Fasteners are a type of mechanical parts that are widely used. They are characterized by a wide variety of specifications, different performance and uses, and a high degree of standardization, serialization, and generalization. Among them, stainless steel fasteners have excellent corrosion resistance, formability, compatibility and strength and toughness in a wide temperature range, and are very popular in heavy industry, light industry, daily necessities industry, building decoration and other industries.
Seizure (or galling) often occurs in fasteners made of stainless steel, aluminum alloys, and titanium alloys. These metal alloys are inherently corrosion resistant. When their surface is damaged, a thin oxide layer forms on the metal surface to prevent further corrosion. As a stainless steel fastener is tightened, pressure and heat generated between the threads destroy this oxide layer, causing interference or shearing between the metal threads and resulting in adhesion. If this continues, the stainless steel fastener becomes completely seized and can no longer be removed or tightened further.
Reasons for the locking phenomenon of stainless steel fasteners:
1, incorrect product selection
Before use, you should first confirm whether the mechanical properties of the product can meet the needs of use (such as the tensile strength of screws and the safe load of nuts). In addition, the length of the screw should be selected appropriately, with one or two pitches of the nut exposed after tightening.
2, the tooth texture is rough or foreign matter is stuck on it
If there are solder joints and other metal shavings mixed between the tooth lines, it will often cause locking.
3, too much force or too fast locking speed
Use a torque wrench or socket wrench whenever possible, and avoid using adjustable wrenches or electric wrenches. Because electric wrenches often cause the locking speed to be too fast, the temperature rises rapidly and locks.
4, the force direction angle is wrong
The nut must be screwed perpendicular to the axis of the screw, never at an angle.
5, washer not used
The use of washers/retaining rings can effectively prevent the problem of over-tightening.
Therefore, understanding the characteristics of stainless steel products and following correct operating methods is the first step to prevent stainless steel fasteners from locking.
Measures and solutions to prevent lock-up:
1, correctly select the grade and performance level of product materials. Matching bolts and nuts of different material grades (such as 304 with 316) can reduce the occurrence of locking, but you must confirm whether the product meets the usage requirements before use.
2, bolt length specifications should be selected appropriately, and one or two pitches of the nut should be exposed after tightening.
3, keep the thread surface clean. Foreign matter in the tooth pattern, solder joints, metal shavings and other inclusions often lead to locking.
4, slowly lock at a fixed speed. Use a torque wrench or socket wrench as much as possible, and avoid using an adjustable wrench or electric wrench as much as possible, because electric wrenches tighten quickly and can easily cause the temperature to rise rapidly and cause locking.
5. The contact surface must be perpendicular to the thread. Uneven stress will increase the probability of locking.
6. When tightening, pay attention to the force direction of the wrench being perpendicular to the screw axis and not tilted.
7. For flanges and other large-area situations where multiple screws are fastened, they should be tightened slowly and gradually in diagonal order.
8. During the tightening process, the safe torque of the screw must not be exceeded, and the force must be applied evenly.
9, it is recommended to lubricate the nuts and bolts before use. Lubricant can use engine oil, butter, molybdenum disulfide, stone grinding, mica, talcum powder, etc. according to the use occasion and conditions to lubricate the internal and external tooth lines to reduce the occurrence of locking.
10, select Coating
The processed nuts or the use of screws and nuts to buckle and stop at the same time can greatly reduce the occurrence of locking.
11. Under high temperature conditions, effective cooling measures should be taken when using stainless steel fasteners, and appropriate high-temperature resistant lubricants should be selected to prevent an increase in the probability of locking.
12. For occasions such as locking flanges that require pre-assembly, if the above requirements have been met and it still cannot be removed after pre-locking, you can suspend the use of carbon steel nuts when pre-locking the flanges and other devices, and then replace them with stainless steel nuts when the final formal locking is completed.
(Source: Baochi Yunze Tianjin Technology Co., Ltd.)