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Common Mistakes in Fastener Selection

Posted by: Fymicohuang 2021-10-10 Comments Off on Common Mistakes in Fastener Selection

Standard fasteners are divided into ten categories. When choosing standard fasteners, it should be determined according to the use occasions and functions of the standard fasteners.

 

1.Coarse teeth instead of fine thread

There are many important connecting parts on the machine, such as drive shaft, and most of the bolts are fine thread. If parts are missing during maintenance, some maintenance personnel will use coarse tooth bolts instead, which should be avoided. Because the fine tooth bolt has large inner diameter, small pitch and external Angle, high strength, good self-locking performance, and strong ability to bear impact, vibration and exchange load. Once the coarse tooth bolt is used instead, it is easy to loosen or take off, disassemble, and even cause mechanical accidents.

 

2.Pore not worthy

Bolts that are subjected to transverse load and shearing force on the machine, such as drive shaft bolts and flywheel bolts, shall be fitted with the bolt holes for transition fit, and shall be sturdy and reliable in assembly and able to withstand lateral force. Someone in the assembly process do not pay attention to check, bolt and bolt hole still continue to install when there is a large gap, which is easy to happen bolt loosening or cutting accident.

 

3.Thickened nuts increase connection reliability

Some people mistakenly believe that thickening the nut increases the number of loops of the thread and improves the reliability of the joint. But in fact, the thicker the nut, the more uneven the load distribution between the threads of each ring, the more easy to loosen the connection.

 

4.Multiple washer for one nut

After the installation is completed, sometimes the bolt is too long, so someone will install extra spring washers on the bolt. In this case, the spring washers will break unevenly during the hardening process, thus reducing the pre-tightening force of the bolt, and may produce eccentric load, reducing the reliability of the bolt connection.

 

5.The tighter the better

Many staff members have such a misunderstanding: that is, the bolt should be “rather tight than loose”, so they deliberately increase the tightening torque, resulting in bolt slip buckle. In addition, some of the important bolts that need to be tightened with torsion, but some people because of the effort to tighten with a movable wrench, the result is insufficient torque, resulting in loose bolts, and even lead to mechanical failure.

6.Too big a gasket won’t hurt

Sometimes there is a lack of washers of appropriate size, and some workers will replace them with washers of larger inner diameter. In this case, the small contact area below the bolt head and the washers will reduce the bearing pressure or locking force of the washers. If there is vibration and impact in the working environment, the bolts are easy to loosen.

 

7.Inappropriate locking

Important bolts should be locked by anti-loosening device after assembly is completed, which is illustrated in four cases. If the opening pin is used for locking, too thin opening lock or half opening lock shall not be used for locking. If the spring gasket is used for locking, the gasket opening is not allowed to be too small; If the lock piece is used to lock, the lock piece shall not be locked at the edges and corners of the nut; If double nut locking is used, thinner nuts shall not be installed outside.

 

8.False strong

If the bolts, nuts or threads are rusty, or scale, iron filings and other impurities, must be cleaned before assembly; The joint surface of the connection has burr, sand and other impurities should also be removed. Otherwise, when tightening the bolt, it appears to be tightened on the surface, but in fact the connection is not really tightened. Under such false firmness, bolts will loosen quickly if vibrations, load shocks, and temperature changes occur.

 

Selection principle and determination of fasteners

Standard fasteners are divided into ten categories.When choosing standard fasteners, it should be determined according to the use occasions and functions of the standard fasteners.

Bolt:bolts are usually matched with nuts (usually with one or two washers) and are used for connection and fastening.

Nut:nuts and bolts are used together.

Screw:the screw is usually used alone (sometimes with washers), usually plays the role of tightening or setting.

Stud bolt:studs are generally threaded at both ends (single-headed studs are single-ended with threads), usually a head of rib firmly screwed into the body of the components, the other end and nut match, play the role of connection and fastening, but to a large extent also has the role of distance.

Washer:the washer is used between the supporting surface of bolts, screws and nuts and the process supporting surface to prevent loosening and reduce the stress on the supporting surface.

Self-tapping screw:the screw hole of the workpiece matched with the self-tapping screw does not need to be tapped in advance, and the internal thread is formed when the self-tapping screw is inserted.

Rivet:the rivet has a head at one end and no thread at the rod. When in use, the rod is inserted into the hole of the connecting piece, and then the end of the rod is riveted and tightened to connect or fasten.

Pin:the pin is set into a work piece when in use, usually for connection or positioning.

