Always think “Safety First!” when working with hydraulic systems and hoses. The Proper Line sizing and thread selection will prevent leaks from occurring.
Several years ago, a farmer died when a cultivator wing fell on him because fluid had leaked from a hydraulic cylinder. Under normal operating conditions, the cylinder would have held pressure and prevented the wing from dropping as the farmer released the transport locks. When the fluid leak occurred, the cylinder lost pressure and could not support the load.
This accident underlines the importance of safety precautions, preventive maintenance, and equipment inspections. High pressures and temperatures seen with hydraulic systems can cause pinhole leaks to appear, an incorrectly installed coupling to blow off, or a loose hose to whip. Leaks occur because of high operating pressure, pressure spikes, corrosion on components and fittings, vibration, and improper installation of fittings that damaged threads or the incorrect selection of a hose or fitting for the application.
Part of establishing a safe work environment involves selecting the proper line sizing and thread selection. Systems may use rigid tubing for longer runs, hoses for moving parts, and end fittings designed for particular applications. Selecting the proper hose and fittings for a hydraulic system involves following the STAMPED criteria.
S.T.A.M.P.E.D.: Size, Temperature, Application, Material Conveyed, Pressure, Ends, Delivery.
What is the S.T.A.M.P.E.D. criteria?
A slow flow velocity causes sluggish system performance while a high flow velocity may cause pressure drops and leaks. A hose must have the proper inside diameter or I.D. to maintain the correct flow velocity. The fluids industry uses a sizing standard called the Dash Size designates the internal diameter of a hose in sixteenths of an inch. For example, a Dash Size of -3 represents a hose that has a 3/16” internal diameter. Manufacturers will provide the specific hose size needed for an application.
We observe the ambient—or outside temperature and the media temperature when selecting a hose. A high or low ambient temperature may cause premature hose degradation. However, the greatest stress on a hose occurs through the temperature of the media flowing through the hose. Media temperature ratings for hoses depend on the type of media used in the system. A hose may have a high temperature rating when carrying petroleum-based fluids but lower temperature ratings when carrying water-based fluids or air.
The application for the system involves the type and amount of media delivered throughout the system and the amount of pressure carried by the system. Each of these factors impacts the selection of the hose and fittings. While the Society for Automotive Engineers, or SAE, establishes the standard for hydraulic hoses and fittings, the fluids industry also applies strict standards for the construction type, size, tolerance, burst pressure, and impulse cycles of rigid tubing and hoses.
A hydraulic hose end fitting consists of a coupling that attaches to the line and a threaded end that attaches to the component port. Permanent couplings require crimping or swaging equipment for connection to the line while different types of field attachable fittings are available.
Selecting the correct end fittings and threads depends on the application and the line size. Manufacturers use the Dash size to indicate the outside diameter of fittings. In addition, never mix and match fittings and lines from different manufacturers. North American standards for hydraulic end fittings include the National Pipe Tapered for Fuels (NPTF), National Pipe Straight for Fuels (NPTS), and National Pipe Straight for Mechanical (NPSM). Internationally-produced equipment may use thread standards based on the country of origin.