Top 5 Common Design Problems with Pallet Conveyor Systems (All Avoidable).

Posted on November 29, 2020 by Esoteric Staffing

Pallet conveyor systems have been used in industrial automation systems for decades. Despite a long history and thousands of applications, there are common design problems that appear frequently on newly installed pallet conveyor systems.

Design fundamentals are old hat to experienced industry veterans. However, as was pointed out in the article "3 automation pitfalls and mistakes to watch out for", more than half the people working in industrial automation have less than 5 years' experience. New engineers (and their customers!) are learning the hard way.

The top 5 common design problems occur due to lack of experience or budget constraints. All of them lead to a pallet conveyor system that doesn't function as expected. Without further ado, the top 5 most common pallet conveyor design problems are:

#1 Insufficient Pallet Guides.

Pallet guides do what they infer. They guide pallets. You'd think that a pallet with 2,000 lbs of product would convey in a straight line on roller conveyor. However, that is not the case. Pallets tend to drift to one side or rotate. Often in a surprisingly short distance.

The difficulty comes when changing directions. The pallet is no longer properly aligned. Depending on the nature of the transition or direction change, this will cause the pallet to get hung up, hit an obstruction, or not convey smoothly in the new direction. The direction change could be via turntable, transfer, from roller to chain. Whatever the case, without sufficient pallet guides, the pallet won't be where expected all of the time, leading to a stoppages and time-consuming recovery.

Pallet guides can be designed to funnel the pallet and guide from both sides or can be single-sided. Pallet guides should be rigidly attached to the conveyor frame yet be adjustable where necessary. Pallet guides should not "flex" or move. Yes, common sense applies. And yet…

It is rare to see a system where guides have been omitted entirely. The common mistake is insufficient pallet guides (quantity and quality). The supplier or customer will fight with twisted pallets, jammed pallets at transfers, and any number of headaches. Yes, they cost money to design, install, and set correctly. Guides are a critical step toward ensuring a smooth operating pallet conveyor system.

#2 Over-reliance on sensors.

Sensor technology is highly advanced and extremely accurate. In combination with variable frequency drives, they can stop pallets within fractions of an inch. Accuracy of stopping does suffer when using motor starters and sensors. However, there are times when physical stops are appropriate. Indeed, physical stops require additional time (and money) to design, install, and test. However, physical stops will increase system reliability. Some examples of where stops should be employed:

First, when changing direction via right angle transfer when the conveyor also continues straight. The purpose of the stop is to stop the pallet precisely and square the pallet – ready to convey in the next direction properly aligned. A temptation is to utilize a sensor only to stop the pallet, which is lower cost and complexity. But the sensor does not allow the pallet to be squared against a positive physical stop. Any deviations in pallet squareness will be perpetuated. Each physical stop is an opportunity to "reset" the pallet location positively.

Second, when stopping pallets on turntables, lifts, or other "end of run" locations. Sensors are highly reliable. Except for the times they aren't! A physical stop prevents the pallet from running off the end of conveyors. If a physical stop is not possible, consider a redundant sensor to stop the pallet before disaster strikes.

Third, stating the obvious, "sensors sense things". Duh! Except in a pallet conveyor system, those sensors are looking for a specific part of the pallet or load which isn't always there. Perhaps the sensor is looking for the leading bottom board on a pallet. What if that bottom board is damaged? Perhaps the sensor is looking at the side pallet stringer or corner block. What if that area is damaged? Perhaps the sensor is looking for load/product. What if that load or product is shifted or missing? In many cases, that means pallets don't start or stop when expected. Physical stops can mitigate these challenges or at least prevent a catastrophe.

#3 Long Drive Sections.

The first question to ask is: are you buying "equipment" or a "system"? If you are purchasing a system, then focus on defining requirements. Check out the Esoteric article "3 Contract Requirements for Automated Warehouse and Logistics Systems". In the case of system purchase, review of conveyor drive lengths should be sanity check – then hold your supplier accountable for performance.

