Pressing concern: innovations in holding devices for industrial presses

Paul Boughton

Industrial presses can potentially present significant dangers for operators and are classified as dangerous in the EU Machinery Directive 2006/42/EG. Because of these risks, a ram drift lock or holding device is mandatory on all mechanical presses used in Europe. This must be operational when work will be undertaken by individuals during press downtime in the area between the table and the ram, such as tool repair and changeover, maintenance and servicing, and repairs.

The holding device ensures that, even in the case of a severe system malfunction (such as an interruption to the supply of compressed air to the balancing cylinder), the ram is held in position and does not present a health and safety risk. The requirements for a holding device are documented in EN 692 Paragraph 5.3.18 [1].

Positive-locking solutions have been the most widely used solution to date, but many of these systems can only lock the upper ram position.

However, frictionally engaged hydraulic clutch/brakes with integrated slide restraint devices are now available which can function effectively irrespective of the ram position and so meet wider range of safety parameters and so fulfil all the requirements of EN 692.

The new systems are BG-tested and take the form of a multi-disc brake which is directly integrated into the existing hydraulic clutch/brake unit, making them a highly cost-effective solution for  both manufacturers and operators.

Fig. 1 shows the new device, offering three functions in a limited space:  primary brake (B), secondary brake (A) and hydraulically activated clutch (C).

The ram is held in place by the secondary brake, which has its own laminated plate stack and actuation system. 

Structure and function

Fig. 2 shows that when the power to the press is switched off, the unit is unpressurised, with brakes A and B closed and the clutch open. The actuator (1) is pressed through the spring assemblies (2), together with the laminated plate stack (3). The spring tension, the friction radius and the number of frictional surfaces, as well as the friction coefficient, yield the so-called holding torque.

When the press is switched on, pressurised oil is delivered to the unit through the cavity (4), ventilating the brakes in parallel. As the pressure is increased, the piston (5) closes the clutch, which moves the shaft and the ram. In the brakes, the pressure from the cylinder chambers can then be released through the press safety valve. The pistons (1) and (5) revert to their original positions above the spring assemblies (2) and (6) again. The clutch is opened and the brakes A and B are closed.

Key benefits for the manufacturer

The inclusion of the new positive-locking holding device solution delivers a number of key benefits for manufacturers. Firstly, being integrated into the existing hydraulic clutch/brake unit creates a significant saving on space, reducing machine footprint.

Design adaptation is simplified too, as the intake for the shaft (diameter, pressure and cooling oil bores) as well as the attachment to the body of the press occur in the same way as with standard clutch/brake units. No additional attachment parts are needed either, as the brake torque is transmitted through the same sprocket. Installation is simplified as the complete unit (clutch with two brakes) can, as with the standard clutch/brake units, be mounted on the shaft.

The system is controlled via the standard press safety valve with no requirement for additional control valves and monitoring. The required accessories - oil supply, press safety valve, damping component, aggregate,  also remain the same.

Benefits for the operator

The positive-locking holding device is activated using the press safety valve during each braking process. This means that the operating space can be accessed immediately after each stop, irrespective of where in the upstroke or downstroke the ram comes to a stop once the press control is locked. This is a key benefit in many applications, for example when using automatic presses in set-up mode in order to check the position of the parts. A wet friction system guarantees largely wear-free operation.

The new system has been used over an extended period on a 2500-ton press with no issues reported, while a 1200-ton press with a positive-locking holding device will soon enter operation.

Systems with holding torques from 10,000Nm to 160,000Nm are available, meaning they are set to become the component of choice for manufacturers and installers seeking equipment offering optimal operator safety alongside rapid access to the operating space.

Lothar Grötzinger is Head of Press Technology, Norgren. www.norgren.com/uk/industrialautomation