Correctly located backstops reduce downtime.
A backstop or holdback is a device that prevents rotation in one direction while allowing free rotation in the other direction. An automatic backstop engages without the use of any external switching mechanisms or sensors.
Automatic backstops are used on bucket elevators and uphill conveyors to prevent rollback in case of an unexpected shutdown or electrical failure. There are several types of backstops, the two most common being either of the roller ramp design or of the sprag design type.
The decision of where to locate the backstop is an easy one. The backstop should be placed directly at the device that you want to protect from reverse rotation. Unfortunately, the decision often becomes one of compromise. There are many points that should be considered and we will investigate four of the main criteria:
* Service Life
* First Cost
There are two main locations for the backstop: the high-speed shaft and the low-speed shaft. The backstop can also be put on an intermediate speed shaft between the high-speed and low-speed shaft. Placing the backstop on an intermediate mediate shaft compromises of the high-and low-speed locations.
Safety: If the system is shut down under normal conditions, the high-speed backstop is quite safe. However, if a component failure occurs such as a coupling or gear, then the backstop will not be able to stop the conveyor because the failed component is between the backstop and conveyor.
Maintenance: Externally mounted high-speed backstops are usually quite easy to maintain. They are accessible and can be handled without difficulty because they are usually relatively small. Most external high-speed backstops do not have oil sight gauges so the oil plugs must be removed to check the oil level. Internal high-speed backstops (mounted in the speed reducer) require partial disassembly of the speed reducer should the backstop require repair.
Most backstops will not tolerate EP (extreme pressure) lubricants. This fact must be considered when using internal backstops since most speed reducer lubricants are of the EP type. If any service must be performed between the high-speed input shaft of the speed reducer and the driven shaft of the conveyor, the high-speed backstop will not hold back the conveyor when couplings or speed reducers are disassembled for maintenance.
Service Life high-speed backstops are of the sprag design. The majority of the sprag types incorporate sprags that drag on the races during conveyor operation. This constant metal on metal rubbing at high speeds will cause short life. There are a few sprag design clutches that have sprags that lift off of the races while operating. These lift-off designs are capable of long life.
First Cost The high-speed shaft usually has the smallest diameter and always carries the smallest torque in the drive train. Since the backstop cost depends on the bore and torque capacity, the high-speed shaft is the lowest first cost location for the backstop.
Safety: The low-speed shaft is the safest location for the backstop. If there is any component failure in the drive train such as couplings or gears, the backstop, being located at the driven pulley will prevent rollback.
Maintenance: low-speed backstops are easily accessible and usually have oil sight gages. However, the size of the low-speed backstop may make simplest removal and repair work cumbersome.
Having the conveyor pulley locked up by the backstop allows components such as couplings and speed reducers to be replaced without concern for the conveyor rolling back.
Service Life: Because of the slow speed of the shaft, this location provides the least wear on the bearings as well as the locking elements (sprags or rollers) and races. A roller design will give longer service life because the rollers are rolling on a thin film of oil. Many roller design backstops have provided more than 30 years of service. In the sprag-type backstop, the sprags are in constant contact with the races resulting in wear and shortened life.
First Cost The low-speed shaft carries the highest torque and usually has the largest shaft diameter. This will dictate the highest first cost for the low-speed backstop.
The location of the backstop necessitates that you look at why a backstop is required. If the backstop is required for ultimate safety and is required to allow service work to be performed on any number of system components without unloading the conveyor, then by all means the low-speed backstop is the proper choice.
If the backstop is required only to prevent rollback under normal conditions, then the high-speed backstop is sufficient.
PHOTO : Marland automatic backstops are a must with inclined conveyors.
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|Title Annotation:||rotation prevention device|
|Publication:||E&MJ - Engineering & Mining Journal|
|Date:||Jul 1, 1991|
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