What is a flat washer

What is a flat washer?

Flat washers are usually thin parts of various shapes to reduce friction, prevent leakage, isolate, prevent loosening or disperse pressure. Many materials and structures have such components to perform various similar functions. Due to the material and process limitations of threaded fasteners, the bearing surface of fasteners such as bolts is small. Therefore, in order to reduce the compressive stress on the bearing surface and protect the surface of the connected parts, washers are used.

The flat washer is basically made by stamping. The edge of one side has radian and the other side has burr. Although it is not specified which side is up, it is better to make the burr side down in terms of the surface pressure mentioned above. However, during fastening, the burr will scratch the fastening object. If you want to avoid peeling off of the coating, it is best to let the curved side face down.

20210816002114 11014 - What is a flat washer

Material for flat washer

Flat washers are often used to reduce friction, prevent leakage, isolate, prevent loosening or disperse pressure. Many materials and structures have such components to perform various similar functions. Due to the material and process limitations of threaded fasteners, the bearing surface of bolts and other fasteners is small. Therefore, in order to reduce the compressive stress on the bearing surface and protect the surface of the connected parts, the bolts are often equipped with flat washers. Therefore, flat washers are essential auxiliary accessories in bolt fasteners.
The flat washer is also made of different materials according to the pressure limit and service environment. The commonly used flat washers are made of iron, carbon steel, stainless steel, copper, plastic, etc. the dimensions of flat washers are generally divided into standard size, enlarged, thickened, enlarged and thickened, etc.

Flat washers without mechanical property requirements are generally made of corner materials, such as Q235A. When there are hardness requirements but not high, they can be calendered with thicker plates to achieve the required hardness. The spring washer and other anti loosening washers are generally made of 65mn70# or s50cm-s70cm and subject to heat treatment (quenching + tempering).

Classification of plain washers

1. Flat washer
The main function of the flat washer is to distribute the bearing surface pressure, clamping load, bolts or nuts on a large surface area to prevent damage.
Gaskets are mainly manufactured according to standard specifications of various sizes, materials and shapes. The flat washer is a thin, flat circular general-purpose washer with a hole in the middle. The inner and outer shapes are usually circular. Depending on the function of the washer, the hole or outer diameter can be asymmetric. The size of flat washers is represented by the shank diameter of fasteners, where they form part of the assembly.
2. Square washer
Square washers for buildings shall replace circular washers for wood buildings and concrete floor applications; They are used with all fasteners for greater load distribution and bolts through walls or wood.
3. Square hole washer
Used in woodworking and fence industry, combined with wheel bolts and body screws with square section under the bolt head; The washer is located on the square below the bolt head.
4. Conical washer
The conical washer is designed for the passage part, under the nut with conical flange, so that the bolt assembly can fit square when tightened.
5. Spring washer splitlock washer
Coil spring washers provide anti vibration function. They are separated by the ring on a ring and bent into a spiral shape, which makes the washer exert elastic force between the head and bearing surface of the fastener, keep the washer hard against the face and the bolt, tighten the nut or threaded hole, and produce greater friction force and rotation resistance. The spring washer is a left-hand spiral that allows the thread to be tightened clockwise with the right hand. When the left rotation movement is applied, the raised edge bites into the underside of the bolt or nut and the part fixed with the bolt to resist rotation. Spring washers are ineffective for left-hand threads and hardened surfaces and should not be used with the flat washer under the spring washer, because this prevents the spring washer from biting into parts that can resist rotation. Spring washers are produced according to various standards, indicating thickness and section size, usually square or rectangular section. Popular spring washers are single coil, but can also be supplied as double coil washers.
6. Rib contact lock washer
A washer with surface rib serrations on one or two flat surfaces designed to bite the connecting material or bolt or nut to change the electrical contact or locking action.
Any type of material that can be stamped, pressed or cut can be made into flat washers. Gaskets are made of a variety of metallic and non-metallic materials, including low and high carbon steel, stainless steel and steel alloys, copper based materials, beryllium copper, phosphor bronze, titanium, aluminum, fiber, mica, polyester film, rubber, nylon, PTFE and phenolic resin.
Gasket materials are important because certain applications require specific materials to ensure that the gasket can withstand environmental conditions. For example, if the washer experiences cyclic loading, strength is an important design criterion, and in the case of spring and disc washers, high alloy steel or spring steel needs to be used. When the gasket bears a light load or a soft surface that may be damaged by the metal gasket, use non-metallic materials, etc.

Standard for flat washers

Washers provide a smooth bearing surface and are used under the head of a bolt and/or a nut.

