Convert from a weighted system to a plate and bolt system

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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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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.