If you are purchasing "equipment", that infers that the customer is responsible for how the system performs in terms of rate, accumulation capacity, etc. There are two scenarios under which it makes sense to purchase equipment rather than a system. 1) the system is very simple 2) you are an experienced designer of pallet conveyor systems and will be responsible for how the equipment performs as a system.

When purchasing a system, often conveyor lengths are not defined at the proposal stage. Complex systems may require months of system design to arrive at drive section lengths. Assumptions are made in the proposal process based on prior experience. This is where "common problems" emerge.

At first glance, it wouldn't seem that long drive sections would affect system performance. You'd think that speed (30, 45, 60 fpm) dictates performance. But that is not true. Sequencing affects performance. Accumulation affects performance. Every drive section costs money. Big money.

System suppliers (fixed bid) are incentivized to have fewer drive sections. Customers generally benefit from shorter drive sections to maximize accumulation and optimize sequencing (within reason of course, in consideration of maintenance, power supplies, etc.). However, much accumulation you've got, you'll probably wish you had more.

There is no rule-of-thumb answer that covers all situations. Each system will have a different set of requirements. If you aren't in a position to understand how drive section length affects performance, then find an independent Expert that will review designs on your behalf.

#4 Field Devices and Field Wiring. Marginal workmanship.

Years ago, field devices such as sensor mounting brackets would be factory designed, with consideration as to proper placement, guarding/protection, and mounting locations. Due to constrained resources, compressed budgets, and what is now common industry practice, field devices have become "field designed and field mounted" using off-the-shelf multi-use brackets. Unfortunately, installation personnel rarely have experience and system knowledge to design these systems well on-site. Invariably, field devices are installed wrong the first time and will need to be fixed.

A few common examples of improper field device installation:

  1. Sensor brackets through-bolted into chain guards, meaning that preventative maintenance requires dis-assembly and subsequent realignment of sensor brackets.
  2. Sensor brackets mounted too close to moving pallets and loads, subjecting them to damage. Sensor bracket location should take into account load overhang, slip-sheets, etc.
  3. Sensor brackets mounted in locations subject to falling boxes/product, twisted pallets, etc. – meaning that simple faults can turn into major recoveries.

Craftsmanship isn't what it used to be. Low-voltage field wiring is critical to reliable system operation. High-quality field wiring simplifies diagnostics and component replacement. Cable management and cable routing should be thoughtfully installed. They rarely are. This is a case where relying solely on written standards is insufficient. The supplier should provide pictures (contract exhibits) of representative work that illustrates installation quality of low-voltage wiring. Make sure it's done properly the first time. If the supplier indicates their first goal is to get the system running, then go back and fix it later – don't allow it. It'll always look like a second-rate installation.

#5 Guarding Installed After Damage Has Already Occurred.

Sufficient guarding is rarely included from the get-go. Why? Because it is easy to say, "we'll do it later". High quality guarding doesn't make the system run faster or smarter. High quality guarding doesn't get more orders out the door. High quality guarding doesn't increase the amount of accumulation. The only thing it does it protect your investment from damage!

For pallet conveyor systems and forklift trucks, guarding needs to be strong. As-in overkill strong. As-in stronger than you think it should be. High quality guarding is expensive (until something gets damaged, then it isn't). Guarding needs to be designed to take a strike from a loaded lift truck, and then in the next moment be removeable for maintenance.

Guarding also happens to be an area where conveniently (sarcasm noted), nobody is an expert. System suppliers will claim their expertise is providing sophisticated equipment, controls, and software and that the customer should know best how to guard the system for their operation. A building contractor will be reluctant to provide guarding for equipment they did not design or install. Nobody claims expertise or responsibility. So, it is omitted until something gets damaged.

Manufacturers of conveyor equipment (conveyor equipment manufacturers that bend, punch, and weld steel) will offer guarding for the equipment they supply. Check with them on a variety of guarding solutions. They'll have options ranging from light-duty all the way through heavy-duty, PRO grade, super extreme. Do the latter.

Are you planning to install a pallet conveyor system? If so, be sure to consider pallet guides, sensor application, drive section length and accumulation, field devices and field wiring, and guarding systems in the design. You need to Find an Expert system designer. Don't start your project without them!