Robotic arm- Moment/bending moment/clamping force

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In summary: Thank you for your input! I'll be sure to consider those factors when designing my next project.In summary, LaTex says that the clamping force would have to cause enough friction between the clamp and the rod to prevent sliding.
  • #1
Aaron Mac
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Homework Statement
Hello there, I'm currently designing a robotic arm, basically a revolute (shaft of rotating axis) and a prismatic joint (which further extends with the help of a motor), for which the necessary sizing has already been completed. As you can see, I previously created a counterweight to counteract the effect of the robotic arm's moment around the shaft while the gripper is holding something (payload) or even owing to the weight of the robotic arm itself. But, after some study, I discovered something called clamping force, which is utilized in the fastener industry, but I'm unable to make the required equations to establish the clamping force owing to my insufficient expertise in this area and maybe eliminating the counterweight. I'd appreciate it if someone could assist or guide me in this regard.
Relevant Equations
Mass of link 1 - 0.8kg
length of link 1 - 0.15m

Mass of link 2 - 1kg
length of link 2 - 0.15m

Mass of payload - 0.3kg
Acceleration due to gravity in this case = 10ms^-2

Counterweight calculation
Moment around the axis of rotation = F*D
=(8*0.15) + (13*0.15)
= 3.15 Nm
Screenshot (153).png
Screenshot (154).png
 
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  • #2
I think the clamping force would have to cause enough friction between the clamp and the rod to prevent sliding.

My LaTex isn't functioning right now, but I think it would be something like Fclamp * mu = mass * gravity.
 
  • #3
osilmag said:
My LaTex isn't functioning right now, but I think it would be something like Fclamp * mu = mass * gravity.
$$\mu F_{clamp} = mg$$
:wink:
 
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  • #4
You are trying to resist the torque of rotating a payload with the arm at the shaft that drives the arm, correct?
 
  • #5
osilmag said:
I think the clamping force would have to cause enough friction between the clamp and the rod to prevent sliding.

My LaTex isn't functioning right now, but I think it would be something like Fclamp * mu = mass * gravity.
Thank you very much. I have already chosen my screws and need to calculate according to this screw clamping force.
 
  • #6
erobz said:
You are trying to resist the torque of rotating a payload with the arm at the shaft that drives the arm, correct?
No not really dear, it is the clamping force to resist deflection due to moment
 
  • #7
Aaron Mac said:
No not really dear, it is the clamping force to resist deflection due to moment
I don’t get it. If you have already specified the fastener based on the design load, you’ve already specified the clamp force. In other words, specifying the bolt specifies the clamp force.

Your question seems odd to me, because the clamp force is what should be determined based off of the design load, and the appropriate fastener selected off of the clamp force required. You seem to be doing the process backwards if you have already selected the fastener as you indicate in post #5?
 
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  • #8
It's hard to tell from the picture, but I believe you are rotating the arm horizontally while it's carrying a load. The friction force at the shaft would have a very small moment arm. That torque is responsible for any angular acceleration the arm + payload experience around the axis of rotation (the shaft). The links and payload have moments of inertia about their own centers of mass, and they are compounded by the being at some distance from the center of rotation. Given the small moment arm of the applied force (from the clamping force of the collar) it's not clear to me which force is larger; simply supporting the weight of the arm and payload vertically, or the force involved in giving the system some modest angular acceleration?

$$ \frac{I_{tot} \alpha }{ r_{shaft}} \quad \text{or} \quad mg $$

In reality, fasteners are cheap, so you typically just select one that is obviously larger/stronger than necessary. Computing basic magnitudes might be more work than it's worth.
 
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  • #9
Can you define your expectations using Laws of Motion for work to be done? e.g. Mass of target, acceleration, velocity , degrees of freedom , maximum radius. Desired minimum resonant frequency, joint stress-strain slack, moment of imbalance, mass of arm.
 
  • #10
Your design seems ok for space without gravity but weak on earth. I would think you want to have stronger bones and muscles.

1684181041562.png
 

FAQ: Robotic arm- Moment/bending moment/clamping force

What is the significance of the moment in a robotic arm?

The moment in a robotic arm is crucial because it determines the arm's ability to exert torque or rotational force around a pivot point. This is essential for tasks that require precise positioning, lifting, or manipulation of objects. The moment affects the robotic arm's stability, accuracy, and overall performance.

How do you calculate the bending moment in a robotic arm?

The bending moment in a robotic arm can be calculated using the formula M = F * d, where M is the bending moment, F is the force applied, and d is the distance from the point of force application to the pivot point. This calculation helps in designing the arm to ensure it can withstand the required loads without failure.

What factors influence the clamping force of a robotic arm?

The clamping force of a robotic arm is influenced by several factors, including the actuator's power, the mechanical advantage provided by the arm's design, the friction between the gripper and the object, and the material properties of the gripper. Proper calibration and material selection are essential for achieving the desired clamping force.

Why is it important to consider the bending moment in robotic arm design?

Considering the bending moment in robotic arm design is important because it helps ensure the arm can handle the expected loads without excessive deflection or structural failure. This consideration is critical for maintaining the arm's precision, reliability, and longevity, especially in applications involving heavy or dynamic loads.

How can you optimize the clamping force in a robotic arm for different tasks?

To optimize the clamping force in a robotic arm for different tasks, you can adjust the actuator settings, use interchangeable gripper designs, and employ sensors to provide feedback on the clamping force. Additionally, software algorithms can be used to dynamically adjust the clamping force based on the object's properties and task requirements, ensuring optimal performance and safety.

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