Vol. 12 Back to Basics Series! Screw Reference Dimensions

Hey there! Nedzigon's back!
Let's be sure we understand screw reference dimensions, the ABCs of screw knowledge.
Lack of understanding may result in placing an order for the wrong dimensions, making a mess! Therefore, pay close attention.
I will also explain the terminology used for the reference dimensions. This is very important when designing, so it is worth a quick read.

Terms used in screw standards

First I will explain the terms used.

①Pitch (P)
The distance between neighboring screw crests.


②Effective diameter
It means the diameter of an imaginary cylinder in which the screw groove width (P/2 in the figure) and screw thread width (P/2 in the figure) are equal.


Effective sectional area
The area of a cylinder calculated with the screw effective diameter. This is required to calculate the stress of the male threads and the guaranteed load stress of the nut.

● Calculating the effective sectional area
The effective sectional area of general-purpose metric screw threads can be calculated with either formula
or formula .
 As =π/4((d2+d3)/2)2
 As = 0.7854(d-0.9382P)2

As: Effective sectional area of general-purpose metric screw thread (mm2)
d: Basic dimensions of male screw outside diameter (mm)
d2: Basic dimensions of male screw effective diameter (mm)
d3: Basic dimensions of male screw valley diameter (d1) minus 1/6 of ridge height H

③Thread angle
The angle between the screw crest and valley. Metric coarse threads, metric fine threads, unified coarse threads, and unified fine threads are all 60°. 

 

④Male screw valley diameter
The diameter of an imaginary cylinder touching the male thread valley.


⑤Male screw outer diameter
The diameter of an imaginary cylinder touching the male thread crest.


⑥Female screw valley diameter
The diameter of an imaginary cylinder touching the female thread valley.


⑦Female screw inner diameter
The diameter of an imaginary cylinder touching the female thread crest.


Nominal

This is a symbol used to express screw dimensions, which often uses the outer diameter of the male thread as the reference dimension.

*However, the pitch is often omitted for screws where there is only one standard pitch for the same nominal diameter, as with metric coarse screws and miniature screws.

Various ways to show screw type and thread shape

Here we introduce some of the ways to show screw type and thread shape, such as metric coarse threads, metric fine threads, and unified coarse threads.

How screw type is shown
Classification Screw Types  Symbol showing
screw type
Examples
of screw types
(Old JIS) Thread angle
 Screws showing
 pitch in mm
 Metric coarse screw thread  M
 
 M6  60°
 Metric Fine Screw Thread  M6×0.75  60°
 Miniature screw thread  S  S0.8  60°
 Metric trapezoidal thread  Tr  Tr10×2  (TM)  60°
 Screws showing
 pitch in TPI
 Tapered Pipe Threads Tapered male thread   R  R1/2  (PT)  55°
Tapered female thread  Rc  Rc3/4  (PT)  55°
Parallel female thread (1)  Rp  Rp3/4  (PS)  55°
Parallel pipe Threads(2) Parallel male thread  G
(Add A or B)
 G1/2A  (PF)  55°
Parallel female thread  G  G1/2  55°
 Unified coarse thread  UNC  1/4-20UNC  60°
 Unified fine thread  UNF  No.10-32UNF  60°
 Whitworth (3)  W  W3/8  55°
 American National Tapered Pipe Thread (4)  NPT  3/4NPT  60° 


(1). Tapered parallel pipe threads are used to join a tapered male thread to a parallel female thread (Rp).
(2). Parallel pipe threads are used to join a male parallel pipe thread to a female parallel pipe thread and show the G symbol in either case, but for male threads there is a symbol showing the class (A or B) added.
   (E.g.) Male screws may show G 1/2A whereas female screws show G 1/2.
(3). Whitworth screws originated in the UK but were abolished in the 1968 JIS standards, though some are still in use. They are imperial screws with 55° threads.
(4). The thread angle of American National Tapered Pipe Threads is the same as unified threads at 60°.

Threads forming a 60° triangular shape

Metric coarse thread, metric fine thread, unified coarse thread, and unified fine thread shapes are as in the figure.

Threads forming a 55° triangular shape形

Tapered pipe threads and parallel pipe threads have a 55° thread angle. See the figure.

Trapezoidal thread shape

The figure shows the shape of metric trapezoidal screw threads.


Other screws

Here some special thread shapes, such as square threads and sawtooth threads, are introduced for reference.

Square thread

Sawtooth thread

Round thread

Lightbulb thread

You probably use the reference dimensions when selecting screws, but if you can remember the terms used by the standards, such as pitch and effective diameter, you can also avoid choosing erroneous numerical values for the dimensions. Consider this for your designs.

That's all for today!


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