The commercial sectional door
In principle, a commercial sectional door is the same machine as the door on a house: hinged sections, rollers in tracks, and a torsion spring counterbalance. In execution, it is a heavier thing altogether.
The sections are commonly an inch and three-quarters to three inches thick, with polyurethane foam bonded between two steel skins. The track is of two-inch pattern on lighter doors and three-inch on heavier ones, in a thicker gauge of steel. The shaft is solid or of heavy-wall tube, running in sealed bearings. Hinges and rollers are of industrial grade, with long-stem rollers at the positions that carry the most load. Springs are larger in every dimension and are offered in cycle ratings many times the residential standard.
The sectional door has become the standard for docks and drive-in bays for a simple reason. Of all the common commercial door types, it insulates best and seals most tightly.
Dock doors and drive-in doors
The two serve different purposes and are proportioned differently.
Dock doors stand at trailer-bed height, roughly four feet above the yard, and open onto the rear of a truck. They need be only a little wider and taller than a trailer. Eight feet by nine or ten, and nine feet by ten, are the usual sizes. Each is commonly paired with a dock leveler in the floor and a seal or shelter on the outside wall.
Drive-in doors stand at grade and admit vehicles into the building. They are larger: twelve by fourteen feet is common, and sixteen feet high or more where trailers and equipment must pass.
A building generally has many more dock doors than drive-in doors, and the dock doors cycle more often.
Track arrangements
How the track is laid out determines where the door goes when it is open, and on a dock, that matters a good deal.
| Arrangement | Path of the door | Where it is used |
|---|---|---|
| Standard lift | Rises and turns back horizontally just above the opening | Low ceilings. The open door and track occupy the space above the dock. |
| High lift | Rises vertically for some distance above the opening, then turns back | Buildings with moderate clear height. Keeps the door clear of forklift masts and racking. |
| Vertical lift | Rises straight up the wall | Buildings with ample clear height. Nothing projects into the room. |
| Follow-the-roof | Track pitched to follow a sloping roof line | Buildings with pitched roofs and limited height at the eaves |
On a dock, we prefer vertical lift or high lift wherever the building permits. Horizontal track above a dock position is exposed to forklift masts and raised loads, and it is the most frequently damaged part of a standard-lift dock door.
High-lift and vertical-lift doors use tapered or stepped cable drums, which alter the leverage as the door rises. The springs must be calculated for the specific drum and lift. An error here produces a door that is heavy at one point of its travel and light at another.
Horizontal track above a dock position is the most frequently damaged part of a dock door.
Features worth specifying on a dock
- Impact-resistant bottom sections The lowest section takes blows from pallets and forks. Reinforced or sacrificial bottom sections, and breakaway designs that release from the track and reset, greatly reduce repair costs.
- Vision lites A small insulated window at eye level allows staff to see whether a trailer is in position before the door is opened.
- Full perimeter seals Jamb, header, and bottom seals, with the bottom astragal sized to close on the leveler lip.
- High-cycle springs Dock doors are among the busiest in any building.
- Track guards Steel protectors at the base of the vertical track, where forklifts strike.
- Locking A slide lock or an interlock that ties the door to the vehicle restraint and leveler.
- Exhaust ports and pass doors Where the use requires them.
The dock as a system
A dock door does not work alone. It is one of several components at each position, and they must be coordinated.
The leveler bridges the gap between the building floor and the trailer bed. Where it is a pit-mounted type, the door closes onto the floor behind it. Where it is a vertical-storing type, the door closes in front of it, onto the pit floor, which gives a better seal.
The dock seal or shelter closes the gap between the building wall and the trailer. Foam seals compress against the trailer. Shelters use curtains that drape over it.
The vehicle restraint holds the trailer at the dock.
Bumpers absorb the impact of the trailer.
When we install dock doors, we establish what is fitted at each position and make certain the door, its track, and its seals are compatible with it.
