- June 01, 2011, By Timothy J. Walker Contributing Editor
In today's match, we have two well-known web guides ready to square off and try to finally decide which is the top web guide.
In one corner — weighing in at what is usually two rollers in a pivoting frame — is the displacement guide (also known as the offset pivot, a positive displacement, or a tabletop guide). In the other corner — sleek and trim in what is usually a single roller on a unique arcing motion — the steering guide.
Both of these famous guides have a long history of being the “right” guide for intermediate corrections of lateral web position. These two mighty guides long ago dismissed the slow, wanna-be end-pivoting roller with their speedy and stable performances.
Let's put these two top guide candidates through some battle scenarios and see if we can crown a champion.
Space | In a tight location, the displacement guide wins hands down. The displacement guide's twist-displace-twist action without web bending (pulling a web in its width direction) can fit in tight locations with spans as short as one-half to one web width.
When a steering guide is shoe-horned into a tight location with short spans, it reacts like a cornered wild boar. In short spans, a steering guide can only make minimum lateral shifts before the force to bend exceeds the traction available or the guide becomes a shear wrinkle factory.
Traction | Steering guides can struggle with traction. Like any roller, they need traction to overcome bearing drag and inertia losses, but they also need lateral traction to bend the web left or right.
Like a race car in a turn, when there isn't enough traction, they lose control. Displacement guides win the traction battle with their easy twisting of the web.
Wrinkling | Bending will create wrinkles at much smaller angles of correction that twisting, so displacement guides would seem to win this one, but steering guides can magnify their correcting angles with long entry spans. Even considering the span multiplying effect, displacement guides will have far fewer wrinkles.
Ease of Setup | A steering guide with one 90-deg wrapped roller seems easier to set up than the two-roller displacement guide, but each system has four rollers that need to be properly set with wrap angle and span lengths to work their best. However, steering guides need to be tuned to the entry span length, need to avoid the strange unstable steering geometry, and by far, get installed incorrectly more often by a 20:1 ratio.
Rate and Range Limits | Both guides can correct as fast as their actuators will move them. Both can be designed for large correction ranges. This one is a tie.
Upstream Corrections | The correction of a displacement guide occurs between the two rollers in the frame (or during the wrap of one large roller in single-roll displacement guides). A steering guide can exert its influence of a long, long entry span. This is where the steering guide excels.
If you have a long process span, such as an oven, you have two choices:
- Wait until the web exits the oven and correct the web with a displacement guide.
- Install a steering guide as the first roller at the end of the long oven span.
The displacement guide will be happy to let your web crash into the side of the oven. A steering guide will start correcting at the first error during startup and work to bend the web in the oven back to centerline. A steering guide has its limits, but it will open the non-crashing window of your long span process.
The winner? If you have anything other than a long-span correction need, the displacement guide easily wins “top guide” honors. However, if you are worried about a long span, the steering guide will do what no other guide can — limit the lateral shifting upstream and put the web on centerline downstream.
Web handling expert Tim Walker, president of TJWalker+Assoc., has 25 years of experience in web processes, education, development, and production problem solving. Contact him at 651-686-5400; firstname.lastname@example.org; www.webhandling.com.