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Thread Inside Diameter Outside Diameter Thickness Standards
M1 1.1/1.25 2.3/2.5 0.2/0.4 DIN433 BS4320A
M1.2 1.3/1.45 2.8/3 0.2/0.4 DIN433 BS4320A
M1.4 1.5/1.65 2.8/3 0.2/0.4 DIN433 BS4320A
M1.6 1.7/1.85 3.2/3.5 0.25/0.4 DIN433 ISO7092
1.7/1.85 3.7/4 0.25/0.4 DIN125A ISO7089
        BS4320A
1.8/2.05 3.25/4 0.2/0.4 ISO7091 ISO7092
M2 2.2/2.35 4.2/4.5 0.25/4 DIN433
2.2/2.35 4.7/5 0.2/0.4 DIN125A ISO7089
        BS4320A
2.4/2.6 3.25/4 0.2/0.4 ISO7091  
M2.5 2.7/2.85 4.7/5 0.45/0.55 DIN433 ISO7092
2.7/2.85 5.7/6 0.45/0.55 DIN125A ISO7089
2.9/3.2 5.3/6 0.4/0.6 ISO7091  
2.7/2.85 6.2/6.5 0.4/0.6 BS4320A  
2.7/2.85 7.7/8 0.7/0.9 DIN9021  
M3 3.2/3.4 5.7/6 0.45/0.55 DIN433 SO7092
3.2/3.4 6.7/7 0.45/0.55 DIN125A ISO7089
3.4/3.7 6/7 0.4/0.6 ISO7091  
3.2/3.4 6.7/7 0.4/0.6 BS4320A  
3/3.2 8.7/9 0.7/0.9 DIN9021 ISO7093
M3.5 3.7/3.9 6.7/7 0.45/0.55 DIN433 ISO7092
3.7/3.9 6.7/7 0.4/0.6 BS4320A  
3.7/3.9 7.7/8 0.45/0.55 DIN125A ISO7089
3.7/3.9 10.6/11 0.7/0.9 DIN9021 ISO7093
M4 3.7/3.9 6.7/7 0.45/0.55 DIN433 ISO7092
3.7/3.9 6.7/7 0.4/0.6 BS4320A  
3.7/3.9 7.7/8 0.45/0.55 DIN125A ISO7089
3.7/3.9 10.6/11 0.7/0.9 DIN9021 ISO7093
M5 5.3/5.5 8.7/9 0.9/1.1 DIN433 ISO7092
5.3/5.5 9.7/10 0.7/0.9 BS4320B  
5.3/5.5 9.7/10 0.9/1.1 DIN125A ISO7089
      BS4320A  
5.3/5.5 12/12.5 0.9/1.1 BS4320C  
5.3/5.5 14.6/15 1/1.4 DIN9021 ISO7093
5.5/5.8 9/10 0.8/1.2 DIN126 ISO7091
M6 6.4/6.6 10.6/11 1.4/1.8 DIN433 ISO7092
6.4/6.6 11.6/12 1.4/1.8 DIN125A ISO7089
6.4/6.7 12/12.5 0.7/0.9 BS4320B  
6.4/6.7 12/12.5 1.4/1.8 BS4320A  
6.4/6.4/ 13.6/14 0.7/0.9 BS4320D  
6.4/6.7 13.6/14 1.4/1.8 BS4320C  
6.4/6.7 17.6/18 1.4/1.8 DIN9021 ISO7093
6.6/7 11/12    1.3/1.9 DIN126 ISO7091
M8 8.4/8.6 14.6/15 1.4/1.8 DIN433 ISO7092
8.4/8.6 15.6/16 1.4/1.8 DIN125A ISO7089
8.4/8.7 16.6/17 0.9/1.1 BS4320B  
8.4/8.7 16.6/17 1.4/1.8 BS4320A  
8.4/8.7 20.5/21 0.9/1.1 BS4320D  
8.4/8.7 20.5/21 1.4/1.8 BS4320C  
M10 10.5/10.8 17.6/18 1.4/1.8 DIN433 ISO7092
10.5/10.8 19.5/20 1.8/2.2 DIN125A ISO7089
10.5/10.9 20.5/21 1/1.45 BS4320B  
10.5/10.9 20.5/21 1.8/2.2 BS4320A  
10.5/10.9 23.5/24 1/1.45 BS4320D  
10.5/10.9 23.5/24 1.8/2.2 BS4320C  
10.5/10.8 29.5/30 2.3/2.7 DIN9021 ISO7093
11/11.5 18.8/20 1.7/2.3 DIN9021 ISO7091
M12 13/13.3 19.5/20 1.8/2.2 DIN433 ISO7092
13/13.4 23.5/24 1.4/1.8 BS4320B  
13/13.4 23.5/24 2.3/2.7 BS4320A DIN125A
        ISO7089
13/13.4 27.5/28 1.4/1.8 BS4320D  
13/13.4 27.5/28 2.3/2.7 BS4320C  
13/13.3 36.4/37 2.3/2.7 DIN9021 ISO7093
13.5/14 22.8/24 2.2/2.8 DIN126 ISO7091
M16 17/17.3 27.5/28 2.3/2.7 DIN433 ISO7092
17/17.3 29.5/30 2.7/3 DIN125A ISO7089
17/17.4 29.5/30 1.8/2.2 BS4320B  
17/17.4 29.5/30 2.7/3.3 BS4320A  
17/17.4 33/34 1.8/2.2 BS4320D  
17/17.4 33/34 2.7/3.3 BS4320C  
17/17.3 49.4/50 2.7/3.3 DIN9021 ISO7093
17.5/18 28.8/30 2.4/3.6 DIN126  
M20 21/21.5 33.4/34 2.7/3.3 DIN433 ISO7092
21/21.5 36/37 1.8/2.2 BS4320B ISO7091
21/21.5 36.5/37 2.7/3.3 DIN125A ISO7089
21/21.5 36/37 2.7/3.3 BS4320A  
21/21.5 38/39 1.8/2.2 BS4320D  
21/21.5 38/39 2.7/3.3 BS4320C  
22/22.5 35.5/37 2.4/3.6 DIN126 ISO7091
20/20.6 54/56 3.4/4.6 DIN9021 ISO7093

