Blinker Door -- Also called a "balanced door" or a "slide glide door." A door (door panel) that is dragged along an approximately 90 degree arc to an an open position against and straddling the hinge side door jamb or in some cases with its latch side at the hinge side door jamb. The motion is vaguely similar to that of a tambour door or a roll top desk cover except that the door system has a single panel (or two panels that part in the middle). Actually the door panel floats (or glides) on a pivoted linkage and there are no hinges.
While the door panel appears to slide into and against the the door jamb, the pivoting linkage is superior to actual sliding door tracks. A door panel with a small width relative to its height would have a tendency to pitch (the leading edge rising or dipping) on sliding door tracks and jam up. Also the bottom track would accumulate debris and also lead to jamming.
(For double doors parting in the middle, the latch side door jamb is imaginary, in the middle of the doorway.)
In the diagrams the blue line shows the approximate position of the pivoting linkage. In addition, a roller at the top of the door panel runs in a track above the doorway, to keep the door panel aligned and not flopping about as it is opened and closed. The doors can be designed so the opened door panel is mostly to the outside of the door jamb or mostly to the inside but after it is installed, each door always opens and closes the same way.
The pivoting linkages (one is at the top of the door also) are rigidly attached to a rotating vertical pole at the "hinge side" of the doorway and the linkages must move in tandem to keep the door from tilting. The linkages support the entire weight of the door. A mechanical door opener can be attached to the pole, which is rotated about 90 degrees to fully open or close the door.
Blinker doors are commonly used on buses and other transit vehicles, especially where there isn't enough space on each side of the doorway for sliding doors. Usually the panels are paired, one pulled to each side of the door opening. Originally the panels were about a foot wide with two or even three pairs of panels used where a wider doorway was desired except that the space was divided into two (or three) passageways each about two feet wide. Nowadays blinker doors for transit vehicles are made in a variety of widths including widths suitable for wheelchairs. The first widespread use was on streamlined streetcars introduced in the 1930's. Starting in the 1960's blinker doors became the predominant door type for new buses.
(Photo credit: W. Chermisino) Typical bus with blinker doors shown open. The "interior" surface faces out. A darker triangular shaped stain that shows the area swept out by the door as it opens can be seen on the bus floor. In normal operation the door panel is held in place by a catch when open so a handrail can be mounted on it and be used safely.
One advantage of blinker doors was not exploited until years later, in the 1960's. Bus doors require glazing so the driver can observe passengers outside at bus stops. It is easier to construct wider panels with glazing and the view is improved compared with up to four narrower panels for folding doors. Meanwhile wider swinging door panels might protrude too much if they opened outward and might not fit into a stepwell if they were to open inward. The blinker door solves this problem in that the door panel in the open position can optionally be partly outside, partly inside the door opening.
Because (for most versions) the door panel in the open position is partly outside and partly inside, it sits in the doorway, detracting from the usable width.. A typical swinging door also detracts from the usable width of the doorway but usually by a smaller amount..
Another configuration has the door panels in the open position recessed into the sides of the passageway so the clear area can be the full width of the door opening. This was done on some Chicago elevated train cars where the bottoms of the door panels curved inward to match the contour of the car body and would otherwise detract from leg room when the doors were open.
(Above left) Closeup of blinker door with moving parts labeled.. The pivoting linkages here are zig-zag shaped to fit around the door edge because the door pivot is on the "outside" and the rotating pole is to the "inside." There are versions with the vertical pole positioned differently and with straighter linkages. (Above right) Example of door opener/closer (door motor) for a bus. This door motor is pneumatically operated and moves both panels of a center parting door setup using crankshafts attached to the rotating poles of the door mechanisms. Incidentally most folding doors (another common door type) on buses are attached to rotating vertical poles instead of being hinged to the door jamb, and use the same type of door motor.
This picture shows blinker doors (balanced doors) on a building. The floor or ground area swept out by the door panel is less than that for a comparable swinging door. An advantage is less crowding and congestion when the doors open onto a sidewalk. The doors shown open mostly (about 2/3) outward for quicker exiting in case of fire, and so they can be easily pushed open from the inside. These doors can also have automatic openers, which usually use overhead push arms. Manually opened doors may have a hollow rotating pole with a spring or other closing device inside.
Given the width and weight of the door panel, the pivoting linkage arms and the rotating pole must be sturdy enough so the door panel does not sag and rub on the floor.
A door panel, particularly one with a latch side that protrudes considerably when open and is opened manually, can be designed to swing both ways but we have not seen any examples of this. The door panel would need cutouts at the top and bottom to allow for the movement of the pivoting linkages and may require more fancy weatherstripping at those points.
Locks
Blinker doors (on buildings) can use the same types of locks as swinging doors. Single door panels can use ordinary knobs or deadbolts halfway up the "latch side." Double doors can use top and bottom deadbolts at the latch side for one panel and can use a knob or deadbolt or panic bar latch halfway up for the other panel.
Transit vehicles do not need high security door locks and latches. Adequate latching is provided by the rotating pole and opener/closer mechanism. Because blinker doors on transit vehicles open mostly inward, there is no increased danger of passengers' falling out from leaning on the doors compared with other types of doors.
Safety Considerations
* A push bar (panic bar) to unlatch the door generally cannot be longer than half of the door panel width. This is to prevent persons from pushing on the portion of the door panel that opens inward, which would impede the opening of the door. For a full width horizontal bar that protects a mostly glass door, an additional handle or paddle should be placed near the latch side to indicate to persons where to push.
* Remote control buttons and other items that are touched by persons should not be installed on the portion of the wall behind the door panel in its open position since the moving door could cause injury..
Mechanical Challenges of Blinker Door Design
When a mechanical door opener/closer is attached to the vertical rotating pole, the various components need to be carefully designed to work together.
In the diagrams above, the black line to the right of the blue linkage arm is not a component. It represents the direction of the force exerted on the door panel when the pole and linkages are rotated (cranked; wrenched) to the right as shown to close the door. The force is directed at the roller that runs in the track above the doorway. The middle diagram shows a situation where force is applied nearly perpendicular to the track. Here it would require great force to get the roller and also the door latch side to move to the right towards the closed position. An auxiliary spring might be installed in the overhead track or on an overhead push arm to get the door started towards the closed position but this may make it more difficult to push a manually opened door to a full 90 degree opened position.
As an alternative, the right diagram shows the roller mounted away from the door edge so force is exerted on the roller at a more shallow angle with the track for closing the door. This requires less force applied to the rotating pole to move the roller and door panel. But when the roller is offset from the door edge, the overhead track must be similarly displaced, away from the door frame.
Sometimes the bottom (and top) pivots on the door panel are displaced slightly to the outside. This also changes the angle of force between the door pivot and the overhead roller to be more shallow relative to the overhead track and make it easier for a pole mounted door closer to move the linkages and close the door. This results in a small unobtrusive bulge on the outside of the door at each pivot.
For transit vehicles there are numerous variations of roller, pivot, and overhead track placement depending on where the door panel is supposed to end up in the open position. Sometimes the overhead roller track can be angled or curved.
What's In a Name?
According to an internet forum (Chicagoland in railroad.net) "blinker door" was first used back in the 1940's where the rapidly opening center parting doors as used on streetcars suggested opening and closing eyelids.
Although the term "balanced door" is widely used for doors of this type, we have not yet found any unique feature or attribute in the door or its mechanism that we would think of as being "balanced.". One might say that the door panel is doing a balancing act on just one point, the end of the pivoting linkage at the floor, although it is far from symmetric in terms of geometry..
Last updated June 14, 2018
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