When working with hydraulics there is always a risk of hose leakage. This issue generates direct costs such as the oil lost to hose leaks, but also indirect safety risk costs, such as slip hazards, failed fittings under pressure and injection injuries. Leaking fittings can also affect machine performance, especially in clamping and load holding applications. In order to prevent hose leakage, there are four key factors that must be performed correctly.
Any hose is only as good as the procedure used to assemble it. Many hoses fail within the first seconds of being pressurized. There are several critical factors to consider when assembling hose, including:
Is the hose pushed all the way into the fitting?
Is the shell compressed correctly to get a proper bite on the hose? A shell that is crimped too much can damage reinforcements, and a shell that is not crimped enough can let loose.
Correct Hose and Fitting Match
Do the series and brand of the hose and fittings match? It is incredibly dangerous to mix and match hose and fittings from different manufacturers, or different series from one manufacturer. These combinations are not tested or designed to work together, and can lead to catastrophic failure.
Is the hose length correct? Incorrect length on assemblies can lead to mechanically stretching hose, which can stress reinforcements. Additional length on hoses can expose them to getting caught in moving parts of your machines.
Angles of Orientation
If there are angle fittings on both ends of the hose, are they aligned correctly for the ports? Correct alignment is critical, because incorrect alignment can lead to torque pressure reinforcements, which can lead to premature hose failure. Misalignments of end fitting angles can also affect the natural bend of your hoses.
STAMP is a simple acronym that is used to ensure that the hoses you choose match their application requirements. You can read about the STAMP process in previous blog posts here.
The hoses that you choose must be the right size for your application. It is important to note that this is not as simple as matching the sizes of your port connections and your hoses must be sized to match flow requirements. Pressure lines can be smaller because they can accept a larger pressure drop. Losing 15psi on a 3,000psi pressure line might have a significant affect, but on a suction line, you could cavitate your pump. Hose manufacturers publish guidelines for pressure, return and suction applications; be sure to follow them closely.
Hoses are affected by both the operating temperature of the oil inside them, as well as ambient temperature. Special blends of rubber are manufactured to withstand temperatures up to 300°F, but as temperatures exceed 300°F, your application will require special materials such as PTFE and stainless steel. Similarly, low-temperature hoses are designed for use in cold climates and refrigerated warehouses.
The applications in which hoses will be used must be considered in order to make the proper choice. Application considerations can include high or low impulse cycles, regulatory compliance (such as DOT, CSA or Coast Guard approvals) and proximity to workers.
Oil and other fluids conveyed by your hoses must also be compatible with your hoses’ inner tubes, fittings and covers. Certain oils, such as Skydrol, can attack standard rubber compounds leading to hose failure and detached rubber flowing downstream into your valves and pumps. Hose manufacturers publish compatibility guides for common fluids in their catalogs and on their websites.
Hoses are rated at both working pressures and burst pressures, with hydraulic hoses generally rated at a 4:1 safety factor between burst and working pressure. Hoses should be specified to operate within their working pressure rating, including spikes in your machine’s cycles.
When assembled properly and used in the correct applications, the correctly chosen hoses can still fail if they are not installed properly. Proper hose routing considers external stresses that can affect your hoses, including:
Hoses can be clamped in order to keep them away from moving components, to allow moving actuators from torqueing hoses and to increase the bend radius of an application. Hose clamps are available to mount to Unistrut, DIN rail and with weld plates. Unlike tube clamps that have ridges to aggressively grab tubes, hose clamps are smooth to keep from damaging the hose cover.
When hoses are routed along machinery, there is an increased risk that they will rub against sharp edges or other hoses. This can lead to abrasion damage to your hose covers, which expose reinforcements and eventually lead to hose failure. In order to prevent against abrasion issues, you can employ the use of tough cover hoses, spring guards, abrasion sleeves and plastic guards. These solutions should be used when proper routing cannot protect the hose.
Hoses are designed with bend radius specifications. Bend radii specify the tightest loop in which hoses can be used. When bent beyond their radii, your reinforcements can be damaged, inner tubes can collapse or your hoses can fail. This is especially critical at fitting crimps, after which a minimum distance of your hoses must remain straight. Fitting crimps are the most common site for hose failures due to excess stress placed on the hose.
As machines are used, original routing and conditions of its hoses deteriorate. It is therefore critical to maintain the care and inspection of your hoses throughout their lifecycles.
Hose cover wear indicates that your inner reinforcements are exposed and need to be replaced. Hoses should also be inspected for rust at their fittings and signs of damage such as flattening and torqueing. Additionally, when inspecting hoses, inspectors must take special care to not expose any parts of their bodies. Injection injuries are often suffered when a hand is used to find leaks in a hydraulic hose, and can result in tissue damage, amputation or death. For more information on injection injuries, read Parker Hannifin’s Safety Solutions guide to injection injuries.
Many manufacturing facilities have instituted hose replacement schedules in order to replace hoses before they fail. This is accomplished by tagging hoses with date codes and replacing them at set intervals. The number of years between replacements is set based on the severity of the application and critical costs of failure.
Overall, the proper use of hydraulic hoses allow for machines with incredible power and mobility. However, these hoses present real dangers if they are not made, used and maintained properly. Exercise care when working with hydraulic hose, and be sure to confirm the parameters of your application with a qualified expert.
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