Check ring:The retaining ring is usually on the shaft or in the hole to limit the axial direction of the workpiece.

Wood screw:Wood screws are used to screw into wood for joining or fastening.

 

Selection principle of variety:

⑴From the processing, assembly work efficiency consideration, in the same machinery or engineering, should minimize the use of standard parts of the variety.

⑵Considering economy, we should give priority to the selection of commodity standard parts.

⑶According to the expected use requirements of standard parts, according to the type, mechanical properties, precision and screw thread and other aspects to determine the selection of varieties.

 

Type

⑴:Bolt

①Bolts have hexagonal head, square head, refer the national standard GB5780 – GB5790, etc.

②Bolts for reaming holes: when in use, the bolts should be tightly inlaid into the holes made of reaming to prevent the dislocation of the workpiece, refer GB27, etc.

③Arrest bolt: there are square neck, with vertebrae, refer GB12-15, etc.

④Special purpose bolts: such as T-slot bolts, refer GB37; Looper bolt, see GB798; For anchor bolts, refer GB799, etc.

⑤Steel structure with high strength bolt connection pair: generally used in construction, bridge, tower, pipe support and lifting machinery and other steel structure friction connection occasions, refer GB3632, etc.

 

⑵Nut

①Nuts for general use: mainly hexagon nuts, other square nuts, many varieties, refer GB41, GB6170-6177, etc.

②Slotted nut: mainly refers to hex slotted nut, namely in the hex nut above the processing of bad. It is used with screw bolts with holes and split pins to prevent the relative rotation of bolts and nuts, refer GB6178 — 6181, etc.

③Lock nut: refers to nuts with locking function, including hexagonal locking nuts with nylon inserts, as GB889, GB6182, GB6183 and all metal hexagonal locking nuts, as GB6184 — 6187.

④Nuts for special use: such as disc nuts, refer GB62; Ring nut, see GB63; Cover nut, refer GB802, GB923; Knurled nuts, refer GB806, GB807 and insert nuts, refer GB809, etc.

 

Screw

①Machine screws: due to the head and groove type of different and divided into many varieties. The head type has a cylindrical head, pan head, countersunk head and half countersunk head several kinds, the head shape is slotted (a word material) and the cross groove two kinds. Refer GB65, GB67-69 and GB818-820, etc.

②Set screw: the end of the set screw is used to prevent the mutual displacement between the workpiece and transfer less torque. Refer GB71, GB73-75, GB77, GB78, etc.

③Socket head cap crew: Hexagon socket screw is suitable for small installation space or where the screw head needs to be embedded. Refer GB70, GB6190, GB6191 and GB2672-2674, etc.

④Special purpose screws: such as setting screws, refer GB72, GB828, GB829; Non-release screws, refer GB827-839, GB948, GB948 and lifting ring screws, refer GB825 etc.

 

Stud bolt:

①Double-headed studs of unequal length: suitable for one end of the screw into the body of the components, to connect or tighten the role of the occasion. Refer GB897-900, etc.

②Double end stud of equal length: suitable for connecting ends and nut matching, play the role of connecting or setting distance. Refer GB901, GB953, etc.

 

Washer:

①Flat washer: used to overcome the unevenness of the workpiece bearing surface and increase the stress area of the bearing surface. refer GB848, GB95-97 and GB5287.

②Spring (elastic) washers: spring washers see GB93, GB859, etc., elastic washers refer GB860, GB955, etc.

③Stop washers: with inner tooth lock washers, see GB861; Outer tooth lock washer, see GB862; Single lug stop washer, see GB854; Double ears stop washers, see GB855; For round nut stop washers, refer GB858, etc.

④Bevel washers: To accommodate the slope of the workpiece bearing surface, oblique washers can be used. For square inclined washers with I-steel, see GB852; For square inclined washers of channel steel, refer GB853.

 

Self-tapping screw:

①Common tapping screw: thread conforms to GB5280 (thread for tapping screw), large pitch, suitable for use on thin steel plate or copper, aluminum and plastic. Refer GB845 — 847, GB5282 — 5284, etc.

②Tapping locking screw: the thread conforms to the common metric coarse thread, suitable for use in the need of vibration resistance. Refer GB6560-6564.

 

Rivet:

①Hot forged rivets: generally large size, used in locomotives, ships and boilers, etc., usually through hot forging to shape the head. Refer GB863-866.

②Cold heading rivets: generally diameter ≤16mm, usually by cold heading to make the head. Refer GB867 — 870, GB109, etc.

