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Greyhound101
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Join Date: Apr 2010
Posts: 3

How to Calculate Hold Down Force?

04/21/2010 11:27 AM

I have a square metal plate that is fastened down to another metal surface by four M6 socket head cap screws in each corner that are torqued to 55"/lbs. I want to build a fixture with a pneumatic cyclinder (Bimba most likely) that has a 'foot' with four legs attached to the ram that will press down equally over the four screw thru holes. The area in each corner that each individual leg will contact will be no more than the area that the head of each M6 screw contacts as it holds down the plate. What force is needed to equal the force that four M6 screws exert so that I can size my pneumatic cyclinder accordingly? Clear as mud?
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passingtongreen
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Join Date: Aug 2009
Location: Glen Mills, PA.
Posts: 2385
Good Answers: 114
#1

Re: How to calculate hold down force?

04/21/2010 1:36 PM

Is the question,"How do I convert torque into bolt tension?"?

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peterg7lyq
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Join Date: Jan 2010
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#2

Re: How to calculate hold down force?

04/21/2010 2:19 PM

I think the first link is the one you need but stuck a few more on just for fun

https://www.engineersedge.com/calculators/torque_calc.htm

https://www.engineersedge.com/fastener_thread_menu.shtml

https://www.futek.com/boltcalc.aspx
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nick name
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Join Date: Mar 2007
Location: City of Light
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Good Answers: 183
#3

Re: How to calculate hold down force?

04/21/2010 2:51 PM

Use the norbar calculator since the others can give you -for your application erroneous values. It seems to me (I could not open the futek one) from the group the only offering the possibility to adapt both coefficients to real conditions.

Define the friction according to the kind of bolt and parts you tight together, do not work with default values which are for quite dry surfaces if they are even slightly oiled.

Friction can vary with >40% from under 0.1 (even 0.08) when lubricated with oil or grease to over 0.18..0.2 if dry. The higher the friction the lower the force for same torque.
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Greyhound101
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Join Date: Apr 2010
Posts: 3
#4
In reply to #3

Re: How to calculate hold down force?

04/21/2010 4:59 PM

Both surfaces will be dry...no oil present. Both parts are fastened together immediately after they're run through a washer with an aggressive degreaser. It looks as if (according to the first engineer's edge link given) that clamping force of a 1/4 bolt at 55"/lbs is 1100 lbs. So, I would assume that my cylinder needs to be capable of 4400 lbs (4 bolts)?
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nick name
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#5
In reply to #4

Re: How to calculate hold down force?

04/22/2010 1:52 AM

Depending of what you want to do take a safety coefficient of at least 1.5 to 2.
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Greyhound101
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Join Date: Apr 2010
Posts: 3
#6
In reply to #5

Re: How to calculate hold down force?

04/22/2010 9:22 AM

Right. I plan to use a 2.5 ton cylinder. Thanks to all for the help.
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34point5
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Join Date: May 2010
Location: in optimism
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#8
In reply to #6

Re: How to calculate hold down force?

04/03/2011 9:12 AM

I'm hoping you have not mixed up lbs and kg as 4x M6 is way under for 2.5 tons

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kirannawathe
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Join Date: Jun 2008
Location: India,Pune
Posts: 46
Good Answers: 1
#7

Re: How to Calculate Hold Down Force?

04/25/2010 1:51 AM

It depends upon the area under the shear.

calculate the area at the root dia of the bolt.

Force of cylinder

= Area of cross section of screw X 4(as you are using 4 screws) X 4.2 ( thumb rule strength)

kiran nawathe

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Users who posted comments:

34point5 (1); Greyhound101 (2); kirannawathe (1); nick name (2); passingtongreen (1); peterg7lyq (1)

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