Today, there are millions of miles of plastic piping with threaded fittings, providing reliable, leak-free service. However, a tiny percentage of those threaded plastic fittings may leak or break. The reason for this is improper assembly of threaded joints. On threaded male PVC fittings each successive thread is slightly larger in diameter than the one before it. Female threads get successively smaller.
All pipe manufacturers voluntarily follow these standards to assure their customers they are receiving quality materials. Because the threads are tapered, additional turns cause the female part to stretch or undergo "strain.
The amount of strain increases as the size of the pipe decreases. Therefore it is easier to split smaller diameter threaded joints than larger ones. It is also easier to over-torque smaller diameter fittings because their resistance to torque is less. Table 1 gives Strain and Tensile Stress levels according to pipe diameter. The resistance of PVC ispounds per square inch psi. The strain per turn past finger tight for one-inch PVC pipe is. Thus, a one-inch threaded PVC joint that is tightened four turns past finger tight will develop a tensile stress of 7, psi.
The joint is bound to fail since the stress exceeds the 7, psi tensile strength of PVC, without even adding the tensile stress caused by the pressure inside the irrigation system up to a maximum of 2, psi. Max Allowable. Hydrostatic Stress. The right way to assemble a threaded PVC joint-Schedule 40 or 80 is finger tight plus one to two turns-no more.
Two turns past finger tight plus the stress of the system pressure is within the tensile strength of one-inch PVC. Teflon tape, Teflon paste and pipe dope is intended for metal pipe and fittings. Metal to metal fitting joints are more difficult to tighten than plastic; the surfaces tend to gall without the aid of such lubricants as Teflon or pipe dope. Plastic fittings do not need this lubrication.
When Teflon tape is wrapped around plastic male threads it adds to the strain and tensile stress. The tendency of most installers is to incorrectly wrap several thickness of tape around the male threads, increasing stain and stress further. Teflon paste and pipe dope, just like Teflon tape, make threaded joints slippery.
Their use on PVC fittings can be an invitation to over-torque. When working with threaded plastic fittings do use a proper sealant. The right sealant for threaded joints is non-hardening, compatible with plastic and doesn't add slipperiness. A non-hardening compound is forced by water pressure into potential points of leakage, thereby performing a true sealing function. Tapes and hardening pastes permit a leak path to develop when a joint is backed off, mechanically flexed, or expands with rising temperatures.
A sealing compound must be compatible to plastics. Many brands of pipe sealant contain oils, solvents or carriers that can damage plastic. A proper sealant must be certified by the manufacturer to be harmless to the fitting material and to not contaminate fluid in the pipe.
Finally, a sealing compound must not lubricate the joint to the point that over-tightening is encouraged. Several sealants on the market meet all these requirements. Don't use Schedule 80 threaded fittings in a Schedule 40 System.
Do use the same Schedule threaded fittings with the same Schedule pipe and fittings. Many plastic piping system installers who encounter problems with splitting assume Schedule 40 fittings are weak.
They conclude that the problem can be solved by switching to "stronger" Schedule 80 fittings. There are several fallacies in this reasoning.The basic height of thread engagement for buttress thread is 0. The buttress thread form has certain advantages for the applications where high stress occurs only in one direction along the thread axis. The buttress form of thread is particularly applicable when tubular members screwed together.
Breech assemblies of large guns, airplane propeller hubs and columns for hydraulic presses are some of the examples of buttress threaded rods and nuts. The standard provides information about preferred diameter-pitch combinations, formulas for calculation pitch diameter tolerances, tolerances for major and minor diameters, a system of allowances between external and internal threads, recommended methods of measuring and gauging, dimensional acceptability of buttress threaded rods and nuts.
Note: For more information on buttress threads including diameter-pitch combinations, thread designation, basic dimensions, formulas, thread tolerances and allowances for easy assemblyplease refer to pages - of Machinery's Handbook, 30th. Note: 1 Tolerance on major diameter of external thread, pitch diameter of external and internal thread, and minor diameter of internal thread. Allowance: An allowance clearance should be provided on all external threads to secure easy assembly of parts.
Major Diameter: The maximum diameter of a thread which is diameter of the crest of a male thread or root of a female thread. Minor Diameter: The minimum diameter of a thread which is diameter of the root of a male thread or crest of a female thread.
Roots of Threads: There are 2 alternatives for thread roots. These are rounded and flat roots. Rounded Roots: Equal radii, at the roots of the external and internal basic thread forms tangential to the load flank and the clearance flank. The rounded root form of the external and internal thread shall be a continuous, smoothly blended curve within the zone defined by 0. The resulting curve shall have no reversals and sudden angular variations, and shall be tangent to flanks of the thread.
There is, in practice, almost no chance that the thread forms will be achieved strictly as basically specified, that is, as true radii. Flat Roots: Equal flat root of the external and internal thread. In instances where absence of root radius is not detrimental to the requirements for strength, and where it is more economical to provide tools which do not produce a radius at root, flat root buttress threads may be specified. Nominal diameter inches - threads per inch.
