Introduction to Spring Steel Wire

　　Spring steel wire is a type of wire used to make springs (SPRING) or wire forms (WIRE FORM). Depending on the use of the spring, there are many types of spring steel wire required for making springs, such as spring steel wire for making mattress springs (referred to as mattress wire) with lower requirements, spring steel wire for making shock absorbers, spring steel wire for making suspension springs, spring steel wire for making engine valve springs, and spring steel wire for making camera shutter springs, etc. Although there is no unified standard classification name, the quality requirements of these wires are different. Spring steel wires can also be classified according to manufacturing processes, such as cold-drawn spring steel wires (not quenched in a lead bath before drawing), lead-quenched spring steel wires, galvanized spring steel wires, oil-quenched spring steel wires, etc.

　　The tensile strength of spring steel wire varies with different types, standards, and specifications, ranging from 1000 to 3000 MPa.

　　The diameter range of round spring steel wire is 0.08-20 mm.

　　The cross-sectional shape of spring steel wire is generally round, but it can also be rectangular, square, or oval. Finished steel wires are usually delivered in coils, but they can also be delivered in straight bars.

　　Springs used in different environments have different special requirements for the wire. For example: springs working in corrosive media require good corrosion resistance; springs in precision instruments require long-term stability and sensitivity; springs in high-temperature environments require sufficient elastic limit and creep resistance.

　　Different types of spring steel wire have different production methods. The common feature is that they require certain strength, high toughness, and good coiling performance.

　　Common production methods for various spring steel wires are as follows:

　　Production process of cold-drawn spring steel wire: (steel) coil -- surface treatment -- wire drawing (mattress wire can use this method)

　　Lead-quenched spring steel wire: Lead bath quenching is performed at the coil size or intermediate specifications of cold drawing, and then surface treatment and wire drawing are performed.

　　Galvanized spring steel wire: Usually hot-dip or electroplating is performed at the finished size. There is also a method of pickling and hot-dipping the coil and then cold-drawing it to the specified size.

　　Oil-quenched spring steel wire: Carbon steel or alloy steel can be used as needed. After surface treatment and cold drawing to the finished size, quenching and tempering treatment is performed. This process is more commonly used in automotive suspension springs and valve spring wires, but it can of course also be used for ordinary springs.

　　Surface treatment: Generally, pickling and phosphating are used to remove iron oxide scale and form a phosphate film; a few also use mechanical methods. The purpose is to meet the requirements of the cold drawing process and obtain a smooth surface. For spring steel wires with high fatigue life requirements, such as valve spring wires, the coil should be peeled to reduce surface defects. If the steel mill can perform grinding on the billet, it will also help reduce defects.

　　Wire drawing: The drawing process of the finished drawing has a great influence on the product performance. Generally, a larger total reduction rate (see area reduction rate) of about 90% and a smaller reduction rate per pass (about 10%-20%) are used to ensure the toughness of the product. For high-strength spring steel wire, the outlet temperature of the wire in each pass during drawing should be controlled below 150℃ to prevent the wire from twisting and cracking due to strain aging. This is the main defect that causes the wire to be scrapped. Therefore, good lubrication and sufficient cooling are necessary during drawing. Using a smaller reduction rate per pass and drawing speed helps reduce the temperature rise of the wire.

　　Heat treatment: Lead bath quenching is commonly used for carbon steel spring steel wire, which can obtain a very fine pearlite structure (sorbite), which is beneficial to improving the deep drawing performance and spring performance. The alternative process of fluidized bed for lead bath has not yet been widely promoted and is only used in some small diameters. Alloy steel wires generally use annealing heat treatment to make the microstructure adapt to drawing deformation. The heat treatment of stainless steel wire uses solution treatment, the purpose of which is also to improve the microstructure to adapt to the needs of drawing. The oil quenching process is used on finished steel wires. Induction heating or gas/oil heating furnaces are used to heat the wire to the austenitizing temperature, hold it for a period of time, quench it, and then perform a medium-temperature tempering. Although it is nominally called oil quenching, water quenching or water quenching with high-molecular materials is basically used.

　　Springs are used within the elastic range, and should return to their original position after unloading. Plastic deformation should be as small as possible, so the wire should have a high elastic limit, yield strength, and tensile strength. The higher the yield strength ratio, the closer the elastic limit is to the tensile strength, thus improving the strength utilization rate and making the spring more elastic.

