A machine builder overcomes the unwanted effects of hydraulic oil compressibility in a precision tablet compaction machine with a novel clamping device that engages automatically at the critical maximum clamping force stage
Huxley Bertram Engineering designs and builds special purpose industrial machines like the ESH tablet compaction simulator, which is used by pharmaceuticals companies to test and verify their manufacturing processes. Customers look for ease of use, simple adjustment of parameters, an ability to produce a wide range of tablets, accuracy and repeatability.
The ESH is two-pillar, computer controlled, fully servo-hydraulic machine, which applies compression forces of up to 50kN. It is capable of mimicking the exact cycle of any tableting process in real time, and recalling all the important parameters during the cycle.
Powder used to make the tablets is loaded into a small hopper and positioned over the tablet die-plate in the centre of the machine. A laser sensor confirms powder level in the die to within 0.1mm. Positioned in line above and below the die are two servo controlled hydraulic actuators comprising 63mm diameter pistons with 45mm rods, each providing a working stroke of 105mm. These are fitted axially with precision load cells, and their position is sensed via LVDTs.
However, at the final compaction stage, when maximum force is applied, the hydraulic oil compresses and introduces a 'bounce' effect. A simple system was thus required to clamp the piston rod and eliminate any inaccuracies introduced by this unwanted effect. Huxley Bertram's solution was to introduce a holding brake that would activate at this critical stage in the process.
The lower actuator is locked by an ETP 'Octopus' locking bush, a Swedish product supplied in the UK by Lenze. Octopus is an hydraulically actuated joint, comprising a cylinder with a separate inner and outer sleeve. Under hydraulic pressure the thin walled inner sleeve flexes and expands inwards to grip the shaft passing through it. For this application, an Octopus bush with a bore of 80mm was necessary to achieve the clamping force, and a split collet transmits the force to the actuator rod to provide a very rigid axial lock.
The bush is designed to transmit both torque and axial forces, but in this case it is used only to close the collet on the piston rod. The action is fast and repeatable with a life of about 500,000 cycles.
With the bottom actuator locked in this way, final compaction is then achieved by the top actuator and the tablet is ejected to an output carousel. The compaction cycle can be pre-programmed to produce tablets of slightly different profiles, or to manufacture multiple tablets of the same profile.
ETP Octopus bushes are available in a standard range to clamp shafts between 30 and 100mm diameter; special designs are also offered. Other application areas include the locking of machine slides and locking tool holders in machining centres. Huxley Bertram director, William Bertram describes it as a simple way to achieve a locking brake. "Octopus can apply the force locally and quickly," he says.