17-4PH into tapped hole in carbon steel plate - slip critical

[J3AV8R]

We design custom machines often used in civil structures. Straddling the mechanical and structural worlds can be a "fun" challenge at times! These projects often require corrosion resistant hardware (galvanized, stainless) and we require slip critical connections as they are subjected to fluctuating loads. Our designs use many tapped holes into large carbon steel plates (A36, A572) finished with slip critical paint to connect clevises etc. As I understand it, galvanized bolts into the tapped holes is not practical. Does anyone have any experience of sufficiently torqueing 17-4PH bolts into tapped holes to achieve the clamp-up equivalent to an A325 slip critical connection? In theory, with the bolt being small in volume compared to the plate, galvanic corrosion should not be a problem.

 

[J3AV8R]

Doing some more research and talking with colleagues, galvanized bolts into non-galvanized plate is likely ok. It would be similar to the galvanized nuts getting their threads cut post-galvanizing. Does that make sense?

 

[BridgeSmith]

To be confident of getting the required tension, I would think you'd have to use a turn-of-the-nut, or in this case turn-of the-bolt, method. Accounting for the movement associated with compression of the galvanizing in the calculation of the required 'stretching' of the bolt may prove difficult, if it's significant. If the plate you're attaching is fairly thick (and therefore the 'stretching length' is fairly long) it may not be an issue.

You would have to be careful to ensure you have enough threads in the plate to resist the tension force, so your plate would have to be significantly thicker than the hardened heavy hex nut normally used for the HS bolts.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[dik]

I think you can calculate the clamping force to give you an equivalence to slip critical, and also check faying surfaces... but, I'm not sure they are technicaly 'slip critical'. I'm not sure what you would call them.

So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[BridgeSmith]

I think you can calculate the clamping force to give you an equivalence to slip critical, and also check faying surfaces... but, I'm not sure they are technicaly 'slip critical'.

The clamping force multiplied by the coefficient of friction of the faying surface is the force resisting slip of the connection. "Slip Critical" is a designation meaning the connection is not supposed to slip, i.e. the forces on the connection (typically at the service load limit) are not supposed to exceed the resistance to slipping.

The hard part in this scenario is how to ensure the required amount of clamping force is achieved. Normally with bolts covered by F3125, the clamping force is assumed to be achieved by applying a certain amount of torque, but the amount of friction between the bearing face of the bolt head, or nut (in the case of twist-off bolts) and the friction between the threads of the bolt and nut, can be estimated fairly accurately. It this situation, I don't think the friction can be estimated with any decent degree of accuracy.

Rod Smith, P.E., The artist formerly known as HotRod10