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 joints
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 with the maximum reachable torque, and the steepness indicates the duration in which this is achieved.

A torque gradient 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

The following application cases result accordingly:

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

For a spring-loaded joint as well as for a soft joint, the maximum tightening torque is lower than for a hard joint. Also, a clearly longer impact duration is required.

Guide values for maximum screw tightening torques
Figures given in Nm; calculated from the tensional cross-section; utilisation 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.