Machine Design

Sizing up Hinges

Real-world analysis prevents premature failure, overdesign, and seal compromise.

John Bates
Middletown, Pa.

The question of hinge strength often arises during the design of an enclosure. Determining the strength of the individual hinges is fairly straightforward, but relating the strength predictions to real-life situations can be a challenge. The dimensions of the door, the position of the center of gravity, the position of the hinges, and the installation tolerances profoundly affect the load-carrying capability of a hinge.

The most common enclosure-door orientation has hinges in a vertical plane. Nontrivial out-of-plane loads render simple tensile testing of single hinges meaningless. When sizing such a door, the engineer must consider vertical load from the weight of the door, load sharing between hinges, horizontal loads caused by door and hinge geometry, gasket force against the door, and shock loading.

Vertical loading
The vertical load from the weight of the door can be easily calculated as the total weight of the door plus anything hanging from it.

W = (md + ms) g

where md = mass of the door, ms = mass of anything suspended on the door, and g = gravitational constant, 9.8 m/sec2.

If we assume the door has two hinges aligned vertically, the vertical force is divided equally between them:

W = FA + FB

Equal load distribution between hinges requires the hinges be made exactly to nominal dimensions and installed precisely. Tolerances in hinge manufacture or placement cause the load to be distributed unequally. In the extreme case shown in the figure, the lower hinge takes all the vertical force because the gap between the two halves of the upper hinge prevents load transfer between the door and the frame at that location.

Neither the hinges nor the door are perfectly rigid, however. Hinge parts or the door itself can deflect under the uneven force so the unloaded hinge may pick up some of the vertical load. Softer hinge materials such as glassreinforced nylon may make up for their reduced strength by sharing load more equally. This permits more leeway for manufacturing tolerance and imperfect installation.

For lift-off hinges, the worst consequence of an installation error is that one hinge is forced to carry the entire weight of the door. If the hinges are captive, the force caused by an installation error cannot be relieved and can permanently deform the hinges and impair door function.

Horizontal loading
Although the main force on the door is vertical, the load on the hinges also has a horizontal component in the plane of the door. The center of gravity of the door is cantilevered off the hinges, resulting in an induced moment. The hinges see this moment as a horizontal load at the hinge points.

Heavier and wider doors, therefore, generate greater horizontal forces on the hinges. A simple rectangular door will have its CG at its center. Varying thickness, cutouts, or suspended loads can move the CG away from the center and affect the moment on the hinges accordingly.

Wider hinge spacing reduces the horizontal force. Because of the nature of the moment, a third hinge at the pivot point will not take a significant amount of the horizontal loading. Similarly, a door CG which is above or below the midpoint between the two hinges will result in uneven horizontal loading.

The moment from the weight of the door is:

M = W x Y

where Y = the distance between the door CG and the hinge line.

The moment M is reacted by horizontal loads C and D on the upper and lower hinges, respectively:

M = (ZC x C) + (ZD x D)

where ZC and ZD = the distances from the pivot point to the upper and lower hinges, respectively.

Gasket forces
The force of the door against a sealing gasket, if one is present, is an out-of-plane horizontal stress on the hinges. Depending on the seal configuration, the loads can be substantial enough to bend the door away from the gasket, defeating its purpose. Multiple hinges and multipoint latching are often used to counteract this bending.

One common seal specification from the National Electrical Manufacturers Association dictates NEMA-4 enclosures for protection of electronics against windblown dust, rain, and hose-directed water.

Compressible foam can be used to seal a NEMA-4 enclosure. The force needed to compress the foam is approximately 40 lb/ft of gasket length. For a 6 X 3-ft door, this translates to a 720-lb force simply to seal the door.

Another option for NEMA-4-compliant sealing is a bubble-type seal where the seal is created through bending of a relatively thin rubber layer as opposed to compression of a monolithic foam. Bending a bubble-type gasket to create a secure seal demands approximately one quarter of the load required to compress foam seals. In the example above of a 6 X 3-ft door, this translates to 180 lb if bubble-type seals are used.

Low-compression force gaskets, stiffer door materials, and deeper return flanges can all reduce the possibility of door bending and seal compromise.

Shock loads
In addition to the static loads detailed above, hinges often experience shock loads from a variety of sources. While the engineer will have incorporated a safety factor when choosing hinges, severe unexpected loading can cause permanent deformation and weakening of the hinges. Care should be taken to protect the door and hinges from electrical panel short circuits, environmental shocks like earthquakes, shipping damage, and rough handling during service, especially since these infrequent overloads are so difficult to quantify.

The loads during shipping can be greatly minimized by latching the door and preventing it from moving relative to its frame during transit. Door stops can solve the problem of overextending the hinge in service.

In earthquake-prone regions and in manufacturing areas with multiple hazards, consider potential shock loads when choosing hinge and door materials.

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A bubble-type gasket requires less force to compress than a monolithic foam gasket. Substantial gasket forces can bend the door or hinges.

TAGS: Fasteners
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