Getting the Right Magnetic Conveyor
Match the characteristics of the conveying system to your requirements
By Brian J. Hogan
The chip conveyor on your machining center or stamping press has just given up the ghost. So where do you go from here? Do you contact the company that built your machining center or press system and buy a replacement, or do you get adventurous and find out who builds the best, to get a worry-free replacement?
Let's say you've decided to see if you can save some money, and begin to look around and find out what types of conveyors are available. What you'll discover is that not all magnetic conveyors are created equal, and even when conveyors are competitively priced, some suppliers could offer you a much better unit compared to others. So what do you look for to get the perfect conveyor for your needs, whether it's to replace one you have or purchase one for a new application?
If your application demands a replacement conveyor, you can get the specifications from the one that's no longer functioning, and shop them around to various magnetic conveyor companies. But if you're in the market for a new conveyor for a machining system, Product Manager Bruce Kiwala from LNS America (Cincinnati), a conveyor manufacturer, says: "Typically you want to look at the chips that are coming from the machine tool to make sure they're small and broken. This is because any long, bushy chips that would breach the magnetic field from one magnet set to the next will short them out. So these types of chips aren't a good candidate for a magnetic conveyor. But any broken ferrous application is perfect for one."
"Jam-ups or slowdowns in conveyor systems can undoubtedly offset machine tool productivity," says Dan Zimmerman of Eriez Magnetics (Erie, PA). "The solution to the problem is a reliable conveyor system that promotes smooth, uninterrupted production flow.
"The most important thing when selecting such a system is cost. In many peoples' minds, cost encompasses three things: initial price, operating expenses, and maintenance requirements. Another important factor that is often overlooked is the cost/performance ratio per dollar of investment. By balancing all the variables, a parts and scraphandling system can be designed or purchased to improve productivity and any costs can be recovered.
"Material dropped on a magnetic conveyor can damage it," Zimmerman adds. "Because all the moving parts are inside the conveyor housing, denting the external housing could cause the magnets to hit the back side of the slider. This could create premature wear of the slider bed and damage to several or all of the magnetic elements. If material falls a considerable distance before contacting the slider bed, an impact plate or diverters might need to be installed to blunt the fall of parts before they hit the slider bed."
For a new application, says Magnetics Product Manager Bob Mount of Storch Magnetics (Livonia, MI), "From an engineering standpoint, the first thing I want to look at is the conveyor size. I want to know what it's handling. Is it handling chips, press stampings, other types of ferrous parts—what's actually being conveyed? Then the next thing I want to know is how much needs to be conveyed.
"A conveyor using magnets has them spaced a certain way to allow them to run at a particular speed. We have a certain number of magnets per minute going by, and each magnet will carry 'X' amount of load. In one instance we would multiply this out and have 60 magnets a minute moving 1 lb [0.45 kg] per magnet, giving us a load-carrying capacity of 60 lb/min [27 kg/min]."
There's another point to consider, says Zimmerman. "With a magnetic conveyor, the material size and shape are very important. Material that can contact more than one of the magnetic elements, spanning across them, might cause magnetic bridging. When this condition exists, the conveyor will not move the material. When the material spans two magnets, it can either move along with the leading magnet or stay back with the trailing one. If bridging occurs, magnet spacing might need adjusting and conveyor capacity could decrease."
"The other thing we want to know is how the product is coming onto the conveyor," Mount states. "Magnetic conveyors can be a lot more sensitive than hinge-belt conveyors. They require a more even flow of product. If you're handling 500 lb/hr [227 kg/hr], for example, you can't just take 500 lb and dump it on a magnetic conveyor and expect to move it. You have to take 500 lb/hr and split it evenly across that time, so that you have an even load."
"Furthermore," says Mount, "we want to know about the application. If the conveyor is going into a machine tool, are there wet or dry chips? If they're wet chips, what is the coolant type? Is it oil or water-soluble? If it's water-soluble, it's good to know what the composition of the solution is so we don't have an interaction with the seal material.
