UNJ vs. UN
UNJS vs. UNS
UNJC vs. UNC
UNJF vs. UNF
UNJEF vs. UNEF
The J series thread is defined in specification ANSI/ASME SAE AS8879 (formerly
MIL-S-8879) and in ANSI/ASME B1.15.
The rounded root of a UNJ external thread greatly improves fatigue strength over that
of a flat root UN thread. It seems that sharp cuts at the minor diameter of the external screw thread create stress points where fractures can begin and which ultimately cause failure of the external thread. An additional benefit is that the rounded root of the UNJ thread reduces the rate of threading
tool crest wear. The internal thread does not get the same rounded root treatment because the internal thread usually has more mass around the thread and is nearly impervious to stress fracturing. If you have a situation where the internal thread is made in a thin wall part and stress fracturing is a concern; there is an internal thread rounded root option available, but it is not officially codified.
Typically and historically the J-series of thread has been Class-of-fit 3A and 3B. This is the only class designated in ANSI/ASME SAE AS8879. Not as common is Class-of-Fit 2A and 2B which are permitted and defined in ANSI B1.15.
If you have a requirement for a J-series thread it would be best to verify the class-of-fit if it is not designated on the product drawing.
INTERNAL SCREW THREAD
The UNJ for internal threads is identical to the UN thread except
that the UNJ minor diameter is slightly larger than the UN minor
diameter. This is to allow the minor diameter of the internal thread
to clear large root radius of the UNJ external thread. If a UNJ
bolt is screwed into a UN nut there will in most cases be an interference
fit at the thread minor diameter. To make the minor diameter larger use a larger size drill bit when preparing for a J-series screw thread. If you have a standard UN-series threaded hole, and you wish to have a UNJ-series threaded hole; select the proper size drill bit for the UNJ-series size and enlarge the minor diameter of the UN-series threaded hole; you now have a UNJ-series threaded hole.
EXTERNAL SCREW THREAD
Both UNJ and UN are used for external threads.
External UNJ threads per SAE AS8879 have a rounded controlled root
radius at the minor diameter. The minimum root radius is calculated
at 0.15011p and the maximum root radius is calculated at 0.18042p.
The root contour shall have a smooth, continuous, non-reversing contour
and shall blend tangentially into the flanks and any straight segment.
The radius is larger than the UNR style.
External UN threads per ANSI/ASME B1.1
Paragraph 1.3: UN threads have "a flat or optional rounded
root contour" at the minor diameter.
Paragraph 2.3.1: UN threads have "a flat root contour"
at the minor diameter, but "a rounded root contour cleared
beyond the 0.25P flat width of the basic profile is optional"..."to
provide for some threading tool crest wear."
UN Bolt into UN Nut = No problems.
UNJ Bolt into UNJ Nut = No problems.
UN Bolt into UNJ Nut = No assembly problems; functional problems
UNJ Bolt into UN Nut = In most cases an interference fit at the
thread minor diameter; functional problems are possible.*
SCREW THREAD WORK PLUG GAGES
UN = UNJ The only difference between the internal UNJ thread and the standard
UN threaded is that the UNJ thread has a larger minor diameter.
The thread work plug gage does not check the minor diameter of the
thread, thus when checking the threads, standard UN style Work Plug
Gages are used. If UNJ gages are ordered from a gage maker, the
gage maker will supply the standard UN gage members with a handle
marked UNJ. The minor diameter of internal threads should be checked
with plain pin gages.
SCREW THREAD RING GAGES
2A or 3A GO: UN ≠ UNJ
2A NOGO UN =or≠ UNJ (Varies by size/pitch; assume ≠.)
3A NOGO UN = UNJ
Because of the large root radius on the minor diameter of the external
thread, the UNJ GO ring gage is supplied with an enlarged minor
designed to clear the root radius. The GO ring gage is the only
gage that is modified specifically for the UNJ specification and
the only difference between the UNJ GO and the UN GO thread gages
is the minor diameter. The UN GO ring gage if used to measure the
UNJ thread the gage will reject the part because of the minor diameter
interference. The UNJ NOGO ring gage is identical to the UN NOGO
ring gage except that it may be marked with a "J". The
minor diameter of the NOGO ring gage is relieved to just below the
pitch diameter, and because of this the standard UN NOGO gage will
clear the minor diameter of both the UN and UNJ external threads.
SCREW THREAD SET PLUG GAGES UN = UNJ
The set plug members used for the UNJ series threads are identical
to the set plugs used for the UN series. If UNJ gages are ordered
from a gage maker, the gage maker will supply the standard UN gage
members with a handle marked UNJ. This is because set plug gages
do not check the minor diameter of the ring gage they are setting.
The minor diameter of the ring gage should be checked with plain
pin gages after the ring has been properly set with the set plug.
HOW TO CHECK THE ROOT RADIUS
Use an optical comparator to check the UNJ external thread form
root radius at the minor diameter. The thread radius must be smoothly blended with the flank angles leaving no sharp groove or edge at the root of the thread.
UN = Basic Unified National thread series
UNJ = Basic Unified National thread series with external thread
controlled root radius
UNS = Special Unified National thread series
UNJS = Special Unified National thread series with external thread
controlled root radius
UNC = Unified National Coarse thread series
UNJC = Unified National Coarse thread series with external thread
controlled root radius
UNF = Unified National Fine thread series
UNJF = Unified National Fine thread series with external thread
controlled root radius
UNEF = Unified National Extra Fine thread series
UNJEF = Unified National Extra Fine thread series with external
thread controlled root radius
You ask: < What is meant by: “Functional problems are possible.” >
This is one of those esoteric things; or alternately said a CYA thing on my part.
Basically the two threads are not designed to be used as a connection, even though they will/may screw together. There are some physical differences in the threads which could in some extreme situation cause a problem. If you choose to use them as such you should conduct an engineering evaluation of the design parameters to assure that some minor detail will not pop-up to cause a failure of your connection. The differences in the threads are related to the minor diameter size and shape which could affect the strength of the material and/or the maximum loading of the thread (torque and/or straight pull) and/or the potential for stress cracking. All of these things are product design related and impossible for me to determine in any general statement.
This data is provided for general information only. The intention
is to provide accurate information; regardless; errors may exist
in the supplied information. If accuracy is critical, base your
final decisions on the data provided in the root documents: B1.1 and AS8879; which are copyrighted documents. To purchase a copy visit
an Authorized Reseller.
Original Posted: 4/26/2005
Last Revised: 6/282018
Refer comments/corrections to firstname.lastname@example.org.