Wood Screw Thread Form

[JLBK56]

Hi all.

I've got a rather simple question that I can't seem to find an answer to in Machinery's Handbook - although perhaps I'm just looking in the wrong sections. My understanding is that wood screws do not conform to the unified series thread form (i.e. UNC, UNF, etc.) since wood screws typically come in a coarser pitch than machine screws. Is there a standard which describes the thread form for wood screws, similar to ANSU/ASME B1.1-1989 (R2008) for unified screw threads? Can you provide any reference to relevant sections in Machinery's Handbook or other sources for further reading?

Along these lines, is the tensile area still calculated the same way for wood threads as for unified threads? Machinery's Handbook (30e) suggests in the section "Torque and Tension in Fasteners / Calculating Thread Tensile-Stress Area" that the equation only applies for unified threads. I have an interest in doing a basic assessment of tensile loading, torque specification, and thread shear for a wood joint, and would appreciate any help.

Thanks in advance.

 

[DBronson]

JMP1098:

Wood Screws are covered by ASME B18.6.1. Note that there are two types of threads - cut thread and rolled thread. Cut threads have tapered shank, rolled threads have a constant shank diameter and taper at the point. The design steps and allowable values for use in wood structures is covered in the National Design Specification for Wood Construction (NDS).

Regards,

DB

 

[MintJulep]

However, there are many wood screws that use proprietary thread designs.

 

[TugboatEng]

I suggest assuming zero strength in tension when designing connections with wood screws.

 

[dik]

For handrails, you often have to use woodscrews in tension. The same is true for guardrail attachments. There is published data for this as well as most wood codes have calculations for this.

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

-Dik

 

[phamENG]

Why is that, Tug? If not for resistance to withdrawal, what's the point of using a screw? Nails are better in nearly every way if withdrawal is not a concern...

 

[TugboatEng]

It's just better practice, in my opinion. If you need to hang a weight from a beam it's better to lap a plate onto the side then to screw a plate into the bottom. Obviously some configurations are impossible to avoid tension on the screws such as a pocket door hinge.

 

[phamENG]

Got it. I agree that withdrawal should be avoided wherever possible. Where necessary, though, wood screws can provide plenty of withdrawal resistance if designed properly.