Convert from a weighted system to a plate and bolt system

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Discussion Overview

The discussion revolves around converting a cooling fixture from a weighted system to a plate and bolt system. Participants explore the calculations and considerations involved in ensuring that the new system can effectively replace the weight while maintaining the necessary pressure and contact area.

Discussion Character

  • Technical explanation
  • Experimental/applied
  • Debate/contested

Main Points Raised

  • Plastixman outlines the parameters of the conversion, specifying the use of 4 - 10mm cap screws and a pressure plate to replace 7.82kg of weight, and provides calculations for the required torque.
  • Plastixman expresses concern that the calculated torque of 0.0383 Nm seems insufficient for the application.
  • One participant suggests conducting an experiment to compare the pressure felt from the weights to that from the bolted plate, indicating that the calculated value might be accurate.
  • This participant also notes that friction could be a significant error source at low bolt torques and recommends ensuring the bolts can spin freely before assembly.
  • Another suggestion is made to use compression springs under the bolts to achieve a specified height instead of relying solely on measuring low torque values.
  • A further point is raised regarding the importance of the stiffness of the white plate in achieving uniform pressure distribution.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the adequacy of the calculated torque or the best method for achieving the desired pressure distribution. Multiple viewpoints and suggestions are presented, indicating ongoing debate and exploration of the topic.

Contextual Notes

There are unresolved assumptions regarding the friction involved in the bolted system and the specific mechanical properties of the materials used, which may affect the overall performance of the system.

plastixman
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Hi all,
I need to convert a cooling fixture from a weighted system to a plate and bolt system. I'd like to use 4 - 10mm cap screws and a pressure plate to replace 7.82kg of weight. The weights are 190mm x 70mm of contact area. The pressure plate will have flanges to accommodate the bolts. But the contact area won't change.
Assuming each bolt must provide 1/4 of the pressure of the weights, each would need to provide 1.955 kg. So, T=KFd and I used 0.2 for the K constant, 10mm for the bolt dia, and 1.955kg (19.17N) for the required Force. no lubricant and I get 0.0383 Nm.

It seems to be too little torque. Would someone be nice enough to check my work?

Thanks!
Plastixman
 
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plastixman said:
Hi all,
I need to convert a cooling fixture from a weighted system to a plate and bolt system. I'd like to use 4 - 10mm cap screws and a pressure plate to replace 7.82kg of weight. The weights are 190mm x 70mm of contact area. The pressure plate will have flanges to accommodate the bolts. But the contact area won't change.
Assuming each bolt must provide 1/4 of the pressure of the weights, each would need to provide 1.955 kg. So, T=KFd and I used 0.2 for the K constant, 10mm for the bolt dia, and 1.955kg (19.17N) for the required Force. no lubricant and I get 0.0383 Nm.

It seems to be too little torque. Would someone be nice enough to check my work?

Thanks!
Plastixman
Welcome to the PF. :smile:

Can you Upload sketches of your system before & after (or photos). Use the Upload button in the lower right of the Edit window to attach a PDF or JPEG file to your posts and replies. Thanks.
 
Here are pictures of the bolted plate concept. I'd drill the white plate with a clearance hole and tap into the bottom plate.
The bottom pic shows the weighted system.
Thanks
 

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  • weighted fixture.jpg
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You can do an experiment to check for yourself. Carefully set the weights on your hand and note the pressure. Then put your hand under the bolted plate and tighten the bolts until you feel the same amount of squish. I suspect that you will find that your calculated value is about right.

The largest error source at low bolt torques is friction. Make sure the bolts spin in and out freely before assembling.

Suggestion: Put compression springs under the bolts and tighten to a specified height instead of trying to measure such a low torque. Bolt torque is not a good way to accurately control preload.
 
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If you need uniform pressure distribution, realize that will depend on the stiffness of the white plate.