Every day, thousands upon thousands of screws and bolts are used to fasten components together. That may sound routine initially. However, specialists also take a closer look. Because if bolts and screws are too loose, components may move. If they are fastened too tightly, components and the fasteners themselves may be damaged.
The objective is to fix components to each other in a way that enables them to fulfil their function. When fastening, the torque must be correct in order to achieve the required clamp force – the force that a fastener uses to hold two components together. This is not always easy. In practice, tightening instructions often specify fitting fasteners with a specific torque. However, the resultant clamp force is dependent on several factors. These include the geometry of the screw and the friction between fastener and components. Surfaces often have a major influence on this. The friction between roughened surfaces is greater than between smooth ones. If fasteners are lubricated, friction is reduced – this applies both with regard to both external lubrication via grease or wax and integrated lubrication via the addition of a lubricant to the coating. Friction effects such as these between fastener and component are detailed with the total coefficient of friction (µges). This comprises the friction in the thread (µth) as well as that between head and head contacts (µb). A very low total coefficient of friction runs the risk of the fastener working itself loose. If the factor is very high, fasteners need to be tightened with a high torque and the efficiency of the fasteners decreases.
In order to achieve the desired clamp force with a specified torque, the tightening instructions must assume a total coefficient of friction of the fasteners. Industrial companies and associations often specify a coefficient of friction window for this. For example, the German automobile industry association (VDA) recommends using fasteners with a µtot of 0.09 to 0.14. Coefficients of friction cannot be specified using measurement technology. They are identified via torque and clamp force. In this it is also necessary to take into consideration the geometry of the fasteners – more precisely, the increase of the thread, its flank diameter and the diameter of the head contact. The torque and pretension of fasteners and bolts are specified globally by the International Organization for Standardization (ISO) in standard ISO 16047. This standard also specifies how these coefficient of friction levels can be calculated. The title of the standard is EN ISO 16047 Fasteners – Torque/clamp force testing.
- A bolt that requires testing is screwed into the test bench and fixed with a reference bolt. The bolt heads lie on a reference base plate or a reference washer.
- When tightening the bolt the clamp force, the rotation angle, the total torque and one of the two partial torques are measured. On some test benches the partial torque is measured in the thread, in others on the base. The respective complementary partial torque can be identified via subtraction.
- These parameters – clamp force, torque and rotation angle – can be used to calculate the three coefficients of friction µth, µb and µtot.
The ISO standard describes reproducible laboratory tests. These are useful, for example in comparing the coefficients of friction of different coating systems. With these tests it is also possible to check the quality of the coating in ongoing production.
The ISO standard enables different reference base plates or reference washers to be used. These bases may be of low or high hardness, uncoated or galvanised. The option of choosing between uncoated and galvanised bases reflects the prevailing procedures in the American and European areas. Galvanised reference parts do not rust in storage. Uncoated reference parts are oiled to protect against corrosion and later cleaned again. However, galvanised reference parts have their own coefficient of friction. For example, brighteners in the galvanised coat serve to lower the friction between reference part and fastener.
- Coefficients of friction calculated according to ISO standard 16047 need to be regarded with caution, however. As a result, the coefficients of friction calculated from torque and clamp force only really apply to the conditions on the test bench.
- For hard bases the standard prescribes a narrow range of hardness, but permits a broad range of hardness for soft bases. Similarly, varying degrees of roughness are possible for both types.
- The testing of fasteners with captive washers is not described in the standard. Nevertheless, coefficients of friction are determined for these parts. In this process the fasteners are screwed to their own washer. However, if the fastener (and with it the washer) is coated with a lubricant, the lubricated base of the fastener is tightened against the lubricated washer on the test bench. This “double lubrication” results in a lower coefficient of friction than if the fastener is screwed to a standardised reference part without its captive washer.
The problem: in practice there is rarely a differentiation between normal fasteners and those with captive washers. Tightening specifications for normal fasteners are also applied for those with washers. These are then tightened with the same torque, although the coefficient of friction of the fasteners with captive washers is lower. From a standardisation viewpoint, this dilemma is unresolved. In practice, there are two ways in which companies deal with this: they accept that the total coefficient of friction lies at the lower limit of tolerance when using lubricated fasteners with captive washers or they specify separate process parameters for their use.
Note: fasteners with captive washers are used to prevent coated surfaces from being damaged during the screwing/bolting process. They can also be used to cover larger holes. With these fasteners a washer is threaded onto the blank; the subsequent cold forming in which the thread is rolled into the surface ensures that this washer cannot fall off.