How to identify hydraulic threads

Different hydraulic systems use different types of fittings and adaptors, and it can be difficult to identify the different thread forms used. The sealing methods can be identified by appearance, but unfortunately thread forms all look the same, making it difficult and time consuming to figure out which is which when it is time for maintenance and repairs. It is critical to know the correct thread when selecting the right replacement parts as choosing the wrong one can cause problems with the pressure holding capacity and seal reliability of the fitting or adapter.

Six types of threads are commonly used for hydraulic tube fittings: UN/UNF, NPT/NPTF, BSPP (BSP, Parallel), BSPT (BSP, Tapered) Metric Parallel and Metric Tapered. To be able to identify which thread is which fours steps can be undertaken:

1: Determine if the thread is tapered or parallel – this can be done through visual inspection. Tapered threads get smaller in diameter towards the end of the fitting while parallel threads maintain the same diameter throughout. If this cannot be done through visual inspection a calliper can be used to make the comparison. NPT/NPTF and BSPT are tapered threads and UN/UNF and BSPP are parallel.

2: Determine the pitch by using a pitch gage for comparison or you can measure and calculate the number of the threads within a given distance. It is easier to determine the pitch against a lighted background with a pitch gage. Due the most pitches being similar, try a few pitch gages before making your final decision.

3: Determining the size of the thread can be done by combining the results of step 1 and 2. There are two methods for determining the thread size:

  1. Compare it with a normal size profile (pipe sizes up to a 2-inch nominal size can be determined by measuring the actual outside diameter, subtracting a quarter inch, then rounding off).
  2. For non-pipe thread, the actual size can be determined by measuring the outside diameter with a calliper.

 4: Designate and define the thread in an industry standard format. This will allow others to understand what type of thread it is.  This will give an indication of the thread size (whether normal or actual), the type and the pitch. Using these four steps a company will be able organise maintenance when it is needed and identify the types of thread without any problem.

Power Team SA is on hand to help with any hydraulic needs. Through years of experience within the industry, Power Team SA understands the importance of having the correct components to complete the job, and that is why they are able to provide expert solutions to all hydraulic queries, including identifying hydraulic thread.