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; utilization of the yield point: 90 % (with friction coefficient µtotal = 0.12). As a control measure, always check the tightening torque with a torque wrench.

Property classes ‌according to DIN EN ISO 898

Standard screws

High-strength screws

3.6

4.6

4.8/5.6

6.8

8.8

10.9

12.9

M16

57

76

96

153

206

302

354

M18

80

106

133

212

295

421

492

M20

112

150

187

300

415

592

692

M22

151

202

252

403

567

807

945

M24

193

257

322

515

714

1017

1190

M27

284

379

474

759

1050

1496

1750

M30

386

515

644

1031

1428

2033

2380

M33

523

697

871

1395

1928

2747

3214

Tips

Before screwing larger, longer screws into hard materials, it is advisable to pre-drill a pilot hole with the core diameter of the thread to approx. 2/3 of the screw length.

Note: Ensure that no metal particles enter the power tool.

After working at a low speed for an extended period, you should operate the power tool at the maximum speed for approximately three minutes without load to cool it down.