　　Springs rely on elastic deformation to absorb impact energy, so spring steel wire does not necessarily need to have very high plasticity, but it must at least have the plasticity to withstand spring forming, as well as sufficient toughness to withstand impact energy.

　　Springs usually work under alternating stress for a long time, so they must have a high fatigue limit, as well as good creep resistance and relaxation resistance.

　　Springs used in specific environments also have some special requirements for the wire. For example: springs used in corrosive media must have good corrosion resistance. Springs used in precision instruments should have long-term stability and sensitivity, low temperature coefficient, high quality factor, small aftereffect, and constant elastic modulus.

　　In addition, the forming process and heat treatment process of spring steel wire should also be considered. Cold-drawn spring steel wire and oil-quenched and tempered spring steel wire are both used to directly wind springs with the steel wire in the as-supplied state. After the spring is formed, it is directly used after stress relief treatment. The tensile strength of cold-drawn spring steel wire is slightly higher than that of oil-quenched and tempered steel wire. Large-sized cold-drawn steel wire has too much elasticity, and it is difficult to wind springs, so the general size of cold-drawn spring steel wire is less than 8.0 mm, and the general size of oil-quenched and tempered steel wire is less than 13.0 mm. In fact, light-drawn spring steel wire is often used for 13.0 mm diameter springs, which are cold-drawn and formed, then quenched and tempered. Steel wires with a diameter of 15.0 mm or more mostly use heating and winding processes to make springs.

　　Springs can be divided into static springs and dynamic springs according to their operating status. Static springs refer to springs with a limited number of vibrations during their service life, such as safety valve springs, spring washers, scale pan springs, constant load springs, mechanical springs, watch hairsprings, etc. Dynamic springs refer to springs with more than 1×10^6 vibrations during their service life, such as engine valve springs, vehicle suspension springs, shock-absorbing springs, coupling springs, elevator buffer springs, etc. When selecting materials for static springs, the main considerations are tensile strength and stability; when selecting materials for dynamic springs, the main considerations are fatigue, relaxation, and resonance performance.

　　According to the load conditions, springs can be divided into three states: light load, general load, and heavy load. Light load refers to springs that withstand static stress, with relatively low stress and small deformation, such as springs used in safety devices and springs used to absorb vibrations. The design service life is 10^3～10^4 times.

　　General load refers to ordinary springs with a design life of 10^5～10^6 times, used under the condition of a vibration frequency of 300 times/min. Within the allowable stress range, the life is guaranteed to be 1×10^6 times; the lower the load stress, the longer the life.

　　Heavy load refers to springs that work for a long time and vibrate frequently, such as valve springs, air hammers, presses, hydraulic controller springs, etc. Their load is high, and they are often used at about 10% below the allowable stress, with a service life of more than 1×10^6 times, usually 10^7 times.

　　The principle of spring material selection is: first meet the functional requirements, then the strength requirements, and finally consider the economic factors.

　　Carbon spring steel is the most widely used and largest-volume steel type among spring steels. The steel contains 0.60%～0.90% carbon and 0.3%～1.20% manganese, without adding other alloying elements, and the cost of use is relatively low. Carbon spring steel wire, after appropriate processing or heat treatment, can obtain high tensile strength, sufficient toughness, and good fatigue life. However, carbon steel wire has low hardenability, poor relaxation resistance and corrosion resistance, and a large temperature coefficient of elasticity modulus (as high as 300×10-6/℃). It is suitable for manufacturing springs with smaller cross-sections and lower operating temperatures (120℃>).

　　Alloy spring steel generally contains 0.45%～0.70% carbon and a certain amount of Si, Mn, Cr, V, W, and B alloying elements. The addition of alloying elements improves the relaxation resistance of spring steel, increases the toughness of the steel, and significantly improves the hardenability and operating temperature of the steel. It is suitable for manufacturing springs with larger cross-sections and higher operating temperatures.

