Hydraulic Fittings: Threads
In this lesson we will describe threads and their components using the correct vocabulary, measure unknown fitting or port threads and classify them correctly, and explain best practices and common mistakes in connecting threaded fittings.
The Problem
A directional control valve has arrived on your test bench. You need to connect a flow meter to conduct a test, but there's a problem; which one of the fittings should you use to attach it?
Luckily, someone has taken the time to label the fittings, but without knowing which thread is used in the valve ports, how do you know which fitting to use?
The "Will It Fit?" Game
It's really tempting to start grabbing fittings from the box, and just start trying them in the port, right?
This is a terrible plan - do not do this.
Even though threads are machined metal, they are still quite delicate, and can be easily compromised. If you ruin the threads inside of this port, and that's easier to do than you might think, you'll be buying a new valve.
The only way to find the correct fitting is to measure the threads in the port in order to learn what they are, and what they require.
You may achieve a seal, but the threads will always be damaged.
If you ever need to take the connection apart, you may be unable to reconnect it again. You also have an unsafe situation where the mismatched fitting may separate under pressure.
Tools
You're going to need some tools. Some fittings manufacturers sell thread identification kits, which include thread gauges, thread profiles and calipers, as well as identification charts for some of the more common thread sizes and styles.
You'll need some version of these tools to proceed.
Vocabulary
Caps and plugs have a single set of threads. Adapters and nipples may have a different thread on each side. You'll need to carefully identify each set of threads.
Close-Up
Let's take a closer look at the threads, themselves. There are a few more terms that you will need to know to discuss threads with accuracy; crest, root, flank, complete and incomplete threads.
Standards
It will quickly become clear that there are a lot of different fitting standards in the world. This module will concentrate on some of the most common standards used in fluid power applications: UN/UNF, NPT/NPTF, SAE (ORB), BSPP (G-), BSPT, Metric Parallel and Metric Taper.
Label It
How to Identify Threads
It's time to start taking measurements.
Step 1: Parallel Or Tapered?
Are the threads parallel, or tapered?
If It's Obvious...
You might be able to detect an obvious taper (such as 1:16) by comparing the threads against a straight edge. If you're not sure, or if you are trying to identify female threads, get out your calipers.
What If It's Not Obvious?
Measure the crest of the first, fourth, and last full threads - if all three measurements are the same, you know it's parallel. Otherwise, if your measurements are progressively growing (male) or shrinking (female fitting), it must be tapered.
Using the calipers, you record 0.763", 0.743", and 0.687".
Since these measurements shrink, the port threads must be tapered.
Try It!
17/32"
and 1.056"
and 69/128"
Step 2: Determine Thread Pitch
Thread pitch can be defined in a couple of different ways. If you are dealing with a metric thread, thread pitch is the distance from one thread peak to the next, in millimeters. In the example shown here, this metric bolt has a pitch of 2 mm.
If you are working with non-metric threads, pitch is stated as the number of threads within 1 inch. The cap shown here has a pitch of 10 threads per inch.
Counting threads can be time consuming; use the pitch gauges that come in your thread identification kit. Be particular about the fit between the threads and the form; some of the forms are very similar; however, the angle of the threads flanks change between some standards. For example, NPT/NPTF thread flanks are at a 60° angle, while BSPP/BSPT thread flanks are at 55°. The difference is slight, but more than enough to compromise a seal.
Take a little time and make sure you get the right fit against the form. This step will also tell you which family the threads belong to.
A screw pitch (NPT) form of 14 threads per inch fits the valve threads correctly.
Try It!
Find the pitch of each set of threads by matching it with the correct form.
Step 3: Gauge Thread Size
This step will change, depending on which family the threads belong to.
You have already established that this port's threads are NPT.
Measuring Tapered Threads
When measuring with calipers, the standard convention is to measure the fourth or fifth complete thread on a taper. (Obviously, the value should be the same on parallel threads, regardless of where you measure.)
BSPP, BSPT or NPT/NPTF
The diameter of these thread standards use the nominal (or average) size of the threads. This means that measuring the major diameter (the crests of the threads) will not give you the correct measurement.
Male Threads
Use a thread profile, or use a size chart to translate the measurement to nominal size if it is a male thread.
Female Threads
If you are measuring female threads, as in our case, you can subtract ¼" from the inside diameter measurement to get the nominal value.
UN/UNF or Metric
Use calipers to measure the major diameter of UN/UNF or Metric threads. No further work is needed; that is the correct thread size.
Measuring with calipers and subtracting ¼" puts this port at ½ inch.
