More than just how heavy
The first question...and often the last that's asked by folks thinking about a pallet conveyor project is the weight of the loads. That's an important question for sure, but not the one that trips most plans up.
Based on the review of thousands of high-speed automatic pallet wrapper project application surveys - and then the complications that arise from incomplete and incorrect data - we've identified 5 specification details that are sometimes overlooked. They include:
- Width vs. weight
- Speed of existing conveyor
- Back pressure of accumulation on gravity conveyor
- Substrate (most often pallet configuration & orientation)
Let's unpack these one-by-one, roughly in order of least to most common.
Width vs. weight
Pallet conveyors have a clear spec for maximum weight - often in the 4-5,000 lb range for standard applications. That weight is calculated based on typical pallet widths. It gets tricky, however, with wider pallets because the maximum load capacity often decreases with increases in effective width.
Pass height and transfers are important, but the palletizer discharge may be radically different.
On the one hand that isn't really a big deal - it's possible to specify conveyor leg heights to adapt....except that by the time you get to a stretch wrapper, the cumulative impact of pass height, load height and process clearance (e.g. lifting the film delivery system high enough) may make a machine structurally impractical or exceed ceiling heights in a facility.
Speed of existing conveyor
That's the actual speed, today. Too often projects are based on a recollection of what the spec was, and people hesitate to measure the actual speed because they assume they need a tachometer or some other instrument.
Here's a perfect application of the KISS principle. No instruments required - simply measure a specific distance of conveyor (say 5 or 10 feet) and time how long it takes for the leading edge of a pallet to advance from the start point to the end point. Then it's a simple calculation to determine the actual speed.
Please don't guess, recall or assume! Check the speed of your existing pallet handling conveyor.
Back pressure of gravity conveyor accumulation
Everyone loves gravity conveyor. Fewer chains to lube, fewer drives to maintain, lower cost, etc. So often we see systems that call for extended gravity discharge for taking off accumulation. Makes sense, right?
Here's the rub. Let's say your pallets are 40 X 48" and weigh 2,500 lbs. each. You want plenty of accumulation to allow your fork lift drivers and warehouse teams time to maneuver, so you calculate that you'd like an extra 20 feet of gravity pallet conveyor for discharge accumulation.
Well, that means you'll have somewhere around 13,000 lbs of product sliding down the conveyor when you drop your pneumatic gate....building momentum and then SUDDENLY stopping when you throw your gate back up. That's not likely to stop gently...imagine the wear and tear on your system, not to mention the possibility that you can't get the gate up in time or there's an inadequate gap between pallets.
The point should be clear - gravity conveyor is appealing for various reasons, but there's a practical limit.
While this is often pallets, occasionally it's slip sheets. So let's look at a range of factors.
Small units (e.g. small individual corrugated cases), especially when column stacked, are susceptible to the instability inherent in traversing normally spaced rollers while interlocked stacking or larger individual units are generally more forgiving. That's made more challenging with slip sheets, but even the leading edge of pallets can dip and rise depending on the width, orientation, and placement of bottom boards.
Generally, when pallet bottom boards run perpendicular to the rollers (e.g. parallel to the direction of flow) stability and transfers are easy. In contrast, when the boards are parallel to the rollers, space between boards and distance between rollers can be critical.
Transfers, however, can be real brain teasers, particularly when considered with pallet type, loading and unloading orientation and position.
Two-way entry pallets, for instance, which are loaded in one direction, traverse pallet conveyor to a reorientation (e.g. 90 degree) transfer and then unloaded from a different direction, can not only impact roller spacing, but can even require orientation to accommodate the loading and unloading directional requirement which may be complicated by plant floor space constraints. In other words, the substrate, or pallet, becomes a factor not just in normal conveying, but even in the transfers, loading, and unloading from pallet handling conveyors.
Project application surveys
These are the most common oversights....experience tells us there are many more details to manage - and many of those seemingly minor details can have huge practical implications during installation and commissioning.
That's why the industry relies on application surveys. Sure they're long and tedious. And often the information may not seem relevant and may be difficult to collect.
Those questions are there for a reason, though, and often the reason is one that is based on a hard learned lesson.
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This post was published on December 19, 2016 and updated on October 2, 2017.