Last week, I had conversations with two customers, and both of these conversations got into the subject of REPEATABILITY, as it relates to products produced on a three roll mill (TRM). As I get these questions often, repeatability will be the subject of this column. As in my previous columns, I would like to offer this invitation and this promise. If you have a question regarding TRMS, please email that question to me (jon.hatz@keithmachinery.com), and I promise to answer it. It is my intention to have your questions drive the content of these columns. And now, here I go with my fourth column.
 
When I refer to repeatability, what I mean is that the product that ships this month is exactly the same as the product that shipped last month. When this subject has to do with a TRM, and although QC checks can vary from one company to another, the standard check is to use a grind gauge. If the criteria for passing the product is for the product to reach a particular fineness of grind, and if both this month’s product and last month’s product reached the same grind, then the product is the same.
 
Or is it? The answer is maybe. We know that the product is no worse than an X grind, but we don’t know exactly how much better than an X grind it is. Another way of looking at this subject is the call that the ink maker may get from a printing customer. The customer calls to complain that the ink that he used last month worked perfectly on his #2 printing press. The problem is that this month’s ink is giving him problems. The reality may be that last month’s ink may have been just a little bit better than this month’s ink, and that can be a big problem.
 
To begin this discussion, let me assume that the raw materials that go into the product did not change. Let me also assume that the formula for this product also did not change. And finally, let me assume that the mixing technique to achieve the pre-mix did not change. If none of these items changed, then something, or some things, have changed on the TRM. So, what can change on a TRM from month to month? The answer to this question is the APRON KNIFE PRESSURE, ROLL SPEED, PRODUCT TEMPERATURE, and ROLL PRESSURE.
 
Apron Knife Pressure
Let me begin with Apron Knife Pressure. Basically, the apron knife should be set at the lowest pressure consistent with scraping all of the product off the apron roll. It

should be noted that as the apron knife wears, the knife edge becomes uneven, and apron knife pressure will need to increase to scrape all of the product off the apron roll. Although not a major product influencer, changing the apron knife pressure will affect the final project. TRMS with manual apron set leaves it to the operator to adjust the apron knife pressure, and this negatively affects repeatability. For repeatability, the aim is to keep the apron knife pressure constant. The standard way to do this is to have a hydraulic or a pneumatic apron set system. These systems will have a gauge to show pressure, and a regulator to control this pressure. The photo at the right shows an electro-hydraulic apron set on a typical TRM.
 
Roll Speed
The next parameter is Roll Speed. There is no question that for some products, roll speed can affect the final product. As far as repeatability is concerned, Roll Speed is not an issue, as even on older TRMS with single or two speed motors, the operator will know what speed the product was run at (High or Low Speed). For more modern TRMS with variable roll speed, there is typically an RPM indicator, and the operator can run the product at the same speed from month to month.
 
Product Temperature
And now we come to the major repeatability parameters. The first one is Product Temperature, and there is no question that processing most products at different temperatures will affect product outcome. In this column I will not discuss optimum temperature for any particular product, as the subject is repeatability. To begin, almost all TRMS with Manual Roll Temperature Control will have three inlet valves, allowing cooling water to flow into the annular space inside the roll. Allowing more cooling water to flow into the roll will lower product temperature, and allowing less water to flow into the roll will lower product temperature.
 
If the operator has a hand held temperature gun, and he is very diligent in monitoring roll temperatures, he will be able to keep product temperature at the desired temperature as the batch is processed. Of course, this procedure requires a very diligent operator, and operator that does nothing else but run his mill. The reality is that since operators do have other things to do, TRMS with manual roll temperature control are not good at repeating Product Temperature from month to month.
 
The “fix” to this is an Automatic Roll Temperature Control System (ATC). These systems quite simple, and since all ANSI 2017 compliant electrical packages will have a PLC, adding this feature is quite easy.
 
With ATC, the operator inputs the desired temperature for each roll, and the system keeps the temperatures at these set points. Although there are quite a few variations when it comes to ATC, I will now describe how the KMC ATC system works. To begin, we monitor the temperature of the water inside each roll. As this temperature increases due to friction, the system allows water to flow into a specially designed water pipe that sprays water upward toward the top of the roll, which is the most efficient way to achieve heat transfer. Once the water temperature inside the roll falls to the low temperature delta, water flow stops.
 
In simple terms, the system maintains product temperature within a very small temperature range of 1°F or 2°F. The KMC ATC system also includes both a water pressure sensor and a flow sensor. The photos below show the water side of a typical remanufactured TRM, as well as an HMI screen shot of the display that deals with the ATC.


























Roll Pressure
And now I come to the main parameter affecting repeatability, and of course this is Roll Pressure. On older or simple configured TRMS that have manual roll set, roll pressure is controlled by “feel” and by “eye.” With a manual roll set TRM, the operator turns a hand wheel, which is connected to a hand wheel screw, and this screw is attached to the bearing block. Turning the hand wheels in the clockwise direction will first close the nip, and turning them more, will pressurize the nip. The operator will “feel” the hand wheel get harder to turn as more pressure is applied. Typically, the operator will look at the bead of material on the feed roll to determine how much pressure is being applied to the feed nip, but obviously, repeating roll pressure from month to month is a major problem.
 
There are a number of systems that have been used over the years, to measure Roll Pressure. The vast majority of these is use hydraulics, and these fall into two distinct categories. In very simplified terms, with Manual-Hydraulic Roll Set, most manufacturers have a hand wheel that “pushes” on hydraulic fluid in a small hydraulic reservoir, and this hydraulic fluid in turn “pushes” on the bearing block, and also on a gauge which registers pressure. Such a system is a four-point adjustment system, utilizing four hand wheels, exactly like a manual roll set TRM.
 
Another type of Manual-Hydraulic Roll Set is found on Day TRMs, and this utilizes two hand pumps that are used to send hydraulic fluid to hydraulic cylinders that close the feed nip, and the apron nip. Such a system is a two-point adjustment system, which does easily not allow adjustment of pressure from side to side on the roll.
 
Although not found on many TRMS, there are also Manual-Load Cell Roll Set systems, and these work in a similar manner to a four hand wheel manual-hydraulic system, except that instead of hydraulic gauges that read pressure, there would be electronic indicators. The KMC standard for these TRMS is to have two HMIs, one for the feed side, and the other for the apron side of the TRM. The photos below show TRM with either Manual-Hydraulic Roll Set or with Manual-Load Cell Roll Set.














































Almost all of modern TRMS have Electro-Hydraulic Roll Set. Although the specifics will vary from manufacturer to manufacturer, all will have a reservoir for hydraulic fluid, and an electrically driven hydraulic pump. The systems will all have four hydraulic cylinders for roll movement, and one or two hydraulic cylinders to pressurize the apron knife. The KMC ELECTRO-HYDRAULIC ROLL SET system is a four-point adjustment system, although it can easily be converted to a two-point adjustment system. Buhler and similar TRMS have two-point adjustment Electro-Hydraulic Roll Set systems. The photos below show TRMS with Electro-Hydraulic Roll Set.