From trfastenings.com

Note: due to the great differences in the application of fasteners, the above information is only for reference and is correct to the best of our knowledge. Customers must be satisfied with the performance of fasteners and the effectiveness of data.

DIN125 flat pad standard

DIN125 flat washer refers to the German standard DIN125-1990, which specifies the non chamfered flat washer with product grade A, thread major diameter of 1.6 ~ 36mm and hardness grades of 140hv and 200hv.
DIN125 flat pad drawing:

20210816003752 17155 - What is a flat washer
Flat pad drawing
German standard DIN125 flat pad specification and standard parameter table:

Thread inner diameter Internal diameter d1 Outer diameter d2 Thickness h
min max max min Nominal max min
1.6 1.7 1.84 4 3.7 0.3 0.35 0.25
2 2.2 2.34 5 4.7 0.3 0.35 0.25
2.5 2.7 2.84 6 5.7 0.5 0.55 0.45
3 3.2 3.38 7 6.64 0.5 0.55 0.45
4 4.3 4.48 9 8.64 0.8 0.9 0.7
5 5.3 5.48 10 9.64 1 1.1 0.9
6 6.4 6.62 12 11.57 1.6 1.8 1.4
8 8.4 8.62 16 15.57 1.6 1.8 1.4
10 10.5 10.77 20 19.48 2 2.2 1.8
12 13 13.27 24 23.48 2.5 2.7 2.3
14 15 15.27 28 27.48 2.5 2.7 2.3
16 17 17.27 30 29.48 3 3.3 2.7
18 19 19.33 34 33.38 3 3.3 2.7
20 21 21.33 37 36.38 3 3.3 2.7
22 23 23.33 39 38.38 3 3.3 2.7
24 25 25.3 44 43.38 4 4.3 3.7
27 28 28.33 50 49.38 4 4.3 3.7
30 31 31.39 56 55.26 4 4.3 3.7
33 34 34.62 60 58.8 5 5.6 4.4
36 37 37.62 66 64.8 5 5.6 4.4

Washers with hardness class 200hv are suitable for:

  • Hexagon head bolts and screws with performance grade up to 8.8 and product grade A and B.
  • Hexagon nuts with performance grade to grade 8 and product grade A and B.
  • Hexagon head bolts, screws and nuts of stainless steel and similar chemical composition.
  • Surface hardened self extrusion screws.

DIN125 flat pad material and performance
Class a flat pad DIN125 is divided into carbon steel flat pad and stainless steel flat pad according to material.
The carbon steel flat pad is divided into 140hv and 200hv according to the hardness grade. The hardness range of 140hv is 140hv-250hv, and the hardness range of 200hv is 200hv-250hv.
08F, 08Al, 10, 10F and other materials can be selected for hardness grade 140hv, and the surface hardness can reach about 140hv, while 45, 50, 60, 70 steel or 65Mn steel shall be selected for hardness grade 200hv, which can meet the technical requirements of 200hv-300hv through heat treatment, quenching and tempering.
The hardness grade of stainless steel flat pad is 200hv, and the hardness range is 200hv-250hv.
Optional materials for stainless steel flat pad: A2, A4, F1, C1 and C4.
The chemical composition of each grade of stainless steel is as follows:

