Fasteners and Threads flashcards

A bolt usually passes through clearance holes and takes a nut. A screw engages threads in a part or forms its own. A stud is threaded at both ends or along its length and stays in one part while a nut goes on the exposed end. A nut is internally threaded.

Metric thread with an 8 mm nominal major diameter (not the hex head width) and a pitch of 1.25 mm, the axial distance from one crest to the next. M10 x 1.5 and M10 x 1.25 are different threads.

$p=\frac{1}{\text{TPI}}$ inches. 1/4-20 means a 1/4 in nominal diameter with 20 threads per inch, so $p=0.050$ in; 16 TPI gives 0.0625 in. TPI is not a torque or a pitch in mm.

Lead $L=np$, n = number of starts. Single-start: lead equals pitch; a two-start 1.5 mm pitch thread advances 3.0 mm per turn. Travel over N turns of a single-start screw is $d=Np$.

A tap cuts or forms internal threads in a prepared hole (tap drill smaller than the major diameter). A die cuts or forms external threads on a rod or bolt.

Preload is the clamping force from tightening: the bolt stretches elastically and the joint compresses. Torque only estimates it, roughly $T\approx KFd$; friction and lubrication change K, so the same torque can give different preload.

No. Matching diameter is not enough; pitch, thread form, handedness and grade must all match. A nut that starts then binds should not be forced; stop and check for wrong pitch or cross-threading.

No. Lock washers do not guarantee vibration resistance; washers cannot fix wrong pitch, length or stripped threads; extra torque can stretch bolts, strip threads or crush gaskets. Use the specified parts and torque.