Small change – large effect

The printing result and the service life of the entire system are influenced significantly by the quality of the clamping shaft interface

If the print image on flexographic printing presses shows strong quality fluctuations or the make-ready process repeatedly comes to a standstill during job changes due to sluggish carbon-fiber adapters, the clamping shaft is quickly identified as a supposed source of error. It is often forgotten that the clamping shaft and carbon fiber adapter are a highly complex overall system. Whoever wants to avoid bad investments should therefore first check the complete interface. Often enormous effects can be achieved with targeted corrections.

For a long time, the interface to the clamping shaft was considered a major unknown factor in flexographic printing operations. And even today, some employees are still not sufficiently aware of its influence on the print quality. The handling of the individual components is accordingly rude: Clamped carbon-fiber adapters are knocked off the expansion shafts with hammer and squared timber, regardless of losses. “Again and again we meet employees who are not aware that a jammed adapter is not a normal condition but an indication of functional errors within the overall system”, explains Oliver Pfeiffer, Managing Director of the clamping technology specialist SWT Services from Ilsfeld. “Provided that all parameters of the overall system fit and the employee has internalized the know-how, adapters can be easily joined even after years”.

• Prevent bad investments

In cooperation with the leading manufacturers of printing machines, adapters and clamping devices, SWT Services developed a program 10 years ago that enables maintenance and service directly on the machine. In this way, around 90 percent of all faults that occur can be rectified directly on site. In the meantime, the service technicians of SWT Services are in demand all over the world. The customer base includes leading suppliers from the food, tobacco, non-food and chemical industries in Europe, China, Malaysia, Singapore, Australia and the USA. In all these companies, SWT Services guarantees trouble-free operation with regular maintenance.

“Time and again it happens that complete expansion shafts are replaced in the event of process malfunctions without the actual problem being eliminated in the long term. On the other hand, five- and even six-figure sums are sometimes spent on new components, even though the existing expansion shafts and carbon-fiber adapters could still provide good service for years after minimal corrections,” reports Oliver Pfeiffer. Very few flexographic printing companies are aware that there is a whole range of effective and, what’s more, extremely economical options for checking the entire system consisting of clamping shaft and carbon-fiber adapter and considerably improving the overall condition of the system with just a few targeted measures, says the clamping technology expert.

• Complex overall system

In order to understand what SWT Services does on site, it is worthwhile to take a look at the entire interface. On the one hand there is the clamping shaft. It usually consists of a high-precision hydraulic clamping system in which a piston is moved by means of an actuation screw. This in turn presses hydraulic oil into an expansion chamber. The increase in pressure in the expansion chamber causes an expansion sleeve connected to the base body to curve in a defined manner, which causes the carbon-fiber adapter above it to be clamped flat and precisely at the clamping points. Hydraulic expansion solutions, such as those used in flexographic printing machines, but also in machine tool manufacturing, are among the most precise clamping technologies available. They are used whenever high precision has to be maintained over the long term with frequent changes and minimal set-up times. No other clamping system combines the factors concentricity, minimization of set-up times and flexibility as perfectly as hydraulic expansion systems. The oil-filled expansion chambers and the expansion bushing also have the effect of damping the vibrations occurring during the running process and absorbing peak loads. “When used correctly, the interaction of the hydraulic expansion shaft and the carbon-fiber adapter is a safe and long-lasting system that guarantees a high degree of process safety,” explains Oliver Pfeiffer.

The adapters, in turn, are generally made of carbon-fiber in modern high-performance printing presses and are pushed and clamped on the expansion shaft up to eight times per shift. In order to ensure perfect functioning, the bore dimension of the adapter must be precisely matched to the diameter of the expansion shaft. Coarse interventions, such as hammer blows, loosen the adapter from the shaft, but at the same time cause damage to the bore. In view of the fact that both the expansion shaft and the adapters are precision-ground in new condition and thus perfectly matched to each other, it quickly becomes clear that longitudinal scoring, punctual damage or deformation can significantly impair process stability.

• Wide range of influence potential

According to SWT Services, the optimum function of the interface depends on three factors: the quality of the expansion shaft, the quality of the adapters and, most importantly, the perfect interaction of the entire system. Errors can only be eliminated if all three factors are checked simultaneously and readjusted if necessary. “It happens again and again that new clamping shafts are purchased in the event of quality fluctuations, but the old, possibly faulty adapters are used,” reports Oliver Pfeiffer. “In such cases, it is everyday occurrence that the process stability again shows fluctuations within a very short time or, what is even worse, the new clamping shafts are damaged by the old adapters.”

SWT Services therefore recommends a different approach: “It is much more economical to check the complete system consisting of clamping shaft and adapter regularly and to correct it specifically if necessary”, advises Pfeiffer. “This avoids bad investments on the one hand and expensive machine downtimes on the other. SWT Services has specifically specialized in ensuring that 90 percent of the tests can be performed directly at the machine, often within 18 to 20 minutes during a job change. Within the last ten years, there have hardly been any cases in which errors could not be corrected immediately on site due to the extent of the damage.

The maintenance process usually takes place in three stages: First, the complete clamping shaft is cleaned and analyzed using high-precision measuring equipment and calculation programs. This involves checking both the absolute dimensions of the clamping diameter and the roundness and clamping force of the expansion shaft. By comparing the values of the adapter and the clamping shaft with the aid of special calculation programs, the complete system can be evaluated, i.e. a statement can be made as to whether the function of the expansion shaft is sufficient for reliable and precise clamping of the adapter and whether process safety is ensured. Afterwards, the service technicians discuss the results on site and point out alternative courses of action. Finally, in the third step, the interface is specifically optimized. SWT Services draws on a whole arsenal of measures to extend the service life of the entire system, starting with the fine adjustment of the clamping forces, the treatment of the clamping surfaces and the optimization of the fit clearance. For this purpose, SWT Services uses over 75 years of service experience in the field of clamping technology. In order to avoid costly machine downtimes, SWT Services also carries out inspection and maintenance work at weekends and public holidays as well as during shutdowns.

• Repair and general overhaul

Before buying new expansion shafts and adapters for major problems, Mr. Pfeiffer advises to consider repair or overhaul for greater sustainability and cost savings. Hydraulic pressure cylinders and anilox rollers can be sent directly to SWT Services and are ready for use again after a few weeks.