Type Group Chemical composition /%
C Si Mn P S Cr Mo Ni Cu
Austenite A2 0.1 1 2 0.05 0.03 15—20 8—19 4
A4 0.08 1 2 0.045 0.03 16—18.5 2—3 10—15 1
Martensite C1 0.09—0.15 1 1 0.05 0.03 11.5—14 1
C4 0.08—0.15 1 1.5 0.06 0.15—0.35 12—14 0.6 1
Ferrite F1 0.12 1 1 0.04 0.03 15—18 1

DIN125 carbon steel flat pad generally needs surface treatment. If the flat pad is not subject to surface treatment, it shall be of natural color and coated with antirust oil or other coatings.
For Quenched and Tempered Flat Washers, appropriate coating or plating process shall be adopted to avoid hydrogen embrittlement. When plating or phosphating flat washers, appropriate treatment shall be carried out immediately after plating or coating to eliminate harmful hydrogen embrittlement.
DIN125 flat pad is not allowed to have irregular or harmful defects, and there shall be no protruding burrs on the surface. All tolerances apply to dimensions before coating or plating.
Stainless steel flat pads are usually natural color and do not need surface treatment.
DIN125 flat pad marking method
Marking of flat washer with standard series, nominal specification of 10mm, hardness grade of 140hv made of carbon steel, without surface treatment and product grade of a:

  • Flat gasket DIN125-10-140HV

The hardness grade of standard series, nominal specification 10mm and A2 stainless steel is 200hv. The marking of flat washer without surface treatment and product grade A:

  • Flat gasket DIN125-10-200HV-A2

Application of DIN125 flat pad
DIN125 flat pad is basically the same as the national standard flat pad GB97. The function of the flat pad is to increase the contact or bearing area, disperse and reduce the compressive stress between the nut and the fastened parts, prevent local compression deformation due to the tightening force, and generally enhance the tightening effect.
Generally, the flat pad can be used only when the load is relatively small and does not bear the vibration load. In case of large load and bearing vibration load, flat pad and elastic pad must be combined.
When using the flat pad, it shall not exceed the design provisions, so as to avoid the loss of resilience due to excessive compression of the flat pad. If the flat pad is damaged during use, it must be replaced with a new one to prevent equipment problems.

GB97 standard for flat pad

The national standard GB/T 97.1-2002 for flat washers specifies flat washers with product grade A, standard series, nominal specification (major diameter of thread) of 1.6 ~ 64mm and hardness grades of 200hv and 300hv.

GB97 flat pad drawing:

20210816003752 17155 - What is a flat washer

Flat pad drawing

GB97 flat pad specification and standard parameter table:

Thread inner diameter Internal diameter d1 Outer diameter d2 Thickness h
min max max min Nominal max min
1.6 1.7 1.84 4 3.7 0.3 0.35 0.25
2 2.2 2.34 5 4.7 0.3 0.35 0.25
2.5 2.7 2.84 6 5.7 0.5 0.55 0.45
3 3.2 3.38 7 6.64 0.5 0.55 0.45
4 4.3 4.48 9 8.64 0.8 0.9 0.7
5 5.3 5.48 10 9.64 1 1.1 0.9
6 6.4 6.62 12 11.57 1.6 1.8 1.4
8 8.4 8.62 16 15.57 1.6 1.8 1.4
10 10.5 10.77 20 19.48 2 2.2 1.8
12 13 13.27 24 23.48 2.5 2.7 2.3
14 15 15.27 28 27.48 2.5 2.7 2.3
16 17 17.27 30 29.48 3 3.3 2.7
18 19 19.33 34 33.38 3 3.3 2.7
20 21 21.33 37 36.38 3 3.3 2.7
22 23 23.33 39 38.38 3 3.3 2.7
24 25 25.3 44 43.38 4 4.3 3.7
27 28 28.33 50 49.38 4 4.3 3.7
30 31 31.39 56 55.26 4 4.3 3.7
33 34 34.62 60 58.8 5 5.6 4.4
36 37 37.62 66 64.8 5 5.6 4.4
39 42 42.62 72 70.8 6 6.6 5.4
42 45 45.62 78 76.8 8 9 7
45 48 48.62 85 83.6 8 9 7
48 52 52.74 92 90.6 8 9 7
52 56 56.74 98 96.6 8 9 7
56 62 62.74 105 103.6 10 11 9
60 66 66.74 110 108.6 10 11 9
64 70 70.74 115 113.6 10 11 9

Washers with hardness class 200hv are suitable for:

  • Hexagon head bolts and screws with performance grade up to 8.8 and product grade A and B.
  • Hexagon nuts with performance grade to grade 8 and product grade A and B.
  • Hexagon head bolts, screws and nuts of stainless steel and similar chemical composition.
  • Surface hardened self extrusion screws.

