Edited by: Michelle DiFrangia • Assistant Editor
Proper component selection is critical to design a safe hydraulic system. Although couplings are a small component, they play a major role in hose assembly safety. A hose end coupling connects the hose with a port, adapter, another coupling, or attachment such as a quick disconnect. A mismatched coupling could spell disaster; therefore, it is imperative that the coupling selected is compatible and approved for use with a given hose. STAMPED, an acronym for the seven major information areas required to provide a quality hose assembly, is an excellent resource in this matter.
Choosing the proper stem and ferrule depends on the specific hose and terminal end being used in the assembly. When using two-piece couplings, it is important to match the ferrule with its appropriate stem and hose. Ensure operations run smoothly by checking the manufacturer’s recommendations and crimp specifications. Keep in mind that hose from one manufacturer is not usually compatible with couplings from another. To avoid potential injury and damage, do not intermix hose and fittings from two manufacturers without prior approval from both parties. Hydraulic fluid leaks can significantly decrease the effectiveness of the system, and pressurized fluid can cause major injection injuries to users.
Follow these parameters when determining coupling selection:
- size of hose it will attach to
- type of hose it will attach to
- terminal end connection—typically threaded or flanged
- coupling type (permanent/field or attachable/clamped)
- coupling configuration (straight, angled, drop dimensions)
- system operating pressure
Properly identifying thread measurement is also key to maintaining a safe system. The proliferation of threads in today’s market makes it easy to mismatch them, resulting in leakage and possibly serious accidents. Correct thread identification begins with the right tools and reference materials, such as a coupling suppliers catalog. The recommended tools are as follows:
- thread pitch gauge
- I.D./O.D. caliper
- seat angle gauge
- reference materials showing different threaded end charts
There are three basic steps to identify the correct coupling/end connection: measure the thread pitch, measure the thread diameter and determine the seating angle, if applicable.
Start by using the thread pitch gauge to determine the number of threads per inch (distance between two threads), trying different thread gauges for the tightest fit. Engage as many threads as possible—the more threads engaged, the more accurate the reading. Hold the fitting and thread gauge up to the light and look for gaps or light appearing between the gauge and threads. Best safety practices call for a tight fit with minimal light coming through. However, take care not to over tighten, as this can distort the threads too much and create a path for leakage around the threads.
The I.D./O.D. caliper is used to measure thread diameter. Male thread diameter is measured on the outside diameter, while female thread diameter is measured on the inside. When measuring male thread, hold the caliper at a slight angle for the most accurate reading; hold it perpendicular to the thread for female. If the connection seals on a flared surface or on an inverted angle seat, the next step is to determine that angle of seal. Place the seat angle gauge on the sealing surface (insert the gauge into the connection first if female). If the centerlines of the connection and gauge are parallel, the correct angle has been determined.
It is also important to keep in mind that the hose assembly (coupling and hose) is only one component of the system. In choosing the correct end terminations for the couplings attached to the hose, formal design standards and sound engineering judgement should be used. If you have questions about what end fittings to use, defer to your product safety guide or NAHAD Hose Safety Institute Distributor.
Special thanks to the NAHAD Hose Safety Institute for providing the material for this article. Information was taken from the Handbook for the Design and Specification of Safe Hose Assemblies.
Filed Under: Fittings, Couplings & Adapters, Fluid Power World Magazine Articles, Hose Assembly Tips