Step 4: Put It Together to Define the Thread
The nominal thread size (obtained from using the calipers and subtracting ¼") at the fourth complete thread) is ½", and the pitch was gauged at 14 threads per inch. Using the fitting labeled ½ - 14 NPT will be correct. Problem solved!
Thread Standards
There are numerous standards of threads throughout the world, including SAE, Metric, JIS, and more, as well as NPT/NPTF and BSPP/BSPT. Sometimes the threads from two standards can have the same diameter and pitch.
For example, tapered threads with a ½" diameter and 14 threads per inch could belong to either the NPTF or the BSPT standard. So a male and female fitting of this size from each standard should be able to create a reliable seal, right?
| Nominal Pipe Size | SAE Dash Size | NPT/NPTF Thread |
|---|---|---|
| 1/8 | -2 | 1/8-27 |
| 1/4 | -4 | 1/4-18 |
| 3/8 | -6 | 3/8-18 |
| 1/2 | -8 | 1/2-14 |
| 3/4 | -12 | 3/4-14 |
| 1 | -16 | 1-11 1/2 |
| 1 1/4 | -20 | 1 1/4-11 1/2 |
| 1 1/2 | -24 | 1 1/2- 11 1/2 |
| 2 | -32 | 2-11 1/2 |
| Nominal Size | BSPP Thread Size | BSPT Thread Size |
|---|---|---|
| 1/8 | G 1/8-28 | R 1/8-28 |
| 1/4 | G 1/4-19 | R 1/4-19 |
| 3/8 | G 3/8-19 | R 3/8-19 |
| 1/2 | G 1/2-14 | R 1/2-14 |
| 3/4 | G 3/4-14 | R 3/4-14 |
| 1 | G 1-11 | R 1-11 |
| 1-1/4 | G 1 1/4 -11 | R 1 1/4 -11 |
| 1-1/2 | G 1-1/2 -11 | R 1-1/2 -11 |
| 2 | G 2-11 | R 2-11 |
There is one more factor to consider, and that's the angle of the thread flanks. In our NPT/BSPT example, while the size and pitch are indeed the same, the angle of the thread flanks is not. The NPT angle is 60°, while the BSPT angle is 55°. You could force these fittings together without too much extra torque, but the seal may not be reliable, and you will probably damage one or both sets of threads.
Best Practices
Calling Out
When you are discussing threads, it is always best to describe each measurement, rather than just the numbers. For example, say, "I need a fitting with a 1 inch pipe size, and 18 threads per inch, NPT-type"; rather than, "Hand me a male 1-18 fitting, please." Using the longer form will avoid the chance of a mistake by the speaker, or the listener.
To Seal, or Not to Seal?
Sealant should always be used with tapered threads, except for NPTF. Sealant should never be used with parallel threads. The reason for this is the difference in how the two thread types seal.
Parallel Threads
Parallel threads do not provide a seal; sealing must be done by another face on the fitting or cap. Parallel fittings incorporate an o-ring, a gasket, or an additional flat sealing surface. If you add sealing tape to this connection, it can be similar to trying to couple two unmatched thread sets. The extra bulk of the tape can warp and damage the delicate threads, and it isn't helpful anyway.
Note the small gap that allows oil to seep out. This path spirals around the fitting.
Tapered Threads
Tapered threads are a different story. The flanks of the threads provide the seal, but there is a spiral that follows the thread around the fitting that is not sealed. (This is because no two thread sets will ever be perfectly matched; there will always be slight imperfections between the male and female pieces.) Tape or liquid sealant is not only helpful, but necessary to fill this spiral, and prevent leakage. Learn to use tape and sealant judiciously; the spiral provides a small cavity for the extra sealant material, but too much can still damage threads.
NPTF Threads
NPTF is in a category by itself. It is a tapered thread, but it has been designed so that the thread crests fit tightly into the opposing roots, to eliminate the spiral leakage path. This design causes the threads to deform in order to create this tight fit, so NPTF fittings may not be appropriate for re-use in your industry. (Check with your supervisor to find out.) When threading two NPTF fittings together, sealant is not necessary.
Other than this slight difference, NPTF threads are manufactured to the same standards as NPT threads, and are considered semi-compatible. They may be able to be threaded together with the use of sealant.
Review
Which statements about threads and fittings are true?
Recap
- Always identify mystery threads before attempting to use them. You can ruin expensive components by forcing the wrong connection.
- Threads are identified by their standard, diameter, and pitch.
- There are multiple thread standards; knowing what you're dealing with is an important part of the thread identification process.
- Sealant should always be used with tapered threads, except for NPTF, and never with parallel.
- Always use fully descriptive terms when calling out a thread. There are too many ways for shorthand to be misinterpreted.