Space-saving development

Photo | Bosch Rexroth AG
Marginal Column

Above: Jørgen Pedersen (left), CEO of Solomatic, and his partner Peter Bonde, alongside the specially engineered machine.

July 2013

 

With the help of a new planetary screw assembly, the Solomatic company cracked a tough nut when developing a special-purpose machine.

 
 

Complex problems are Solomatic’s bread and butter. This Danish company specializes in developing and manufacturing custom-engineered machinery – especially for industries that impose the most exacting demands in terms of functionality, design and documentation. To achieve that, the developers really don’t even have to know the application right down to the last detail in advance. One current example demonstrates this – a machine that punches, cut and bends gasket strips and rubber seals and mounts them in aluminum profiles. “If, for instance, holes are to be punched in steel instead of aluminum, then that’s not a problem for us,” emphasizes CEO Jørgen Pedersen.

To maintain this degree of flexibility, his team constantly surveys the technology markets for appropriate solutions. There Solomatic A/S discovered a new type of planetary screw assembly. It is far superior to the alternative, a purely hydraulic system, which had been considered up to then. “Given the pressures and the distances that the machine’s eight cylinders have to deal with, we would have needed 2,500 liters of hydraulic fluid,” Petersen reckons. The planetary screw assemblies, being purely electromechanical, require no fluid at all. What’s more, they can be programmed independently one of another. This makes it easy to adapt the configuration to suit the final product.

Powerful and compact

The planetary screw assemblies offer a number of advantages in this job, compared not only with hydraulic systems, but also with recirculating ball screws. In planetary screw assemblies the rotating, threaded rollers – the so-called planets – substitute for the recirculating balls. Held by a threaded nut, they rotate around a threaded spindle, their axes parallel. The many, large contact surfaces make for far higher load-bearing capacities, when compared with ball screws. Martin Bunk, in Factory Automation Sales at Bosch Rexroth in Denmark, explains what that means to the application. “Planetary screw assemblies are smaller. Even though they occupy less space, they can transmit the same amount of power.” The eight planetary screw assemblies used in the machine, each with a spindle diameter of just 48 millimeters, move ten tons each.

Photo | Bosch Rexroth AG

Bosch Rexroth sales engineer Martin Bunk appraises the application of planetary screw assemblies.

 

A ball screw would have required a spindle diameter of 80 millimeters. It is clear that large forces can be handled by a relatively small spindle. That is something Jørgen Pedersen is quite happy about. “For those of us in mechanical engineering, it is important that the design be as compact as possible, since space is always a problem in machinery. At the same time, the design is much improved.” This was another reason why the IndraDrive Cs compact drive fit the application perfectly. Working with a synchronous servomotor, it makes for high torque densities at the planetary screw spindle – without occupying unnecessary space.

Useful additional features

This custom-engineered Danish machine profits – over and above the efficiency level of the planetary screw assemblies – from other optional Rexroth features. “Of particular benefit to us is a special ‘Advance to stop’ function which is integrated into the drive,” Pedersen confirms. There the motor, controlled in response to position or speed, drives the system deliberately against a stop and then continues to apply torque at a defined level. This provides the pressure needed for clamping, for example. In addition to this helpful function, safety is of major interest at Solomatic. Two of the eight planetary screw assemblies are located within a protected area.

The operator enters this space only in order to load the system with material. Solomatic was able to implement this protected zone using the safety functions in the IndraDrive C. The company selected two features – “safely limited speed” and “safely monitored position” – from the “Safety on Board” options by Rexroth. “These safety features, integral to the drive, were another aspect that swayed our decision in favor of Bosch Rexroth,” Petersen summed up. These functions, together with high power density and compact dimensions, show what electromechanical drive concepts can do today.