Pivotal Advice: When hanging a door, it's wise to obey the laws of physics
Gravity. Momentum. Inertia. Resistance. Energy.
These terms hold much clout in the world of physics. They are the unseen but easily understood forces that, quiet literally, make the world go round. The world of doors and hardware is no different; gravity, momentum, inertia, resistance and energy are always at play. Unfortunately, when it comes time to hang a door the rules of physics are not always followed, leading to doorways that are warped, malfunctioning, and in the worst case scenario, in need of replacement.
Life is much easier when the laws of physics are obeyed. And of all the hardware available to hang a door, none is more obedient to Newtonian principles (Principia Mathematica) than pivots.
Pivot offer a desirable means for hanging a door. They are designed to work with the laws of physics to provide long-lasting performance and reliability. The weight of the door is supported entirely by the bottom arm, which is directly connected to the pivot spindle. This method of hanging provides several important advantages:
- Fasteners on the door and frame are in shear rather than tension (as with hinges) and are less likely to pull out over time. This creates less stress on the frame assembly, prevents door sagging and allows the door to swing with less resistance from friction.
- A door supported in this manner relies on the strength of the floor to carry the weight, not the frame. This allows extremely heavy doors to be hung in an opening and allows door adjustment throughout the life of the building to compensate for settlement.
To better understand the benefits offered by pivots, it helps to analyze the gravitational forces that are placed on a doorway.
Principia Patefacio (Principles of Opening)
First, an overview of the physics behind a door hung with hinges. When the door is attached to the frame jamb with standard full mortise hinges, the weight or gravitational pull of the door is placed on an angular trajectory down and away from the hinge/frame connection points. These connection points are made with screws. Thus, the gravitational pull is down and away from the screws, meaning they are held in tension. The tension is actually the potential energy generated by the door as it is held slightly above the floor.
Over time, the constant tension causes stress that can loosen the screws or, in a worse case scenario, bend the frame and cause the doorway to fail. When this happens, a common remedy is to replace the mortise hinges with continuous hinges.
Continuous hinges run the length of the door and are secured to the frame jamb with self-tapping screws. Because there are more connection points, continuous hinges are able to absorb more abuse than a standard full mortise hinge. But, like their full mortise counterparts, the screws of a continuous hinge are held in tension and the weight of the door is borne by the frame, rendering it vulnerable to bending or warping.
Additional stress can be placed on hinges by another common force-momentum. Newton's first law of motion-sometimes called the law of inertia-states the natural tendency of an object in motion is to remain in motion. When a door is opened, momentum wants to continue pulling the door along its path of movement. This force, calculated as mass multiplied by velocity, is absorbed at the hinge connection point, placing further stress on the screws and frame.
Yet another principle of physics that applies to doorways is friction. A standard butt knuckle hinge operates when the knuckles swivel over ball bearings between the joints. Since the weight and potential energy of the door is placed on the hinge, it generates friction on the ball bearings. This resistance decreases the efficiency of the hinge operation and increases the force needed to move the door. There is a direct correlation between the door weight and hinge efficiency; the greater the weight of the door, the less efficient a hinge becomes. A hinge, therefore, can be the limiting factor in determining how large a door can be used on an opening.
Decreased hinge efficiency also impacts the performance of door closers. A door that requires greater force to move will, in turn, demand increased exertion by the door closer to ensure the door is latched shut. The extra closing force is generated by increasing the amount of resistance during the opening cycle. ADA codes limit the maximum opening force to 5 lbs for a handicap accessible interior doorway. A heavy door hung on hinges could have difficulty meeting this requirement.
Fighting the laws of physics can easily become a loosing battle. Instead, it's best to avoid potential problems by incorporating Newtonian principles into doorway design. With their blind obedience to physics, pivots can increase the efficiency and reliability of doorway operation.
What do pivots have that make them, physics-wise, more sensible than hinges? A firm footing on the ground, for one.
As previously mentioned, hinges are mounted on the door jamb. Pivots, on the other hand, are placed on the top and bottom of the door. The top pivot keeps the door aligned and the bottom pivot is mounted into the floor and supports the entire weight of the door. An intermediate pivot can also be installed on heavier doors to help keep the door in alignment.
While hinges transfer the weight of the door to the jamb, the bottom pivot passes this burden onto the floor. And that one disparity makes a world of difference. With little weight to bear, pivots are largely freed to perform a single task: facilitating movement of the door.
A quick examination of the physics behind pivot operation reveals how this is made possible. Since the bottom pivot is mounted onto the floor, it serves as an extension of floor. This alignment directs the potential energy generated by the weight of the door in a straight downward trajectory and onto the floor. Thus, instead of any weight or tension being placed on the frame, the potential energy is transferred to the floor.
The benefits of this arrangement are numerous. The most obvious advantage is the reduction of wear and tear on the frame. Eliminating the weight of the door prevents bending and extends the life of the frame. In addition, the screws securing the pivots in place are held in shear and are devoid of the angular tension that is placed on hinge screws. Screws, minus tension, equal pivots that will stay in place without loosening.
Reduced tension also translates into less friction, making the task of opening a door-even one weighing thousands of pounds-nearly effortless. Pivots allow almost limitless doorway design possibilities; door weight and size are not limiting factors. (See article).
Extra heavy doors, such as a hospital lead-lined door or a specially designed oversized door, are ideal applications for pivots. Their ability to withstand stress also makes pivots desirable for high frequency doors that subjected to continuous use, or areas that are subject to abuse, including delivery entrances and back of house areas.
Pivots can also be hidden from view, making them a popular choice for glass or other aesthetically designed openings.
Vertical adjustment capabilities are another feature unique to pivots. If a door hung on hinges becomes slightly askew from settling of the building or some other factor, little can be done to compensate for the change, other than re-hanging the door. Pivots do not suffer from this deficiency. If the door shifts due to settlement, the pivot can be vertically adjusted to realign the door.
Pivots can accommodate other common doorway applications, such as fire-rated openings or electromechanical access control. In the case of the latter, an electrically wired transfer pivot acts in the same manner as a transfer hinge.
The science of physics is the invisible hand that dictates the movement of a door. These forces can be harnessed by incorporating pivots into doorway design.