Thread Tolerance Class 1. Class 2 Class 3.Buttress Thread is the thread profile that is made on pipelines and wellbore tools. This thread profile is also used in liner tubular and well casings as well. The profile of these threading is usually in square cut shape. When the thread comes in contact with the mating thread, a hydraulic seal is created through interference fit which allows the buttress-threaded equipment to lock firmly.
In oil field tubing, a Buttress Thread profile is usually designed for superior hydraulic seals. Any equipment that is designed with Buttress Threads has a transmission of force which is parallel to the axis of rotation of the equipment.
There are various types of Buttress Thread profiles that are used for various equipment, pipelines, well casings and tubular, they include:. Toggle navigation Menu. Home Dictionary Tags Upstream Fabrication. Buttress Thread. Definition - What does Buttress Thread mean?
Petropedia explains Buttress Thread In oil field tubing, a Buttress Thread profile is usually designed for superior hydraulic seals. Share this:. Related Terms. Related Articles. The Bunkering of Marine Oil Tankers. Related Tags.
Upstream Fabrication Hydrocarbon Exploration Engineering. Connect with us. Please Wait Latest Articles. MORE Newsletters.Threads are used as mechanical means to hold the neighboring joints together during axial tension or compression. For all casing sizes, the threads are not intended to be leak resistant when made up. Also for STC coupling, if under huge external pressure and axial tensile stress, when heavy impact or bent happens, the threading will have more chance to slippage.
API standard threading coupling primarily seal the pipe by thread grease, metal plating and thread engagement of interface. STC thread root to tip clearance is 0. Bottom to tip is 0. The internal yield pressure is the pressure that initiates yield at the root of the coupling thread. The coupling internal yield pressure is typically greater than the pipe body internal yield pressure.
The internal pressure leak resistance is based on the interface pressure between the pipe and coupling threads because of makeup.
This equation accounts only for the contact pressure on the thread flanks as a sealing mechanism and ignores the long helical leak paths filled with thread compound that exist in all API connections. In round threads, two small leak paths exist at the crest and root of each thread. Buttress threads have a much larger leak path along the stabbing flank and at the root of the coupling thread.
API connections rely on thread compound to fill these gaps and provide leak resistance. The leak resistance provided by the thread compound is typically less than the API internal leak resistance value, particularly for buttress connections. The leak resistance can be improved by using API connections with smaller thread tolerances and, hence, smaller gapsbut it typically will not exceed 5, psi with any long-term reliability.
Applying tin or zinc plating to the coupling also results in smaller gaps and improves leak resistance. The round-thread casing-joint strength is given as the lesser of the fracture strength of the pin and the jump-out strength. The fracture strength is given by.
Where Acme & Buttress Thread Forms Are Used and Why
These equations are based on tension tests to failure on round-thread test specimens. Both are theoretically derived and adjusted using statistical methods to match the test data. For standard coupling dimensions, round threads are pin weak i. The buttress thread casing joint strength is given as the lesser of the fracture strength of the pipe body the pin and the coupling the box.
Pipe thread strength is given by. These equations are based on tension tests to failure on buttress-thread test specimens. They are theoretically derived and adjusted using statistical methods to match test data. When performing casing design, it is very important to note that the API joint-strength values are a function of the ultimate tensile strength. This is a different criterion from that used to define the axial strength of the pipe body, which is based on the yield strength. If care is not taken, this approach can lead to a design that inherently does not have the same level of safety for the connections as for the pipe body.
This is not good practice, particularly in light of the fact that most casing failures occur at connections. This discrepancy can be countered by using a higher design factor when performing connection axial design with API connections. If API casing connection joint strengths calculated with the previous formulae are the basis of a design, the designer should use higher axial design factors for the connection analysis.
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Invalid email. This is required. Login to Your Account. Remember Me? Register Forgot password? What's New? Results 1 to 7 of 7. Thread: Buttress Thread. Thread Tools Show Printable Version. Buttress Thread Hey guys, I have a part that has come in for repair. The male part is being re made however the female is not. The thread on it is a buttress thread. The problem I'm having is 1, I have never cut a buttress thread before.
I do have an insert for the pitch I need though. This is a very odd size. Can you still use wires? I measured the ID of the female and its measuring 2. Something doesn't seem right as that would only leave.Acme and Buttress threads are extremely useful for the lead screw shaft of various assemblies.
Specifically, for assemblies thatmove efficiently either for load bearing or accuracy. We can see the lead screw shafts in some equipment; however most are hidden from sight. For example, lifting pallets, marking and labeling machines, medical diagnostics and personal mobility, off road equipment, and machine tools, all require load bearing or accurate movement which can be provided by devices that use a lead screw mechanism.
The thread forms normally required for lead screws are shaped to offer high strength and accuracy. Acme is the most popular trapezoidal thread form worldwide for lead screws.
Although square threads are the most efficient, they remain difficult to produce. They can be produced with multiple start threads, left hand threads and self-centering threads. Regal offers standard sizes of Acme taps and manufactures many variations of special purpose taps usually coarse pitch Acme forms.
The buttress thread form is designed to handle extreme high axial thrust in one direction. Regal manufactures this style to customer specifications in twelve days or less. Contact us today to find out more information. Click here to see a larger image. Whether you are a tool user or a distributor, subscribe to our newsletter for the latest metal cutting tool resources.
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