The right tool for zero-defect production

The achievement of zero defect production has the highest priority in industrial assembly. Time and again we hear of product recalls due to defective screw joints. The associated material damage and loss of brand image is the dread of every industry. And when it relates to safety-relevant components such as in the automotive industry then the manufactures make every conceivable effort to ensure maximum process reliability throughout the installation process. The selection of the correct tightening tool for industrial mass production is the cornerstone in creating a high quality fault free production system.

A threaded fastener is intended to clamp components together so that external forces cannot cause them to come apart and the parts essentially behave like a single unit. The designer calculates how high the applied clamping force must be so that the screwed or bolted component can withstand the most demanding mechanical stress. However, the question is whether the predetermined clamping force is correctly achieved during the tightening process. The trickiest problems concerning screw driving technology are unknown screw seating and assembly/component-related variables.

Since the clamping force generated during mass production assembly is difficult and, when feasible, expensive to measure, the applied torque and angle of rotation are accepted as the governing process variables in screw driving technology. The more precisely an industrial screwdriver applies the predetermined torque or angle of rotation the more likely that the goal of zero-defect screw/bolt assembly will be achieved.

There is no universal answer to the much debated question on the most appropriate drive for the stationary screwdriver spindle. The correct answer lies in finding the best screwdriver for the particular application. Full service providers such as DEPRAG SCHULZ GMBH u. CO. in Amberg/Bayern offer both: pneumatic screwdrivers and screwdrivers using EC Electric motor technology. These specialists in screw driving technology have made a name for themselves with their technically advanced and economical stationary screwdriver spindles.

DEPRAG experts competently advise their clients in choosing the proper screwdriver for the task. Gerd Zinn, head of the DEPRAG development department for production machines quotes an example: "Stationary screwdriver spindles with EC drive from our MICROMAT-EC/MINIMAT-EC range with low maintenance brushless EC motors are always particularly suitable when it comes to achieving and documenting different tightening torques or sequences in one assembly system."

The brushless electric motors which DEPRAG have developed for their screwdriver spindles are practically free of wear parts and therefore guarantee a long service life. The EC motor is able to deliver high torque even at low speeds. With its compact, slim design and high energy density it is ideally suited for the demands of seating threaded fasteners.

DEPRAG EC screwdriver technology is available in two versions: For programmable screw tightening there is the MICROMAT-EC Servo Screwdriver Spindle or the MINIMAT-EC Servo Screwdriver Spindle both with an integrated direct measurement system and covering torque ranges from 0.2 to 500 Nm. An EC Servo Screwdriver spindle is used in applications with high security requirements (critical screw joints) where a direct measuring system is specified. The associated AST30 sequence controller can be operated easily by an industrial PC or PLC. From here the required parameters are selected for each fastening application to be applied in the assembly. In automatic mode the device is controlled via the PLC or industrial PC system.

It is possible to apply a number of different torques and or angle sequences in one assembly cycle, for example, if a fastening application requires the screw to be tightened in two steps. In this case the screw is first applied with full speed at a low pre-tensioning torque. This is followed by the second step with reduced speed at the final tightening torque. Monitoring the angle of rotation further enhances the accuracy of the results. The tasks stored in the AST30 are sequentially processed. The strain gauge transducer integrated in the MICROMAT-EC Servo/MINIMAT-EC Servo determines the tightening torque which is documented for evaluation by quality management.

The second version is the MICROMAT-EC/MINIMAT-EC. It is operated by the programmable sequence controller AST10 which can be programmed by the operator directly from the controller's keyboard or by untilizing embedded software and any PC equipped with a web browser and an Ethernet port. Offering high accuracy, low noise and precision, the stationary screwdriver spindle carries out the programmed assembly process by referring to specified tightening parameters which can be assembled from four basic programs. The AST10 also facilitates the monitoring and documentation of each joint.

A functionally reliable system can be quickly and easily created from individual EC spindles and their respective controllers. The DEPRAG Standard screw function module (SFM) manages the drive stroke for automatic screw assembly in single or multi-spindle screw driving applications. This automatic screwdriver spindle slide, is designed to be mounted to any substructure . From a variety of options the DEPRAG specialists will quickly determine the model which is most suitable for your particular application.

The second screwdriver motor version for which the name DEPRAG is famous is the air motor. Developer Gerd Zinn describes its advantages as follows: "Our pneumatic screwdrivers are particularly well suited for tightly spaced screws. Their small size and excellent power to weight ratio speak in favour of this type of drive". The robust and simple design of the vane motor is the basis for heavy duty screw and nut runners designed for continuous use in industry. For torque ranges of 0.008 to 180 Nm the MICROMAT/MINIMAT family of pneumatic screwdrivers offers high screw assembly speed, repeatability and a low system price achieving excellent value for money.

Regardless of air pressure fluctuations or screw joint characteristics the DEPRAG screwdrivers deliver a torque repeatability well below plus / minus three percent standard deviation, maintained over millions of cycles. That is the advantage of the tried and tested MICROMAT/MINIMAT Auto shut-off clutch.

Its function prevents uncontrolled torque variations. The shut-off clutch transfers torque at the pre-set value to the screw then decreases the torque despite the continued rotation of the drive and stops the air motor at 50% of the formarly applied torque thus minimizing the effects of intertia over the screw joint. Speed fluctuations between hard and soft screw joints and fluctuations in compressed air pressure have no influence on the tightening torque. This enables the screwdriver torque to be reliably set without having to consider the individual joint. This high precision is a pre-requisite for the industry norm machine capability study and which is available on request for each of these screwdrivers.

DEPRAG assembly tools are also available for various special applications. For instance, with self tapping screws the thread forming torque can be higher than the final seating torque. The tool of choice here is the specifically developed SENSOMAT® screwdriver spindle. It drives the screw home with full torque - right up to the point where the screw head begins clamping the materials. At this point the highly accurate shut-off clutch is engaged and over-tightening of the screw is prevented. The basic model is the MICROMAT/MINIMAT, whose integrated function control port monitors the screwdriver's on and off condition and signals that the set torque has been achieved when the clutch disengages and stops the motor. The depth of screw insertion is also monitored by means of length comparison using a proximity sensor array.

Jürgen Hierold, DEPRAG sales manager: "In choosing the right screwdriver you must focus on the individual application". If process parameters have to be identified and documented for the quality management process then an EC screwdriver or an EC Servo screwdriver is the right tool. Electronically operated, programmable screwdrivers are also suitable where high flexibility is needed, e.g. where screw assembly parameters vary or there are frequent product changes on a flexibly assembly line. When data collection and flexibility are not clearly required then the pneumatic torque control screwdriver with function port is the most economical and technically superior solution.

The requirement for flexibility and process control are not the only parameters which must be considered in the selection of a suitable fastening system for your application. Consideration must also to be given to the necessary torque accuracy, requirements for data collection and statistical process control, purchasing, operating and maintenance costs, the desired power range, the service lifetime and the availability of the drive media for the tool used. The decision on the drive system may depend on a combination of several of these different influencing factors or it may come down to just one. For instance, the absence of compressed air immediately excludes the use of a pneumatic screwdriver. In summary one can assume that the selection of an optimum system for a particular application is only possible in consultation with experts and through extensive preliminary tests such as screw joint analysis.

Sales Manager Jürgen Hierold: "Our DEPRAG specialists will help you to select exactly the right assembly tool and will also advise on the configuration of the complete system."