John Bates
EMKA Inc.
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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EMKA USA, emkausa.com