Tightening Methods of Flange Bolts and Nuts
The flange bolts and nuts is one type of fastener mainly composed of a hexagonal head and a flange plate (the gasket under the hexagon is fixed together with the hexagon), and a screw (a cylindrical body with external threads). It needs to be matched with a nut to fasten and connect two parts through holes.
Flange bolts and nuts are widely used in highway and railway bridges, including industrial and civil construction, cranes, excavators and other heavy machinery, with a wide range of applications.
Common types of flange bolts and nuts
There are two types of hexagonal flanges, one is flat and the other is concave. The surface is coated with white, military green, colored yellow, and the other is Dacromet, which never rusts.
According to the different usage locations of flange screws, the size requirements of the disc vary. There are also flat bottomed and toothed types, with toothed ones providing anti-slip function.
According to the force distribution of the connection, there are ordinary and hinged holes used. The flange screws used for hinged holes should match the size of the hole and be used when subjected to lateral forces. In addition, to meet the needs of locking after installation, there are holes in the rod that can prevent the bolts from loosening when subjected to vibration. Some flange bolts without threads need to be made thin, called thin rod flange bolts. This type of flange bolt is beneficial for connecting under variable forces.
There are specialized structure. -strength bolts on the steel structure. The head will become bigger, the size has also changed.
Common materials for flange bolts and nuts
The material of > carbon steel C%>0.45% hexagonal flange bolts and nuts is currently not widely used in the market.
Medium carbon steel 0.25% hex head screws are commonly referred to as 35 # and 45 # steel in China, and 1035, CH38F, 1039, 40ACR, etc. in foreign countries. It is mainly used for grade 8 nuts, grade 8.8 bolts, and grade 8.8 hexagon products.
Low carbon steel with a C% ≤ 0.25% is commonly referred to as A3 steel in China. It is commonly referred to as 1008101510181022, etc. It is mainly used for products without hardness requirements such as grade 4.8 bolts, grade 4 nuts, and small screws. (Note: Drill tail nails are mainly made of 1022 material.)
Alloy steel adds alloying elements to plain carbon steel to enhance its special properties, such as 35 and 40 chromium molybdenum SCM435,10B38. Hexagonal flange bolts mainly use SCM435 chromium molybdenum alloy steel, which is mainly composed of C, Si, Mn, P, S, Cr, and Mo.
Methods and requirements for tightening flange bolts and nuts.
The torque free ring wrench or hammering wrench is suitable for general equipment and pipeline flange fastening, and is selected according to the screw size and flange pressure rating. The fastening requirements are as follows.
1. The maintenance unit shall develop a tightening plan, symmetrically tighten the flanges, and number them in the tightening sequence.
2. Use 4 screws as washers to position, ensuring that the center of the wound washer is within the edge of the flange.
3. Tighten the positioning bolts and nuts by hand, then insert other stud screws and tighten them to balance the load, ensuring that at least two threads are exposed on each end of the nut.
4. Based on the on-site equipment and flanges, calculate the number of tightening cycles (at least three times) and the impact load (force) for each tightening cycle. Tighten the impact load (force) in ascending order from opposites to large (such as 50%, 80%, 100% increasing), and do not load too quickly or too opposites to prevent gasket sealing failure.
5. The sequence of tightening each time with a torque free ring wrench or impact wrench.
Tighten the two radially opposites to the predetermined hammering load (force) of the screws.
Tighten another pair of bolts and nuts that are approximately 90 degrees apart along the circumference from the previous two bolts and nuts.
Continue tightening until all other screws are tightened to the predetermined hammering load.
6. Finally, tighten all bolts and nuts in a clockwise or counterclockwise direction with 100% hammering load (force).
Torque wrenches are suitable for high temperature, high pressure, flammable and explosive important equipment and pipeline flanges. The fastening requirements are as follows.
1. The maintenance unit needs to develop a tightening plan, set appropriate torque, and conduct design review based on parameters such as screw strength, initial sealing pressure and working sealing pressure of the gasket, medium pressure, etc., to prevent screw breakage and gasket loss of elasticity due to excessive clamping force, leading to sealing failure.
2. Tighten the flanges symmetrically and number them in the tightening sequences.
3. Use four screws as washers to position, ensuring that the center of the wound washer is within the edge of the flange.
4. Tighten the positioning screw by hand, then insert other stud screws and tighten them to balance the load, ensuring that at least two threads are exposed on each end of the nut.
5. Based on the on-site equipment and flanges, calculate the number of tightening cycles (at least three times) and the tightening torque for each cycle. Tighten the torque in ascending order from small to large (such as 50%, 80%, 100% increasing), and do not load too quickly or excessively to prevent gasket sealing failure.
6. The sequence of tightening each time with a torque wrench:
Tighten the two radially opposite bolts and nuts to their predetermined torque.
Tighten another pair of bolts and nuts that are approximately 90 degrees apart along the circumference from the previous two bolts and nuts.
Continue tightening until all other bolts and nuts are tightened to the predetermined torque.
7. Finally, tighten all bolts and nuts in a clockwise or counterclockwise direction according to the 100% torque value.
8. Record the torque value as a reference for future maintenance.
Screw tensioners are suitable for important equipment such as high temperature, high pressure, flammable and explosive materials, as well as pipeline flanges. The fastening requirements are as follows.
1. The maintenance unit shall develop a tightening plan, establish appropriate tensile force, and conduct design review based on parameters such as screw strength, initial sealing pressure and working sealing pressure of the gasket, medium pressure, etc., to prevent screw breakage and gasket loss of elasticity due to excessive clamping force, leading to sealing failure.
2. When using a screw tensioner to individually (step by step) tighten screws, follow the principle of uniform tightening of screws and refer to the tightening sequence of a torque wrench for tightening.
3. The process of tightening and stretching with a screw tensioner involves setting a reasonable number of tightening and stretching times. The pressure should be applied gradually from small to large (such as 50%, 80%, 100%), evenly increasing the pressure. For each increase in pressure, it should be stabilized before increasing again to avoid excessive impact tension that may affect the pre-tightening effect of the bolts and nuts.
4. Record the pressurization value as a reference for future maintenance.
