To say that hose is an important part of a hydraulic system is a huge understatement. Hydraulics have become progressively more complicated. Pressures keep getting higher while hose dimensions become smaller so they can be routed into increasingly tighter spaces. Suppliers offer hundreds of different types and styles of hydraulic hoses and thousands of different fittings to choose from. When it comes time to replace a hydraulic hose assembly, how do you know which hoses and fittings are the right ones? Read on to learn the most essential tips to consider when building a hose assembly.
The fluid power industry measures hydraulic hoses by the inside diameter (I.D.) of the hose. This is also known as the dash size and it is measured in sixteenths of an inch. The inside diameter can be measured manually or it can be found on the layline of the hose. The I.D. is needed to determine the proper flow velocity for the system. A flow that is too high will cause pressure drops, system damage, and hose leakage, while a flow that is too slow will cause sluggish performance. Our flow capacity nomogram helps determine the proper hose I.D. for your application. Parker’s Hose Products Division offers hoses in sizes -4 through -40 to fit a variety of flow rates.
When determining which hose is right for your application, temperature is a critical factor. There are two aspects of temperature to look at: ambient and media temperature. Ambient temperature is the temperature outside of the hose, while media temperature is the temperature of the media being carried through the hose. The temperature rating of the hose should exceed the higher of the two temperatures of the application. Also consider the media type when selecting a hose. Some media types can increase or decrease the effects of temperature. For this reason, some Parker hoses have different temperature ratings for different fluids. Parker offers high- and low-temperature hoses for specific applications.
When selecting a hose, it is important to consider how the hose will be used. To help with this, start by answering a few questions:
Industry standards set specific requirements involving construction type, size, tolerances, burst pressure, and impulse cycles of hoses. Parker strives to meet or exceed standards such as:
Parker products also adhere to the requirements set by Government agencies to regulate certain standards. These agencies regulate standards for industries such as the U.S. Coast Guard and ABS. Hose selection must meet the legal requirements as well as the functional requirements of the application.
The minimum bend radius of a hose refers to the minimum radius that the hose may be bent through while operating at the maximum allowable published working pressure. Bending radius provides enhanced flexibility and eases routing. However, bending the hose below the minimum bending radius leads to loss of mechanical strength and hence possible hose failure. A minimum straight length of 1.5 times the hose’s outside diameter (OD) shall be allowed between the hose fitting and the point at which the bend starts. For tight bend radius applications, Parker offers two hoses, 787 and 797 that have half the bend radius of a standard SAE 100R15 hose.
When selecting a hose, you need to consider external conditions for an application. Is there risk of abrasion? Most hose has a neoprene outside, but if you have hoses that will inevitably rub against materials, against machinery, against the jib or boom, any application where it can rub back and forth, a high abrasion carcass is a good way to go. Hoses tend to fail by rubbing on the outside until they wear through the outer wall into the braiding and start wearing that away, and eventually, enough so that the hose can no longer contain pressure. At that point the hose will burst. External abrasion is probably the number one cause of hose failure. Parker recommends using Tough Cover (TC) for harsh environments and SuperTough (ST) for the extremely abrasive environments.
Hoses should be selected with media compatibility in mind. The media being conveyed through the hose should not only be compatible with the inner tube, but also the, cover, fittings, and O-rings as well. Incompatible media will cause several problems to hose assemblies and hydraulic systems. In order to prevent hose failures, check hose compatibility by using our chemical resistance chart.
When considering hose pressure, be sure to consider the system working pressure as well as any surge or spike pressures. These pressures should be below the rated maximum working pressure of the hose. Although Parker hydraulic hoses have been tested to meet a 4:1 pressure safety factor, they are not intended to be used at higher than published working pressures.
It is important to note, when considering hose assembly pressure, the maximum working pressure of the system is the lowest rated piece of the assembly. For example, if the fittings are rated to a lesser pressure than the hose, the maximum working pressure of the entire assembly is the working pressure of the fittings.
Hose pressures can be found on the engineering specifications for each hose as well as our hose overview chart and the hose laylines. To find the pressure rating for fittings, please see the Pressure Rating of Hose End Connectors chart.
The fitting-to-hose mechanical interface must be compatible with the hose selected. The proper mating thread end must be chosen so that connection of the mating components will result in leak-free sealing.
There are two general categories of fittings to connect most types of hose: the permanent type (used primarily by equipment manufacturers, large-scale rebuilders, and maintenance shops) and the field-attachable type.
Another component of fitting selection is thread type. For hydraulic hose assemblies, there are several different types of thread types:
At the end of the day, all fittings are only as good as assembler who installs them. There are proper assembly procedures for each of the fitting styles, and strict adherence to those steps makes the difference between a solid connection and a problem waiting to happen. To assure the correct identification, the threads must be measured and compared to the tables listed here.
Parker Hannifin has some great resources for hydraulic hose, fittings and equipment including online literature that outlines the proper hose assembly techniques. The Parker HoseFinder mobile app is also a useful tool for on-the-go assistance for choosing the right hose and fittings for any application.
Article contributed by Kyri McDonough, marketing services manager at Hose Products Division, Parker Hannifin.
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