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Photo | Bosch Rexroth AG

Stihl makes use of head contact detection.

Photo | Bosch Rexroth AG

Self-tapping screws, ready for use in bins on the assembly line.

Safe and sure tightening

The powered machines built by Stihl have to stand up to real punishment. Tightening systems by Rexroth ensure that they do just that.

 

Chain saws, stone cutters and cut-off machines – the gasoline-engine powered devices made for professional use by Stihl, a German company – deliver one main thing: a great deal of power. And the customer counts on complete safety when working in forests or gardens or on construction sites. An important element: threaded fittings. They ensure high precision and uninterrupted functioning. That is why they have to stay tight over the long term.

At Stihl, this is a central requirement. “Especially when we want to join plastic parts with metal components, we need systems with carefully controlled final tightening,” explains Gerhard Stübs, manufacturing planner at the Stihl factory in Waiblingen. “The screw, torque and rotation angle have to be exactly matched to each other. Only then can the screw connection satisfy quality expectations in the final product both dependably and safely.” When selecting the operating resources, equipment safety is at the top of the list.

A focus on process reliability

The Rexroth ErgoSpin handheld nutrunner satisfy these requirements and keep products safe. Stihl uses self-tapping screws. This saves time and reduces the number of steps, since tapping and cleaning the female threads are eliminated. At the same time, this kind of screw offers great protection against loosening due to vibration. In addition, shorter, smaller-diameter screws can be used.

When tightening them down, self-tapping screws are quite demanding. Their ideal tightening torque may vary, depending on the process. Here is where head contact detection by Rexroth helps. The screws are tightened down in several phases. Here the controls recognize the screw’s position, determines the required tapping torque, and calculates the final snugging torque required to achieve the specified clamping power.

Short cycle times, optimized processes

Using optimized tightening processes has also let Stihl shorten its cycle times. The shorter time between the start of tightening and achieving final torque pays off in particular, says Stübs: “During assembly, even hundredths of a second can have a significant influence on the cycle time and output rate.”

What’s more, the company saves money. The operating costs for the ErgoSpin handheld nutrunner are up to 50 percent lower than for air-powered tools. And Stihl uses the statistics and evaluation functions integrated into the Rexroth BS350 operating system right from the initial trial runs. The purpose here is to optimize processes for mass production.

These power nutrunners, generating forces of from 1 to 220 newtonmeters, cover almost every tightening application and keep the Stihl cut-off machines, the bike-handle brush cutter, and the new MS 661 chain saw good and tight.