In the field of industrial pipe connection and sealing technology, the question of whether RC threads can achieve sealing
often troubles technicians and design engineers. In fact, as an important branch of the imperial system conical pipe
thread, RC threads possess excellent sealing performance. Their taper design and thread shape make them perform
exceptionally well in medium and low-pressure systems below 1.6 megapascals. After you understand the structural
principle and correct application methods of RC threads, you will find that the sealing technology of RC threads plays
an irreplaceable role in the fields of petrochemicals, mechanical manufacturing, hydraulic and pneumatic systems, etc.
I. Basic Structure and Sealing Principle of RC Threads
1. Geometric characteristics of conical threads: RC threads adopt a standard taper ratio of 1:16. That is, the diameter
of the thread decreases by 1 unit for every 16 units of length. This taper structure enables radial compressive force
to be generated between the inner and outer threads during tightening, thereby achieving effective sealing.
According to the ISO 7-1 standard, the helix angle of RC threads is 55 degrees, and the pitch is determined according
to the imperial system. For example, the pitch of RC1/2 is 1.814 millimeters, and the pitch of RC3/4 is also 1.814
millimeters.
2. Metal-to-metal sealing mechanism: The RC thread seal mainly relies on the wedge-shaped compression effect of
the thread profile to achieve sealing. When the internal and external threads are tightened together, the radial force
generated by the taper causes the top of the thread teeth to closely adhere to the bottom of the teeth, forming
a continuous sealing line. Theoretically, properly machined RC threads can withstand a working pressure of
0.8-1.0 megapascals without leakage when no sealant is used.
3. The influence of thread accuracy on sealing performance: The sealing effect of RC threads directly depends on
the processing accuracy and surface quality. The tolerance of the thread's major diameter should be controlled
within ±0.05 millimeters, and the surface roughness should be below Ra1.6 micrometers. When the thread accuracy
is insufficient, poor tooth contact will lead to sealing failure. Using high-precision RC threads in combination with
thread sealant can increase the sealing pressure to above 2.5 megapascals.
II. Performance Comparison of RC Threads with Other Sealing Threads
1. Differences between RC Threads and NPT Threads: Both RC threads and NPT threads belong to conical pipe
threads, but there are differences in the tooth angle and pitch standards. The tooth angle of NPT threads is 60 degrees,
while that of RC threads is 55 degrees; NPT threads are manufactured according to the American standard,
while RC threads follow the British standard. Under the same specifications, the sealing performance of RC threads
is slightly better than that of NPT threads, and it performs more stably when subjected to pulsating pressure.
2. Comparison with straight pipe thread sealing scheme: Compared with straight pipe threads such as G threads, which
require sealing gaskets to achieve sealing, RC threads can achieve self-sealing of the threads, simplifying the connection
structure. The sealing pressure of the straight pipe thread connection mainly depends on the performance of the gasket,
while RC threads achieve sealing through metal deformation, and have better reliability in high-temperature
environments.
Experimental data shows that under a temperature of 150 degrees Celsius, the sealing life of the RC thread connection
is 3 to 5 times longer than that of rubber gasket sealing.
3. Enhanced effect of sealant combination: Although the RC threads have self-sealing capabilities, in practical
applications, they are usually used in combination with thread sealants to improve the sealing effect. Materials such
as polytetrafluoroethylene tape and liquid thread locking agents can fill microscopic gaps and increase the sealing
pressure from 1.0 megapascal to over 3.0 megapascal. When choosing a sealant, it is necessary to consider
the characteristics of the medium and the working temperature to ensure long-term sealing reliability.
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For more details, please stay tuned for the next article.
