Air Screw Driver Choice

How does a screw connection work?

A screw connection is a releasable connection between two parts that behave like one part under load. The following graphic illustrates schematically the forces that act on the connection when a screw is tightened. The applied tensile force leads to an elongation of the screw; this tensile force is achieved by the torque and is given in [Nm]. The effort to reset this elongation causes the assembly clamping force, and this acts as a resistance to shear forces (transverse load). These forces are measured in N, the tightening torque is given in Nm.

The graph shows how the bolt is stretched by tightening. This elongation is limited by the so-called point of plastic deformation of the bolt material - this is the point at which a bolt shank permanently deforms when unloaded and no longer allows any recovery. As a rule, the permissible strain only occurs in the elastic strain range to ensure recovery. For this purpose, bolts in the mechanical engineering sector are marked with strength classes from which the permissible torque ranges can be derived. The maximum values should be strictly adhered to.

The situation is different in applications where the strength is specified by the material to be screwed, e.g. when screwing plastic parts or wood. Here, tightening is only allowed up to the deformation limit of the material. These different applications make it necessary to choose different coupling systems. An exception here are controlled screwdriving systems (EC screwdriving systems), which will not be discussed further here.

Which coupling is the right one?

Screwdriver with shut-off clutch

With this type of coupling, the tightening torque increases very steeply (see diagram) and then drops off abruptly when the preset cut-off torque MN is reached. The cut-off torque is set by a preloaded spring (red arrow). The clutch claw (green arrow) slips over the clutch claw correspondingly lighter (low tightening torque) or heavier (high tightening torque). The drive definitely switches off at this moment. For electric screwdrivers, the power supply is interrupted.

Advantages:

• Energy-saving, as switch-off is ensured when the switch-off torque is reached.
• High switch-off accuracy, approx. ±5%.
• High repeatability, thus high process reliability

Important: Fluctuations in the compressed air supply or voltage fluctuations can lead to malfunctions and thus make unsafe production processes possible. Screwdrivers with this special mechanics have their price.

Screwdriver with slip clutch

With this clutch, the power supply is not switched off when the preset torque MN is reached. The upper clutch claw slips over the lower one and the tool continues to rotate, this leads to a certain impact effect. This allows the torque to be increased until a constant torque is achieved. This is also reflected in the following graph.

Advantages:

•  The impacting torque when the cut-off torque is reached is advantageous, especially in soft bolting applications
• Good for use when screwing self-tapping screws (sheet metal screws): A higher torque is required for cutting than for the subsequent tightening torque.
• Cut-off accuracy approx. ±10 - Good repeatability (process reliability)
• Inexpensive screwdriver solution

Important: Due to the higher stress on the clutch, wear and, if necessary, maintenance must be taken into account here. More compressed air is required than with the shut-off screwdriver.

Screwdriver with direct coupling

This type of coupling is also described as a stall coupling, which also very accurately describes the behavior inside the coupling system. The stalling torque - i.e. the torque at which the nutrunner comes to a standstill - is determined only by the air pressure present. The figure shows that there is practically no flattening of the torque curve compared to the previously described systems. This system is used especially for materials without structure, such as wood, plastic, and rigid foam. The worker has a direct influence on the screwing process, which is unavoidable with different strengths (for example wood), he determines when the screwing process should be interrupted.

What is the difference between hard tightening, soft tightening and structureless tightening?

The screw joint describes how quickly the tightening torque increases when the head is in contact with the surface of the part to be screwed.

Hard screw joint

A bolted joint without intermediate layers or washers, spring washers, disc springs and the like.

Characteristics:

• Small resistance during tightening
• Rapidly increasing tightening torque
• The angle of rotation between the shut-off torque and the first head contact is less than 30°

Soft bolting

A bolted joint with intermediate layers and/or washers, spring washers, disc springs, etc.

•  Small resistance when screwing in
• Slowly increasing tightening torque
• The angle of rotation between the shut-off torque and the first head contact is approx. 720° (approx. 2 turns).

Structureless bolting

A bolted joint with intermediate layers and/or washers, spring washers, disc springs, etc.

• Slowly increasing, relatively low insertion torque
• Highest torque when reaching the head contact

Find optimum starting system

• Push start: The rotary movement is started by axial pressure of the screwdriver on the fastener
• Combination of push start with lever/push button: Start after placing the tool by actuating the pushbutton/valve lever AND axial pressure on the screw joint
• Pushbutton, lever, valve start: Start after placing the tool on the screw joint by actuating the pushbutton/valve lever.
• Direct start: The screwdriver is started via an external compressed air control (electrically controlled valve, e.g. with our screwdrivers for installation).

Further selection criteria:

• Shut-off accuracy
• Adjustable torque range
• Type of construction: rod, pistol, pistol with indented handle, handle on top

Explanation of the screwdriver designations:

Example: SD 30/5 A

• SD – Screwdriver, Rotary
  
• SW – Screwdriver, Angle
• 30 – Power [Watt] Code x 10 = 300 Watt
• 5 – Torque [Nm]
• A – Shut-off screwdriver

Other types:

How do I set the torque?

Our torque wrenches with shut-off and slip clutch are equipped with replaceable clutch springs. A clutch wrench is included with the torque wrench for adjustment. There is a sleeve on the wrench that releases the adjustment range when turned. Now you can tighten or widen the clutch spring. You can use a torque gauge to check whether the tool is shutting off correctly. Never screw the spring together to its complete block length. This will damage the screwdriver and it will no longer be able to shut off. If you would like to receive your screwdriver preset, please contact us and we will take care of it.

Screwdrivers for very high shut-off torques:
impact and pulse screwdrivers

For applications with high shut-off torques, air nutrunners can be equipped to a certain extent with support devices such as our EPA40 handling system, linear arms or other torque supports to absorb recoil torques. However, there are also limits to this. For higher torques, self-supporting screwdrivers such as impact or impulse wrenches are recommended.