Belt conveyors are relatively simple machines that, in some ways, haven’t changed since their invention more than a century ago.
They still use a wide flexible belt continuously moving from load point to discharge carrying material. A pulley on each end directs the belt into a continuous loop being moved by an electric motor at the discharge pulley and tensioned with take-ups at the tail pulley. The material carrying run is typically supported in a trough by idler sets spaced along the length, and the return belt run is also intermittently supported by idler rolls.
While this simple description is true, it doesn’t do justice to the decades of refinement and innovation found in modern conveyors. Today’s world of higher inflation, skilled worker shortages and increased competition is generating additional interest in finding ways to improve the effects of these trends.
Contact seals
An area where this manifests is reviewing the power used by a conveyor.
Portable equipment is often used in dirty, dusty and wet environments with frequent high-pressure washdown. Historically, North American-designed idlers have anticipated these challenging environments and included contact seals in the standard design.
Usually, this includes contact seals built into the bearing and an additional auxiliary contact seal with the external labyrinth seal. Idler designs without contact seals exist, and they do use less power.
Lab testing and field experience shows designs lacking contact seals struggle with contamination, especially in pressure wash situations. Contact seal designs have proven to regularly provide multiple seasons of maintenance-free operation, even in washdown applications, without significant power difference to the conveyor.
To put it in context, a typical 100-ft. conveyor will have about 100 rolls on it, and about 1.5 hp will be used to turn rolls designed with contact seals. Depending on your electric rate, that’s about $1 to $1.50 per eight-hour shift.
Designs without contact seals may cut this power use in half. That’s a large difference for the idler but in portable conveying, the difference in real power use by the conveyor is relatively small.
The most economical and ecological choice is to use an idler designed with contact seals, which will minimize maintenance cost, reduce safety risks from fewer changeout events and put fewer failed rolls in the landfill.
These low-drag designs have a place, typically on very long, flat overland conveyors. In these cases, there may be about a five- to 10-mile flat overland conveyor where the difference may be 300 to 600 hp.
In these situations, the capital expenditure and power use differences will be significant, and less effective sealing for those gains is worth considering.
Unfortunately, it can be difficult for equipment users to determine which idler design is used in the equipment they are considering. No standards of construction exist, and all idler manufacturers have options available.
In general, most North American-made idlers include contact seals, but not all. Idlers from other areas of the world tend to design without contact seals, but often provide contact seal designs when requested.
The only sure way to know is to discuss it with the conveyor OEM or idler supplier.
Tackling take-ups
We also see these macro trends influencing pulley designs.
The tensions in the belt are the primary load on pulleys. Most portable conveyors are tensioned by manual screw take-ups. Screw take-ups are economical, easy to use and effective at tensioning the belt, but can be too effective.
Once the belt is taught, a relatively small amount of take-up travel can drastically increase belt tensions, and the mechanical advantage of the screw in most take-ups is often capable of creating much higher tensions than needed to drive the belt and more than the components can withstand.
Pulley designs have been improved to reduce sensitivity to this variability of the belt tensions.
Originally, pulleys were directly welded to the shafts, and screw take-ups could easily create overloads resulting in fatigue failures at the shaft welds. This led to designs with single-tapered bushings with matching hubs welded to a plate end disc in the pulley. Designs using many different bushing styles were created, and many still exist.
By eliminating the shaft weld, a significant life improvement was realized, but failures at the hub weld persisted. Designs eliminating the hub weld are now commonly available and have all but eliminated failures from the variability created by screw take-ups.
In the interest of reducing cost, we occasionally see conveyor designers having interest in these historical designs without fully realizing the trade-off in performance being made.
Closing thoughts
The seemingly simple conveyor can quickly get more complicated as we look deeper. The demands of our world are pushing us to continuously innovate, improve and cost reduce the products we design and use. To avoid unintended consequences, it is important to consider all aspects of these decisions.
Tim Wolf is vice president of engineering at PPI.