The strength class of a bolt is not an indication of general bolt quality. It is a precise technical specification of two mechanical properties: tensile strength and yield-to-tensile ratio. A bolt marked 8.8 and a bolt marked 10.9 are not variants of the same product with different quality. They are structural elements with fundamentally different load capacity and different requirements for bolt torque at installation. Choosing the wrong strength class in a bolted joint is a specification error, not a purchasing decision. The consequences appear during operation, not at installation.
Strength class reference table: ISO 898-1
|
Strength class |
Min. tensile strength (MPa) |
Min. yield strength (MPa) |
Typical application |
|
4.6 |
400 |
240 |
Light construction, non-critical applications |
|
8.8 |
800 |
640 |
Standard structural and mechanical connections |
|
10.9 |
1000 |
900 |
High-load structural, flanges, compressor housings |
|
12.9 |
1200 |
1080 |
Precision high-load: limited use in corrosive environments |
What do the numbers mean?
The strength class for bolts is defined in ISO 898-1 and is stated as two numbers separated by a period. The first number is one tenth of the nominal tensile strength in MPa. The second number is one tenth of the ratio between yield strength and tensile strength expressed as a percentage. Strength class 8.8 means a nominal tensile strength of 800 MPa and a yield ratio of 80 percent, which gives a yield strength of 640 MPa. Class 10.9 gives 1000 MPa tensile strength and 900 MPa yield strength. These are not gradings of bolt quality from good to better: they are separate specifications for different load conditions.
Bolt quality is always assessed as a combined specification in which bolt, nut and bolt torque are directly interlinked. The bolt that is technically correct for the application is the one that meets the design's clamping force requirement, not the one with the highest numerical value.
Bolt torque and clamping force per strength class
The strength class directly determines the clamping force a bolt can generate at a given bolt torque. An M20 bolt in class 8.8 gives around 125 kN clamping force at 70 percent of the yield strength. The same bolt in class 10.9 gives around 177 kN: a difference of 40 percent. Tightening a 10.9 bolt to the torque value specified for an 8.8 bolt gives insufficient clamping force. Torque values must always be strength-class specific.
The table below gives indicative torque values for dry, uncoated joints (μ ≈ 0.14) at 75 percent of the yield strength as target preload. The values are indicative: the actual torque is calculated for the specific lubricant and surface treatment.
|
Diameter |
Class 8.8 (Nm) |
Class 10.9 (Nm) |
Class 12.9 (Nm) |
|
M8 |
24 |
34 |
40 |
|
M10 |
48 |
67 |
81 |
|
M12 |
83 |
117 |
140 |
|
M16 |
206 |
291 |
350 |
|
M20 |
402 |
565 |
679 |
|
M24 |
696 |
982 |
1180 |
Nuts have their own strength classes
Nuts have their own strength classes defined in ISO 898-2: class 8, class 10 and class 12. The principle is that the nut must not fail before the bolt in the event of over-tightening. A class 8 nut is correct for an 8.8 bolt. A class 10 nut is correct for a 10.9 bolt. A 10.9 bolt combined with a class 8 nut can cause thread stripping at a bolt torque below the bolt's capacity limit. The strength class for bolt and nut must be specified together on every purchase order.
An incorrect combination is not visible at installation, but it weakens the load capacity of the joint and entails a risk of failure under operational load. This is particularly critical in high-load joints and flange systems where bolt torque is controlled against the design specification.
SAE grades and imperial bolts
In Norwegian oil and energy and offshore, imperial bolts occur frequently on older installations and on American-made rotating machinery. SAE J429 applies to SAE imperial bolts. SAE and ISO state yield strength values by different methods: direct numerical comparison is not technically valid. Replacing an SAE Grade 8 bolt with an ISO 10.9 bolt without engineering approval is a specification error.
Marking and identification at goods receipt
ISO 898-1 requires that bolts in strength class 8.8 and above are marked with the strength class number and manufacturer identification directly on the head. A bolt without such marking is by definition not in conformity with the standard. At goods receipt, the strength class marking and manufacturer identification should be checked against the purchase order and the material certificate.
A supplier certified to ISO 9001 must have systems for incoming inspection and traceability that ensure the bolt quality in the delivered batch corresponds to the specified strength class. This is a minimum requirement when selecting a supplier for critical industrial applications.
Why does standard locking fail under vibration?
Clamping force is not automatically maintained over time in joints exposed to vibration. Standard spring washers and nylon lock nuts rely on friction, which is progressively reduced under dynamic load and settling. When the preload falls below the design's minimum requirement, the joint faces begin to move. This movement generates further preload loss and accelerates the failure of the joint.
In critical applications: compressor stations, turbine mountings, flange joints under pressure: preload retention over time is a design requirement, not a bonus. The strength class alone does not guarantee that preload is maintained during operation.
Why are NORD-LOCK washers necessary?
NORD-LOCK wedge-locking washers maintain the clamping force mechanically through the wedge-locking principle, independently of friction in the thread surfaces. Two washers with wedge-shaped undersides clamp so that vibration and dynamic load cannot rotate the bolt. This system is verified for offshore installations, rotating machinery and energy infrastructure where traditional locking methods lose their effect.
NORD-LOCK washers are designed to work across strength classes: 8.8, 10.9 and 12.9, and are available in a wide range of dimensions. INDUSTRISALG carries the full range of NORD-LOCK wedge washers in stock.
CHECKLIST: BEFORE YOU ORDER
- Is the strength class stated explicitly on the purchase order — not just dimension and thread standard?
- Has the nut class been checked against the strength class of the bolt (8.8 → cl. 8, 10.9 → cl. 10, 12.9 → cl. 12)?
- Are the bolt torque tables in the maintenance documentation strength-class specific — not generic?
- Are imperial fasteners (SAE) and metric ISO kept separate in stock and in specification?
- Is the supplier ISO 9001:2015 certified and able to present a 3.1 material certificate per batch?
- Have the strength class marking and manufacturer identification been checked against the certificate at goods receipt?
Conclusion
Strength class is a design parameter, not a purchasing choice. 8.8, 10.9 and 12.9 state tensile strength and yield strength, and directly determine which bolt torque is required and which clamping force the joint can maintain. The strength class of nuts must be matched to the class of the bolt. SAE and ISO are not directly interchangeable without conversion verification. Bolt quality is always assessed as a combined specification in which bolt, nut and bolt torque belong together.
RELATED READING
- How to specify fasteners correctly: size, thread, length, material, coating, standard, strength
- Screw heads and drive types: what withstands industrial torque? With torque table and failure mode analysis
Get in touch
Send us the application description with diameter, strength class and environmental requirements — and we will confirm the correct specification and bolt torque values for your joint. INDUSTRISALG carries 8.8, 10.9 and 12.9 in a wide range of dimensions with 3.1 material certificate and full traceability documentation as standard. We have NORD-LOCK wedge washers in stock in all common dimensions.