　　Current national and industry recommended standards for carbon spring steel wire are divided into two types: Chang Le County Shengda Wire Products Co., Ltd. | Shandong Spring Steel Wire | Steel Wire Manufacturer | Shandong Wire Rope | Wire Rope Manufacturer

1. Cold-drawn spring steel wire

　　One type is cold-deformed strengthened wire, also known as cold-drawn spring steel wire. Cold-drawn carbon spring steel wire is first subjected to lead quenching treatment to obtain a sorbite structure, and then phosphated on the surface. It is then drawn to the finished size with a large reduction rate. The steel wire structure is fibrous, with high tensile strength and elastic limit, and good bending and torsion performance. Cold-drawn spring steel wire has high dimensional accuracy, smooth surface, no oxidation and decarburization defects, and relatively stable fatigue life. It is the most widely used spring steel wire.

2. Oil-quenched and tempered wire

　　Another type of carbon spring steel wire is martensite-strengthened wire, also known as oil-quenched and tempered wire. Carbon steel wire, through quenching and tempering treatment, can obtain good comprehensive mechanical properties. When the steel wire specification is smaller (φ≤2.0mm), the various strength indicators of oil-quenched and tempered wire are lower than those of cold-drawn wire after sorbite treatment. When the steel wire specification is larger (φ≥6.0mm), it is impossible to use a large reduction rate to obtain the required strength indicators for sorbitized wire, while oil-quenched and tempered wire can obtain higher performance than cold-drawn wire as long as it is completely quenched. Under the same tensile strength conditions, martensite-strengthened wire has a higher elastic limit than cold-deformed strengthened wire. The microstructure of cold-drawn wire is fibrous, with obvious anisotropy, while the microstructure of oil-quenched and tempered wire is uniform tempered martensite, which is almost isotropic. At the same time, the relaxation resistance of oil-quenched and tempered wire is better than that of cold-drawn wire, and the operating temperature (150～190℃) is also higher than that of cold-drawn wire (≤120℃). Large-specification oil-quenched and tempered wire has a trend of replacing cold-drawn wire.

　　Below, we introduce the application range and process characteristics of various types of carbon spring steel wires according to standards.

(1) YB/T5220-93 "Carbon spring steel wire for non-mechanical springs"

　　This standard is applicable to carbon spring steel wire for non-mechanical springs such as sofa cushion springs, seat cushions, backrest springs, snap rings, and clamping springs. According to different tensile strength requirements, the standard divides the wires into nine groups: A1, A2, A3…A9. Each group of wires is supplied within a strength range regardless of size, and the tensile strength deviation is ≤200Mpa. A1, A2, and A3 groups are used to manufacture springs with lower stress. A3, A4, and A5 are used to manufacture springs with general stress. A7, A8, and A9 are used to manufacture springs with higher stress. Mattress springs generally use A3 and A4 groups.

　　From the analysis of the usage status, the steel wire in this standard basically belongs to static springs, and only the tensile strength, winding, and single-bend performance of the finished steel wire are tested. See Table 3 for details. Table 3 YB/T5220-93 Carbon spring steel wire for non-mechanical springs

Note:

　　① Wires with Φ≤4.0mm are wound two turns on a 2d mandrel without cracks or breakage.

　　② Wires with Φ>4.0mm are subjected to a bending test. The specimen is bent 90o in different directions along an R=10mm arc, and there should be no cracks or breakage at the bending point.

　　Wire ropes in groups A1-A3 are generally made of 45～70 steel, those in groups A3～A6 are made of 65Mn or 70 steel, and those in groups A7～A9 are made of 70 or T8MnA (82B) steel. Since the wire ropes supplied according to this standard are mainly used for making static springs, the requirements for fatigue life are relatively relaxed, and the wire ropes can be directly drawn from controlled rolling and controlled cooling coils. The pre-processing of wire ropes can also be replaced by normalizing instead of lead bath treatment. The use of converter-killed steel as a raw material is also permitted.

(2)GB/T4357-89 "Carbon Steel Spring Wire"

　　This standard is a general standard for cold-drawn carbon steel spring wire, mainly used for making static springs working under various stress conditions. According to the working stress state of the spring, the wire can be supplied in three grades: Grade B is used for low-stress springs, Grade C for medium-stress springs, and Grade D for high-stress springs. The finished wire is tested for four performance indicators: tensile strength, torsion, winding, and bending. The mechanical properties of common specifications are shown in Table 4. Table 4 GB/T4357-89 Carbon Steel Spring Wire

Note:

　　①Grade D wire with Φ≤4.0mm and Grade B and C wire with Φ≤6.0mm are wound 2 turns on a mandrel equal to the wire diameter; the surface of the wound sample must not have cracks or breaks.

