Square or round bolt pattern for even compression between plates 1

[skarf2]

Hello all,

I am currently in the process of designing an analytical fuel cell. This assembly consists basically of several square blocks of various materials, stacked on top of one another and held together by a series of bolts which seat on the top plate and thread into the bottom plate. The assembly basically consists of 5 plates, 4 of which are about an inch thick. The 5th "plate" is actually a membrane electrode assembly (MEA), and can be treated as a gasket. It is highly beneficial for the MEA to see even compression across its center. To achieve this, I have ensured that the surrounding plates are very stiff and precision ground. My question is, should I employ a circular bolt pattern, or a square/rectangular pattern? I would assume a larger number of bolts is beneficial.

Any tips would be appreciated.

Thanks

 

[CoryPad]

A circular pattern of many closely spaced bolts would produce the most uniform compression.

 

[Compositepro]

If you are compressing square blocks you should use a square bolt pattern to keep the distance between bolts to a minimum. The more important issue is the stiffness of your plates.

 

[unclesyd]

I worked on a EHD (Electrohydrodimerization) project for many a year where we used rectangular pilot plant and bench scale cells with a membrane or without. The secret was to use very stiff outside plates, in our case CS. We used studs instead of bolts, many small instead of a few large. It was imperative that the studs be tightened in the proper order and specific torque values.
On some cells where the clamping force was necessarily small we used polished studs and nuts. The thread tips were also rounded.
The reason for rectangular cells was for better flow patterns that were impossible in round cells.

 

[TheTick]

If I had my druthers, I would use dowel pins to align the plates and then put the whole thing in a hydraulic press.

 

[TheTick]

There is film you can get that will tell you the resulting pressure.

http://lmgtfy.com/?q=pressure+film

 

[dinjin]

I would think that you would want to use 4 as a prime
factor in your number of bolts in your circular or rectangular pattern would give you the most uniform even pattern and that 4 of these would be in line with the corners. Two sets of 4 bolts with the second set being spaced inward and between the other 4 bolts might be ideal.
How much surface area are you talking about verses the bolt diameter being used? I think ideally you want each bolt to have the same surface area about its centerline.

 

[Tmoose]

I think For really uniform compression a force applied to the center of each end of the stack would be hard to beat.

http://www.traderscity.com/board/userpix24/21453-oval-manhole-cover-manway-door-pressure-1.jpg

 

[skarf2]

Unclesyd, it sounds like you had a very similar situation as I do. If you don't mind telling me, did you employ a rectangular or circular bolt pattern on your assembly? After some further digging through my textbooks I definitely see the benefits to your other suggestions, but I am still unable to decisively determine which pattern would benefit me the most.

Dinjin, I like your suggestion as well of using a concentric pattern. However, due to space limitations and flow channel orientation in my cell, this may not be possible. I may try to come up with some other geometry to account for this.

Thanks for the suggestions

 

[unclesyd]

There were some early testing of circular cells but they were quickly abandoned as unworkable for our process.
Our standard cell for bench scale work is rectangular with varying length to widths ratios, the largest is 6' x 6". The largest cells (2) run about 3 months and are then broken down for inspection and cleaning.
We use as tightening pattern modeled after the tightening sequence on an engine head with dedicated torque wrenches. We are clamping the anode and cathode using PP spacers to prevent electrical contact with the CS end plates. The cells are hinged in the middle to allow the cell to be serviced in horizontal position. The design philosophy on the bolting was many small instead of a few large fasteners. The fasteners are A193 B7 studs with 2H nuts lubricated with colloidal graphite,dip. We try to stay away from SS studs. We tried bolts and set in studs to apply the clamping force but were dropped in favor of studs and 2 nuts.
As our process operated at a low pressure, 30 psig, achieving a tight system depends on having dedicated technicians.
As stated before vertical operation of our cells is required, not only from a flow point of view, but we generate H2 and O2 in sufficient quantities if allowed to accumulated could be trouble in 1 compartment cell.

 

[macthenife]

If the top and bottom plates are thick enough a single 8.8 hex hd bolt and nyloc nut through the middle.

Shim each contact plane.This controls the membrane squeeze
and allows wrenching to full tightening torque(the bolt).

A roll pin in one corner will stop rotation.

A tower nut(used on the RR Merlin head)or a simple bush can be used to increase free shank length(increases fastener elasticity).

I cannot recommend a bolt size but a 1/4" cap screw grd 10.8 can hold a ton!

 

[unclesyd]

Checkout this site as a lot of the pictures of stacked cells will enlarge.

http://www.ginerinc.com/home.php