Precautions for using locknuts
The anti-loosening effect of locknuts not only relies on their own structural design, but also is closely related to the installation specifications, working condition suitability, and maintenance methods, the specific precautions are as follows.
I. Selection and specification matching
Adaptability to working conditions: Different types of locknuts have significant differences in adaptability to the scene. For example, although RN Precision Lock Nuts are suitable for small spaces, the anti-loosening effect in high-frequency vibration scenarios is not as good as that of F-type Bearing Retaining Nut; R-type radial locknuts rely on radial contraction force, and should not be used for bolts with a plated surface (e.g., thick chrome-plated layer) of the connection, otherwise the plated layer may be squeezed off, which affects the locking force. The selection should be based on the type of force (axial / radial), frequency of vibration, dimensions of the space and other parameters.
Matching of thread accuracy: The thread accuracy of lock nut and bolt should be consistent (e.g. 6g-grade nut with 6h-grade bolt), if there are defects such as knocking and slipping of bolt threads, it will result in the nut not being able to be uniformly loaded, and even if it is tightened, it may be partially loosened. Before installation, it is necessary to check the accuracy of the bolts with a thread gauge and replace them if they are not qualified.
Material compatibility: in wet or corrosive environments, if the bolt is made of carbon steel, should avoid matching with stainless steel lock nut (e.g. 304 material), otherwise it may be due to electrochemical corrosion to accelerate the bolt corrosion; at this time, you need to choose the same material or plating compatibility of combinations (e.g., carbon steel bolts with zinc-plated lock nut).
Second, the preparatory work before installation
surface cleaning: the threaded surface of the bolt and nut should be free of oil, iron filings, paint and other impurities. Oil will reduce the thread friction, resulting in preload attenuation; iron filings may be embedded in the thread gap, resulting in stalling during tightening, or even scratching the teeth. Use a wire brush to clean up rust stains, wipe the oil with a solvent (e.g., alcohol, kerosene) to ensure that the surface is dry and clean.
Tool selection: Use the appropriate wrenches (such as plum wrenches, torque wrenches), avoid the use of movable wrenches and other tools that are easy to slip, to prevent damage to the surface of the nut (especially the corners of the RN square locking nut, once the wear and tear will affect the subsequent disassembly). For scenarios requiring precise control of the preload force (e.g., precision mechanical connections), a torque wrench must be used to set the torque according to the design requirements (e.g., M10 locknuts usually require 25-35N・m), and should not be tightened by feel.
Condition check of components: If there are deformations or depressions on the contact surfaces of the connected parts, it is necessary to level them out first, otherwise additional torque may be generated due to uneven force after tightening, which may accelerate the loosening of the nut. For the lock nut with elastic structure (such as the elastic retaining ring of K-type axial lock nut), it is necessary to check whether the elastic element is intact or not, if there are cracks, aging and other problems, it is prohibited to use.
Third,Standardized operation during installation
Tightening method: apply force smoothly along the bolt axis to avoid eccentric tightening. Eccentricity will lead to uneven engagement of the threads of the nut and bolt, part of the teeth deformed by excessive force, reducing the overall anti-loosening capacity. For the symmetrical connection of multiple locknuts (such as equipment flange circumferential distribution of bolts), need to be tightened step by step according to the diagonal order, each step tightened to the target torque of 1/3, and gradually incremental to the specified value, to prevent the connected parts warping.
Torque control: Different types of locknuts have different sensitivities to torque; if the torque of F-type tooth web locknuts is too large, it may lead to excessive extrusion and deformation of the teeth, which may weaken the effect of wedge loosening; R-type radial locknuts need to be tightened at a small torque of the end locking screw (usually 1/5-1/4 of the main torque of the nut), and excessive tightening will cause the open groove to fracture. Refer to the product manual for torque parameters and do not overtorque.
Avoid secondary adjustment: After the lock nut is tightened, if it is necessary to fine-tune the position, it should not be directly knocked or pried, but should be loosened to a half-tight state, and then re-tightened to the specified torque. Forced adjustment will destroy the friction distribution of the threaded vice, resulting in a decline in anti-loosening performance.
Fourth, the special response to the working environment
Temperature fluctuation scenarios: in high-temperature environments (such as boiler piping, engine compartment), you need to choose high-temperature-resistant materials (such as Inconel alloys) lock nut, while reserving a margin for thermal expansion and cold contraction -- installation of preload can be appropriately lowered by 10% -15% to avoid overload fracture of the nut caused by the expansion of the bolt after temperature rise. 15%, to avoid overload fracture of the nut due to expansion of the bolt at elevated temperatures. In low-temperature environments (e.g., refrigerated equipment), it is necessary to ensure that the elastic components of the lock nut (e.g., K-type retaining ring) remain tough at low temperatures to avoid embrittlement and failure.
Vibration impact scene: in high-frequency vibration equipment (such as vibrating screen, press), in addition to the choice of F-type impact lock nut, can be added between the nut and the connected parts of the elastic gasket (such as nitrile rubber pads), to further buffer the vibration energy. However, it should be noted that the thickness of the gasket should not exceed 1mm, otherwise it may affect the preload transfer of the nut. Dusty and humid environments: For locknuts exposed to dust or water vapor, it is necessary to apply antirust grease (e.g. lithium grease) to the exposed portion of the threads after installation and install a dust cover (e.g. plastic protective cap) to prevent impurities from intruding into the thread gap. In food processing and other scenes that require cleanliness, food-grade lubricants (such as white oil) need to be used to avoid ordinary grease contamination products.
Five, maintenance and replacement guidelines
Regular inspection: running equipment needs to be checked according to the cycle (such as once a month) to check the status of the lock nut, focusing on observing whether there are signs of loosening (such as the nut and the connected parts of the gap between the appearance of the operation of the abnormal noise). A torque wrench can be used to retighten the locknuts. If the torque drops by more than 20% during retightening, the threads may be worn out and need to be replaced in a timely manner.
Restriction on reuse: Most locknuts have a limited number of times of reuse - R-type radial locknuts are recommended to be reused no more than 3 times due to plastic deformation of the open grooves; F-type web locknuts can be reused 5 to 8 times if there is no obvious wear of the teeth, but the web needs to be cleaned of any residual impurities before each use. For the key parts (such as machine tool spindle, crane boom) of the lock nut, even if it has not reached the number of times of use, if there is a slight deformation should be replaced with a new product.
Disassembly precautions: When disassembling, use the same specification wrench as the installation, and avoid violent disassembly. For rusted locknuts, apply loosening agent (e.g. WD-40) to moisten them for 10-15 minutes, and then loosen them slowly, and do not hit them with a hammer to prevent the nuts from breaking or the bolts from breaking. After disassembling, check whether the threads are intact or not, if there is any damage, replace the bolts synchronously.
By strictly following the above precautions, the anti-loosening performance of locknuts can be fully utilized to avoid connection failure due to improper installation and lack of maintenance, thus guaranteeing the operation safety and stability of the equipment.
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