③Hollow and semi-hollow rivets: Hollow rivets, see GB976; Semi-hollow rivets, refer GB873-875, etc.

④Special purpose rivets: there are headless rivets, refer GB1016; Tubular rivets, refer GB975; For label rivets, refer GB827, etc.

⑤Blind rivets: single riveted rivets, widely used in recent years, is a new type of rivets, rapid, safe, convenient, and has the advantages of waterproof, anti-leakage.

 

Pin:

①Cylindrical Pins: Cylindrical Pins with or without thread, see GB119; Cylindrical pin with internal thread, refer GB120; Cylindrical pin with external thread, refer GB878; Elastic cylindrical pin, refer GB879; Cylindrical pin with hole, refer GB880, etc.

②Taper pin: cylindrical pin with or without thread, refer GB117; Cylindrical pin with internal thread, see GB118; Screw taper pin, refer GB881; Open taper pin, refer GB877.

③Slotted pin: generally matched with screw bolts with holes and slotted nuts to prevent loosening of bolts and nuts. Refer GB91.

 

Check ring:

①Resilient retaining ring: The elastic retaining ring with holes. refer GB893; Elastic retaining ring for shaft. Refer GB894 and GB896 for shaft opening retaining ring.

②Steel wire retaining ring: steel wire retaining ring with holes, see GB895.1; Wire retaining ring for shaft, refer GB895.2 and wire locking ring, see GB921.

③Lock retaining ring for shaft parts: retaining ring with taper pin locking, refer GB883; ReferGB884, GB885 and so on for retaining rings with screw locking.

④Shaft end retainer ring: For shaft end retainer ring fastened with screws, refer GB891 and for shaft end retainer ring fastened with bolts, refer GB892.

 

Wood screw:

It is divided into many varieties because of the head shape and groove shape. Head type has a round head, countersunk head, half sunk head several kinds of head groove for slot (a word slot) and cross slot two. Refer GB99-101, GB950-952, etc.

 

 Bolt,screw,stud bolt and nut

Sheet 1-1:property classes of bolts,screws,studs and nuts

 Property classes nut 4 5 6 8 9 10 12
boltscrew

stud bolt

3.6,4.6 4.8 5.6 5.8 6.8 8.8 9.8 10.9 12.9
 Application Low strength and low torque  High strength and high torque

Set screw

 

Property classes 14H 22H 33H 45H
Application Low strength and low torque High strength and high torque

 

Self-tapping screws and self-tapping locking screws

Ordinary tapping screws are not classified according to mechanical properties. As long as the appropriate specifications are selected, refer 3.6 and Table 3 in GB3098.5), they can meet the general use requirements.

 

The mechanical properties of self-tapping locking screws are divided into two grades: A and B. Grade A is high strength grade; Class B is a low strength grade.

 

Stainless steel bolt,screw,stud and nut

sheet 1-3

Material categories Property classes
Austenite —— A1—50A2—50

A3—50

—— A1—70A2—70

A3—70

A1—80A2—80

A3—80

Martensite —— C1—50C4—50 —— C1—70C4—70 C3—80
Ferrite F1—45 —— F1—60 —— ——
Applicable occasions Low strength and low torque High strength and high torque

 

The precision of standard parts (fasteners) is determined by the product grade.

Sheet1-4:Product grade and accuracy of standard parts (fasteners)

Position Product level
A B C
Thread,rod and support surface tight tight loose
Other position tight loose loose
Accurate degree accurate a little accurate inaccurate

Tolerance class of thread

Tolerance class of thread for standard parts (fasteners)

Sheet 1-5

 Tolerance class of thread  Corresponding product grade  Applicable
inner thread 6H A、B Threads for general machinery,appliances,etc.
outer thread 5g16g
inner thread 7H C Threads for general machinery appliances,etc.
outer thread 8g

Note:1)Thread tolerances for only class 45H set screws and class 12.9 hexagon socket screws can be in class 5g.

 

Selection of pitch

Comparison of coarse thread and fine thread

sheet 1-6

 coarse thread fine thread
Has good versatility and economy, suitable for fast fastening and repeated loading and unloading. Has good anti-looseness, high fastening force and strength

 

Confirm specification

①The first series of values should be chosen as far as possible for the diameter specification. The values in parentheses in the product standard size table and the values in the specification table should be avoided as far as possible.

②Length specifications generally do not adopt specifications other than those specified in the product standard. General bolt or nut matching connection, the thread end exposed (0.2-0.3) D length is appropriate.

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