What Are Threaded Fasteners?
Threaded fasteners are mechanical parts with internal or external threads. The traditional thread detection method is to use a thread gauge for contact or use a universal tool microscope for manual measurement. The workload is large, the working efficiency is low, and the measurement results are easily affected by the operator.
According to the shape of plane graphs, threads can be divided into triangles, rectangles, trapezoids, and serrated threads.
According to the winding direction of the helix, threads can be divided into left-hand thread and right-hand thread.
It is stipulated that when the thread is upright, the helix rises to the right into a right-hand thread, and rises to the left into a left-hand thread.
Characteristics of Threaded Connection
①Great shaft force can be generated when threads are tightened;
②Threaded fasteners connection can easily realize self-locking;
④It is simple to manufacture and can maintain high precision.
Threaded Fasteners Type Selectivity
Used to connect two thinner parts. There is a seam between the rod and the hole of the ordinary bolts, the processing requirement of the through-hole is relatively low, with a simple structure, easy to disassemble, and widely used.
It is used where one of the connected parts is thick and not suitable for bolt connection, and the thicker connected parts have poor strength and need to be disassembled frequently. Threaded holes are processed on thick parts and smooth holes are processed on thin parts. Bolts are screwed into threaded holes, and a nut is used to tighten the thin part.
Bolts(or screws) are screwed directly into the threaded hole of the connected parts without nuts. Its structure is simpler and more compact than the stud. A screw connection is used for the thicker one between the two connected pieces and does not need to disassemble often, so as to damage the threaded hole.
Set Screw Connection
Screw into the threaded hole of the end of the thread against the surface of another part or into the pit of another part, in order to fix the relative position of the two parts. The structure of the set screw connection is simple, and some can change the part’s circular or shaft directions position randomly, so as to adjust, such as the fix of the electrical device rotary switch.
Fastening Principle of Threaded Fasteners
A thread can be defined as an inclined surface around a cylinder and is equivalent to an inclined surface. Conversely, the unfolding of a thread is an inclined surface. The self-locking phenomenon refers to that if the action line of the net force Q acting on the main force of the object is within the friction Angle, no matter how big the force is, there will always be an all-reaction force R in balance with it, and the object will remain static.
Conversely, if the action line of the resultant force Q is outside the friction Angle, no matter how small the force is, the object cannot keep balance. This kind of equilibrium condition which is independent of force but related to friction Angle is called the self-locking condition.
The object in this situation still keeps equilibrium, which is called the self-locking phenomenon. The resultant force R of the friction angle and the normal reaction force N of the self-locking phenomenon and the friction force F are called the total reaction force of the support facing the object.
That is when the friction force F up to its F maximum value, at the same time, the included Angle an also reaches its maximum value b which is called the Angle of friction.
Used for the situation where the thread diameter is less than 10mm and the strength requirement is not high. The head of the screw or part of it sinks into the connected part. This structure is mostly used on occasions that require a flat outer surface, such as an instrument panel.
Self Tapping Screw
It’s applicable to situations where connection strength is not required. But generally, the bottom hole should be made in advance. If self-drilling and tapping screws with drill bits are used, there is no need to prefabricate bottom holes for non-ferrous metals, wood, etc.
Wood Screw Connection
Usually applied in the wood structure connection. Depending on the hardness of the material and the length of the wood screw, wooden parts may not be prefabricated or prefabricated with a certain size and depth.
Threaded Fasteners Component Parts Connection
Washers and threaded fasteners with external thread are manufactured and supplied in complete sets by a standard workshop professional factory. These kinds of connected parts are convenient to use, time-saving, safe, and reliable, and are usually used in connected situations where threaded fasteners are closely connected.
Thread Forming Screws Connection
The thread is an arc triangular interface, the screw surface is hardened, and can be screwed into the metal material of the prefabricated hole, and extrusion to form an internal thread. The internal thread formed by extrusion is more than 30% stronger than that by cutting. The small tensile strength of the screw is 800MPa. Thread forming screw with low tightening torque and high locking performance, which is widely applicable to household appliances, the electrical industry, and the automobile industry.
