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 Precise
Holes
If the
holes are precise and the same in both components, such as honed
or reamed holes with a tolerance of .0008" or .002mm, then
the length of the bushing need only receive minor consideration
for purposes of precise relative location. We recommend using
the minimum specified hole in these situations. The bushing will
assume the diameter of the initial installation hole and the
unsized
diameter of the normally exposed end would compensate for the
tolerance difference between the holes if any. If no interference
whatsoever is acceptable when assembling the mating component
over
the exposed bushing, then it is recommended to keep the exposed
down length to a minimum, or if practical, to push the dowel
length
to a minimum, or if practical, to push the dowel through the
initial component to size the exposed end. In any event it is
recommended
to install at least two thirds of the total dowel length into
the initial hole so as to permanently fix the dowel position.
 Maximum
Tolerance Holes
The maximum
allowable tolerance is one-half the total recommended tolerance.
This is still within the normal production hole tolerance for
drilled or cored holes. The smaller hole, that is the hole with
the minus tolerance, should be the hole into which the dowel is
initially installed. The larger hole, that is the hole in the
mating component, should have a plus tolerance. To illustrate:
The total recommended hole tolerance for an 8 mm dowel is 8.00
to 8.13. Take the approximate midpoint and split the tolerance.
The smaller hole would be 8.00 to 8.06, the larger 8.06 to 8.13.
The smaller hole used for the initial installation will size the
dowel but the protruding unsized length of the dowel remains larger,
with the diameter increasing as the distance from the hole increases.
It normally requires a protruding length equal to 1-1/2 times
the dowel diameter for a dowel installed in a minimum hole to
have a protruding diameter greater than the maximum hole. For
an 8 mm dowel in a 8 mm hole, that would require a protrusion
of 12 mm to have a dowel diameter at the protruding end greater
than 8.13 mm. The smaller hole in the initial installation helps
in fixing the location of the dowel but it is still recommended
that the greater length of engagement be in the smaller initial
hole. Therefore, in the example used here to illustrate the maximum
hole tolerance situation, the dowel would be BUSH 8 x 30 BK SD
200.
 Centerline
Tolerancing
If more than one dowel is used, centerline tolerancing
for hole positioning becomes an issue. In situations with precision
holes requiring precision locating, the centerline tolerancing
needs to be accurate and similar to tolerancing used for solid
dowels. A tolerance of .0006" or .0015 mm is recommended.
When a dowel is installed in a minimum hole, which is recommended
in these cases, the dowel gap is butted and further spring action
is very limited, if any.
Hole
tolerance can be increased to provide for relaxed positioning
tolerances with some sacrifice of rigidity. The centerline tolerance
can be increased to the tolerance of the holes, or the smallest
tolerance if the tolerances of the holes are different. In the
8 mm dowel example used, the centerline tolerance can be .06 mm.
The net hole at maximum misalignment cannot be less than the smallest
recommended hole; in the example, 8 mm. The misalignment will
normally distribute itself between the dowels.
If it is a dowel bushing application with a bolt passing through
the dowel into a threaded component, the clearance between the
minimum inside diameter of the dowel bushing and maximum bolt
diameter needs to be enough to compensate for misalignment. If
these guidelines are used, the standard clearance will always
be adequate at maximum misalignment.
Blind
Holes and Stepped Holes
Blind and stepped holes can be used for dowel location and stepped
holes are generally used for dowel bushings used in conjunction
with bolts. Since blind and stepped holes only fix the dowel location
in one direction, it is still recommended that the dowel be fixed
into location by using the smaller hole and greater length of
engagement.
Joint
Integrity
Loss of joint integrity due to rotational loosening is triggered
by vibration. Loads perpendicular to the axis of the bolt, particularly
cyclic loading cause slip at the bolt head or the nut which translates
into rotational loosening. Dowels, particularly dowel bushings,
reduce or even eliminate rotational loosening. In this instance,
the use of the smallest hole possible within the tolerance range
is recommended to reduce dowel flexibility after insertion. The
shear strength also needs review. In static loading or a long
cycle time between loads, maximum load should not exceed of 75%
of the minimum shear strength. When the loads are in the form
of severe vibration, 50% is recommended.
Spirol
applications specialists are available to make recommendations
based on your requirement or to review your application.
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