Guideline for the Selection Screwdriving Technology
Product- and processing engineers are familiar with this situation: The screwdriving technology, known as one of the most comprehensive connection technologies, surprises with variables, which were not considered during the design- and specification phase. A solution is found by the engineer that has already in the design phase selected the right screwdriving technology.
This guideline offers help and tips for the selection of the best-suited screwdriving system in an industrial assembly area.
DEPRAG offers a large line of screwdrivers for the most varying applications. Tools for the assembly of sophisticated products with a high demand for process reliability, have to correspond to different considerations as for example impact tools, which are used in a rugged environment.
THE RIGHT TOOL FOR YOUR APPLICATION!
Overview of the different screwdrivers and their applications
A: Screwdriving technology for the industrial assembly
The classification of those tools are based on different criteria, such as
- manual or stationary use
- the drive principle pneumatic or electronic
- the design execution: inline, angle-head design, pistolgrip
- the requirement in regards to process reliability, flexibility, documentation, etc.
Many times in the industrial assembly process it is not only the screwdriver, but the complete screwdriving system that has to be adapted to the application, the tightening process and the required process-reliability. This means that everything has to be coordinated: the screwdriver, torque- and angle-measuring equipment, the feeding technology up and including to the fully-equipped manual workstation or the fully-automatic assembly machine.
This area help you get started with the selection of a suitable screwdriver. However: A determination for the correct screwdriving technology that fits your specific application, can only be made by performing a targeted application analysis (such as a screw-joint analysis done directly with your parts).
B: Screwdriving Technology for special applications
DEPRAG offers a selection of diverse tools for special applications, such as:
The screwdriver with locked-out mechanical clutch for the assembly of thread-tapping, thread-cutting or self-drilling fasteners.
The screw is run down using the full motor power of the tool. Shortly before the seating of the screw, the shut-off clutch is engages and the driver shuts off at the exact preset torque.
- Impulse screwdrivers with automatic shutoff / Impulse screwdrivers without automatic shutoff
The screwdriver (available with and without torque-shut-off) that incorporates a hydraulic impulse unit, which is used for a fast assembly without any noticeable torque-reaction.
- Impact Tools
For a fast rundown with a torque range of 90 - 1200 Nm
The Depth-Stop screwdriver with precise shut-off at a preset screw-depth; used for the wood- and plaster processing industries
The all-rounder for the wood industry
The flexible tool for the repair- and recycling operation that simplifies the loosening of fasteners.
- Flat Head Wrenches
The solution for limited space conditions and screw sizes from M3 to M8.
The selection of a screwdriver for the industrial mass-production is demanding. Therefore, we have included many details into the following steps of this guideline.
SELECTION OF THE ASSEMBLY TECHNOLOGY BASED ON OUTPUT
Basically, it is important to know what production output is required for your assembly. If relative low volume, short product cycles and a high variety of products are the norm, then a handheld screwdriver is best for a manual assembly.
Such a handheld screwdriver can be equipped – when production output increases – with an automatic screwfeeder.
The opposite situation would mean that your assembly requires large volume outputs and long product cycles. For those situations, we can offer semi- and fully automatic assembly machines.
SELECTION OF DRIVE – ELECTRIC OR PNEUMATIC?
We basically offer two different drive methods: electricity of compressed air. Electronically or pneumatically powered screwdrivers distinguish themselves through clear qualities, that are selection decisive for individual application cases.
The following factors should be taken into account:
|Criteria||Electronic Drive||Pneumatic Drive|
|A: Flexibility (varying tightening parameter)||high||low|
|B: Process Reliability||high||medium|
|C: Torque Accuracy||high||high|
|D: Operating data acquisition and statistical process control||high||medium|
|E: Documentation Obligation||high||medium|
|F: Investment needs||high||low|
|G: Operating Cost||low||low|
|H: Energy Supply||available||generally available|
Individual factors or combinations also influence the selection of the drive mediums. i.e. For instance, the absence of compressed air immediately excludes the use of a pneumatic screwdriver. Or: the requirement calls for a flexibility of varying tightening parameter, which is a clear case for the use of an electric, freely programmable screwdriver.
Criteria A: Flexibility
If there is the possibility or necessity of changing parameter (torque, angle, speed, turn-direction or screw-depth), then we can recommend the use of our electronic screwdriving systems. The DEPRAG EC-screwdriving systems (MINIMAT-EC-Servo Screwdriver, MICROMAT-EC Screwdriver, MINIMAT-EC-Screwdriver) are freely programmable for any above mentioned parameter. Changes to the screwdriving process can easily be achieved.
Criteria B: Process Reliability:
Process reliability is a broad term, that has a decisive effect on the scope and cost of the screwdriving system to be selected. The requirements for the process reliability have to be clearly defined ahead of time.
While some applications require only the output of an OK/NOT OK signal, other application require the exact recording, evaluation and documentation of every assembly step.
OK/NOT OK signal output is possible with both Air Screwdrivers and EC-Screwdrivers . For the recording, evaluation and documentation of assembly step, we recommend the use of screwdriving systems based on an electronic EC system. Alternatively, the use of air-operated screwdrivers with integrated torque/angle sensors is possible.
