1. Composition of super nut
The super nut is also called a multi-stage push nut. It consists of a center nut, a push bolt with a circle of small satellite bolts around it, and a hard washer, as shown in Figure 1.
There is another type of super bolt, which consists of a center bolt, a push bolt with a circle of small satellite bolts around it, and a hard washer, as shown in Figure 2.
Sometimes the first type of super nut is also called a super bolt. It is officially defined as a push tensioning device. It can be directly used to replace the existing hexagonal nut for easy differentiation. This article calls the type 1 in the figure as a super nut and the type in the figure 2 as a super bolt.
2. The specific working principle of super nut
(1) By tightening the push bolt, a larger axial push force is generated. The pushing force acts directly on the hard washer. The friction diameter of the push bolt is small, so a smaller torque can be used to achieve a larger push force;
(2) The load is transferred to the nut body which has been manually installed on the main bolt;
(3) Hard washers are used to transmit thrust while protecting the flange surface;
(4) The axial thrust of several jack bolts and the reaction force of the main bolt head together produce a huge clamping force on the flange;
(5) The axial thrust of the jack bolt produces a pulling force of the same magnitude on the main bolt.
Excessive tightening of conventional bolts commonly causes thread galling, or metal adhesion. Observation shows that adhesion usually occurs on the lower two or three nut threads because of stress concentration from excessive preload. Stress relief is another key feature of a Supernut. Interaction between the jackbolts and main bolt creates a bending moment, distributed circumferentially around the Supernut, so the resulting stress is circumferential.
Circumferential stress causes the diameter of the superbolt to increase at the bottom and decrease at the top. This telescopic movement plays a role in releasing stress concentration, increasing static load, and preventing thread seizure (metal adhesion).
3. How to use super nuts and precautions
Due to the structural characteristics of the super nut, installation and disassembly must also be strictly performed in accordance with technical requirements. The steps are as follows:
1.Preparation
(1) Confirm that molybdenum disulfide or other grease is used on the push bolt;
(2) Confirm that the pressing ends of all push bolts do not protrude from the bottom of the super nut;
(3) Lubricate main bolt threads;
(4) Install the hard spacer on the main bolt;
(5) Lubricate the surface of the hard gasket or the compression end of the jack bolt with grease.
2. Fastening of super nut
Step 1: Screw the super nut onto the main bolt by hand until the bottom contacts the hard washer, then rotate 45±15° in reverse to ensure that the main thread is completely free;
Step 2: According to the positions shown in the figure 5, use the rated torque of 50% to symmetrically tighten the two sets of screws at positions 1-1 and 2-2 respectively;
Step 3: At the positions shown in the figure below, use 50% of the rated torque to tighten the four groups of jack bolts symmetrically in sequence at positions 3-3, 4-4, 5-5 and 6-6;
Step 4: Repeat step 2 using the rated torque of 75%;
Step 5: Repeat step 3 using the rated torque of 75%;
Step 6: Repeat step 2 using the rated torque of 100%;
Step 7: Use the rated torque of 100% to tighten the remaining jack bolts in a clockwise direction;
Step 8: After tightening all jack bolts, review the tightening degree of all jack bolts. The final tightening torque requirement is 100%-105% of the rated torque.
3.Removal of super nut
When disassembling the super nut, attention should be paid to gradually removing the torque on each push bolt. It is never allowed to remove all the torque of a certain push bolt at one time. Grease needs to be added during disassembly to aid disassembly. Starting from the 1 screw, in a circular sequence, loosen each push bolt by a maximum of 1/4 turns. Do not tighten to the point of looseness. At least three times, completely relieve the pressure on the jacking bolts. After all the push bolts of the super nut are loosened, the super nut can be removed by hand.
During the installation and disassembly process, care should be taken to prevent scratching the main threads. If necessary, heating can be used to speed up the disassembly.
4.Maintenance and other precautions
When tightened correctly, the super nut's pre-tightening force will not loosen even after years of use. However, we still recommend checking the pre-tightening force of the nut every time you perform maintenance. Use a torque wrench adjusted to 100% of the allowable tightening torque. It is not allowed to tighten a single jack bolt of a super nut to the rated torque at one time. After disassembling the super nut, you may sometimes find a few hundred millimeter dents on the hard washer, which is normal. The hard washer can still be used, just flip it over.
References:
[1] Zhang Yanfang, Lun Xiaobo, Qiang Xianghong. Application of super bolts [J]. Gansu Metallurgy, 2013, 35(04): 121-122.
[2] Zhu Pengfei, Zhao Peifeng, Wang Liquan. Application of super nuts [J]. Coal Mining Machinery, 2019, 40(04): 122-123.
[3] Dai Fang. Super bolts and their characteristics [J]. Equipment management and maintenance, 2009, (08): 33-34.
[4] Ma Tianjiao, Liu Lei, Zhang Qiuying, Gao Yulin. Application of super nut technology in the piston structure of reciprocating compressors [J]. China Science and Technology, 2019, (10): 46-47
Source: Hydrogenation Equipment Engineer
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