"There are certain coolants that actually inhibit the discharging of chips on a magnetic conveyor. It's very rare, but it does happen. They stick right to the slider plate, and when the magnet goes around the head and releases the chips, the chips don't want to let go of the conveyor's beltless slider plate."
Once you have a good idea of the size conveyor you'll need, the next question is how it's built. Because magnetic conveyors should be sealed, you need to know what the internals are all about to know if you're getting one that can give you 20 or more years of service, and possibly even be rebuilt.
In most magnetic conveyors, an electric motor drives a chain that rides in guides along the conveyor's length. The chain drives magnets along a stainless steel faceplate, and the ferrous material is moved across that face plate. Magnets, usually a long-life ceramic type, are spaced along the chain at certain distances that depend on the load they are conveying. They don't contact the faceplate, but are positioned at a preset gap beneath it. The guides make sure this gap is properly attained and is uniform along the conveyor's length. Typically made from 11 to 16-gage stainless steel, the conveyor's faceplate is also called the top plate or slider plate. It allows ferrous materials to slide along its surface, and is usually the main wear item.
Larry Farr, president of Livonia Magnetics (Livonia, MI), notes that some companies build environmentally friendly conveyors using plastic internal guides and mechanisms. He says that although the design sounds like it might be good for the environment, this type of construction is "not working out very well, because the plastic tends to wear out, and these manufacturers are defeating the objective of giving customers a long-life conveyor."
Both Livonia Magnetics and Storch Magnetics use an oil-bath system. The conveyor housing is an oil-tight unit, and the chains and sprockets are run through a lightweight-oil bath. The seals used should never wear out, because there isn't anything within the conveyor that will cause wear on them.
Kiwala mentions that his company produces a conveyor that should offer a customer no short-term maintenance. "The way we design our conveyors to achieve this is by using a self-lubricating track. The track is actually a Teflon-impregnated plastic material, so it's self-lubricating. Because the Teflon is impregnated into the plastic, the track acts like a nice, slick surface for the chain to ride in. So we don't need to add any lubrication to the inside of the conveyor frame."
Storch's construction is a bit different than that used by most conveyor producers, says Mount. "We have a double C-track construction, and the C-track is bolted to the conveyor body, so that makes it readily replaceable. We lasercut all of our conveyors, and we have an electronic file on the conveyor. If the customer damages the tracking in the conveyor, it's just a matter of pulling up the file, recutting the pieces, bending them, and shipping them out. Or we replace them in our own shop.
"If you don't have precision-cut components, then it becomes a manual process to fit all the pieces to the conveyor. If the customer is not experienced with fitting track into a conveyor, they're not going to get the right gap between the magnet and the slider plate, and that will cause problems. We bolt the slider plate and the side guides to the conveyor body with grade eight fasteners rather than with self-tapping screws. Doing so provides a better fastening method. If you have an over-zealous fabricator who is using self-tapping screws and they're too tight, they'll strip out, but the fabricator is just going to leave them in the conveyor. What will happen, especially if you have a conveyor that's under water or under coolant, the self-tapping screw is going to provide a leak area, because there's not a tight seal like you would get using a nut and bolt.
"Self-tapping screws can also create a chip that spins and will grab, tug, or tear at the seal within the layers between the conveyor"s body and the slider plate, possibly causing a leak."
The top plate of the conveyor should be a 300-series stainless steel, according to Mount, although Farr says that some magnetic conveyors today use manganese on the slide face. "This material work-hardens, and it's a lot harder than the conventional stainless slide face," Farr explains. "Therefore, you can go to a 3/16" [4.8-mm] thick manganese sheet instead of a 16 or 11-gage piece of stainless and not have to use an impact plate, because the manganese offers good wear. It's a lot more expensive, but it allows you to skip replacing the stainless steel skins."
