Overrunning clutches and backstops play a fundamental role in heavy industry such as mining and iron and steel, but also in food processing and all applications in which goods and materials are moved by conveying equipment. Their principal task is to prevent reverse running and uncontrolled acceleration of the system in the event of a drive failure (e.g. loss of supply voltage). It is important, therefore, that these components are specified and designed with a high degree of precision.
Backstops are a fundamental safety component for preventing an uncontrolled change of direction and speed under the influence of gravity. A fully loaded, blocked belt conveyor system can impose huge torques on the drive. However, with the right know-how and an in-depth knowledge of the prevailing conditions of use, it is possible to design the backstop in a compact and cost-efficient manner and still safeguard the drive train reliably against overload. Backstops also merit closer attention from the point of view of technical aspects of occupational safety and health and accident prevention.
Design principles
To design the backstop correctly, the plant designer requires exact data regarding the torques present on the conveyor line. In cases of doubt, many customers and designers work with a larger safety factor, although it is naturally desirable to avoid over-dimensioning and unnecessary costs.
Plant operators should therefore give the job of determining the torques occurring to experienced design engineers, who are able to calculate these very accurately. This information helps the overrunning clutch manufacturer in turn, working in tandem with the customer, to coordinate the performance data of the backstop precisely to the torques to be expected.
In the ideal case, the backstop is designed in parallel with the drive unit, because the motor and gearbox play a key role for its dimensioning and performance parameters.
Backstops can either be flanged directly onto the transmission housing on the fast-running transmission shaft or mounted externally on the shaft end of the slow-running transmission shaft. For reasons of greater ease of servicing – the transmission can be dismantled without having to strip down the belt – some users prefer to have a transmission mounted separately from the conveyor line. In this case an external backstop between the conveyor line and the transmission is the best solution.
The component must have a suitably high torque capacity here, which is reflected in the construction size and possibly the procurement costs. If the aforesaid advantages for maintenance can be sacrificed, a transmission-mounted design of backstop is commonly recommended. Since the torque loading is lower here than with external mounting, it can normally be of smaller dimensions and be purchased at a lower price.
Load balancing and torque limiting
The number and configuration of backstops are dependent, for the most part, on the application. On conveyors with multiple drives and a corresponding number of backstops without torque limiting, it should be assumed that only low load balancing takes place. The reason for this is the delayed engagement of the various backstops (RS) in consequence of tolerances in the stops, different belt elongations at the stops, different friction states (efficiency rates) in the belts etc.
Figure 1 clarifies this scenario. It is shown that RS1 has to transmit considerably more torque than RS2 – in an extreme case, even the entire reverse torque. In practice, safety factors of up to 3 times the drive torque result from this. Load balancing between RS1 and RS2 does not take place in this situation. The point of RS2 can then only be a safety backup, which at the same time means, however, that RS2 must be designed with the same safety factor as RS1. It isn’t hard to see that both RSs turn out to be disproportionately large in this way.
