pipe clamp design

[Shafty1]

Hello.

I have a pipe to clamp and am copying an existing design. It is an 'arch' design with 2 bolts. either side. see picture.

I'm checkign section for bending. at the smallest part of the arch (middle) . i am treating the arch as a beam with max moment as PL/4 (where P is 2 x the bolt force).

The current design is checked as failing (2000+ MPa) but i know it has been in use for AGES.

I am using the clamp load from tables for stainless bolts (lurbricated at about 56 kN). i suspect this high load is making the section fail. The pipe clamp is re-used, and hand tightened. over and over. What would be a reasonable bolt force? Or am i oversimplifying my asessment (or making fundamental error!!)?.



sketch below:

 

[Doug Hunter]

Shafty, don't use the rated load for the screws, but rather convert it from the installation torque that's been used. Use T = 0.2dF, where d is the nominal screw thread diameter. The 0.2 is friction dependent.

 

[Stress_Eng]

If you’re copying an existing design, do you have a copy of the stress calculations for it? Sounds like you’re doing a new set of stress calculations. Can you show a picture of your free body diagram (FBD)? I’m assuming there’s another clamp block on the underside. If there is, is there a gap between them once assembled (so you can see the bolt) or are the clamps in contact. I’m interested to see how you are including the pipe contact reaction forces in your FBD.

 

[dvd]

What beam equation are you using? It seems like this will act like a beam on an interrupted elastic foundation.

 

[3DDave]

When you say "fail" is that because the stress exceeds the elastic limit? If so, so what? The stress gets better distributed as a plastic hinge and the conformance to the pipe gets better, unless this is a material with very little elongation before fracture.

 

[Stress_Eng]

There are a number of ways you can model this clamp. It also depends on what software tools you want to use for the calculations.

If you wanted to do this by hand, I would suggest Mathcad. It also depends on how much simplification you want to apply.

This is just one suggested approach. The easiest is as a beam but with a varying cross section. As it’s symmetric, you can take your datum where the vertical axis of the hole goes through your clamp. With distance from your axis, you could model the hole radius as a function of x. Where a 45 degree line is in contact with the hole, then continue the modeling of the cross section using the line. This is where the fun begins! Some may say to apply a cosine function to represent the contact reaction from the pipe. You could include reaction point forces normal to and evenly distributed about the hole surface (say every 10 degrees) and calculate them by applying zero displacement at the contact points.

This is one out of many ways. You could use a curved beam with point reactions, or as suggested, include an elastic foundation (pipe radial displacement stiffness). Be mindful that you are dealing with a non-prismatic beam (changing cross section and shear distribution), contact interface loading and stress variations based on curved (non-linear stress through thickness) / thick wall conditions. I suggest looking at Roark on these subjects, you may be able to factor them in (table in Roark for curved beam).

The choice depends on what calculation software you want to use and to what level of detail you want to apply. Hope these comments give you some ideas.

 

[mfgenggear]

Hello.

I have a pipe to clamp and am copying an existing design. It is an 'arch' design with 2 bolts. either side. see picture.

I'm checkign section for bending. at the smallest part of the arch (middle) . i am treating the arch as a beam with max moment as PL/4 (where P is 2 x the bolt force).

The current design is checked as failing (2000+ MPa) but i know it has been in use for AGES.

I am using the clamp load from tables for stainless bolts (lurbricated at about 56 kN). i suspect this high load is making the section fail. The pipe clamp is re-used, and hand tightened. over and over. What would be a reasonable bolt force? Or am i oversimplifying my asessment (or making fundamental error!!)?.



sketch below:

View attachment 6883

AI Overview



+9

To calculate pipe clamp force, you need to consider the clamping mechanism (e.g., bolt torque, band tension), the friction between the clamp and the pipe, and the potential for slippage under load.

Here's a breakdown of the factors and how to approach the calculation:
1. Understanding the Clamping Mechanism:

• Bolt Torque:
  If using bolts, the clamping force is related to the torque applied to the bolt. A common formula is: Clamping Force (P) = Torque (T) / (K * D), where K is a bolt friction constant (around 0.15-0.2) and D is the bolt diameter.
  
  
• • Band Tension:
    For band clamps, the clamping force is directly related to the tension in the band. The force exerted by a band clamp is greater than the applied tension by a factor of 2π, so F = 2πT.
    
  • Toggle Clamps:
    The clamping force is a ratio of the force applied to the clamp handle to the force exerted on the clamped object. For example, a 10:1 ratio means 10 units of clamping force for every unit applied to the handle.
    

2. Friction and Slippage:

• • Coefficient of Friction:
    The force required to prevent slippage depends on the coefficient of friction between the clamp and the pipe material. A higher coefficient of friction means less clamping force is needed to prevent slippage.
  • Safety Factor:
    Always use a safety factor to account for variations in friction, vibrations, and other factors that could reduce the clamping force. A safety factor of 2.0 to 10.0 is common.
  • Slippage Calculation:
    To determine if a clamp will slip, calculate the maximum possible frictional force (Fmax = μ * N, where μ is the coefficient of friction and N is the normal force) and compare it to the load or force trying to cause slippage.
    

3. Example Calculation (Bolt Torque):

• • Given: Bolt diameter (D) = 1 inch, Torque (T) = 100 inch-pounds, K = 0.15
  • Calculate: Clamping Force (P) = 100 / (0.15 * 1) = 666.67 pounds.
    
  • Safety Factor: If using a safety factor of 2, the required clamping force would be 666.67 * 2 = 1333.34 pounds.
    

4. Tools for Measurement:

• • Force-Torque Sensors:
    These devices measure the force and torque applied to bolts, allowing for accurate determination of clamping force.
    
  • Clamping Force Gauges:
    These tools measure the clamping force directly, especially useful for band clamps and other types of clamps.
    

5. Important Considerations:

• • Material Compatibility:
    Ensure the clamp material is compatible with the pipe material to prevent corrosion or galvanic reactions.
    
  • Environmental Factors:
    Consider the operating environment (temperature, pressure, etc.) and its effect on the clamp and pipe materials.
    
  • Documentation:
    Always document your calculations and the chosen clamping force to ensure repeatability and traceability.