"We're using an oversized roller chain that rides in internal tracks on our conveyors," Farr adds. "Some conveyor manufacturers use a RC chain that is similar to bicycle chain. Our magnet attachments are on the magnets rather than the chain. What this allows us to do is service a conveyor, or add or subtract or move magnet centers, without breaking the internal chain. With a RC chain you have to find a connecting link or cut the chain, and add a half link or master link to move the magnets around, because these attachments are on the chain not on the magnet."
When it comes to the type of magnet, Kiwala says that the magnet must have the correct amount of magnetic attractive force, so it won't wear down the faceplate material. "We're trying to balance the difference between the pull-down of the chips to the slider face with the amount of friction that it takes to pull them off. Doing so will create a nice balance, so we really don't need to worry about the slider face wearing out."
In some applications, says Farr, Livonia Magnetics uses neodymium magnets that offer 30–40% greater magnetic attraction than conventional ceramic material. The advantage is that these magnets are so much stronger that the user can employ a small conveyor for a large part.
Although magnetic conveyors all look the same on the outside, it's the inside that really counts when you are looking for a quality product. The wider a conveyor becomes and the longer it is, the more important the kind of tracking and chain becomes, according to Mount. "As the chain becomes wider and longer, it becomes a critical wear component. What happens is that the magnet is longer and weighs more, and it's also carrying more load on the conveyor's surface. Even though the chain is the same size, it has to carry a heavier burden. The heavier load just magnifies the wear. Whether it's an 8" [203-mm] wide conveyor or a 20" [508-mm] wide one, or 5' (1.5-m) long or 25' [7.6-m] long, you still have the same track and magnets."
To compensate for this greater weight, some conveyor manufacturers use a heavier-duty chain and a thicker slider plate for the magnets and the conveyed parts. Mount also says: "The gap between the magnet and the slider plate gets very critical with larger conveyors. If that gap is too great, then the capacity of the conveyor is reduced. If it's too small, then anything that forces the slider plate to move inward slightly will allow the magnet to rub against it. Eventually, the plate can cave in and jam the conveyor."
Further, according to Mount, the speed of the conveyor has a lot to do with wear. Obviously, the faster a magnetic conveyor is run, the shorter its lifespan. "We try to run them as slow as we can in an application, and compensate by magnet centers to carry a particular load. So there are things we can do with magnet centers and the speed of the conveyor to handle particular applications."
As to conveyor speeds, Kiwala notes that some companies offer standard magnetic conveyor models with a variable-speed control. This design allows belt speed to range from about 2–9 fpm (0.6–2.7 m/min), and allows the user to adjust belt speed to match the chip flow that's created. What's important about this capability is that the slower the conveyor runs while still maintaining good chip removal, the more time the chips have to drain the coolant down the incline of the conveyor, reducing coolant loss.
In summary, Mount asserts that what differentiates the product's service life is its construction. One way to extend the service life is to make sure that all the parts on the conveyor have a tight, close tolerance, and that the conveyor is sealed properly. If you don't have a seal and the conveyor is submerged, coolants might get in, and metal chips can infiltrate and abrade the track or chain.
Another conveyor-chain issue, in addition to the track-size of the roller on the chain, is that different grades of chain are used for conveyors. Some are just a hollow-pin-style chain, some are hollow-pin and prestretched, some use a solid pin, and some a prestretched solid pin. The advantage of a solid pin over a hollow pin is it has a much higher tensile strength. Therefore, if you have a conveyor jam, it could snap a hollow pin chain, while a solid-pin chain will hold better. Also solidpin chains are prestretched before installation.
The manufacturers mentioned above say that they rebuild conveyors that have already provided a long service life. A well-made conveyor can be rebuilt for extended service, but before you buy, make sure that all the points mentioned in this article are considered. Purchasing the best magnetic conveyor for your job can save you money, increase both productivity and safety, and give you a conveyor with a long service life that can also be rebuilt.
This article was first published in the December 2008 edition of Manufacturing Engineering magazine.