Editor's Note: This is the second in a four-part series of articles on web guiding. Part 1 covered understanding web-guide accuracy and how to achieve it.

What's the best compliment you could ever say about a web guide? There are several answers to this, but the best is, "We put it in and never had to worry about it again."

For a small package, a web guide is quite a complex system. As far as the web guide's components are concerned, sensors and controllers are pretty hard to kill; and the likely weakest link is the actuator. If the actuator fails, your worries will soon return.

The driving force behind any web guide is its actuator. The actuator needs to generate a force to move the web and the web guide. Just like torque and RPM are used to determine a motor's size, thrust force and speed determine the correct web-guide actuator size.

An actuator needs to move at a rate equal to or greater than the web's lateral error velocity. Typical required actuator speeds based on web line speeds are:

Thrust is the force the actuator must exert to move the web guide. An actuator's required thrust is found by adding up all of the force demands, including inertia, friction, and gravity.

F(thrust) = F(inertia) + F(friction) + F(gravity)

Inertia is a big component of actuator sizing because web guides are constantly moving and reversing direction. The force to overcome inertia, F(inertia), is the system's mass times acceleration.

The acceleration side of this equation is determined by the top desired speed, shown above, and the desired response time. A standard versus high-response system, the time to reach two-thirds of the maximum velocity is 150 millisecs or 80 millisecs, respectively.

The second term of thrust, F(friction), is the force to overcome all the frictional losses of the web guide. These frictional losses include drag from bearing motion and other real-world drag factors, such as drag from seals, misalignment, lubrication, and contamination.

Frictional forces are usually described as (mass of the object) X (coefficient of friction, COF). The advertised COF of anti-friction bearings ranges from 0.005 to 0.01, but these ideal values don't account for the real-world drag factors. Common conservative COF values are 0.075 to 0.1 for anti-friction bearings and 0.25–0.3 for sliding surfaces.

Thrust estimates need to consider gravity if the web guide moves out of the horizontal plane. The force required to overcome the gravitational load will be the (weight) X (the sine of the off-horizontal angle, Θ).

Reviewing the thrust force and its components:

F(thrust) = F(inertia) + F(friction) + F(gravity)

F(inertia) = (weight/gravity) * acceleration

F(friction) = weight*cosθ * COF

F(gravity) = weight*sinθ

Combining these equations, the required thrust is:

F(thrust) = (weight) *

[(acceleration/gravity)+(cosθ * COF)+(sinθ)]

If the load moves horizontally, θ = 0, the cosθ = 1, the sinθ = 0, and the equation becomes:

F(thrust) = weight * [(acceleration/gravity)+(COF)]

Thrust and speed estimates are used to select individual components. Each component's life is calculated using what is known as the L10 life. All the rotational components have a known life under the predicted running conditions. If the low advertised coefficient of friction is used to calculate thrust, the actuator will be undersized and fail. A conservative COF accounts for the unknown loads and ensures a life of four to five years or more.

For any application, whatever your process speeds or the size of your equipment, you can trust that a knowledgeable web-guide supplier can deliver the right actuator for your application. The right actuator will ensure the accuracy and reliability you expect from your web-guide system and keep your worries from becoming reality. You weren't worried, were you?

For an expanded version of this article with additional formulae, go to www.convertingmagazine.com