Additional requirements may be necessary for the assembly safety-related components. This may include the acquisition and evaluation of torque by an independent sensor-array or even the need for redundancy, which we can offer in our EC-Servo-Screwdriving Technology in connection with the use of an additional torque-measuring system.
Criteria C: Torque Accuracy
DEPRAG Air Screwdrivers of the series NANOMAT, MICROMAT or MINIMAT are equipped with a highly accurate mechanical shut-off clutch, which offers the already proverbial accuracy of +/- 3 % standard deviation – and that at millions of cycles.
The standard deviation is however only one statement in regards to the precision of the DEPRAG screwdrivers. The machine capability index of Cmk is the decisive value to the qualification of a tool for an industrial application. DEPRAG air-operated screwdrivers with a shut-off clutch operated at suitable surrounding conditions a Cmk value of ≥ 1.67 at ± 10 % tolerance based on 6 Sigma according to ISO 5393. In another word, the Cmk value of ≥ 1.67 corresponds to 0.6 defects per one Million assemblies.
The DEPRAG screwdrivers based on our EC- or EC-Servo technology can do even better: Depending on the array and programming of the tightening sequence, these screwdriver allow a smaller standard deviation of ± 1 % or based on the machine capability index, a Cmk value of ≥ 1.67 at 5 % tolerance based on 6 Sigma according to ISO 5393. This means the same low default rate of 0.6 errors per one Million assemblies, but at half of the tolerance window.
Criteria D: Operating data acquisition and statistical processing
For operating data acquisition and statistical processing it is necessary to use screwdriving systems, which can communicate with a host process-controller. For this, we can offer our EC- or EC-Servo technology as solution.
Criteria E: Documentation Requirement
Both systems allow the recording of the acquired torque- and angle values. While this process is standard for the EC- or EC-Servo technology, a pneumatic screwdriver has to be specially equipped with torque- and angle sensors.
The initial economic advantage of an air-driven system is therefore reduced.
Criteria F: Investment Requirement
The multitude of possibilities and functions of the EC-/EC-Servo technology is reflected in the height of the investment. Basically the rule is: The higher the requirements on a screwdriving system (process reliability, functional variety, etc.), the higher the cost. Here, the well-tried air-tool offers clear cost-advantages.
Criteria G: Operating cost
To clearly determine operating cost, a multitude of factors have to be considered, such as
- comparison of energy cost
- maintenance effort and maintenance cost: While an air-operated screwdriver can generally be serviced by an internal technician, the EC-Screwdriver has to be sent to the manufacturer.
- Training expenditure for specialized personnel (i.e. screwdriver software training)
- Calibration requirement for measuring equipment (Quality Standard EN ISO 9001)
- Insensitive to external influences (dust, humidity,....)
- Investment cost
Criteria H: Supply of Energy
The absence of compressed air immediately excludes the use of a pneumatic screwdriver.
Criteria I: Durability
DEPRAG screwdrivers are designed for the continued industrial use and have a long life span. This applies for both drive technologies.
THE CORRECT DESIGN-FORM FOR YOUR APPLICATION
Screwdrivers are made available having a variety of design forms.
The classic, manual, inline screwdrivers is to be used in a vertical screw-axis while the pistolgrip screwdriver should be used for a horizontal application. Additional designs are the angle-head screwdrivers for limited space conditions or exceedingly high torque output. Screwdrivers with a cylindrical housing are well suited for the integration into automatic assembly machines and then there are a multitude of customized tool.
TORQUE, ANGLE AND SPEED
The goal of an assembly is generally the connection of two or more parts to a defined pre-load force. However, direct methods to measure the target pre-load in the industrial mass-production are not economical. Thus, the torque counts as the determining process value in the screwdriving process, as it behaves proportionally to the pre-tension.
As a result, the accuracy of the applied torque is in direct combination with the achieved pre-load force and the associated quality of the assembly: The more accurate a screwdriver, the more reliable is the screw-connection.
A defined angle is measured from the threshold to the final torque. If both measuring results (torque and angle) are within the specified tolerance ranges, an O.K. assembly is most likely assumed.
Also, the speed is a defined medium that needs to be determined. For example, when using high speeds for self-forming plastic assemblies, a high-friction is generated, which negatively influences the shaping of the screw thread.
Settling conditions, surface finish and dimensional accuracy of the parts to be assembled, as well as the positioning accuracy of the parts with each other, also have an influence and should be considered for the montage process. Settling conditions as they occur for example during a pump assembly with an integrated gasket, require a different solution. A dual- or multi-step tightening process or a torque stand-by time can solve this problem. Other approaches can be found in the combination of different tightening parameter. For example, the friction-torque tightening process allows the recognition of the seating of the screw and the therefore clearly defined final-tightening by torque and/or angle.
Comprehensive screw-joint analysis are perfect for establishing the required tightening parameter and to solve assembly problems.
Centuries of experience in the Screwdriving Technology, a professionally equipped analysis- and calibration laboratory (DAkkS-accredited) and a large team of application specialists is at your disposal.
Our application consultants in Germany and around the world are standing by to support you with determining your specifications. If you prefer, you may also speak with one of our associates in the corresponding department.