3.3.2.7 Screws
The importance of screws in the design of metal-sealed connections, cannot be underestimated. Screws also have application limits before they break. Improperly installed screws are potential sources of risk for leaks, especially with cyclic thermal stresses.
Two important mechanical factors must be considered when mounting screws: tensile strength Rm , which describes from which tensile stress upwards it results in screw breakage, and the yield point Rp 0.2, which indicates from which stress upwards the tension force remains the same for the first time or becomes less, despite increasing elongation of the screw. This represents the transition between the elastic and plastic range. Screws should not be subjected to stresses that are higher than the 0.2 % yield point Rp 0.2. The reference values for the tightening torque of screws are therefore values that take a 90 % utilization of the 0.2 % yield point into account.
The tensile strength and yield strength of steel screws at room temperature can be found in the specification under their strength classes, i. e. a two-digit number combination. The first number 1/100 [N/mm2] indicates the tensile strength. The multiplication of both numbers results in 1/10 [N/mm2] of the yield point. Example: strength class 8.8, Rm = 8 · 100 N/mm2 = 800 N/mm2, Rp 0.2 = 8 · 8 · 10 N/mm2 = 640 N/mm2.
In the designation of stainless steel screws, the material quality and the tensile strength are indicated. These are: A for austenitic,1 to 5 for the alloy type, and the strength class: -70 for strain hardened or -80 for high strength. The strength class equals 1/10 N/mm2 of the tensile strength. Example: specification A2-70, A2 equals austenitic, alloy type 2, 70 equals Rm = 70 · 10 N/mm2 = 700 N/mm2.
The nuts used, should have at least the same strength as the screws. For steel nuts, a number is indicated which equals 1/100 [N/mm2] of the test tension. Example: the number 8 equals Rm = 800 N/mm2. For stainless steel screws, a nut with either the same or a higher material quality and property class must be used. Caution: nuts, with a lower height than 0.8 times the screw diameter (flat design), have a restricted load- bearing capacity.
| Type of screw | 0.2 % yield point Rp 0.2 [N/mm2] |
Tensile strength Rm [N/mm2] |
Material |
|---|---|---|---|
| Stainless steel, A2-70 | 450 | 700 | Stainless steel, 1.4301, 1.4303, 1.4307 |
| Stainless steel, A4-80 | 600 | 800 | Stainless steel, 1.4401 |
| Steel, strength class 8.8 | 640 | 800 | Carbon steel, quenched and tempered |
Table 3.6: Mechanical characteristics and material of screws at room temperature
To determine the tightening torque and the preload stress, it is necessary to know the friction coefficient µtotal of the screw connection. Due to the variety of surfaces and lubrication conditions, it is impossible to provide reliable values. The scattering is too large. For this reason, only scattering ranges for the friction coefficient can be provided. To determine the correct torque, a test under operating conditions is recommended.
The friction coefficients can be reduced by the use of lubricants, however, the large scattering range remains the same. It should be noted that a smaller friction coefficient leads to a lower maximum torque. Hence: Use of lubricants → friction coefficient µtotal drops → less torque is necessary or may be applied.
For stainless steel screws, the friction values in the thread and on the supporting surfaces are substantially greater than with tempered steel screws. The scattering ranges of the friction values is also much larger (up to above 100 %). Due to the high edge pressure, they also tend to get stuck. A lubricant can generally help in this case. Alternatively, silver plated screws or nuts can be used.
| Screw | Nut | µtotal without lubrication |
µtotal with MoS2 paste |
µtotal greased |
|---|---|---|---|---|
| A2 or A4 | A2 or A4 | 0.23 – 0.50 | 0.10 – 0.20 | – |
| Steel, electrolytically galvanized | Steel, electrolytically galvanized | 0.12 – 0.20 | – | 0.10 – 0.18 |
Table 3.7: Friction coefficient for stainless steel and zinc-plated steel screws
Properties of screws at elevated temperatures
When using screws at elevated temperatures, it must be noted that the tensile strength and yield point are reduced. In addition, the creep strain or heat resistance must be considered as a basis for assessing the mechanical strength. Therefore, information on yield strength serves only as a guide. For critical or safety-related applications and other mechanical parameters and all influencing factors must be considered.
| Type of screw | 0.2% yield point Rp 0.2 [N/mm2] at | ||||
|---|---|---|---|---|---|
| 20 °C | 100 °C | 200 °C | 300 °C | 400 °C | |
| Stainless steel, A2-70 | 450 | 380 | 360 | 335 | 315 |
| Stainless steel, A4-80 | 600 | 510 | 480 | 450 | 420 |
| Steel, strength class 8.8 | 640 | 590 | 540 | 480 | - |
Table 3.8: Temperature dependence of the 0.2% yield point for stainless steel and steel screws with diameters ≤ M24
If a through-hole screw connection is tightened by the rotation of the nut, a tensile force is created in the threaded bolt and an equal compressive force between the plates. As a result, the screw is elongated and the component compressed. Through the elongation of the screw, the preload stress is created. The clamping force is produced by the compression of the parts, and without any additional force on the connection is the same size as the preload force.
During tightening of the screw, friction between the contact surfaces is created. With a growing preload force, the friction moments in the thread and the nut contact area increase. The maximum preload force represents the sum of the friction moments the majority of the total tightening torques. With lubricated screws (small friction coefficients) the friction proportion is lower, so that the screws produce a higher preload stress with the same tightening torque. It should be noted that the maximum permissible tightening torque with lubricated screws is lower than with non-lubricated screws.