　　②Grade D wire with Φ>4.0mm is wound 2 turns on a mandrel of 2 times the wire diameter; the surface of the wound sample must not have cracks or breaks.

　　③Wire with Φ>6.00mm should be subjected to bending test; the sample is bent 90° in different directions along an R=10mm arc; no cracks or breaks should occur after bending.

　　Grade B and C wires are generally made of 70 (67A, 72A) or 65Mn (67B), and Grade D is made of T9XtA and T8MnA (82B).

　　The wire in this standard is mainly used for making static mechanical springs. Its service life vibration frequency is higher than that of non-mechanical springs, and it has certain requirements for fatigue life. The finished product also increases the test of torsional performance. Therefore, the coil for wire should be smelted by electric furnace or electric furnace + out-of-furnace refining method, with P≤0.030% and S≤0.020% in the coil. If the finished wire contains free ferrite in the microstructure, it will reduce the fatigue life of the spring. Generally, there is 5%～1.5% free ferrite in the controlled rolling and controlled cooling coils, so it is generally not suitable to use the coil to directly produce the finished wire. The wire supplied according to this standard should be lead-bath treated before forming in principle to eliminate the free ferrite microstructure. The microstructure of the finished wire should be fibrillar sorbite microstructure.

(3)GB/T4358-1995 "Carbon Steel Spring Wire for Important Applications"

　　The wire supplied according to this standard is mainly used for making dynamic springs working under various stress conditions. According to the working stress state of the spring, the wire is supplied in three groups: Group E is suitable for medium-stress dynamic springs, Group F is suitable for high-stress dynamic springs, and Group G is suitable for dynamic springs with high fatigue life. The finished wire is tested for five performance indicators: tensile strength, torsion, winding, bending, and decarburization. The mechanical properties of common specifications are shown in Table 5. Table 5 GB/T4358-1995 Carbon Steel Spring Wire for Important Applications

Note:

　　①Wire with Φ<4.0mm is wound 5 turns on a mandrel equal to the wire diameter, and wire with Φ≥4.0mm is wound 5 turns on a mandrel of twice the wire diameter; there should be no cracks or breaks.

　　②Bending test is conducted on wires with Φ>1.0mm; the sample is bent 90° in different directions along the R arc; there should be no cracks or breaks at the bending point. For Φ≤4.0mm, R=5mm; for Φ>4.0mm, R=10mm.

　　③The decarburization layer of Group G wire ≤1.0d%.

　　Since the wire supplied according to this standard is used for making dynamic springs working under medium and high stress conditions, the finished wire must not only maintain a high elastic limit and good toughness indicators, but also consider the fatigue limit and fatigue life of the spring. Therefore, higher requirements are put forward for the purity of the steel, the content of non-metallic inclusions and gases, the content of ferrite, and the degree of surface decarburization. The coil for wire must be smelted by electric furnace + out-of-furnace refining method, and higher requirements are put forward for the chemical composition of the coil: P≤0.025%, S≤0.020%, Cr ≤0.10%, Ni≤0.15% (0.12%), Cu≤0.20%. In actual production, in order to improve fatigue life, Mn is often controlled at the upper limit. Group E uses 70 or 70Mn (72B), Group F uses T8MnA or T9RtA, and Group G uses 65Mn (Mn can be adjusted to 0.9-1.2%) or 67B. The purpose of reducing the content of P and S in steel, increasing the content of Mn, and using out-of-furnace refining is to reduce the content of non-metallic inclusions in steel, improve the morphology of inclusions, reduce the gas content, and improve the fatigue limit and fatigue life. If the microstructure of the wire contains free ferrite, it will significantly reduce the fatigue life. The wire supplied according to this standard must be lead quenched before forming.

　　Group G wire is used to make valve springs working under severe vibration conditions, and the requirement for fatigue life is extremely high, so 65Mn coil with better toughness is used. Although the tensile strength is somewhat reduced, the fatigue life is more guaranteed. The decarburization of the wire surface forms a ferrite microstructure, which seriously affects the fatigue performance. The standard increases the decarburization layer test for Group G wire, stipulating that the total decarburization layer depth must not be greater than 1.0%d. However, for larger specifications of wire (Φ>4.0mm), due to the limitation of the reduction rate, the decarburization layer brought by hot-rolled coils is difficult to completely eliminate. The standard also supplements the provision: "With the consent of the purchaser, wire with a decarburization layer not exceeding 1.5%d of the diameter can be supplied."

