Brakes provide reliable overload protection for blow moulding

The manufacture of PET bottles is an impressive process. During the so-called stretch blow moulding, a preform, which looks similar to a test tube with a threaded neck, is heated in an oven. Then a metal mould encloses the hot blank. This blank is first stretched longitudinally using a horizontal bar, and then it is inflated with an air pressure of up to 36 bar and so pressed into the mould: The result is one of the familiar plastic bottles. This process is repeated up to 80,000 times per hour on large, modern stretch blow moulding machines - meaning that approximately 20 bottles are created per second.

This high production rate in combination with the high pressures involved present operators with challenges regarding precision and safety. The processes are so tightly synchronised that only minimal machine tolerances are permitted. Otherwise, machine standstills or even serious damages are highly likely to occur. For example, if the mould does not close completely because the preform is not fitted accurately into the mould, then in worst case scenarios the high blow pressure will cause the entire mould to explode - and the machine will suffer severe damage or even destruction. Interruptions have particularly serious consequences on stretch blow moulding machines because all the components are linked to one drive line. This means that a synchronous process is guaranteed for the numerous moving parts, but the failure of any individual components will halt all production processes immediately.

Too fast for electronics

At such high speeds, any electronic control systems would be completely overburdened. It might still be possible to trigger an alarm, but the reaction would be too late. Therefore, mechanical torque limiters such as the EAS-Compact by mayr power transmission represent the best way to having safe stretch blow moulding machines. They are able to open the drive line and decouple masses within fractions of a second, for example on the large toothed belt pulleys via which the stretch blow moulding machines are frequently driven. Clutches in smaller construction sizes can protect moulds against destruction: They interrupt the process if a mould cannot close due to an incorrectly clamped preform. When the torque limiter is triggered, a proximity switch emits a signal and the drive is interrupted immediately, before the compressed air is blown into the mould. In this way, torque limiters reliably prevent expensive damages. In addition, by opening the drive line it is possible to remove an incorrectly clamped and possibly destroyed preform, for example, easily and without complications. In this way, it is not necessary for the production process to backtrack on itself, and fewer rejects are produced. Torque limiters therefore make restart easier, increase the machine availability and help to minimise reject costs.

The EAS-Compact torque limiters belong to the load-disconnecting clutches. They transmit the torque through positive locking. The torque itself is generated through the pre-tensioning of cup springs.

Load-disconnecting torque limiters disconnect the input and output in case of overload or collision, i.e. they interrupt the torque or force transmission. EAS-Compact clutches in ratchetting design are immediately ready for operation after elimination of the fault. Ratchetting clutches re-engage automatically, for example every 15 degrees, or at a defined position depending on the application, and synchronous clutches after 360 degrees. As a result, the latter are often the preferred choice for machines with rotary tables or carousels such as those used on stretch blow moulding machines.

Benefits of disengaging clutches

The EAS-Compact torque limiters in disengaging design, however, disconnect completely and almost residual torque-free. They remain in this disconnected position until they are manually re-engaged or are re-engaged using a device. Disengaging torque limiters are therefore particularly suitable for high-speed drives or drives with large mass moments of inertia - well suited for the high-speed discipline stretch blow moulding.

The EAS torque limiters by mayr power transmission can be adjusted accurately and work with high repeat accuracy. During fault-free operation, they transmit the torque backlash-free and therefore ensure a high service life and minimal wear. In this way, they also ensure the short reaction times necessary in order to disconnect the torque transmission in the stretch blow moulding machines quickly enough in case of overload. The company also provides special designs and constructional designs made, for example, from stainless steel with seals or a housing, for fields of application such as the food industry where wet conditions and cleaning liquids are part of the daily ambient conditions.

Torque limiters provide effective protection against damage to machines, downtimes and unnecessary rejects during the production of PET bottles. However, in order to reliably protect against overload damage, a torque limiter must optimally match the respective application; mayr power transmission therefore provides a wide, sophisticated product range. The family-run company can call on decades of experience, extensive expertise in development and design and rely on the most modern testing possibilities. And besides torque limiters, mayr power transmission also develops and manufactures reliable safety brakes.

The safety brakes are also suitable for use in stretch blow moulding machines. For example, high mass moments of inertia develop due to the weight of the metal moulds and the fast rotational movements in the carousels. If an emergency stop is triggered, safety brakes have to stop the moving parts rapidly and reliably and hold them in position. Safety brakes of various designs are available from mayr power transmission for the purpose, including a sealed version featuring the appropriate IP Protection, or a rustproof version for use in food production, for example.

mayr power transmission stands for complete solutions, and provides know-how and service from a single source. Learn more at www.mayr.co.uk.