Huxley Bertram Engineering design and make special purpose industrial machinery. When their new tablet compaction simulator needed a holding brake, the need was met by an ETP Octopus locking bush, which gave a simple mechanical solution.
Mr William Bertram, Director of Huxley Bertram, described the requirement The hydraulic oil is too elastic in the final stages of compaction and we need a simple solution to clamp the piston rod .
The ETP Octopus works with the hydraulic pressure available on the machine and gives a compact assembly that is easy to connect and disconnect. The ESH tablet compaction simulator fills a need in the market for a flexible research and development tool for formulation scientists, usually working in the major pharmaceutical companies.
It can be used either for the development of new commercial drugs or to mimic the performance of production compaction machinery, in which role it can be used to investigate production problems. Typical output rates are 1-2 tablets per minute.
Customers for these machines look for ease of use, simple adjustment of parameters, the ability to produce a wide range of tablets and accurate repeatability. Huxley Bertram, based at Cottenham near Cambridge, developed their new 2-pillar machine from an earlier 4-pillar design.
The ESH is computer controlled and fully servo-hydraulic with feedback giving controllable compression forces up to 50kN. The machine is capable of mimicking the exact cycle of any tableting process in real time, and recalling all the important parameters during this cycle.
Alternative types of tableting simulator on the market use cam driven compression tooling, so a change of tablet type requires mechanical resetting of the machine. The servo-hydraulic ESH gives advantages of flexibility and a fast change to a different tablet is achieved simply by calling up a new programme.
Also the open two- column design gives easier access for the operators, easier cleaning and a smaller footprint on the laboratory floor.In operation, the powder used to make the tablets is loaded into a small hopper and positioned over the die-plate in the centre of the machine.
The shaped die and upper and lower punches, which are easily removable, match the finished dimensions of the tablet. Positioned in line above and below the die are two servo controlled hydraulic actuators capable of delivering controllable force of up to 50kN from a hydraulic supply of 280 bar.
The actuators have 63mm diameter pistons with 45mm rods and a working stroke of 105mm, and are fitted axially with precision load cells. A position feedback signal comes from two LVDT (linear variable differential transformers) which are variable reluctance devices giving a high-resolution hysteresis free voltage signal according to distance travelled.
When the hopper is moved over the die, powder falls into the die; when the hopper is retracted the powder is level with the die table. This level is confirmed by a laser sensor capable of resolution better than 0.1mm.
At the final compaction stage where maximum force is applied, a locking brake is needed to get around the tendency of the hydraulic oil to compress and give a bounce effect. The lower actuator is locked by means of a locking bush called Octopus from ETP, a Swedish company who specialise in hydro-mechanical connection technology.
The Octopus is a hydraulic actuated joint, looking like 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. In this case an Octopus bush with a bore of 80mm was necessary to achieve the clamping force and a split collet transmits the force to the 45 mm diameter actuator rod.
The split collet has a flanged end which is fixed rigidly to the rear of the lower actuator. As the tablet is compressed and prior to final compaction, the 280-bar supply is applied to the Octopus causing the inner sleeve to contract and grip the actuator rod through this collet, providing a very rigid axial lock.
The action is fast and repeatable with a life of about 500,000 cycles. The Octopus bush itself is rated at 67kN axially, safely above the maximum piston force of 50kN. The bush is designed to transmit both torque and axial forces, but in this case it is being used only to close the collet onto the piston rod.
The result is an axially very rigid brake working on a smaller rod diameter but with the same axial clamping rating.With the bottom actuator locked, final compaction can be carried out using the top actuator and feedback signals from the load cell and the LVDT.
After compaction the tablet is ejected into an output carousel. This is sized according to the tablets that are being produced and a typical carousel would have 36 pockets. It is positioned by a stepper motor with integrated encoder so that the pocket where individual tablets are located is known and they can be tracked if required.
The ESH machine gives exact control that can be fine-tuned to match process requirements. The compaction cycle can be pre-programmed to enable either each tablet to be manufactured to a slightly different profile, or to manufacture multiple tablets from the same profile.
The usability of the Huxley Bertram machine is further enhanced by having up to three hoppers so that multi-layer tablets can be produced. These are driven into position by pneumatic cylinders and even fitted with a vacuum port to suck away excess powder and keep the die plate clean.
Hoppers can be removed by hand for cleaning, and if required loaded in safe areas. They are gravity fed and may have variable speed stirring paddles. The ETP Octopus bushes are supplied in the UK through drive and automation specialists Lenze Ltd. They are available in a standard range to clamp shafts between 30 and 100mm and special designs are possible.
Other application areas include the locking of machine slides and locking tool holders in machining centres. William Bertram describes it as A simple way to achieve a locking brake. The Octopus can apply the force locally and quickly .