Threaded Fasteners Marking
General Purpose Screw Threads
1) Coarse thread pitch of the common thread is not marked, fine thread pitch must be injected. If M20 is a coarse tooth, pitch 2.5mm is not marked; if M20×2 is a fine tooth, pitch 2mm should be marked.
2) Note LH for the Left-hand thread, not for the right-hand thread.
3) Tolerance zone codes for thread include middle diameter and top diameter tolerance zones, lowercase letters for outer thread tolerance zones, and uppercase letters for inner thread tolerance zones. For example, m20-5G6G indicates that the middle diameter tolerance zone of the external thread is code 5g, and the top diameter tolerance zone is code 6G. If the middle diameter and the top diameter tolerance zone code are the same, such as M20-7H means that the inner thread middle diameter and top diameter tolerance zone are 7H.
4) The screw length of ordinary thread is specified as short (S), medium (N), and long (L), and the medium screw length (N) need not be marked, such as M24-5G6G-L, M20×1.5-6H-S-LH, or M20×1-6g. If the spinning length is a specific value, it can be directly marked, for example, M20-5G6G-40 indicates that the spinning length is 40mm.
Acme Thread Form
Labeled with common threads, such as Tr40×7LH-7H-L represents trapezoidal threads, nominal diameter of 40mm, pitch of 7mm, left rotation, tolerance zone code for 7H, and long screw length.
55° Unsealed Pipe Thread
55° unsealed pipe threads, whose outer threads have two tolerance classes A and B, shall be marked out; Internal threads have only one tolerance class and do not need to be noted. For example, G1/4A refers to the non-threaded sealing pipe thread, which is the external thread, the size code is 1/4, and the tolerance grade is A; G1/4 means non-threaded sealed pipe thread, is an internal thread, dimension code 1/4.
Common Threaded Fasteners
Detachable connections can be made with bolts, screws, and nuts.
Bolts and Screws
Head bolts and screw heads come in a variety of shapes and can be used as clamp bolts, pair bolts, or mating bolts.
Used for fixing cover plates, flange plates, press plates, and other machine parts. Screw in-depth as a double-headed stud. Cylindrical bolts with inner hexagons can be countersunk, but can still be tightened like hexagon bolts.
To be secured with nuts and washers. Pay attention to the flat contact surface of the bolt head and nut and washer.
Used for fixing the position of machine parts or bearing transverse forces. This connection requires mating (H7/ K6), so it is more expensive. It is more economical to combine one or two matching pins with pair bolts.
Slotted Head Cap Screws
The screw heads of slotted screws come in various shapes. This kind of screw can only be tightened with a screwdriver. The screw head of the Phillips screw has no slot through it, so the strength will not be weakened. Phillips screw is characterized by high strength, can be screwed more tightly, and the shape is good-looking.
Used for fixing company signboard and guard board on an iron plate.
The stud is composed of the screw into the end (chamfered), screw, and nut end (top). The purpose is the same as clamping studs. The advantage is that the clamping stud will damage the thread after repeated loosening, while the screw end of the stud can always be screwed in. Remove the studs by removing the nuts. Screw in depth should conform to the strength of the screw material, so as not to break the screw.
The screw is threaded on the whole length, and there is a groove at the end for screwing and tightening, which is used to fix the positioning ring, shaft sleeve, and other machine parts to prevent movement or torsion dislocation. A stud is a variant of a screw used to secure removable parts.
The figure below shows several standard nut shapes. The length of the nut thread is important for the strength of the nut screw connection. Thread length determines the number of thread bearings.
For most nuts, the nut height is equal to the thread length. The exception is blind nuts. Height of standard nut =0.8 x nominal diameter of the thread. Height of the flat nut ﹤ 0.5× d, only used for small force or loose place. If the force is large, super-high nuts must be used, and the height is >1+d.