(4)GJB1497-92 "Specification for Carbon Steel Spring Wire for Special Purposes"

　　In some specific occasions, springs need to have elasticity beyond conventional requirements, such as firearm springs. For easy carrying and use, the space occupied by the spring is very small, and the elasticity must be large enough. Wire with extremely high tensile strength and good toughness must be used to make the spring. The "Carbon Steel Spring Wire for Special Purposes" standard meets this need.

　　This standard specifies that steel wire is supplied in three groups: A, B, and C. Group C is suitable for high-stress springs, Group B for high-stress springs, and Group A for ultra-high-stress springs. The standard recommends using T9A, T10A, and T8MnA steel wire. In practice, the tensile strength of the steel wire increases with increasing carbon content, while the torsional properties (toughness index) decrease with increasing carbon content. Considering these factors comprehensively, Dalian Sanda Steel Wire Company of Liaoning Special Steel Group uses rare-earth-added T9A steel wire to produce Groups A, B, and C steel wires. The finished steel wire is assessed for six performance indicators: tensile strength, torsion, torsion fracture, winding, tensile strength uniformity, and dimensional uniformity. The mechanical properties of common specifications are shown in Table 6.

　　It should be pointed out that the ultra-high tensile strength of the steel wire is achieved at the cost of some plasticity and fatigue life, and is only suitable for making springs with simple shapes and not very high fatigue life requirements. This standard is not recommended for general use. Table 6 GJB 1497-92 Special Purpose Carbon Spring Steel Wire Specification

Note:

　　① The difference in tensile strength between the two ends of each coil of steel wire shall not exceed 100 MPa.

　　② When the steel wire is twisted, no visible cracks or delamination shall occur within the specified number of twists.

　　③ During the winding test, the steel wire shall not break or crack after winding 1 to 5 turns around the mandrel. The mandrel diameter is equal to the wire diameter.

(5) YB/T5103-93 Oil-quenched and tempered carbon spring steel wire

　　For oil-quenched and tempered steel wire, the wire is first produced to the finished size and then oil-quenched and tempered. It is delivered with a tempered martensitic structure. Structural uniformity is an important indicator of the performance of oil-quenched and tempered steel wire. Due to the limited hardenability of carbon spring steel wire, for larger specifications, the core of the wire cannot be completely transformed into martensite after oil quenching and tempering. Therefore, the standard stipulates that the diameter of oil-quenched and tempered carbon spring steel wire is less than 12.0 mm.

　　Compared with cold-drawn carbon spring steel wire, the tensile strength and elastic limit of oil-quenched and tempered steel wire with a diameter ≤2.0 mm are lower than that of cold-drawn wire. However, for large-size steel wire (Φ≥6.0 mm), after sorbite treatment, it is impossible to obtain high tensile strength by large-reduction drawing. However, as long as the oil-quenched and tempered steel wire is completely quenched, it can obtain higher tensile strength than cold-drawn steel wire. Even under the same tensile strength conditions, the elastic limit of oil-quenched and tempered steel wire is higher than that of cold-drawn steel wire.

Cold-drawn spring steel wire has significant anisotropy, while oil-quenched and tempered steel wire is almost isotropic. The fatigue life and stress relaxation resistance of oil-quenched and tempered steel wire are much better than cold-drawn steel wire, the working temperature is higher (150～170℃), and the creep resistance is better. In addition, the oil-quenched and tempered steel wire has good straightness, and the wire straightens after the coil is opened, with almost no bending, and has better forming performance when winding springs. Therefore, in developed countries, large-size oil-quenched and tempered carbon spring steel wire has almost replaced cold-drawn carbon spring steel wire.

(6) YB/T5102-93 "Oil-quenched and tempered carbon spring steel wire for valves"

　　The relationship between YB/T5102-93 and YB/T5103-93 is similar to that between GB/T4358-1995 and GB/T4357-89. The former is used for making dynamic springs, while the latter is a general standard, mainly used for making static springs. Since dynamic oil-quenched and tempered springs have stricter requirements for organizational uniformity, and the hardenability of carbon steel is limited, YB/T5102 stipulates that the supplied specifications are smaller (diameter ≤6.0 mm).

