Welcome to vulcanhammer.info, the site about Vulcan Iron Works, which manufactured the durable air/steam line of pile driving equipment for more than a century. Many of its products are still in service today, providing reliable performance all over the world. There’s a lot here, use the search box below if you’re having trouble finding something. Also look at the end of an article, there are helpful links to more information with every post.
An Overview of Tapered Pipe Threads, and Their Application at Vulcan
It’s hard to imagine that much of our technology is underpinned by very old, basic standards that year after year simply “do their job” without much regard. One of those is tapered pipe threads. This is a brief overview of same, and specifically the “National Pipe Taper” or NPT threads. Much of this material comes from the American Machinists’ Handbook by Fred Colvin and Frank Stanley, Second Edition (1914).
Most screw threads are “straight threads,” i.e., the diameters of the threads (outside, pitch, inside) are constant along the length of the threads. Tapered threads by definition can only work for a limited length, but when pipes are connected, that’s fine. Like any other taper lock, tapered threads have an additional wedge effect, which means that they can seal fluids in the pipe (or outside of it.)
Originally these pipe threads were referred to as “Briggs standard threads” after Robert Briggs who came up with them. In 1886 these were adopted as a standard by the American Society of Mechanical Engineers and various manufacturers. They have varied little since that time. They have been a durable standard for leak-resistant, permanent (and semi-permanent) connections ever since.
An overview of the “Briggs standard thread” is below.
As noted above, only the “perfect” threads (in one way or another) contribute to the sealing/joining of the pipe thread.
The overall dimensions of the various sizes of tapered pipe threads are shown below, with a diagram showing the types of gauges used to check the threads.
The tapered reamer was one item Vulcan seldom used; the usual procedure was to tap drill the hole and then use a tap for the threads in question to put the threads in the hole. Below are some tap sizes for NPT (National Pipe Taper, or Briggs) threads.
A more detailed treatment of the threads as the pipe and hole threads interface is shown below.
The pipe taper standard was wildly successful, and is used in everything from home plumbing to high-pressure hydraulics. In the oilfield the standard was so successful that it’s widely used even in places where metric standards are the norm
As far as Vulcan is concerned, Vulcan used the standard in many of its products, both the air/steam hammers and later the hydraulic vibratory hammers, where they were used for pressures up to 5000 psi. This was due to their durability, ability to resist vibration (a must with any Vulcan product) and their flexibility in radial orientation. With a pipe thread there is a point where it’s “tight” but it can generally be tightened a little further, thus allowing some flexibility in the orientation of parts. One thing Vulcan learned with pipe threads was, although they are designed to seal with their taper, the use of some kind of “pipe dope” or sealant is very important.
Below are some applications of pipe threads in Vulcan hammers.