Though hopefully few and far between, when collisions do occur in a dynamic servo-controlled machine, the mechanical torque-limiting clutch should be the first line of defence, where the protection of servo axes is concerned. This is the case even if the control unit includes an anti-collision monitoring function. No machine is entirely immune from collisions, which can incur costly repairs, unscheduled downtime and loss of production. And then there are less obvious collisions that go unnoticed, often causing damage that can result in loss of accuracy. These insidious problems – with no obvious cause – can be cumulative and their negative impact can ruin the reputation of a machine builder.
Some servo controls manufacturers believe that mechanical torque limiting clutches are no longer necessary. The collision monitoring in the machine controls is intended to take over the protective function. Collision monitoring systems recognise an increase in torque and respond with countermeasures. However, this only works under certain conditions and can only provide effective protection in rare cases, such as less dynamic applications suffering ‘soft’ collision events.
Machine tool builders, faced with demands for higher productivity and greater accuracy from their machines, take the opposite view. When it comes to collisions, there is no alternative to a reliable mechanical torque-limiting clutch and the protection it affords against expensive overload damage.
Machines are no longer designed to produce one and the same part during their entire service lifetime. The increased complexity of modern machines, with several, simultaneously moving axes, multiplies the risk of collision. The demand for greater productivity requires faster axis movements and stronger motors, which possess a higher inertia. Higher speeds and larger mass moments of inertia translate to considerably increased kinetic energy.
Machine tools that offer higher accuracy require stiffer frame and drive axis designs. When a collision occurs, the torque increases so rapidly that electronic monitoring systems can no longer react as fast and effectively as they should. Mechanical torque limiting clutches, on the other hand, are able to separate the drive line within several milliseconds of the preset maximum torque being reached, and are therefore able to offset the majority of the destructive kinetic energy effects.
While mechanical torque-limiting clutches have proved their worth thousands of times over, machine manufacturers continue to shun such protective devices in many servo axes, due in large part to concerns about additional mass moments of inertia, loss of torsional rigidity, and higher costs. The latest EAS-smartic branded torque limiting clutches from Mayr address these concerns, and are specially adapted to the requirements of machine tool axes.
These backlash-free, positive-locking devices are extremely compact and offer a high performance-to-mass ratio. The outer diameter is equivalent to that of a standard shaft coupling, and it is only around10 to 15mm longer. This results in a very low mass moment of inertia – lower, in fact, than a steel servo coupling. Hardened components, based on the tried and tested ball-detent principle, plus special cup springs ensure a high degree high torsional rigidity. The clutch is supplied factory preset to approximately 75% of the maximum torque – a value that is easily changed by the user. Mounting is also straightforward and is simply a matter of tightening a single screw.
During normal operation, the clutch transmits torque from the drive shaft to a drive element such as a sprocket or pulley, or (in combination with an elastic coupling) to an output shaft. If the pre-set torque is exceeded, the clutch disengages. The torque immediately drops, and a limit switch registers the disengagement movement and switches off the drive. After the cause of the malfunction has been removed, the clutch re-engages automatically and is again ready for operation.