　　The application range and quality control requirements of oil-quenched and tempered carbon steel wire for valves are basically the same as those of Group G in GB/T4358-1995, and will not be elaborated here.

Alloy Steel Wire

　　Alloy spring steel wires in China are divided into three categories: alloy spring steel wire, oil-quenched and tempered silicon-manganese spring steel wire, and oil-quenched and tempered chromium-silicon alloy spring steel wire for valves. From the current situation of alloy spring use in developed countries, the proportion of oil-quenched and tempered steel wire is increasing. The Japanese Industrial Standard (JIS) stipulates that all alloy spring steel wires are supplied in the oil-quenched and tempered state.

(1) GB/T5218-1999 "Alloy spring steel wire"

　　This standard merges the original three standards GB5218-85 "Silicon-manganese spring steel wire", GB5219-85 "Chromium-vanadium spring steel wire", and GB5221-85 "Chromium-silicon spring steel wire" into one standard.

　　This standard is applicable to alloy spring steel wire used for making high and medium stress springs. The steel wire needs to be quenched and tempered after spring coiling before use.

　　The standard lists three steel grades: 60Si2MnA, 50CrVA, and 55CrSiA. It also stipulates that other grades of steel wire can be supplied according to the requirements of the purchaser.

　　The steel wire is generally delivered in the cold-drawn state. For finished diameters ≤5.0 mm, the tensile strength σb ≤1035 MPa is inspected, and for diameters >5.0 mm, the HB ≤302 is inspected, which is equivalent to the lightly drawn state. In production, the reduction rate of large-specification (Ф>8.0) finished cold-drawn products does not exceed 25%, and the reduction rate of small-specification steel wire (Ф≤5.0) does not exceed 30%. The annealed steel wire is too soft and is prone to dead bending, often resulting in poor spring shape and uneven pitch during coiling. When the cold-drawing reduction rate is too large, the steel wire is too hard, and the rebound is large during coiling, making it difficult to form thick springs. Therefore, steel wire for direct coiling is most suitable for delivery in a lightly drawn state. For users who need to perform cold processing (such as flattening, rolling into special-shaped sections, etc.) after purchasing the steel wire, annealed steel wire is recommended.

　　To ensure coiling performance, the finished steel wire is subjected to a winding test. Steel wire with a diameter ≤5.0 mm is wound 6 turns on a mandrel with a diameter equivalent to 1 to 2 times the wire diameter, and shall not crack or break.

　　Decarburization and severe surface defects significantly reduce spring fatigue life and must be strictly controlled. Especially for 60Si2MnA and 55CrSi, which have higher silicon content, decarburization is very easy during annealing. For wire rod spheroidizing annealing and semi-finished product recrystallization annealing, low-temperature, long-time annealing processes are recommended. As mentioned earlier, springs with high fatigue life requirements must use polished steel wire, but it is necessary to distinguish between annealed polishing and cold-drawn polishing. When polishing 50CrV soft steel wire, the grinding dust is easily attached to the surface of the wire, forming irregular white spots, so cold-drawn polishing should be used as much as possible to reduce surface "white spots".

　　Under the condition of surface quality assurance, the inclusion content becomes the most important factor affecting fatigue life. Therefore, the standard stipulates that if the purchaser has requirements, non-metallic inclusions and graphite carbon inspection can be added.

(2) YB/T5104-93 "Oil Quenched and Tempered Silicon Manganese Alloy Spring Steel Wire"

　　After using oil-quenched and tempered alloy spring steel wire to make springs, it is not necessary to perform quenching and tempering treatment again. Just like cold-drawn carbon spring steel wire, stress relief treatment can be used, which is very popular. Alloy springs with a diameter of less than 14.0 mm are increasingly using these two standards.

　　YB/T5104-93 supplies 3 categories. Class A is suitable for static springs with medium loads; Class B is suitable for dynamic springs with medium loads and medium fatigue life; Class C is suitable for dynamic springs under higher stress states, such as automotive suspension springs.

(3) YB/T5105-93 "Oil Quenched and Tempered Chromium Silicon Alloy Spring Steel Wire for Valves"

　　YB/T5105-93 is suitable for valve springs of engines that withstand severe dynamic loads.