Practical advice

The torque depends on the impact duration. The maximum achieved torque results from the sum of all individual torques achieved through impact. Maximum torque is achieved after an impact duration of 6–10 seconds. After this duration, the tightening torque increases only minimally.
The impact duration is to be determined for each required tightening torque. The tightening torque actually achieved should always be checked with a torque wrench.

Screw applications with hard, spring-loaded or soft seats
When the achieved torques in an impact series are measured during a test and transferred into a diagram, the result is the curve of a torque characteristic. The height of the curve corresponds to the maximum achievable torque, and the steepness indicates the time in which this is achieved.

A torque characteristic depends on the following factors:

• Strength properties of the screws/nuts
• Type of backing (washer, disc spring, seal)
• Strength properties of the material being screwed/bolted together
• Lubrication conditions at the screw/bolt connection

Consequently, the following applies in each case:

• A hard seat is used for metal-to-metal screw applications that use washers. After a relatively short impact duration, the maximum torque is achieved (steep characteristic curve). An unnecessarily long impact duration only causes damage to the machine.
• A spring-loaded seat is also a metal-to-metal screw application but uses spring lock washers, disc springs, studs or screws/nuts with conical seats. It is also called a spring-loaded seat when extensions are used.
• A soft seat is a screw application of e.g. wood on wood or metal on wood or a screw application that uses soft backing such as lead washers or fibre washers.

For a spring-loaded seat, as well as for a soft seat, the maximum tightening torque is lower than for a hard seat. A much longer impact duration is also required.

Guide values for maximum screw tightening torques
Figures given in Nm, calculated from the tensional cross-section; utilization of the yield point 90 % (with friction coefficient µ_total = 0.12). As a control measure, always check the tightening torque with a torque wrench.