Simple crush washer
Simple crush washer
(OP)
I have to use a copper crush washer to seal against coolant. The bolt is an M12x1.5 simple DIN 908 zinc plated bolt. There is a small interference of 0.1mm between the copper washer and the bolt (worst case), where the bolt surface starts to radius. I need to somehow model or calculate the crush of the material and ensure a proper seal. I'm having even a hard time finding any data supporting the amount of clamp load needed to seal this joint in the first place.
I'm not quite sure I can model the plastic deformation of the copper accurately given pen and paper (excel too I suppose).
Even on just a simple crush washer gasket with a single bolt, I'm not coming up with much in terms of basic equations to understand the clamp load needed to seal the joint using a simple copper washer gasket. How much do I need to compress the copper to fill the voids in the mating materials to ensure a proper seal, or am I thinking about this the wrong way?
I'm not quite sure I can model the plastic deformation of the copper accurately given pen and paper (excel too I suppose).
Even on just a simple crush washer gasket with a single bolt, I'm not coming up with much in terms of basic equations to understand the clamp load needed to seal the joint using a simple copper washer gasket. How much do I need to compress the copper to fill the voids in the mating materials to ensure a proper seal, or am I thinking about this the wrong way?





RE: Simple crush washer
That's why copper crush washers are typically modest in their radial dimension.
As for crushing it enough to fill voids of arbitrary size, the calculation is probably a waste of time. In practice, the mating surfaces have to be very close to perfect, and most practitioners will also use a sealant even if none is specified.
Mike Halloran
Pembroke Pines, FL, USA
RE: Simple crush washer
RE: Simple crush washer
I don't know if they still do this, but in the Sixties, Ford used copper washers under the heads of the nuts holding the differential carrier to the 'banjo' of the rear axle, saving them the cost of spotfacing the sand cast carrier face around the bolts. The nut torques were sufficient to hugely distort the copper washers, and also to provide a sufficient pressure on the gasket sandwiched between the carrier and the banjo.
To sort of answer your question, I think the torque to yield the copper is just a minimum. You have to develop enough strain in the bolt (considered as a spring) to maintain that yield force. Then the washer will 'belly out' until the stress falls to just below the yield point.
How about omitting the copper washer and substituting a gob of Loctite Pipe Sealant with Teflon?
Mike Halloran
Pembroke Pines, FL, USA
RE: Simple crush washer
The other awesome part is that the copper we are using has a varying yield strength between 65 and 330MPa and based on my simple bolt calculations (25nm bolt torque, 12mm fastener and the 0.2 condition factor) I get about 10.4 KN of force, which when I use the area of the copper gasket translates to about 125MPa of stress. So, it looks like in some cases it would easily crush properly, but if the yield was higher on some off batch, it wouldn't crush at all and potentially not seal.
I must be doing something wrong here converting the axial load into the flange clamp load.
RE: Simple crush washer
http://www.seridium.com/downloads/Petrochemical_Ga...
soft copper seating stress 13,000 psi
RE: Simple crush washer
Now it's time to retrieve some washers that leak, and some washers that don't leak, and try to estimate their respective yield points. You'll need to do a fair number to have statistically significant data, of course. Then you try to figure out how the purchasing spec got to be as wide as it is, and use your data to talk someone into narrowing the spec sufficiently to solve the problem within the narrow solution space to which you have been constrained.
Mike Halloran
Pembroke Pines, FL, USA
RE: Simple crush washer
Yep, I think that Mike is right, you are shoveling someone else BS.
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P.E. Metallurgy, Plymouth Tube
RE: Simple crush washer
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: Simple crush washer
The housing spot face was machined at an angle to the thread. Or maybe the thread was punched into the housing at an angle. I forget. Anyhow Some work with Dykem or probably a black sharpie revealed the severe angle mismatch, maybe 30% contact, and the intended mating surfaces were torn up to boot. At first I thought the head on the plug was crooked. By smoothing the surfaces and adding compliance (multiple softer washers) I was eventually able to get a fairly serviceable seal. If that had not worked I would have re-spot faces the housing, but first probably would have set the sealing washer in a bed of plastic steel type epoxy.
I'd be real interested in testing the contact patterns a few of the known leakers.
RE: Simple crush washer
I am struggling with the yield strength. The copper is C11000. Yield strength seems to be all over the place from 69MPa to 375MPa. At the lower end I crush it, at the upper end I'm not even close to plastically deforming it. I have noted some washers come back with obvious plastic deformation, others do not, which would explain localized plastic deformation based on the plug having runout out of spec.
ugh
RE: Simple crush washer
RE: Simple crush washer
Bolt head face runout to thread ( or vice versa ) or head flatness or finish or ???
This seems to suggest DIN copper sealing washers should be 45 HB maximum.
http://mdmetric.com/tech/DIN7603tdc.pdf
Just wondering.
regards,
Dan T
RE: Simple crush washer
http://www.aircraftspruce.com/catalog/eppages/an90...