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Assembly lines for gearbox shafts

Press plant to automatically press sprockets onto gearbox shafts

Defect free final assembly in gearbox operations requires cycle time coordinated provisioning of preassembled gearbox shafts. The key functional and quality criteria for these tasks are characterized by particularly high requirements for the various process steps, specifically when pressing sprockets onto shafts. In order to ensure the required reliability, and to maintain the specified load and accuracy parameters, a well-known gearbox manufacturer relies on an automatic process sequence and the highly sensitive control of hydraulic presses.

 

For cost and efficiency reasons, processing machinery for forming and joining applications are increasingly integrated into an automated process sequence. For this reason, Dunkes, a manufacturer of hydraulic presses and riveting machinery, has long since expanded its product line by task oriented automation and material handling solutions. Not only can customers rely on high-quality machinery with the latest technology, but they are also provided with an automation concept that is tailored to their specific application.  Due to the modular design of the base elements, automation concepts can be quickly and cost-effectively implemented, ensuring unbureaucratic communications and decision-making paths due to the mid-sized structure of the Kircheim-based machinery and plant builder, and facilitating flexible, cost and time optimised operations.

 

A joint venture between an automobile manufacturer and a gearbox manufacturer is leveraging these technical and organizational strengths for automating gearbox shaft assembly operations. The cycle time and quality were the driving criteria in this subassembly step of the operation. The press fitting operation of respectively three sprockets onto different gearbox shafts, including the required supplementary and inspection work, needed to be reorganized and automated. For this task, Dunkes recommended to hydraulic presses, equipped with task oriented manufacturing and in inspection facilities, and incorporated into the continuous material flow of a segment conveyor. Significant demands were placed on the flexibility and know-how of the supplier already during the planning phase, since the specification document required that the plant design provided for a stepwise transition into the automated sequence at a later point – on the basis of a manual feed and retrieval mechanism.

 

Standard products combined with custom solutions

 Out of the broad product line of the Swabian machinery builder, the hydraulic single column presses HZ 10 and HZ 16 with press forces of 100 kN and 160 kN met the technical joining specifications. Each press is positioned along the interior narrow side of a rectangular shaped assembly line, providing both lines with sufficient access for the manual operation selected during the startup phase and the subsequent installation of the various automation facilities. Workpieces are fed using a segmented conveyor with 25 workpiece pallets developed by Dunkes that are each equipped with swappable adapters for various shaft shapes. Hydraulically controlled wedged pushers lift the workpiece pallets in the  workspace of the presses in order to position the vertically arranged shafts in a precisely defined starting position. The conveyor system is arranged around the HZ 10 press. At the start of the assembly process, the operator positions one shaft and one sprocket each into the workpiece pallet After the sprocket is automatically pressed onto the shaft, the operator transfers the shaft to a workpiece pallet of a welding station, where the  sprocket/shaft press connection is secured with a laser weld seam. A CCD camera inspects the presence and quality of the connection. The operator rotates the part by 180° for a subsequent process, and places it into a workpiece pallet of a second assembly plant with the press of type HZ 16. The sprocket assembly has already been automated on this plant. A material handling unit retrieves the sprocket from a storage container and locates it on the shaft positioned directly below the press die. After the sprocket is pressed onto the shaft, the third sprocket is automatically positioned and pressed onto the shaft, which has been maintained in the same position. The shaft has now been fitted with three sprockets and is cycled out of the workspace, retrieved by the operator, and deposited for transportation to the final assembly operation.

 

Control with pressfit monitoring

Both hydraulic presses are equipped with a user-programmable NC control. This facilitates convenient adjustments of the stroke motion, press forces, rapid advance and press speeds to the values dependent on the respective shaft. However, the pressfit monitoring is the key for the 100% inspection. A force/distance measurement facility determines whether the press forces are within the specified tolerances as a function of the pressing on travel. An error message is displayed when the threshold values are exceeded, and the manufacturing process is stopped. Defective parts are now no longer required to undergo an elaborate repair process at a different location, but can be directly reworked at the workstation. To do so, the operator places the defective parts into a special workpiece pallet and moves the pallet into the workspace of the HZ 16 press, where the sprockets can be pressed off the shaft with a tool. This repair process can be performed immediately after the error message, or at a later point, without impacting the cycle time.

 

Flexible cycle time optimisation

The primarily manually fed assembly plants already attained cycle times of 15 to 30 seconds after a brief period of operation. These times were significantly below the specifications, and will be improved further with the stepwise implementation of the automation level of the assembly line, at which point they will be increasingly less dependent on the operator. This brings the user increasingly closer to his ultimate objective - the fully automated assembly of gearbox shafts. Dunkes has created the prerequisites for a stepwise increase of the level of automation. In spite of the extremely short installation period – only eight months were needed from the order award to delivery – the various interfaces of the plant concept are configured such that a flexible response is possible to changes of the customer's production sequence. 

The gearbox shafts supported in the work piece pallets are transferred into the workspace of the hydraulic press with a conveyor belt, where the sprockets are pressed onto the shafts. A feeder unit is installed to the left of the press. It retrieves the sprockets from a storage container, and positions these on the shaft and.
Inspection of the laser weld seam on the shaft/sprocket connection joint. During the semi-automatic shaft assembly, certain sprockets are manually positioned on the front receiver of the toolholder and then retrieved by the feeder unit.
Continuous conveyor belt with staged tool palettes and pre-assembled shafts. The wedge pusher to lift the tool pallet is visible in the press centre to the left of the conveyor belt.