How to code screws


Screws are classified and coded in accordance with the rules regarding to nominal thread size, the thread grade and the winding direction of the thread, as follows:
[Nominal thread size] – [Thread grade] – [Winding direction of the thread]
In general use of standard screws, the latter two are omitted.
The nominal thread size is defined by using a symbol that represents the type of threads, the number of threads per 25.4mm, or pitch and number representing the nominal size or diameter.
In the case of screw generally called mm-system, it is represented as follows:
[Symbol that represents the type of screw thread] [number representing the nominal thread diameter] × [pitch]
However, if the pitch defined for the thread is only one, its pitch notation omitted.

Strength classification of the screws


To represent the strength of screws, the following 10 stages of strength classes are defined in JIS: 3.6, 4.6, 4.8, 5.6, 5.8, 6.8, 8.8, 9.8, 10.9, 12.9 The number before decimal point (3-12) represents the number of 1/100 of the nominal tensile strength expressed in N/mm2. The numbers to the rear side (6-9) shows the value of 10 times as the ratio of the nominal tensile strength to the nominal lower yield point. Thus, the product of these two numbers would represent a 1/10 of the nominal lower yield point expressed in N/mm2.

Caution when tightening screws


① Shortage of tightening will lead to loosening, but too much tightening also cause loosening. Thus, you must make the proper tightening. Tightening strength to be aimed is determined by the type of screw threads, the strength of the screw, the friction of the seating surface and screw surface, and tightening procedures. The tightening procedure is specified in JIS B1083. And determination procedure of tightening strength to be aimed in case of tightening works is specified in JIS B1084.

② The heat conductivity of stainless steel is bad. Therefore screws made of stainless steel easily seize and adhere together, when contact surface of the internal thread and external thread is not lubricated. Please either apply the appropriate lubricant to the threaded portion before tightening or use pre-lubricated screws. Tightening slowly has also the effect to prevent the seizure.

Proper tightening


In order to control the tightening procedure, there are “torque method”, “rotation angle method” and “torque gradient method”. “Torque method” is the most widely used among them. The method is, however, intended to meet the conditions of the case which is somewhat limited. Please use as a guide rather than as an absolute value. When you tighten with “torque method”, approximate tightening torque value to be aimed can be obtained by the following formula: 　目標締め付けトルク（Ｎ･ｍ）ＴｆＡ＝0.001＊Ｋ＊ｄ＊Ｆｆ／（１＋0.01＊ｍ） 　Tightening torque to be aimed (N • m) TfA = 0.001 * K * d * Ff / (1 +0.01 * m)
k: torque coefficient (minimum value obtained in the tightening test of JIS B 1084)
d: nominal diameter of screw thread (mm)
Ff : assumed as 85% of the product strength of male screws thread (or yield point)× As (N/mm2):
As: effective cross-sectional area of the screw threads (mm2)
m: torque accuracy of the tightening tool (%)

Example: When M10 bolt of strength class 8.8 is tightened, if obtained k = 0.195 and m = ±5%, then tightening torque to be aimed (TfA) can be calculated by the following equation. 　TfA = 0.001 × 0.195 × 10 × 0.9 × 640 × 58 / (1 +0.05) = 63 (N • m)

Loosening of screws


In case that screws are used for fastening, the loosening of them can cause various problems. From such reason many coping methods are considered. There are following types of loosening:

① Loosening caused by rotation Screws once tightened may be loosed when rotating in the direction to loosen for any reason. When changing load or impacting force is repeatedly applied, the loosening due to rotation can occur.

② Loosening that does not depend on rotation Preload force loaded to the screw to be tightened will be lost due to the following reasons and may lead to loosening:

[Initial Loosening]Regarding to the load surface of screws, seating surface or contact surface of fastening members, surface irregularities at the time of processing may be deformed under fastening pressure, and preload force may be lost.

[Depression]If the member to be fastened is a soft material, the surface of the member to be fastened may be depressed plastically and the loosening of the members can occur through loss of preload force.

[Fretting Wear]Fretting between the fastened contacting faces may lead to the loosening due to wear with it or the friction.

Methods to prevent loosening


Various fastening method of bolts, nuts, washers, etc. have been devised to prevent loosening of screws. Typical methods used relatively widely are showed as follows:

① Increased friction of screw thread surface The representative of it is High-Lock nut and High-Lock Ad, the screw surface of which is slightly deformed ensuring the proper friction force between the female and male thread.
As other methods for increasing the frictional force, there are a method in which a disk spring or a Nylon is incorporated in a nut and engaged with the male thread, or a method in which a coil spring is inserted into the screw groove and the frictional force is increased with increasing diameter, when it rotates in the loosening direction of the screw. However it becomes more expensive because the number of components increases.

② Adhesion Locking methods with adhesive system are used as an inexpensive method. Some are superior in its effect, but the adhesive is a chemical product. Please be careful, because there is such notice (safety use guide) that exists not in the metal ware.

③ Double Nut By acting (restricting and locking) each other in the opposite direction, the two nuts create a clamping force in the axial direction in the thread surface and prevent loosening.
The purpose can be achieved by using the two normal nuts. However there are some nuts, which are provided with a tapered portion or eccentric part engagement and tighten the screw surface further in axial or radial direction. It becomes of course more expensive.

④ Increased friction force of nuts or bolt seating surface This corresponds to flange nut, the seating contact surface of which is enlarged. There are some that have been modified further with serrated face on the seating surface.

⑤ Washer The simplest is a spring washer, and aims to prevent loosening by providing a preload in the axial direction to the nut or screw, but the effect is limited. Effective one is a washer engagement made up of two plates, one side of it is constructed as a concavo-convex surface that each other engages, and the opposite side is provided with serrated face to bite the mounting surface or the bolt or nut seating surface.

⑥ Others In terms of cost, the above mentioned method aims washers or nuts. There is a bolt, which itself prevents the loosening. However because of the almost specific screw form, it is expensive and its marketability is limited.

As a troublesome but reliable special way, there is also a method to wind a wire in a counter direction of rotation or a method that a washer is bit into the cutout provided in the nut.