Impact doors are simple devices. Still, there are significant differences between models. Consider these features:
• closing and centering force. The door must have a centering force that is strong enough to overcome the resistance of the seals, and to resist wind and pressure differential. Most impact doors are centered by gravity. In the closed position, a cam on the door side of the hinge rests in the bottom of a V-notch fitting that is installed on the frame. See Figure 2. When the door is opened, it rides up the side of the notch. When the door is released, gravity pulls the door back into the center position. A spring may be installed on a hinge rod to increase the centering force. Some manufacturers allow the purchaser to select the centering force by providing notch fittings of different depth or springs of different stiffness.
• swing pattern. By using a hinge installed on the corner of the frame, some models can open as much as 180° in one direction. In other words, the door folds flat against the wall. The door can open 90° in the other direction. See Figure 3. If the wider opening angle is in the direction of the vehicle’s travel, the vehicle can make a turn even before it clears the door. This can be a valuable feature,especially in tight quarters.
• sealing features. The edge seals of the door are an important factor in stopping air leakage. Look at all four sides of the door. Manufacturers use severa types of seals, all of which can block infiltration almost completely. The gap between the two doors is typically closed with soft hollow tube seals. Figure 4 shows typical seals for the door gap and the hinge side. The top seal is a special design because the door rises as it opens. This requires clearance between the top of the door and the door frame. To fill this gap and minimize resistance, many models have a soft seal on the top of the door that mates with a soft seal installed in the top of the frame. The bottom of the door can be sealed effectively with a rubber sweep. The door rises as it opens, so the sweep seal does not drag on the floor except when the door is near the closed
position.
• visibility. Impact doors have enough weight and stiffness to be dangerous if they are slammed open. The primary safety feature of an impact door is a large window that allows vehicle drivers to see that the space on the other side of the door is clear. Unfortunately, the window cannot extend below the top of the bumper portion of the door, which is typically at about half the door height. This leaves the danger of hitting a low object on the other side of the door. Get the largest window that does not interfere with the strength of the door.
• resistance to impact. Impact doors use bumpers to absorb the shock of vehicles running into them. Select bumpers that are appropriate for the severity of the application. Select the height of the bumpers for the nature of the vehicles and goods passing through the door. For example, a stack of goods that is being carried on a forklift truck may strike the door before the truck itself. At the same time, do not make the bumper so tall that it interferes with visibility through the window.
• insulation. Impact doors are relatively small, so conductive heat loss through the door is not a major problem. Even so, it is worth insulating the door. You can get models that are filled with foam insulation. Typical door construction allows for
somewhat more than one inch (about three centimeters) of insulation, which provides an overall insulation value of about R-4.