1.5 is the same as adding 50% to the thread engagement length.

  • When all parameters are set, the calculation will begin automatically.  

An approximation of minimum* thread engagement can be calculated with the following formula:

*Minimum engagement length is often less than fastener guidelines recommend. Use safety factor to get appropriate engagement length for your design.

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Friction

kN Preload: Nm Tightening torque [M]: mm Outer dia. bearing surface [D]: mm Inner dia. bearing surface [d]: mm Screw diameter: mm Pitch [P]: ° Countersink [V]:

Instructions

  • Insert the clamp load that you aim for. In this app it is named “Preload” to indicate that the load immediately after tightening is completed will decrease due to relaxation etc.

  • Put in the tightening torque used for the joint.

  • Specify the bolt head friction diameter by inserting outer diameter and inner diameter of the bearing surface. Note that only inner and outer diameter for the surface that is actually in contact with the bolt head is applicable.

  • Enter the angle of the bolt head. If flat, press button “flat” or enter 180. The steeper the counter sink, the smaller the angle. 180-0.

  • Enter bolt diameter and pitch. E.g. M14x2 has diameter 14 mm and pitch 2mm.

  • When all parameters are set, the calculation will begin automatically.   


About the calculation 

In this app tightening torque is calculated with the following formula:

Where Db is the bearing surface friction diameter, d2 is the thread friction diameter, µ is the friction.

Thread friction diameter, d2, is considered equivalent to pitch diameter.

Bearing surface friction diameter, Db, is the mean diameter of the screw head surface in contact with the underlying surface.




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