Washers with hardness class 300hv are suitable for:

  • Hexagon head bolts and screws with performance grade up to 10.9 and product grade A and B.
  • Hexagon nuts with performance grade up to 10 and product grade A and B.

GB97 flat cushion material and performance
The flat pad is divided into carbon steel flat pad and stainless steel flat pad according to the material.
The carbon steel flat gasket is divided into 200hv and 300hv according to the performance grade. 45, 50, 60, 70 steel or 65Mn steel shall be selected for 200hv and 300hv, which can meet the technical requirements of 200 ~ 300hv and 300 ~ 400hv through heat treatment, quenching and tempering.
The performance grade of stainless steel flat pad is A2 and A4, and SUS304 stainless steel and SUS316 stainless steel can be selected.

The technical conditions and reference standards of GB97 flat pad are shown in the table below:

Material Type Carbon steel Stainless steel
Group A2、A4、C1、F1、C4
Standard GB/T3098.6-2000
Mechanical properties Hardness grade 200HV 300HV 200HV
Hardness range 200HV-300HV 300HV-370HV 200HV-300HV
Tolerance Product grade A
Standard GB/T 3103.3-2000
Surface treatment Without surface treatment, i.e. the gasket shall be of natural color and coated with antirust oil or coating agreed by the supplier and the buyer. Without surface treatment, i.e. the washer shall be natural color.
The technical requirements for electroplating shall be in accordance with GB/T 5267.1-2002.
The technical requirements for non electrolytic zinc sheet coating shall be in accordance with GB/T 5267.2-2002.
For hardened and tempered gaskets, appropriate coating or plating process shall be adopted to avoid hydrogen embrittlement. When plating or phosphating gaskets, appropriate treatment shall be carried out immediately after plating or coating to eliminate harmful hydrogen embrittlement.
All tolerances apply to dimensions before coating or plating.
Surface defect Irregular or harmful defects are not allowed on parts. The gasket surface shall be free of protruding burrs.
Acceptance and packaging GB/T 90.1-2002、GB/T90.2-2002

The hardness test shall be in accordance with the provisions of GB/T 4340.1-2009.

Test force: HV2 for nominal thickness h ≤ 0.6mm, hv10 for nominal thickness 0.6mm<h≤ 1.2mm, and HV30 for nominal thickness h>1.2mm.
GB97 marking method of flat pad
Marking of plain washers with standard series, nominal specification of 8mm, hardness grade of 200hv, hot galvanizing and product grade of a made of carbon steel:

  • Flat pad GB97-8 galvanized

The hardness grade of standard series, nominal specification 8mm and A2 stainless steel is 200hv. The marking of flat washer without surface treatment and product grade A:

  • SUS304 flat gasket GB97-8

Application of GB97 flat pad
GB97 flat pad can increase the contact area between screw and machine. Limited by the material and process of threaded fasteners, the bearing surface of bolts and other fasteners is small. Using it with flat pad can reduce the compressive stress on the bearing surface and protect the surface of the connected parts. If the tightening axial force of some parts is large, it is easy to press the washer into a disc shape. In this case, it can be solved by using materials and improving hardness.
The flat pad is used with the spring pad to eliminate the damage of the spring pad to the machine surface when removing the screw. When in use, it must be a spring pad and a flat pad. The flat pad is next to the machine surface, and the spring pad is between the flat pad and the nut.
20210816011554 37839 - What is a flat washer

How to choose the flat washer?

Then the following aspects can be used as a reference for selecting flat washers.
The inner diameter of the flat washer shall be selected according to the larger diameter of the thread or screw. The connected material shall be soft (such as composite material) or matched with the spring washer, and the larger outer diameter shall be selected.
Steel washers shall be used for important bolts with large diameter or to increase the extrusion resistance. Steel washer shall be used for tension bolt or tension shear composite bolt connection.
When selecting flat washer materials, attention should be paid to the electrochemical corrosion of different metals. The material of the flat washer is generally the same as that of the connected parts, usually steel, alloy steel, stainless steel, aluminum alloy, etc. Copper and copper alloy can be selected when there are conductive requirements.

Source: Network Arrangement – China Flat Washer Manufacturer – Yaang Pipe Industry Co., Limited (www.pipelinedubai.com)

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

If you want to have more information about the article or you want to share your opinion with us, contact us at sales@pipelinedubai.com

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