Preload force and torque cause tensile and torsional stresses in the screw. Both influences must be simultaneously considered during the calculation of the screw load . An alternative for the calculation are tables, as they appear in the VDI guideline 2230. If a 90 % utilization of the 0.2 % yield point is considered acceptable, the maximum permissible tightening torque and the associated preload stresses for the different friction coefficients can be found there. However, the information only represents non-binding values for guidance. For critical or safety-related applications, all influencing factors that may be required for a screws calculation must be considered. Excerpts from the tables for stainless steel and steel screws are listed below.
| Dimension and strength class |
Max. tightening torque [Nm] for µtotal = | Max. preload force [kN] for µtotal = | ||||||||
|---|---|---|---|---|---|---|---|---|---|---|
| 0.10 | 0.14 | 0.20 | 0.30 | 0.40 | 0.10 | 0.14 | 0.20 | 0.30 | 0.40 | |
| M4, A2-70 M4, A4-80 |
1.7 2.3 |
2.2 2.9 |
2.6 3.5 |
3.0 4.1 |
3.3 4.4 |
2.97 3.96 |
2.73 3.64 |
2.40 3.20 |
1.94 2.59 |
1.60 2.13 |
| M5, A2-70 M5, A4-80 |
3.4 4.6 |
4.2 5.6 |
5.1 6.9 |
6.1 8.0 |
6.6 8.8 |
4.85 6.47 |
4.47 5.96 |
3.93 5.24 |
3.19 4.25 |
2.62 3.50 |
| M6, A2-70 M6, A4-80 |
5.9 8.0 |
7.4 9.9 |
8.8 11.8 |
10.4 13.9 |
11.3 15.0 |
6.85 9.13 |
6.31 8.41 |
5.54 7.39 |
4.49 5.98 |
3.70 4.93 |
| M8, A2-70 M8, A4-80 |
14.5 19.3 |
17.8 23.8 |
21.5 28.7 |
25.5 33.9 |
27.6 36.8 |
12.6 16.7 |
11.6 15.4 |
10.2 13.6 |
8.25 11.0 |
6.80 9.10 |
| M10, A2-70 M10, A4-80 |
30.0 39.4 |
36.0 47.8 |
44.0 58.0 |
51.0 69.0 |
56.0 75.0 |
20.0 26.5 |
18.4 24.8 |
16.2 21.7 |
13.1 17.5 |
10.80 14.4 |
| M12, A2-70 M12, A4-80 |
50 67 |
62 82 |
74 100 |
88 117 |
96 128 |
29.1 38.8 |
26.9 35.9 |
23.7 31.5 |
19.2 25.6 |
15.8 21.1 |
| M16, A2-70 M16, A4-80 |
121 161 |
150 198 |
183 245 |
218 291 |
237 316 |
55.0 73.3 |
50.9 67.9 |
44.9 59.8 |
36.4 48.6 |
30.0 40.0 |
All data are approximate values for room temperature – see VDI 2230.
For hexagon screws (ISO 4014 and 4017), hexagon socket screws (ISO 4762) and nuts (ISO 4032) with standard thread at a 90 % utilization of the 0.2 % yield point Rp 0.2.
Table 3.9: Maximum tightening torque and maximum resulting preload force for stainless steel screws
| Dimension and strength class |
Max. tightening torque [Nm] | Max. preload force [kN] for µtotal = |
||||
|---|---|---|---|---|---|---|
| 0.10 | 0.12 | 0.14 | 0.10 | 0.12 | 0.14 | |
| M4, 8.8 | 2.6 | 3.0 | 3.3 | 4.5 | 4.4 | 4.3 |
| M5, 8.8 | 5.2 | 5.9 | 6.5 | 7.4 | 7.2 | 7.0 |
| M6, 8.8 | 9.0 | 10.1 | 11.3 | 10.4 | 10.2 | 9.9 |
| M8, 8.8 | 21.6 | 24.6 | 27.3 | 19.1 | 18.8 | 18.1 |
| M10, 8.8 | 43 | 48 | 54 | 30.3 | 29.6 | 28.8 |
| M12, 8.8 | 73 | 84 | 93 | 44.1 | 43.0 | 41.9 |
| M16, 8.8 | 180 | 206 | 230 | 82.9 | 80.9 | 78.8 |
All data are approximate values for room temperature - see VDI 2230. For hexagon screws (ISO 4014 and 4017), hexagon socket screws (ISO 4762) and nuts (ISO 4032) with standard thread at a 90% utilization of the 0.2% yield point Rp 0.2.
Table 3.10: Maximum tightening torque and maximum resulting preload force for steel screws of strength class 8.8
If screws are screwed into blind holes, an enclosed hollow space is created at the end of the threaded hole. Such dead volume is drained under vacuum only very slowly and leads to prolonged outgassing, which manifests itself in the same way as a leak. It is therefore also called a virtual leak. Under high vacuum and particularly UHV conditions, dead volume of this type should be constructively avoided or if that is not possible, it must be vented.
An alternative are vacuum screws which offer a comfortable means of venting. Their core has drilled holes (degassing bores). The screw head area has also a radial milling groove (degassing reduction), through which the area of the feedthrough hole of the screw connection is vented. The mechanical parameters for screws are not applicable to vacuum screws, as the vent hole leads to mechanical weakening.
