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What are Backstops?

9/19/2012 7:49:22 AM | by Anonymous

Backstops

Also known as holdback devices, backstops are anti-runback devices that prevent the backward movement of conveyors used in manufacturing or other processes. Commonly used on inclined conveyors, these devices can also be used for overland conveyors in order to avoid the unusually severe shock loading that the conveyor would experience upon start-up, when the belt is not tight between the idlers and sags. The backstops placed in flat conveyors are essential as they aid the shock experienced by the belt upon start-up, preventing it from sustaining any damage. In inclined conveyors, backstops or holdbacks are important as they allow the belt to move in one direction but resist movement in the other direction. Therefore if the belt was to fail, the backstops would prevent the belt and the load on it from rolling backwards. This article would focus on backstops installed in inclined conveyors.

 

If an inclined conveyor was not installed with a backstop, in a situation where the belt fails, it could cause the belt to operate in a reverse direction due to the forces of gravity upon its loads. A reversing conveyor can cause several damages and losses to manufacturers, therefore most conveyors are preinstalled with backstops. If the conveyor was to rotate in reverse direction, the load that it would be carrying can fall off the conveyor as reversing conveyors can gain great speeds fast, causing the damage to the load and the conveyor system. It can even cause injury to any personnel operating or around the system as the load on the belt could fly off and hit anyone around, or the belt could break and hit anyone nearby. Not only that, the mechanical system of the device could also get damaged, requiring costly repairs. Industries that manufacture liquid substances and use conveyors to transport these around the factory would suffer massive spillages if the conveyor carrying these items was to fail. Backstops or holdbacks are essentially safety devices that not only ensure the safety of the products that they transport but also the safety of the people around it.

 

Backstops used in industrial conveyors are usually classified into low-speed or high-speed use. Low-speed backstops are typically used in industrial conveyors whose motor power exceeds 30kW to 40kW. The three basic low-speed backstop designs that have been or are being used in conveyors are rachet and pawl, differential handbrake and the overrunning clutch design. Each of these different designs offers different advantages and disadvantages. The rachet and pawl and differential handbrake designs are simpler with a lower price range. These designs may not be found on heavy duty conveyors used in the industry today as they offer some disadvantages such as being affected by dirt, have a backlash and subject to wear. Differential handbrake may also not be compatible with high stress concentrations and may malfunction. Due to these disadvantages, the prices of these designs tend to be lower as compared to over-running designs.

 

The over-running design backstops are created to provide precision operation. They are automatically engaging to transmit torque when the motion is against the direction of movement and freewheels when the rotation is in the direction of the movement. These designs are also able to provide wider range of torque as compared to the simpler designs of rachet and pawl and differential handbrakes. The two basic types of over-running designs are roller clutch and sprag clutch. Both these designs have minor variations to offer different function, but overall can support similar application providing similar benefits. The sprag clutch design is subject to wear unlike the roller clutch design. Both these designs are more costly as compared to the rachet and pawl and differential handbrake designs. However, they are able to function better than those designs as they are not affected by dirt, do not require any adjustments and can sustain high stress concentration.

 

Summary

Depending on the design a manufacturer chooses, the benefits and applications that are supported vary. Backstops are very crucial components of conveyor systems as they ensure the safety of operators as well as provide a sense of security that the goods would not get damaged.

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