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Written by G.L. Huyett Marketing Department on 02/25/2021 with 0 comments

Grease Fitting Threads

Understanding grease fitting pipe threads

Grease fittings are a common component found on nearly every mode of transportation or piece of machinery. Pipe threads, which are formed onto the grease fitting's shank, allow a large portion of fittings to securely attach to their mating component.

What are the different elements of a pipe thread?

Pipe threads are comprised of several elements:

  • Truncation - The material moved from the theoretical V‑thread triangle of the thread form.
  • Crest - The highest point of a thread, opposite the root.
  • Root - The lowest point of a thread, opposite the crest.
  • Flank - Thread portion joining the thread root and crest.
  • Angle - The angle between the adjacent thread flanks.
Elements of Pipe Thread

Various configurations of these elements can result in a wide variety of potential thread styles. During the mid to late 1800's, pipe thread types started to become standardized across the globe, thus providing manufacturers a means for creating compatible and interchangeable parts.

What are the different pipe threads used on grease fittings?

There are ten common types of pipe thread standards used for grease fittings. SAE‑LT, NPTF, PTF, UNF, UNEF, ISO Metric, BSF, BSPP, BSPT, and German Thread are the most popular.

SAE‑LT (Society of Automotive Engineers) threads possess SAE pipe thread dimension of 1/4"‑28 taper. Grease zerk fittings in the automotive industry as well as many other industrial machines and standard duty applications use SAE‑LT threads. These threads are considered self‑sealing and do not require additional sealing compound for a leak free connection.

NPTF (National Pipe Taper Fuel) threads are a tapered thread that provides a pressure tight seal without the need for applying additional sealing compound. This is often referred to as "dry sealing" as the interference fit between mating threads prevents fluid leakage.

PTF (Pipe Thread Fuel) threads are similar to NPTF threads featuring an interference, dry sealing fit. The primary difference between PTF and NPTF threads, however, is that PTF threads are shorter in length by one thread turn. Other PTF thread variations include PTF‑SAE Short, PTF‑Special Short, and PTF‑Special Extra Short.

NPTF and PTF Threads

UNF (Unified National Fine) threads follow an inch standard and are used in applications with liberal clearance tolerances. These are a common thread designation for general applications. UNF threads are considered a parallel thread and should be accompanied with a sealing compound to provide a leak resistant connection.

UNEF (Unified National Extra Fine) threads are similar to UNF threads but with a tighter profile. These are ideal for tight tolerance applications and allow for finer tension adjustment than UNF threaded fittings. Like UNF threads, UNEF threads are parallel and require a sealing compound to create a leak resistant connection.

ISO Metric (International Organization for Standardization) threads are one of the most widely used metric threads. These self‑sealing, tapered threads are one of the first standards originally agreed upon when the International Organization for Standardization was created.

BSF (British Standard Fine) was introduced in 1908 by the British Engineering Standards Association. BSF threads share the same form as BSW (British Standard Whitworth) threads and is still used today throughout Europe. BSF threads are a parallel thread that requires a sealing compound if leak proof connections are necessary.

BSPP (British Standard Pipe Parallel) is a coarse, Whitworth thread sometimes referred to as a BSPF (British Standard Pipe Fitting) thread. BSPP threads feature a parallel profile and is sometimes denoted with a "G" indicating a constant diameter. Both male and female threads are parallel and require a sealing compound for applications requiring leak proof connections.

BSPT (British standard Pipe Tapered) is British standard thread. These self‑sealing threads can be used in low pressure applications for plumbing. While the male threads with this designation are always tapered, the female threads are typically parallel. BSPT threads are mostly used throughout Europe and the UK.

German Thread pipe threads are a DIN standard thread established in Germany after the introduction of BSPT threads. Even though it identifies by a different name, it possesses all of the same attributes as BSPT threads. Like BSPT threads, German threads are also tapered to provide an interference fit to reduce the risk of spiral leakage.

Common Grease Fitting Thread Types

Common Thread Types

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