Do you Need a Low Back Pressure Conveyor?

By: Franklin

Although I am far from being a mechanical engineer I do have some experience troubleshooting conveyor applications, some of which include lineshaft conveyors of various manufacture. The most common problem seems to be a loss of drive and sometimes this is caused by load variations over and above the original design of the conveyor, however other culprits can prove to be the drive transmission components. But perhaps I'm getting ahead of myself, lets first discuss a more powered product in a specific environment to understand why someone might use this particular type of conveyor.

In a powered roller application, the roller turns and the product moves. Some examples are chain driven live rollers where a direct transfer of drive is achieved through transferring the torque from the drive to the rollers via a chain to one of two sprockets mounted directly on the roller (think of a bicycle sprocket and chain configuration). The second sprocket on the roller then transfers the drive to an adjoining roller and so on down the line. This type of direct drive configuration is usually used for heavy product as these rollers have high torque and do not stop turning unless the conveyor is stopped altogether.
There is also belt driven conveyor that allows for some roller stoppage but they maintain a pretty high roller turn pressure as well. In this configuration there is either a multi-ply flat belt or "V" belt that drives the rollers from beneath through a direct pressure method and allows for the rollers to stop if there is excessive accumulation on the rollers above. They may not necessarily be designed for product accumulation however, due to the pressure of the drive belt against the rollers.

Given the above examples, let's imagine a company that ships various products out in one container. In a typical pick facility, the container moves down a conveyor having occasion to stop and start multiple times to be loaded with different items and then finally to a shipping area where the cartons accumulate until they are removed by a shipping clerk. This provides two excellent applications wherein a minimum of roller pressure is necessary. In the pick zones the carton needs to stop and remain stationary momentarily or longer and in a direct drive conveyor this would require the whole system to be turned off and on each time to prevent undue back pressure (the cartons behind pressing against the stopped carton). The second example is the shipping area where the filled cartons accumulate waiting to be palletized or otherwise removed from the conveyor. A direct drive system would transfer the accumulating carton pressure to the forward most carton making it difficult for the clerk to remove or possibly crushing the carton. But a system that allows the rollers to stop turning eliminates this likelihood. It is for purposes of this type that lineshaft conveyors prove invaluable.

The way a line shaft conveyor works is that there is a steel shaft that runs the length of the conveyor perpendicular to the rollers above and is usually orientated to one side of the bed. This shaft is directly driven my a motor / gearbox assembly and much like the chain driven live roller conveyor mentioned above it does not stop turning, but that's where the resemblance stops. The term for this conveyor type is minimum pressure accumulation and as outlined, the purpose of this conveyor configuration is to allow for stops, starts and accumulation to occur without unduly affecting a throughput process. To allow for this effect, the torque must be transferred from the shaft to the rollers with enough force to restore movement of individual products once they have stopped but also to allow for the rollers to stop turning once accumulation occurs. The methodology is that under each drive roller there is a pulley on the shaft that turns freely with a polyurethane circular band that connects the pulley to the roller above. As the band is under pressure this allows for transfer of drive but if the roller is stopped, the pulley simply slips on the driveshaft until the pressure on the roller is relieved and then the pulley will begin to rotate transferring drive and moving the product.
The rollers on this conveyor are typified by multiple grooves orientated directly over the shaft that the drive bands travel in. One band will carry the torque from the shaft to the roller and another band carries the torque from the drive roller to the adjoining roller(s). Transmission from one roller to another is limited as they will lose torque but it is possible for multiple rollers to function in this manner dependant on the loads.
This conveyor is very useful but also much maligned and it is my experience that those unhappy with these units are usually experiencing a loss of drive due to a driveline mis-configuration or a modified application that the unit was not originally designed for.

Some common lineshaft issues may concern the drive pulleys that are located directly under the rollers that maintain their position via spacers. If the pulley and rollers are not directly aligned this may cause a strong pull for a while but the user will see the shaft components wear faster than normal. Worn or incorrectly sized drive bands can cause loss of drive and a common practice is to deliberately undersize the bands as to provide greater torque. This is a temporary fix and can ultimately cause premature failure of the band, wear of the lineshaft pulleys, distortion of the lineshaft, premature wear of the lineshaft bearings, and premature wear of the roller bearings as well as belling of the roller retention holes in the conveyor frame. A better solution is to confirm that the current usage is consistent with the original conveyor configuration and then back into an examination of the drive train.

An additional challenge for the maintenance crew dealing with these conveyors is the downtime required for replacement of the lineshaft components. These are wearable parts and will require replacement eventually which entails dropping the lineshaft and sliding the old pulleys and o-rings off and sliding the new ones in place (as well as u-joints, sprockets, chain couplers and lineshaft spacers). For this reason quick replacement drive bands and split pulleys have been developed to allow for quick changes and maximum uptime until the next scheduled maintenance interval.

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