Locking bush positions with torque limiting
Locking bushes from ETP have replaced mechanical clamping elements in a critical application on PET blow moulding machines. Connecting the lever arms that open and close the moulds, the ETP-Express bushes allow slip on the shaft of there is a stuck perform or bottle. This avoids damage to the machine and reduces downtime. Resetting is a quick and simple process compared to the old solution of a mechanical clamping bush which had seven screws to be torque tightened in sequence.
The ETP-Express, available in the UK from Techdrives, is a self contained bush that connects shafts to hubs without keyways. A single radial fixing screw pressurises an internal chamber and expands the walls, gripping the hub and shaft. Accurate positioning both radially and axially is easy as the screw is tightened. Once the screw reaches a travel stop, the bush is set and full torque is transmitted by friction evenly over the surfaces. Up to 1000 mountings are possible.
PET (polyethylene terephtalate) is widely and increasingly used as beverage containers because it is light, unbreakable, stable and taste free. It is also 100% recyclable so it is environmentally friendly. Blow moulding machines start with a 'preform' that is pre-heated to achieve elasticity. The performs are transported by conveyors and feed wheels into the blow moulds. Depending on the machine type, there can be from 4 to 36 of these blow moulds on a rotating assembly. Materials used are mostly aluminium to reduce inertia and achieve higher speeds - up to 72000 bottles per hour are possible.
The individual blow moulds are opened and closed by a lever arm, and these arms are fixed to the actuator using the ETP-Express bush. Under normal operation the bush acts as a backlash free connection. Should there be a jam from a misfeed, the bush slips at a predetermined torque without damaging the actuator shaft. Once the machine operators have cleared the jam, it is only a matter of seconds to reposition the mould by loosening the single screw of the ETP-Express. Expensive downtime is minimised compared to the previous mechanical locking bush that had 7 screws to be tightened. Furthermore the low and even surface pressures of the ETP-EXPRESS avoid shaft damage and aid a consistent slip torque.
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